Chapter 5 Ballistic Missile Defense Technology: Weapons

Chapter 5

Ballistic Missile DefenseTechnology: Weapons, Power,

Communications, andSpace Transportation

CONTENTSPage

I n t r o d u c t i o n . . . . . . . . . . . . . . . . . . . . . . . 1 0 5W e a p o n s . . . . . . . . . . . . . . . . . . . , . . . . . . 1 0 5

Kinetic-Energy Weapons (KEW) ....106Directed-Energy Weapons . .........123

Power and Power Conditioning . ......142Space Power Requirements . ........142Space Power Generation Technology .143Power Conditioning . . . . . . . . . . . . .145

Communication Technology . .........14660-GHz Communication Links .. ....147Laser Communication Links . .......148

Space Transportation . ..............148Space Transportation

Requirements. . . . . . . . . . . . . . . . . . .149Space Transportation Alternatives ...149Space Transportation Cost

Reduction . . . . . . ................153Conclusions . . . . . . . . . . . . . . . . . . . . . . . .153

Weapon Technology Conclusions ....153Space Power Conclusions. . .........155Space Communications Conclusion. ..l55Space Transportation Conclusions ...156

FiguresFigure No. Page

5-1.5-2a.

5-2b.

5-2c.

5-2d.

5-3a.

5-3b.

5-4.

5-5.

5-6a,

Orientation of SDI to RV .. ....111Space-Based Interceptor Mass v.V e l o c i t y . . . . . . . . . . . . . . . . . . . . . 1 1 4Number of Satellites v. SBIV e l o c i t y . . . . . . . . . . . . . . . . . . . . . 1 1 4Number of Space-BasedInterceptors v. Velocity.. .. ....114Constellation Mass v. SBIV e l o c i t y . . . . . . . . . . . . . . . . . . . . . 1 1 4Number of Projectiles v. SBIV e l o c i t y . . . . . . . . . . . . . . . . . . . . . 1 1 5SBI Constellation Mass v. SBIV e l o c i t y . . . . . . . . . . . . . . . 4 . . . . . 1 1 5Total SBI Constellation Mass inOrbit v. Booster Bum Time ....116Boost and Post-Boost KillEffectiveness (l,400 ICBMs v.“Realistic ’ ’ SBIs) . . . . . . . . . . . . .116Boost and Post-Boost KillEffectiveness (500 single-RVICBMs at three sites)... . ......117

5-6b.

5-7.

5-8.

5-9.5-10.

5-11.5-12.

5-13a.5-13b.

Table5-1.

5-2.

5-3.

5-4.

5-5.

5-6.

5-7.

5-8.5-9.

5-10.

Boost and Post-Boost KillEffectiveness (500 single-RVICBMs atone site) . ...........117Boost and Post-Boost KillEffectiveness (200 “medium-bum-booster” ICBMs at onesite). . . . . . . . . . . . .............118Schematic of an ElectromagneticLauncher (EML) or “Railgun” ..119Lightweight Homing Projectile. .l2lIllustration of the RelationshipsBetween Laser Parameters andPower Density Projected on aTarget . . . . . . . . . . .............125FEL Waveforms . . . . . . . . . . . . . .125Schematic of a Neutral ParticleBeam Weapon.. . . . . . . . . . . . . . .129Annual Space Launch Capacity .152Space Transportation . .........152

TablesNo. PageKey Issues for ElectromagneticLaunchers (EML) . . . . . . . . . . . . . .120Characteristics of a Ground-BasedFEL Weapons System . .........124Characteristics of an HF LaserWeapons System. . .............127Characteristics of Directed EnergyWeapons Against a FullyResponsive Soviet Threat . ......130Possible Beam Steering andRetargeting Requirements forBoost-Phase Engagement . ......134Key Issues for Free ElectronLasers (FEL) . . . . . . . . . . . . . . . . . .136Key Issues for the HF ChemicalLaser . . . . . . . . . . ...............138Neutral Particle Beam Issues ....139Estimated Power Requirementsfor Space Assets . ..............142Current U.S. Space LaunchInventory . . . . . . . ..............150

BallisticWeapons,

Chapter 5

Missile Defense Technology:Power, Communications, and

Space Transportation

INTRODUCTION

This chapter reviews weapon technologies weapons, power systems, and communicationrelevant to ballistic missile defense (BMD). It systems of most interest for the Strategic De-emphasizes the chemically propelled hit-to-kill fense Initiative (SD I). Finally, it considers theweapons most likely to form the basis of any new space transportation system essential forfuture U.S BMD deployment in this century. a space-based defense.The chapter also covers the directed-energy

WEAPONS

A weapon system must transfer a lethal doseof energy from weapon to a target. All exist-ing weapons use some combination of kineticenergy (the energy of motion of a bullet, forexample), chemical energy, or nuclear energyto disable the target. The SDI research pro-gram is exploring two major new types ofweapon systems: directed-energy weapons andultra-high accuracy and high velocity hit-to-kill weapons. Not only have these weaponsnever been built before, but no weapon of anytype has been based in space. Operating manyhundreds or thousands of autonomous weap-ons platforms in space would itself be a majortechnical challenge.

Directed-energy weapons (DEW) would killtheir prey without a projectile. Energy wouldtravel through space via a laser beam or astream of atomic or sub-atomic particles. Speedis the main virtue. A laser could attack an ob-ject 1,000 km away in 3 thousandths of a sec-ond, while a high-speed rifle-bullet, for exam-ple, would have to be fired 16 minutes beforeimpact with such a distant target. Clearly,

Note: Complete defi”m”tions of acronyms and im”tial.z”smsare h“sted in Appen&”x B of this report.

DEW, if they reach the necessary power levels,would revolutionize ballistic missile defense.

DEWS offer the ultimate in delivery speed.But they are not likely to have sufficient de-ployed power in this century to destroy ballis-tic missiles, and they certainly could not killthe more durable reentry vehicles (RVs). Inhopes of designing a system deployable beforethe year 2000, the SDI research program hasemphasized increased speed and accuracy forthe more conventional kinetic-energy weapons(KEW), such as chemically propelled rockets.With speeds in the 4 to 7 km/s range, and withterminal or homing guidance to collide directlywith the target, these KEW could kill a sig-nificant number of today’s ballistic missiles.With sufficient accuracy, they would not re-quire chemical or nuclear explosives.

Although DEWS will not be available forhighly effective ballistic missile defense dur-ing this century, they could play a significantrole in an early 1990s decision on whether todeploy any ballistic missile defense system.That is, the deployment decision could hingeon our ability to persuade the Soviets (and our-selves) that defenses would remain viable forthe foreseeable future. Kinetic-energy weapons

105

106

work initially against the 1990s Soviet missilethreat. But Soviet responsive countermeasuresmight soon render those weapons ineffective.Thus, a long-term commitment to a ballisticmissile defense system would imply strong con-fidence that new developments, such as evolv-ing DEW or evolving discrimination capabil-ity, could overcome and keep ahead of anyreasonable Soviet response.

Strategic Defense Initiative Organization(SDIO) officials argue that perceived future ca-pabilities of DEW might deter the SovietUnion from embarking on a costly defensecountermeasures building program; instead,the prospect of offensive capabilities might per-suade them to join with the United States inreducing offensive ballistic missiles and mov-ing from an offense-dominated to a defense-dominated regime. To foster this dramatic shiftin strategic thinking, the evolving defensivesystem would have to appear less costly andmore effective than offensive countermeasures.

Today, the immaturity of DEW technologymakes any current judgments of its cost-effectiveness extremely uncertain. It appearsthat many years of research and developmentwould be necessary before anyone could statewith reasonable confidence whether effectiveDEW systems could be deployed at lower costthan responsive countermeasures. Given thecurrent state of the art in DEW systems, a well-informed decision in the mid-1990s to build anddeploy highly effective DEW weapons appearsunlikely.1

Kinetic-Energy Weapons (KEW)

Today’s chemically propelled rockets andsensors could not intercept intercontinentalballistic missiles (ICBMs) or reentry vehicles.-—— —

‘The Study Group of the American Physical Society concludedin their analysis of DEW that “even in the best of circumstances,a decade or more of intensive research would be required to pro-vide the technical knowledge needed for an informed decisionabout the potential effectiveness and survivability of DEW sys-tems, In addition, the important issues of overall system in-tegration and effectiveness depend critically upon informationthat, to our knowledge, does not yet exist.” See American Phys-ical Society, Science and Technology of Directed Energy Weap-ons: Report of the American Physical Society Study Group,April, 1987, p. 2.

(RVs) in space. No currently deployable projec-tile system has the accuracy or speed to con-sistently intercept an RV traveling at 7 km/sat ranges of hundreds or thousands of kilome-ters. The SAFEGUARD anti-ballistic missile(ABM) system built near Grand Forks, NorthDakota in the early 1970s, and the existing So-viet Galosh ABM system around Moscow bothwould compensate for the poor accuracy oftheir radar guidance systems by exploding nu-clear warheads. The radiation from that explo-sion would increase the lethal radius so thatthe interceptors, despite their poor accuracy,could disable incoming warheads.

The goal of the SDI, however, is primarilyto investigate technology for a non-nuclear de-fense. This would dictate the development of“smart” projectiles that could “see” their tar-gets or receive external guidance signals,changing course during flight to collide withthe targets.

The following sections discuss proposedKEW systems, KEW technologies, the currentstatus of technology, and key issues.

KEW Systems

Four different KEW systems were analyzedby SDI system architects, including space-based interceptors (SBIs, formerly calledspace-based kinetic kill vehicles or SBKKVs),and three ground-based systems. All four sys-tems would rely on chemically propelledrockets.

Space-Based Interceptors (SBIs).–Each sys-tem architect proposed-and the SDIO “phaseone’ proposal includes—deploying some typeof space-based projectile. These projectileswould ride on pre-positioned platforms in low-Earth orbits, low enough to reach existingICBM boosters before their engines wouldburn out, but high enough to improve the likeli-hood of surviving and to avoid atmosphericdrag over a nominal seven-year satellite life.The range of characteristics for proposed SBIsystems is summarized in the classified ver-sion of this report.

It would take a few thousand carrier satel-lites in nearly polar orbits at several hundred

107

km altitude to attack effectively a high per-centage of the mid-1990s Soviet ICBM threat.There was a wide range in the number of in-terceptor rockets proposed by system archi-tects, depending on the degree of redundancydeemed necessary for functional survivability,on the number of interceptors assigned toshoot down Soviet direct-ascent anti-satelliteweapons (ASATs), and on the leakage rates ac-cepted for the boost-phase defense.

In late 1986, the SDIO and its contractorsbegan to examine options for 1990s deploy-ment which would include constellations ofonly a few hundred carrier vehicles (CVs) anda few thousand SBIs. This evolved into thephase-one design which, if deployed in the midto late 1990s, could only attack a modest frac-tion of the existing Soviet ICBMs in theirboost and post-boost phases.

Exe-atmospheric Reentry Interceptor System(ERIS).–The ERIS would be a ground-basedrocket with the range to attack RVs in the latemidcourse phase. Existing, but upgraded, ra-

dars such as BMEWS, PAVE PAWS, and thePAR radar north of Grand Forks, North Da-kota might supply initial track coordinates toERIS interceptors.2 (These radars might be theonly sensors available for near-term deploy-ments.) Alternatively, new radars or opticalsensors would furnish the track data. Up-graded radars would have little discriminationcapability (unless the Soviets were to refrainfrom using penetration aids); moreover, a sin-gle high altitude nuclear explosion could de-grade or destroy them.

Optical sensors might reside on a fleet ofspace surveillance and tracking system (SSTS)satellites or on ground-based, pop-up probesbased at higher latitudes. Such sensors mightsupply early enough infrared (IR) track data

..—‘The range of planned ground-based radars such as the Ter-

minal Imaging Radar (TIR), which could discriminate RVS fromdecoys, might be too short to aid ERIS long-range intercep-tors; the TIR was planned for the lower HEDI endoatmosphericsystem. A longer-range Ground-based Radar (GBR) system hasalso been proposed. This system may be capable of supportingERIS interceptors.

Photo Credit: Lockheed Missiles and Space Company

How ERIS would work.—The ERIS vehicle would be launched from the ground and its sensors would acquire and tracka target at long range, ERIS would then maneuver to intercept the target’s path, demolishing it on impact.

— —

108


ERIS kill vehicle concept.—The Integrated AvionicsPackage (IAP) computer (top left) receives interceptorposition data from the on-board Inertial MeasurementUnit and target position data from the seeker, orinfrared sensor. The seeker acquires and tracks theincoming warhead. The IAP sends guidance com-mands to the two transverse and two lateral thrusters,which maneuver the vehicle to the impact point. Heli-um is used to pressurize the fuel tanks and also as apropellant for the attitude control system at the aftbulkhead. The lethality enhancement device woulddeploy just before impact to provide a larger hit area.

to take full advantage of the ERIS fly-outrange. 3 If deployed, an airborne optical system(AOS) could give some track data late in mid-course. None of these sensors has been built,although the AirborneOptical Adjunct (AOA),a potential precursor to the AOS airborne sys-tem, is under construction and will be testflown in the late 1980s.

Anon-board IR homing sensor would guidethe interceptor to a collision with the RV inthe last few seconds of flight. This homing sen-sor would derive from the Homing Overlay Ex-periment (HOE) sensor, which successfully in-tercepted a simulated Soviet RV over thePacific on the fourth attempt, in 1984.

No major improvements in rocket technol-ogy would be necessary to deploy an ERIS-like system, but cost would be an importantfactor. The Army’s Strategic Defense Com-mand proposes to reduce the size of the launchvehicle in steps. The Army has proposed—

Whe ERIS, as presently designed, requires a relatively hightarget position accuracy at hand-off from the sensor. The BSTSwould not be adequate for this.


ERIS Functional Test Validation (FTV) v. baseline ERISconcept.–Sizes of the FTV vehicle and baseline ERISconcepts are compared to a 6-foot-tall man. ERIS isdesigned as a ground-launched interceptor that woulddestroy a ballistic missile warhead in space. The FTVvehicle is 33 feet tall, large enough to carry both anobservational payload to observe the impact with thewarhead and the telemetry to relay information to theground during the flight tests. The baseline interceptorconcept is less than 14 feet tall, more compact becauseit will not require all the sensors and redundancies

that are demanded by flight tests.

partly to reduce costs–to-test this system witha Functional Technical Validation (FTV) rocketin 1990-91. This missile would have approxi-mately twice the height, 10 times the weight,and twice the burn time of the planned ERISrocket. The planned ERIS rocket system hasa target cost of $1 million to $2 million per in-tercept in large quantities. Research is proceed-ing with a view to possible deployment by themid-1990s.

Much development would be necessary toupgrade the experimental HOE kinetic kill ve-hicle technology for an operational ERIS in-terceptor. The IR sensors are being radiation-hardened. Since the operational sensor couldnot be maintained at the cryogenically low tem-peratures required for the HOE experiment,higher operating-temperature sensors are be-ing developed, with cool-down to occur afteralert or during rocket flight.

High Endo-atmospheric Defense Interceptor(HEDI).–The HEDI system would attack RVsthat survived earlier defensive layers ofground-based, high-velocity interceptor

109

rockets. HEDI would take advantage of thefact that the atmosphere would slow downlight-weight decoys more than the heavierRVs. Since it would operate in the atmosphere,HEDI might attack depressed trajectory sub-marine-launched ballistic missile (SLBM) war-heads that would under-fly boost and mid-course defensive layers—provided it receivedadequate warning and sensor data.

According to one plan, an AOS would trackthe RVs initially, after warning from the boost-phase surveillance and tracking system (BSTS)and possible designation by SSTS (if available).The AOS would hand target track informationoff to the ground-based terminal imaging ra-dar (TIR). The TIR would discriminate RVsfrom decoys both on shape (via doppler imag-ing) and on their lower deceleration (comparedto decoys) upon entering the atmosphere.Interceptors would attack the RVs at altitudesbetween 12 and 45 km. The HEDI system thuswould combine passive optics (IR signature),atmospheric deceleration, and active radar(shape) to distinguish RVs from decoys.

The penalty for waiting to accumulate thesedata on target characteristics would be theneed for a large, high-acceleration missile. TheHEDI would have to wait long enough to pro-vide good atmospheric discrimination, but notso long that a salvage-fused RV would deto-nate a nuclear explosion close to the ground.To accelerate rapidly, the HEDI 2-stage mis-sile must weigh about five to six times morethan the ERIS missile.

The key technology challenge for the HEDIsystem would be its IR homing sensor. Thisnon-nuclear, hit-to-kill vehicle would have toview the RV for the last few seconds of flightto steer a collision course.4 But very high ac-celeration up through the atmosphere wouldseverely heat the sensor window. This heatedwindow would then radiate energy back to theIR sensor, obscuring the RV target. In addi-tion, atmospheric turbulence in front of thewindow could further distort or deflect the RV

——4The HEDI interceptor would probably include an explosively

driven “lethality enhancer. ”

image. No sensor has been built before to oper-ate in this environment.

The proposed solution is to use a sapphirewindow bathed with a stream of cold nitrogengas. A shroud would protect the window untilthe last few seconds before impact. Since reen-try would heat the RV to temperatures abovethat of the cooled window, detection would bepossible. Recent testing gives grounds for op-timism in this area.

Fabrication of the sapphire windows (cur-rently 12 by 33 cm) would be a major effortfor the optics industry. These windows mustbe cut from crystal boules, which take manyweeks to grow. At current production rates,it would take 20 years to make 1,000 windows.Plans are to increase the manufacturing capa-bility significantly.

The HEDI sensor suite also uses a Nd:YAG5

laser for range finding. Building a laser rangerto withstand the high acceleration could bechallenging.

As with ERIS, plans call for testing a HEDIFunctional Technical Validation missile, whichis 2 to 3 times larger than the proposed opera-tional vehicle. The proposed specifications ofHEDI are found in the classified version of thisreport.

Flexible Light-Weight Agile Experiment(FLAGE).–The weapon system expected toevolve from FLAGE research would be the lastline of defense, intercepting any RVs whichleaked through all the other layers. Its primarymission would be the defense of military tar-gets against short range missiles in a theaterwar such as in Europe or the Middle East. TheFLAGE type of missile would intercept RVsat altitudes up to 15 km. The homing sensorfor FLAGE would use an active radar insteadof the passive IR sensor proposed for on allother KEW homing projectiles.

5“Nd:YAG” is the designation for a common laser used inresearch and for military laser range-finders. The “Nd” repre-sents neodymium, the rare element that creates the lasing ac-tion, and “YAG” stands for yttrium-alumin urn-garnet, the glass-like host material that carries the neodymium atoms.

110

The FLAGE system was flown six times atthe White Sands Missile Range. On June 27,1986, the FLAGE missile successfully collidedwith an RV-shaped target drone which wasflown into a heavily instrumented flight space.The collision was very close to the planned im-pact point. Another FLAGE interceptor col-lided with a Lance missile on May 21, 1987.

The FLAGE program ended in mid-1987with the Lance intercept. A more ambitiousExtended Range Intercept Technology (ER-INT) program succeeds it. The ERINT inter-ceptors will have longer range and “a lethal-ity enhancer. ” FLAGE was a fire-and-forgetmissile; no information was transmitted fromany external sensor to the missile once it wasfired. The ERINT missiles are to receive mid-course guidance from ground-based radars. Sixtest launches are planned at the White SandsMissile Range.

KEW Technology

Three types of KEW propulsion have beenproposed for SDI: conventional projectilespowered by chemical energy, faster but lesswell-developed electromagnetic or “railgun”technology, and nuclear-pumped pellets. Allsystem architects nominated the more maturechemically propelled rockets for near-termBMD deployments.

How Chemical Energy KEWs Work.–Thereare three different modes of operation proposedfor chemically propelled KEWs:

● space-based rockets attacking boosters,post-boost vehicles (PBVs), RVs, anddirect-ascent ASATs;

● ground-based rockets attacking RVs inlate mid-course outside the atmosphere,and

● ground-based rockets attacking RVs in-side the atmosphere.

Two or more rocket stages would acceleratethe projectile toward the target. The projec-tile would be the heart of each system andwould entail the most development.

The smart projectile for the space-based mis-sion would need some remarkable features. It

would be fired at a point in space up to hun-dreds of seconds before the actual intercep-tion.6 After separation from the last rocketstage, the projectile would have to establishthe correct attitude in space to “see” the tar-get: in general the line-of-sight to the targetwould not correspond with the projectile flightpath. If it had a boresighted sensor that staredstraight ahead, then the projectile would haveto fly in an attitude at an angle to its flightpath to view the target (see figure 5-1).7

The projectile would have to receive and exe-cute steering instructions via a secure commu-nications channel from the battle manager.Usually just a few seconds before impact, theprojectile would need to acquire the target–either a bright, burning booster or a much dim-mer PBV—with an on-board sensor. It wouldthen make final path corrections to effect a col-lision. Fractions of a second before impact, itmight deploy a “lethality enhancement device’–like the spider-web structure used in theArmy’s Homing Overlay Experiment (HOE)–to increase the size of the projectile and there-fore its chance of hitting the target.

The SBI projectile must have these com-ponents:

an inertial guidance system,a secure communications system,a divert propulsion system,an attitude control system,a sensor for terminal homing (includingvibration isolation),a lethality enhancement device (optional?),anda computer able to translate signals fromthe sensor into firing commands to the di-vert propulsion system in fractions of asecond.

The on-board sensors envisaged by most sys-tem architects for more advanced “phase-two”

6A computer in the battle management system would esti-mate the actual interception aim-point in space by projectingthe motion or track of the target using the sensor track files.

7For non-accelerating targets, this look angle would notchange, even though the target and the projectile were travel-ing at different velocities. In this “proportional navigation”mode, the projectile orientation would be fixed once the sensorwas aimed at the target.

111

Figure 5.1 .—Orientation of SDI to RV

(SBI)

Orientation of the space-based interceptor (SBI) to the reen-try vehicle (RV) during the homing phase of the flight. (Thisdrawing shows a sensor bore-sighted with the axis of the SBI,which is common for guided missiles operating in the atmos-phere. For space-based interceptors, the sensor could justas well look out the side of the cylindrical projectile.) TheSBI sensor would have to be aimed at the RV so that its line-of-sight would not be parallel to the SBI flight path (exceptfor a head-on collision.) For a non-accelerating RV, the an-gle from the sensor line-of-sight to the SBI flight path wouldbe fixed throughout the flight. Since targets such as ICBMboosters and post-boost vehicles do change acceleration dur-ing flight, then this look angle and hence the orientation ofthe SBI would have to be changed during the SBI flight.SOURCE Office of Technology Assessment, 1988,

space-based interceptors may be particularlychallenging because they would perform sev-eral functions. They would track not only theICBM during the boost phase, but also thePBV, RVs, and direct-ascent ASAT weaponssent up to destroy the BMD platforms. EachSBI would, ideally, kill all four types of targets.

In the boost phase, a short-wave infrared(SWIR) or medium-wave infrared (MWIR) sen-sor with existing or reasonably extended tech-nology could track a hot missile plume. An SBIwould still have to hit the relatively cool mis-sile body rather than the hot exhaust plume.Three approaches have been suggested for de-tecting the cooler missile body: computer al-

gorithm, separate long-wave infrared (LWIR)sensor, or laser designation.

A computer algorithm would steer the SBIahead of the plume centroid by a prescribeddistance that would depend on the look angleof the SBI relative to the booster and on thebooster type. Predicting the separation be-tween the plume centroid and the booster bodyunder all conditions might be difficult or evenimpractical if that separation varied from onebooster to the next.

A separate LWIR sensor channel might ac-quire and track the cold booster body.8 Onedesigner proposed a single detector array, sen-sitive across the IR band, in combination witha spectral filter. This filter would move me-chanically to convert the sensor from MWIRto LWIR capability at the appropriate time.Finally, in some designs a separate laser onthe weapon platform or on an SSTS sensorwould illuminate the booster. In this case anarrow-band filter on the interceptor’s sensorwould reject plume radiation, allowing the SBIto home in on laser light reflected from thebooster body.

In the post-boost and mid-course phases ofthe attack, the SBI would have to track hotor warm PBVs and cold RVs. Therefore eitherSBIs would need to have much more sophisti-cated LWIR sensors, or they would need some-thing like laser designators to enhance the tar-get signature. This laser illumination need notbe continuous, except possibly during the lastfew seconds before impact. But intermittentillumination would place another burden on thebattle manager: it would have to keep trackof all SBIs in flight and all SBI targets, theninstruct the laser designator at the right timeto illuminate the right target.

An SBI lethality enhancer might, for exam-ple, consist of a spring-loaded web which ex-

‘There is also a possibility that an SWIR or MWI R sensorcould acquire a cold booster body. At 4.3 p, for exaznple, theatmosphere is opaque due to the C02 absorption, and the upperatmosphere at a temperature of 2200 K would be colder thana booster tank at 3000 K. As an SBI approached a booster, thelatter would appear to a 4.3 ~m sensor as a large, warm targetagainst the background of the cool upper atmosphere,

112

panded to a few meters in diameter or an ex-plosively propelled load of pellets drivenradially outward. Weight would limit the prac-tical diameter of expansion. System designerswould have to trade off the costs of increasedhoming accuracy with the weight penalty ofincreased lethality diameter.

Ground-based KEW capabilities would re-semble those of space-based interceptors. Exo-atmospheric projectiles that intercept the RVsoutside the Earth’s atmosphere would useLWIR homing sensors to track cold RVs, orthey would employ other optical sensors totrack laser-illuminated targets. These intercep-tors would be command-guided to the vicin-ity of the collision by some combination ofground-based radars, airborne LWIR sensors(AOS) or space-borne LWIR sensors (SSTS,BSTS, or rocket-borne probes). Long-wave in-frared homing sensors in the projectile wouldhave to be protected during launch throughthe atmosphere to prevent damage or over-heating.

Current Status of Chemically PropelledRockets.–No interceptor rockets with BMD-level performance have ever been fired fromspace-based platforms. Operational IR heat-seeking interceptor missiles such as the air-to-air Sidewinder and the air-to-ground Maver-ick are fired from aircraft, but both the rangeand the final velocity of this class of missilesare well below BMD levels.

The SDIO’s Delta 180 flight test includedthe collision of two stages from a Delta rocketafter the primary task of collecting missileplume data was completed. However, thesetwo stages were not interceptor rockets, werenot fired from an orbiting platform, did nothave the range nor velocity necessary forBMD, and were highly cooperative, with thetarget vehicle orienting a four-foot reflectortoward the homing vehicle to enhance the sig-nal for the radar homing system. Note that thistest used radar homing, whereas all SBI de-signs call for IR homing or laser-designatorhoming. This experiment did test the track-ing algorithms for an accelerating target, al-though the target acceleration for this nearly

head-on collision was not as stressing as itwould be for expected BMD/SBI flight trajec-tories.9

Engineers have achieved very good progressin reducing the size and weight of componentsfor the proposed space-based interceptors.They have developed individual ring laser gyro-scopes weighing only 85 g as part of an iner-tial measuring unit. They have reduced theweight of divert propulsion engines about 9kg to 1.3 kg. Gas pressure regulators to con-trol these motors have been reduced from 1.4kg to .09 kg each. The smaller attitude con-trol engines and valves have been reduced from800 g each to 100 g each. Progress has alsobeen made on all other components of a SBIsystem, although these components have notas yet been integrated into a working proto-type SBI system.

Ground-based interceptor rockets are one ofthe best developed BMD technologies. TheSpartan and Sprint interceptor missiles wereoperational for a few months in the mid 1970s.Indeed parts of these missiles have been recom-missioned for upcoming tests of SDI ground-based weapons such as the endo-atmosphericHEDI. The production costs for these missileswould have to be reduced substantially tomake their use in large strategic defense sys-tems affordable, but no major improvementsin rocket technology are needed for ground-based interceptors, other than a 30 percent im-provement in speed for the HEDI missile. Asdiscussed in chapter 4, however, major sensordevelopment would be necessary for these in-terceptors.

Key Issues for Chemical Rockets.–Chemicalrocket development faces four key issues, allrelated to space-based deployment and all de-rived from the requirement to design and makevery fast SBIs.

Constellation Mass. —The overriding issuefor SBIs is mass. The SBIs must be so fast

‘Previous tests of IR guided projectiles such as the HomingOverlay Experiment against a simulated RV and the F-16launched ASAT test against a satellite, shot down non-accelerating targets.

113

that a reasonably small number of battle sta-tions could cover the entire Earth. But, for agiven payload, faster rockets consume muchmore fuel-the fuel mass increases roughly ex-ponentially with the desired velocity. Thedesigner must compromise between many bat-tle stations with light rockets or fewer battlestations with heavier rockets.10

These trade-offs are illustrated in figure 5-2, which assumes a boost-phase-only defensewith three hypothetical rocket designs: a state-of-the-art rocket based on current technology;a “realistic” design based on improvementsin rocket technology that seem plausible bythe mid-1990s; and an “optimistic” design thatassumes major improvements in all areas ofrocket development. The key parameters as-sumed for SBI rocket technology appear in theclassified version of this report. In all casesanalyzed, OTA assumed the rockets to be“ideal”: the mass ratio of each stage is thesame, which produces the lightest possiblerocket. 11 The first chart in figure 5-2a showsthat rocket mass increases exponentially withincreasing velocity, limiting practical SBI ve-locities to the 5 to 8 km/s range for rocketsweighing on the order of 100 kg or less.

For analytic purposes, OTA has consideredconstellations of SBIs that would be necessaryto intercept virtually 100 percent of postulatednumbers of ICBMs. It should be noted thatsince the system architecture analyses of 1986,SDIO has not seriously considered deployingSBIs that would attempt to intercept any-where near 100 percent of Soviet ICBMs andPBVS.12 This OTA analysis is intended only

‘“Projectile mass might not be as critical for ground-basedas for spacebased KEW projectiles, since there would be nospace transportation cost. However, the projectile mass shouldstill be minimized to reduce the over-all rocket size and cost,and to permit higher accelerations and final velocities.

llThe ma99 fraction for a rocket stage is defined = the ratioof the propellant mass to the total stage mass (propellant plusrocket structure). The mass fraction does not include the pay-load mass. For the calculations reported here, an ideal rocketis assumed: it has equal mass ratios for each stage, where massratio is defined as the initial stage weight divided by the stageweight after burn-out (both including the payload; it can beshown that the rocket mass is minimized for a given burnoutvelocity if each stage has the same mass ratio.)

“AS indicated in chapters 1,2, and 3, SDIO argues that thedeterrent utility of defenses far more modest than those neededfor “assured survival” would make them worthwhile.

to give a feel for the parameters and trade-offsinvolved in a system with SBIs.

Deployment of a system of “state-of-the-art”SBIs intended to provide 100 percent cover-age of Soviet ICBMs would entail 11.7 millionkg of CVs; waiting for the development of the“realistic” SBI would reduce the mass to or-bit by a factor of two.

Figure 5-2b shows the number of SBI car-rier platforms and figure 5-2c shows the num-ber of SBIs for a 100 percent-boost-phase de-fense as a function of SBI velocity. The lastchart (figure 5-2d) shows the total constella-tion mass as a function of velocity. The num-ber of CVs was calculated initially to optimizecoverage of existing Soviet missile fields: theorbits of the CVs were inclined so that the CVspassed to the north of the missile fields by adistance equal to the SBI fly-out range.13 EachCV therefore stayed within range of the ICBMfields for a maximum period during each orbit.

The “optimal” number of CVs resulting fromthis calculation was so low as to endanger sys-tem survivability (see ch. 11), calling for up to100 SBIs per carrier to cover the existing So-viet ICBM threat: such concentrations wouldprovide lucrative targets for the offense’sASATs. To increase survivability, the num-ber of CVs was therefore increased by a factorof 3 for the data in figure 5-2. Some polar or-bits were added to cover the SLBM threat fromnorthern waters.

The number of SBIs was calculated initiallyto provide one SBI within range of each of1,400 Soviet ICBMs sometime during theboost phase. The booster burn time was takenas similar to that of existing Soviet missiles,with a reasonable interval allotted for cloud-break, initial acquisition, tracking, and weap-ons launch.

One SBI per booster would not do for a ro-bust (approaching 100 percent coverage) boost-phase defense. A substantial number of SBIs

*The locations of Soviet missile fields are estimated from mapsappearing in U.S. Department of Defense, Sow”et Mih”tary Power,1987 (Washington, D. C.: Department of Defense, 1987), p. 23.See adaptation of this map in chapter 2 of this OTA report.

114

Figure 5-2a. -Space-based Interceptor Mass v. Velocity

Velocity (km/see)

The SBI mass versus SBI velocity. These data assume 100°/0coverage of the current Soviet threat of 1,400 ICBMs. It shouldbe noted that the SDIO currently proposes a substantiallylower level of coverage for SBIs. Therefore, the absolute num-bers in the OTA calculations are not congruent with SDIOplans. Rather, the graphs provided here are intended to showthe relationships among the various factors considered. Itshould also be noted that numerous assumptions underlyingthe OTA analyses are unstated in this unclassified report, but,are available- in the classified version.

SOURCE: Office of Technology Assessment, 1988.

Figure 5-2b.-Number of Satellites v. SBI Velocity

1

I I I I4 6 10 12

Velocity~km/see)

The number of SBI carrier satellites v. SBI velocity.

SOURCE: Office of Technology Assessment, 1988

Figure 5-2c. -Number of Space-Based Interceptorsv. Velocity (Inclined orbits + SLBM polar orbits)

40

30 —

20 —

4 6 10 12Velocity ~km/see)

The number of space-based interceptors (SBIs) required toprovide one SBI within range of each of 1,400 existing SovietICBMs before booster burnout. ‘


Figure 5-2d. -Constellation Mass v. SBI Velocity

4 6 8 10 12Velocity (km/see)

The total constellation mass in orbit (SBIs and carrier vehi-cles, excluding sensor satellites) v. SBI velocity. The mini-mum constellation mass for the “realistic” SBI to be in po-sition to attack all Soviet boosters would be about 5.3 millionkg. Faster SBIs would permit fewer carrier vehicles and fewerSBIs, but the extra propellant on faster SBIs would result ina heavier constellation. For reference, the Space Shuttle canlift about 14,000 kg into polar orbit, a 5.3 million kg constel-lation would require about 380 Shuttle launches, or about 130launches of the proposed “Advanced Launch System” (ALS),assuming it could lift 40,000 kg into near-polar orbit at suit-able altitudes.


115

would fail over the years just due to electronicand other component failures. The number ofSBIs in figure 5-2 was increased by a plausi-ble factor to account for this natural peacetimeattrition. In addition, during battle, some SBIswould miss their targets, and presumably So-viet defense suppression attacks would elimi-nate other CVs and draw off other SBIs forself-defense.

Given the above assumptions, figure 5-2 rep-resents the SBI constellation for nearly 100percent coverage of the existing Soviet ICBMfleet in the boost phase, with modest surviva-bility initially provided by substantial SBIredundancy, degrading to no redundant SBIsas “natural” attrition set in.

Note that for each type of rocket there is anoptimum velocity that minimizes the totalmass that would have to be launched intospace; lower velocity increases the number ofsatellites and SBIs, while higher velocity in-creases the fuel mass. In OTA’s analysis, theminimum mass which would have to be launchedinto orbit for the “realistic” rocket is 5.3 mil-lion kg (or 11.7 million lb); the mass for a con-stellation of “optimistic” SBIs would be 3.4million kg.

The data for figure 5-2 all assume boosterburn times similar to those of current Sovietliquid-fueled boosters. Faster-burning rocketswould reduce the effective range of SBIs andwould therefore increase the needed numberof carrier satellites. The same SBI parametersare shown in figures 5-3a and b with an assump-tion of ICBM booster burn time toward thelow end of current times. The minimum con-stellation mass has increased to 29 million and16 million kg, respectively, for the “realistic”and “optimistic” rocket designs.

Several studies of “fast-burn boosters” con-cluded that reducing burn-time would imposea mass penalty, so the Soviets would have tooff-load RVs (or decoys) to reduce burn timesignificantly. But these same studies showedthat there is no significant mass penalty forburn times as low as 120 s. About 10-20 per-cent of the payload would have to be off-loadedfor burn times in the 70 to 90 s range.

Figure 5-3a. -Number of Projectiles v. SBI Velocity(160 second burn-time)

04 6 8 10 12

Velocity (km/see)

The number of space-based interceptors v. SBI velocity forreduced booster burntime (within currently applied tech-nology).SOURCE: Office of Technology Assessment, 1988.

Figure 5-3b. -SBI Constellation Mass v. SBI Velocity(160 second burn-time)

100

4 6 8 10 12Velocity (km/see)

The total constellation mass (carrier vehicles and SBIs) versusSBI velocity for reduced booster burn times, assuming oneSBI within range of each of 1,400 boosters before burnout.


If the Soviet Union could reduce the burntime of its missiles below that of any currentlydeployed ICBMs, then the total SBI constel-lation mass necessary for boost-phase inter-cept would increase dramatically. The mini-mum constellation mass to place one SBIwithin range of each ICBM during its boostphase is shown in figure 5-4 as a function of

116

Figure 5-4.-Total SBI Constellation Mass in Orbitv. Booster Burn Time

Realisticrocket -

120 160 200 240 280 300Booster bum time (seconds)

The effect of Soviet booster burn time on SBI constellationmass. If we consider 40 million kg as a maximum conceiva-ble upper bound on constellation mass (corresponding to2,800 Shuttle flights or 1,000 launches of the proposed ALSsystem), then booster times of 120 to 150 seconds would se-verely degrade a 100%-boost-phase defense with chemicallypropelled rockets. The ability of smaller constellations of SBIsto achieve lesser goals would be analogously degraded bythe faster burn times.

All assumptions are the same as for the previous figures,except for the burn-out altitude, which varies with burn-time.


booster burn time for the three canonical rocketdesigns.

The masses described above for a boost-phase-only defense are clearly excessive, par-ticularly for a responsive Soviet threat. Add-ing other defensive layers would reduce theburden on boost-phase defense. The next layerof defense would attack PBVs, preferably earlyin their flight before they could unload anyRVs.

A PBV or “bus” carrying up to 10 or moreRVs would be more difficult to track and hitthan a missile. A PBV has propulsion enginesthat emit some IR energy, but this energy willbe about 1,000 times weaker than that froma rocket plume.14 A PBV is also smaller andless fragile than a booster tank. In short, a PBVis harder to detect and hit with an SBI. How-ever, a PBV is still bigger and brighter than

Whe first stage of an ICBM might radiate 1 million W/sr,the second stage 100,000 W/sr, while a PBV may emit only 100W/sr. On the other hand, the RV radiates only 5 W/sr, so thePBV is a better target than an RV.

an RV; sensors might acquire the PBV if itsinitial trajectory (before its first maneuver) canbe estimated by projecting the booster track.

The effectiveness of a combined boost andpost-boost defense in terms of the percentageof RVs killed is estimated in figure 5-5 for the“realistic” SBI rocket. The calculation as-sumes that 1,400 missiles resembling today’slarge, heavy ICBMs are spread over the exist-ing Soviet missile fields.

The net effect of attacking PBVs is to re-duce the number of SBIs needed to kill a givennumber of RVs. For example, to destroy 85percent of the Soviet RVs carried by ICBMs,a boost-only defense system would requireabout 26,000 SBIs in orbit. Adding PBV in-terceptions reduces the number of SBIs neededto about 17,000.

A defensive system must meet the expectedSoviet threat at the actual time of deployment,not today’s threat. For example, the SovietUnion has already tested the mobile, solid-fueled SS-24 missile, which can carry 10 war-

Figure 5-5.—Boost and Post-Boost Kill Effectiveness(1,400 ICBMs v. “Realistic" SBIs)

100

0o 40

Number of space interceptors (SBIs)(thousands)

Percentage of reentry vehicles (RVs) killed as a function ofthe number of space-based interceptors (SBIs) deployed inspace. This calculation assumes a threat of 1,400 ICBMsspread over the Soviet missile fields. The SBIs have a plau-sible single-shot probability of killing a booster and a slightlysmaller chance of killing a PBV; a substantial fraction of theSBIs are used for self-defense (or are not functional at thetime of attack).

SOURCE: Office of Technology Assessment, 1988,

117

heads. There is no reason to doubt that theSoviets could deploy this kind of missile inquantity by the mid-1990s. Such a fleet wouldparticularly stress a space-based defense if de-ployed at one or a few sites, since more SBIswould be needed in the area of deployment con-centration.

The effects on the combined boost and post-boost defense of clustering 500 shorter-burn-time, multiple-warhead missiles at three exist-ing SS-18 sites are shown in figure 5-6a. Itwould take about 23,000 SBIs to stop 85 per-cent of these 5,000 warheads. If the assumed500 ICBMs were concentrated at one site (butstill with 10 km separation to prevent “pin-down” by nuclear bursts), then 30,000 SBIswould be needed (see fig. 5-6b).l5

Finally, the Soviets might deploy 200 (ormore) current-technology, single-warhead mis-siles atone site, as shown in figure 5-7. In thiscase, no reasonable number of SBIs could in-tercept 85 percent of these 200 extra warheads(50,000 SBIs in orbit would kill 70 percent).Twice as many RVs are destroyed in the post-boost period as the boost-phase. Once this con-centrated deployment was in place, the defensewould have to add about 185 extra SBIs andtheir associated CVs to achieve a 50 percentprobability of destroying each new ICBM de-ployed.

SBI Projectile Mass.—The constellationmasses shown above assume that the mass ofthe smart SBI projectile (including lateral di-vert propulsion, fuel, guidance, sensor, com-munications, and any lethality enhancer) canbe reduced to optimistic levels. Current tech-nology for the various components would re-sult in an SBI with a relatively high mass. Thusmass reduction is essential to achieve the re-sults outlined above; total constellation masswould scale almost directly with the achiev-able SBI mass.

“Concentrating 500 missiles atone site would have disadvan-tages for an offensive attack: timing would be complicated toachieve simultaneous attacks on widely separated U.S. targets,and Soviet planners may be reluctant to place so many of theiroffensive forces in one area, even if the missiles are separatedenough to prevent one U.S. nuclear explosion from destroyingmore than one Soviet missile.

Figure 5-6a.— Boost and Post-Boost Kill Effectiveness(500 single.RV ICBMs at three sites)

I(“Realistic” SBIs)

1

- o 10 20 30 40Number of space-based interceptors (SBIs)

(thousands)

The percentage of RVs from modestly short-burn ICBMs killedas a function of the number of SBIs deployed in space. Thiscurve corresponds to 500 such ICBMs deployed at 3 exist-ing SS-18 sites. All SBI parameters are the same as in previ-ous figures.


Figure 5-6b.—Boost and Post-Boost Kill Effectiveness(500 singIe.RIV ICBMs at one site)

- 0 10 20 30 40Number of space-based interceptors (SBIs)

(thousands)

This curve assumes that all 500 shorter-burn ICBMs are de-ployed at one site (but still with 10 km separation to preventpin-down). All other parameters are the same as figure 5-6a.


Rocket Specific Impulse. —Similarly, the spe-cific impulse of the rocket propellant wouldhave to be improved from current levels. Thespecific impulse, expressed in seconds, meas-ures the ability of a rocket propellant to changemass into thrust. It is defined as the ratio ofthrust (lb) divided by fuel flow rate (lb/s).

118

Figure 5-7. - Boost and Peat-Boost Kill Effectiveness(200 “medium-burn-booster” ICBMs at one site)

100

90 — (“Realistic” SBIs)

I

00 10 20 30 40

Number of space-based interceptors (SBIs)(thousands)

Percentage of single-warhead ICBM RVs killed as a functionof number of SBIs in space. The 200 single-warhead ICBMsare deployed at one site with 10 km separation. All SBI pa-rameters are as in previous figures.SOURCE: Office of Technology Assessment, 1988.

The specific impulse of current propellantsvaries from 240 to 270 s for solid fuel and upto 390s at sea level for liquid oxygen and liq-uid hydrogen fuel. Assuming that BMD weap-ons would utilize solid fuels for stability andreliability, then the specific impulse for cur-rent technology would be limited to the 270-srange. 16 One common solid propellant, hy-droxyl-terminated polybutadiene (HTPB)loaded with aluminum, has an impulse in the260-to-265-s range. This can be increased to280s by substituting beryllium for the alumi-num. Manufacturers of solid propellant saythat further improvements are possible.

Rocket Mass Fraction. —Finally, the massfraction-the ratio of the fuel mass to the stagetotal mass (fuel plus structure but excludingpayload) –would have to be raised to meet SDIobjectives. Large mass fractions can beachieved for very big rockets having 95 per-cent of their mass in fuel. It would be moredifficult to reduce the percentage mass of struc-ture and propulsion motor components for verysmall SBI rockets.

—IThe SBI divert propulsion system in the final projectile stage

would probably use liquid fuel, and some have suggested thatthe second stage also use liquid fuel.

The mass fraction can be increased by re-ducing the mass of the rocket shell. New light-eight, strong materials such as carbon graph-ite fiber reinforced composite materials orjudicious use of titanium (for strength) and alu-minum (for minimum mass) may permit in-creased mass fractions for future rockets.

How Electromagnetic Launchers (EML) Work.–Electromagnetic launchers or “railguns” useelectromagnetic forces instead of direct chem-ical energy to accelerate projectiles along a pairof rails to very high velocities. The goal is toreach higher projectile velocities than practi-cal rockets can. This would extend the rangeof KEW, expanding their ability to attackfaster-burn boosters before burn-out. Whereasadvanced chemically propelled rockets of rea-sonable mass (say, less than 300 kg) couldaccelerate projectiles to at most 9 to 10 km/s,future EML launchers might accelerate smallprojectiles (1 to 2 kg) up to 15 to 25 km/s. SDIOhas set a goal of reaching about 15 km/s.

In principle, chemical rockets could reachthese velocities simply by adding more propel-lant. The efficiency of converting fuel energyinto kinetic energy of the moving projectile de-creases with increasing velocity, however: therocket must accelerate extra fuel mass that islater burned. A projectile on an ideal, stagedrocket could be accelerated to 15 km/s, but only17 percent of the fuel energy would be con-verted into kinetic energy of the projectile,down from 26 percent efficiency for a 12 km/sprojectile. Since a railgun accelerates only theprojectile, it could theoretically have higherenergy efficiency, which would translate intoless mass needed in orbit.

In practice, however, a railgun system wouldnot likely weigh less than its chemical rocketcounterpart at velocities below about 12 km/s,since railgun system efficiency would probablybe on the order of 25 percent at this velocity .17

17ms assumes 50 ~rmnt efficiency for converting fuel (ther-mal) energy into electricity, 90 percent efficiency in the pulseforming network, and 55 percent rail efficiency in convertingelectrical pulses into projectile kinetic energy. The SDIO hasa goal of reaching 40 percent overall EML system efficiency,but this would require the development of very high tempera-ture (2,000 to 2,500° K) nuclear reactor driven turbines. Thetotal system mass might still exceed that of a comparable chem-ical rocket system.

119

Therefore a railgun system would have to carryas much or more fuel than its chemical rocketequivalent-in addition to a massive rocket-engine generator system, an electrical pulse-forming network to produce the proper elec-trical current pulses, and the rail itself.

The conventional “railgun” (see figure 5-8)contains a moving projectile constrained bytwo conducting but electrically insulated rails.A large energy source drives electrical currentdown one rail, through the back end of the mov-ing projectile, and back through the other rail.This closed circuit of current forms a strongmagnetic field, and this field reacts with thecurrent flowing through the projectile to pro-duce a constant outward force. The projectiletherefore experiences constant acceleration asit passes down the rail.

The final velocity of the projectile is propor-tional to the current in the rail and the squareroot of the rail length; it is inversely propor-

Figure 5-8.-Schematic of an ElectromagneticLauncher (EML) or “Railgun”

Schematic of an electromagnetic launcher (EML) or “Rail-gun. ” In operation, a strong pulse of electrical current formsa circuit with the conducting rails and the projectile. This cur-rent loop generates a magnetic field. The interaction of thisfield with the current passing through the moving projectileproduces a constant outward force on the projectile, acceler-ating it to high velocities.

SOURCE’ Office of Technology Assessment, 1988.

tional to the square root of the projectile mass.High velocity calls for very high currents (mil-lions of amperes), long rails (hundreds of m),and very light projectiles (1 to 2 kg).

For the BMD mission, the projectile mustbe “smart”. That is, it must have all of the com-ponents of the chemically propelled SBIs: asensor, inertial guidance, communications, di-vert propulsion, a computer, and possibly alethality enhancement device. The EML pro-jectile must be lighter, and it must withstandaccelerations hundreds of thousands timesgreater than gravity, compared to 10 to 20“g’s” for chemically propelled SBIs.

Researchers at Sandia National Laboratoryhave proposed another type of EML launcherwhich would employ a series of coils to propelthe projectile. Their “reconnection gun” wouldavoid passing a large current through the pro-jectile, eliminating the “arcs and sparks” ofthe conventional railgun. The term “reconnec-tion” derives from the action of the movingprojectile: it interrupts the magnetic fields ofadjacent coils, and then these fields “recon-nect” behind the projectile, accelerating it inthe process.

Current Status of EMLs.—Several commer-cial and government laboratories have builtand tested experimental railguns over the lastfew decades. These railguns have fired verysmall plastic projectiles weighing from 1 to2,500 g, accelerating them to speeds from 2to 11 km/s. In general, only the very lightprojectiles reached the 10 km/s speeds.

One “figure of merit,” or index, for railgunperformance is the kinetic energy supplied tothe projectile. For BMD applications, SDIOoriginally set a goal of a 4 kg projectile acceler-ated to 25 km/s, which would have acquired1,250 MJ of energy. SDIO officials now statethat their goal is a 1 kg projectile at 15 km/s,which would acquire 113 MJ of kinetic energy.The highest kinetic energy achieved to datewas 2.8 MJ (317 g accelerated to 4.2 km/s), orabout 50 to 400 times less than BMD levels.

Finally, there have been no experiments withactual “smart” projectiles. All projectiles havebeen inert plastic solids. Some (non-operating)

120

Photo credit: Contractor photo released by the U.S. Department of Defense

Electromagnetic launcher.—This experimental electro-magnetic launcher at Maxwell Laboratories, Inc., San

Diego, CA, became operational late in 1985.

electronic components, including focal planearrays, have been carried on these plasticbullets to check for mechanical damage. Re-sults have been encouraging.

Key Issues for EMLs.—Much more researchmust precede an estimate of the potential ofEML technology for any BMD application.The key issues are summarized in table 5-1.There is uncertainty at this time whether allthese issues can be favorably resolved.

Table 5-1 .—Key Issues for ElectromagneticLaunchers (EML)

Low-mass (2 kg or less), high acceleration (severalhundred thousand g) projectile development.High repetition rate rails (several shots per second forhundreds of seconds).High repetition rate switches with high current (severalmillion A versus 750,000 A)Pulse power conditioners (500 MJ, 5 to 20 ms pulsesversus 10 MJ, 100 ms pulses)EfficiencyMassHeat dissipation


EML Projectile. –If based on current tech-nology for sensors, inertial guidance, commu-nications, and divert propulsion systems, thelightest “smart” projectile would weigh over10 kg. The total mass must shrink by at leasta factor of 5, and the projectile must withstandover 100,000 g’s of acceleration. If the projec-tile could only tolerate 100,000 g’s, then therailgun would have to be 112 m long to imparta 15 km/s velocity to the projectile. Higher ac-celeration tolerance would allow shorter rail-guns. (200,000 g’s would allow a 56-m long gun,etc.)

The SDIO has consolidated the developmentof light-weight projectiles for all kinetic energyprograms into the “Light-weight Exo-atmos-pheric Projectile” (LEAP) program. Althoughresearchers first saw a need for light-weightprojectiles for railguns, the primary initialusers of LEAP technology are to be the chem-ical rocket KEW programs (SBI, ERIS,HEDI). The phase-one LEAP projectile wouldweigh about 5 kg according to current designs(see figure 5-9), if all component developmentsmet their goals. This projectile would weightoo much for any railgun, and it will thereforenot be tested at high acceleration. This tech-nology might evolve into a 2-kg projectile bythe early 1990s. In any case, there are no plansnow to build a gun big enough to test even thephase-two 2 kilogram projectile.

High Repetition Rate.–A railgun wouldhave to fire frequently during an attack, en-gaging several targets per second. The penaltyfor low repetition rates would be additional rail-guns in the space-based constellation to cover

121

Figure 5-9.— Lightweight Homing Projectile

L E A P P H A S E 1

WEIGHT S’

LEAP PHASE 1PAYLOAD (1 600)

SEEKER 295.0IMU 285.0INTEGRATED PROCESSOR 400.0COMMAND RECEIVER 100.0

POWER SYSTEM 325.0STRUCTURE & MISC. 195.0

PROPULSION (3,059.0

VALVES & NOZZLES 570.0

CASE & INERTS 551.0

PROPELLANT 1818.0VALVE DRIVERS 120.0

PROJECTILE TOTAL 4659.(

L E A P P H A S E 2

STATEMENTS

LEAP PHASE 2PAYLOAD (670)

SEEKER 150.0IMU 70.0

INTEGRATED PROCESSOR 150.0COMMAND RECEIVER 25.0POWER SYSTEM 175.0STRUCTURE & MISC. 100.0

PROPULSION 0 ( 1 3 2 5 . 0VALVES & NOZZLES 375.0CASE & INERTS 215.0PROPELLANT 690.0

VALVE DRIVERS 45.0

PROJECTILE TOTAL 1995,0

Illustration of planned projectiles for the Lightweight Exoatmospheric Projectile (LEAP) program. This program is developingprojectile technology for both the rocket propelled and electromagnetic launcher (railgun) programs. However, the phase 1projectile at 5 kg and even the more conceptual phase 2 projectile with a mass projection of 2 kg are too heavy for any existingor planned railguns. There are no plans to test either projectile at the 100,000s of g acceleration necessary for rail gun opera-tion. These projectiles will benefit the SBI, ERIS, and HEDI programs.


the threat. Most railguns to date have beenfired just once: the rails eroded and had to bereplaced after one projectile. Newer systemscan fire ten shots per day, and at least one ex-periment has fired a burst of pellets at a rateof 10/s. Researchers at the University of Texasplan to fire a burst of ten projectiles in 1/6 ofa second, or a rate of 60/s.

Key issues for high repetition-rate guns arerail erosion,18 heat management, and high repe-tition-rate switches to handle the million-ampere current levels several times per second.Conventional high repetition-rate switches can

‘*New rail designs have shown promise of minimum erosionin laboratory tests; it remains to be proven that rails would sur-vive at weapons-level speeds and repetition rates.

122

handle up to 500 A today, although one spe-cial variable resistance switch tested by theArmy carried 750,000 A. An Air Force test suc-cessfully switched 800,000 A, limited only bythe power supply used. EML systems wouldhave to switch 1 to 5 million A.

EML Power. –An EML would consume highaverage electrical power and very high peakpower during each projectile shot. Considerfirst the average power requirements: a 1 kgprojectile would acquire 112 MJ of kineticenergy if accelerated to 15 km/s. Assuming 40percent efficiency and 5 shots per second, thenthe EML electrical system would have to de-liver 280 MJ of energy per shot or 1.4 GW ofaverage power during an attack which mightlast for several hundred seconds. For compar-ison, a modern nuclear fission power plant de-livers 1 to 2 GW of continuous power.

The SP-100 nuclear power system being dis-cussed for possible space application wouldproduce only 100 to 300 kW of power. The onlyapparent near-term potential solution to pro-viding 2.5 GW of power for hundreds of se-conds would be to use something like the SpaceShuttle Main Engine (SSME) coupled to a tur-bogenerator. Assuming 50 percent electricalconversion efficiency, then one could convertthe SSME 10 GW of flow energy into 5 GWof average electrical power while the engineswere burning.

High average power would not suffice. Theelectrical energy would have to be further con-centrated in time to supply very short burstsof current to the railgun. For example, a 112-m long railgun with 100,000-g accelerationwould propel a projectile down its length inabout 15 milliseconds (ins) to a final velocityof 15 km/s. The peak power during the shotwould be 50 GW.l9 And the EML system de-signer would like to shorten the 112-m railgunlength and increase acceleration, which wouldmean further shortening the pulse length andincreasing peak power.

Several techniques are under considerationto convert the average power from somethinglike the SSME into short pulses. One labora-tory approach is the homopolar generator: thisdevice stores current in a rotating machinemuch like an electrical generator and thenswitches it out in one large pulse. Existinghomopolar generators can supply up to 10 MJin about 100 ms; therefore, energy storage ca-pacity must increase by a factor of 50 and thepulse length shorten by a factor of 5 to 20.

EML Mass to Orbit.–The mass of an EMLsystem based on today’s technology would beexcessive. A homopolar generator to supply280 MJ per pulse would weigh 70 tonnesalone. 20 The rails would have to be long to limitacceleration on sensitive “smart projectiles,which would have to be very strong (massive)to resist the outward forces from the high railcurrents. The platform would have to includean SSME-type burst power generator, a ther-mal management system to dispose of theenergy deposited in the rails, divert propulsionto steer the railgun toward each target, andthe usual satellite communications and con-trol functions.

Given the early stage of EML research, esti-mates of total platform mass could be in errorby a factor of 10. At this time, a total massof about 100 tonnes would seem likely, mean-ing that each EML would have to be launchedin several parts, even if the United States de-veloped an Advanced Launch System (ALS)that could carry about 40 tonnes maximum perflight to high inclination orbits. It is conceiv-able that, in the farther term, superconductiveelectrical circuits could significantly reduce themass of an EML. Lighter compulsators (seebelow) might also reduce EML mass.

Nuclear-Driven Particles.–A nuclear explo-sion is a potent source of peak power andenergy. If even a small fraction of the energyin a nuclear explosion could be converted intokinetic energy of moving particles, then an ex-tremely powerful nuclear shotgun could be im-

lgFor a fi~e of referen~, consider that the total power av~-able from the U.S. power grid is several hundred gigawatts.

20Assuming today’s energy density for homopolar generatorsof 4 kJ/kg.

123

agined. These particles could be used for in-teractive discrimination as described above,since the particles would slow down light de-coys more than heavy RVs With more power,nuclear-driven particles could conceivably de-stroy targets. This concept is discussed in moredetail in the classified version of this report.

Directed-Energy Weapons

Directed-energy weapons (DEW) offer thepromise of nearly instantaneous destructionof targets hundreds or thousands of km away.While a KEW system would have to predicttarget positions several minutes in the futureand wait for a high speed projectile to reachthe intended target, the DEW could—in prin-ciple—fire, observe a kill, and even order a re-peat attack in less than a second.

DEW Systems

Although no DEW are planned for phase-one BMD deployment, both ground-based andspace-based DEW systems are possible in thenext century .21 Candidate DEW systemsinclude:

●

●

●

●

●

free electron lasers (FEL) (ground-basedor space-based),chemical lasers (space-based),excimer lasers (ground-based),x-ray laser (pop-up or space-based), orneutral particle beam (space-based).

The FEL is the primary SDIO candidate forground-based deployment (with the excimerlaser as a back-up). The hydrogen-fluoride (HF)laser and the neutral particle beam weapon arethe primary candidates for space-ased DEWS,although a space-based FEL or other chemi-cal laser concepts might also be possible.

Ground-Based Free Electron Laser (GBFEL).–A GBFEL system would include severalground-based lasers, “rubber mirror” beam di-rectors to correct for atmospheric distortionsand to direct the beams to several relay mir-rors in high-Earth orbit, and tens to hundreds

ZISDICI a9Wrt9 that some versions of DE W could be deployedlate in this century. It is examining designs for “entry level”systems with limited capabilities.

of “battle-mirrors’ in lower Earth orbit to fo-cus the beams on target. It would take sev-eral laser sites to assure clear weather at onesite all the time. Several lasers per site wouldprovide enough beams for the battle. Ideallythese lasers should beat high altitudes to avoidmost of the weather and atmospheric turbu-lence. But the FEL, as currently envisioned,requires very long ground path lengths forbeam expansion and large quantities of power.

The logical location for relay mirrors wouldbe geosynchronous orbit, so that the ground-based beam director would have a relativelyfixed aim point. The effects of thermalblooming 22 may best be avoided, however, byplacing the relay mirrors in lower orbit: the mo-tion of the laser beam through the upper atmos-phere as it follows the moving relay mirrorwould spread the thermal energy over a largearea.23

Adaptive optics would correct for atmos-pheric turbulence. The optical system wouldsense turbulence in real time and continuouslychange the shape of the beam-director mirrorto cancel wave-front errors introduced by theair. A beacon would be placed just far enoughin front of the relay satellite that the satellitewould move to the position occupied by thebeacon in the time it took for light to travelto the ground and back. A sensor on the groundwould detect the distortions in the test beamof light from the beacon, then feed the resultsto the “rubber mirror” actuators. With itswave front so adjusted, the laser beam wouldpass through the air relatively undistorted.

—‘zThermal blooming occurs when a high-power laser beam

passes through the atmosphere, heating the air which disturbsthe transmission of subsequent beam energy. See the sectionbelow on key DEW issues for details.

23 For exmple, a 1O-m dimekr laser beam which tracked arelay mirror at 1,000 km altitude would pass through a clean,unheated patch of air at 10 km altitude after 140 ms. If thermalblooming resulted from relatively long-term heating over a fewseconds, then scanning across the sky could ameliorate its ef-fects. While beam energy at altitudes below 10 km would takelonger than 140 ms to move to unheated patches of the atmos-phere, lower altitude blooming could be more readily correctedby the atmospheric turbulence compensation systems proposedfor ground based lasers: atmospheric compensation works bestfor “thin lens” aberrations close to the laser beam adaptive mir-ror on the ground.

124

This concept is discussed further below, un-der the heading of “Key DEW Issues.”

Table 5-2 compares the characteristics of cur-rent research FELs with those needed forBMD operations, as derived horn elementaryconsiderations in the American Physical So-ciety study .24 The key figure of merit is beambrightness, defined as the average laser out-put power (watts) divided by the square of thebeam’s angular divergence. Brightness is ameasure of the ability of the laser beam to con-centrate energy on the target (see figure 5-10).Another important figure of merit is the retar-get time—the time needed to switch from onetarget to another.

~iAmerican Physical Society, op. cit., footnote 1, chapters 3and 5.

Existing FELs operate in a pulsed mode: theenergy is bunched into very short segments,as illustrated in figure 5-11 for the radio fre-quency linear accelerator (RF linac) and for theinduction linear accelerator, two types of ac-celerators proposed for the FEL. The powerat the peak of each pulse is much higher thanthe average power. In the proposed inductionlinac FEL, peak power might exceed averagepower by 60,000 times. But it is the averagepower that primarily determines weapons ef-festiveness. 26

The RF linac experiments to date haveproduced 10 MW of peak power at 10 µm wave

“Short pulses of energy may foster coupling of energy intoa target, however, so the average power required from a pulsedlaser could, in principle, be less than the average power of acontinuous wave (CW) laser. This will be the subject of furtherSDI research.

Table 5-2.—Characteristics of a Ground-Based FEL Weapons System

Operational requirements againstCurrent status a fully responsive Soviet threata

Free Electron Laser. . . . . . . . . . . . . . . . . . . . . . . RF InductionNumber of laser sites . . . . . . . . . . . . . . . . . . . — 5-8Wavelength µm). . . . . . . . . . . . . . . . . . . . . . . . 9-35 8,800 .8 to 1.3Average power (MW) . . . . . . . . . . . . . . . . . . . . .006 .000014 100 to 1,000Peak power (MW). . . . . . . . . . . . . . . . . . . . . . . 10 1,000 b

Beam diameter (m) . . . . . . . . . . . . . . . . . . . . . (4)C (4) 10 to 30Brightness (W/sr) . . . . . . . . . . . . . . . . . . . . . . . 3.6x 1014 1 x 106 several x1022Peak brightness (W/sr) . . . . . . . . . . . . . . . . . . 6.3x 1017 4.9 x 1013 d

Beam director:Diameter (m) . . . . . . . . . . . . . . . . . . . . . . . . . . . (4) 10’Number of actuators. . . . . . . . . . . . . . . . . . . . (103) 103 to 104

Frequency response (Hz) . . . . . . . . . . . . . . . . 103 hundreds

Relay mirrors:Number of mirrors. . . . . . . . . . . . . . . . . . . . . . — 3-5?Diameter (m) . . . . . . . . . . . . . . . . . . . . . . . . . . . – 10 or moreAltitude (km) . . . . . . . . . . . . . . . . . . . . . . . . . . . – tens of thousandsSteering rate (retargets/s). . . . . . . . . . . . . . . . – 4-1o

BattIe-mirrors:Number of mirrors: . . . . . . . . . . . . . . . . . . . . . 30-150Diameter (m) . . . . . . . . . . . . . . . . . . . . . . . . . . . (4) 10Altitude (km) . . . . . . . . . . . . . . . . . . . . . . . . . . . – 1,000-4,000Steering rate (retargets/s). . . . . . . . . . . . . . . . – 2-5

operational requirements are taken from American Physical Society, Science and Technology of Dkected-Energy Weapons: Report to the American Physical Societyof the Study Group, April 19S7. SDIO dlsagreea with some of the numbers, but their disagreements are classified and may be found in the classified version of thisreport. Further, SDIO has identified BMD missions other than dealing with a fully responsive Soviet threat. An “entry-level” syatem (with a brightness on the orderof 10*0), might be developed earlier than the one with the above characteristics and would have less stressing requirements.

bSagment5 of ~ S.meter ~tive minor have baen built, and a A-meter, T.segmmt mirror is under construction. Parentheses in this table indicate that the mirror t=hnOlO-gy exists, but the mirrors have not yet been integrated with the laser.

CA weapons Systam would require the average ~wer levels listed above. The FEL is a pulsed laser—the powar of each pulsa iS much higher than the average power

when the pulses are both on and off. Depending on how targets and pulses Intaract, these short pulses might be lethal even with lower average power.dpeak brightness, like peak power, is not the relevant measure Of weapons lethatity.eThe American physl~ai s~lety, op. cit., footnote a, estimated that brightnesses on the order of 10” Wlsr might be necessary Io counter a responsive threat. A lo-meter

diameter mirror would be requirad for the lower power (1 OOMW) FEL module to raach 10’* WLsr brightness. The more probable approach would be to combine thebeams from ten lo-meter mirrors in a coherent array.

SOURCE: Office of Technology Assessment, American Physical Society Study Group, and Strategic Defense Initiative Organization, 19S7 and 19SS.

125

Figure 5-10. -Illustration of the RelationshipsBetween Laser Parameters and Power Density

Projected on a Target

Laser

D

Laser output power = P (watts)Beam diameter = D (meters)

For a diffraction-limited beam,

wavelength)

Brightness = B

Power density on target = I (watts/square cm)

R 2

Illustration of the relationships between laser parameters andpower density projected on a target. The key figure of meritfor any laser is its brightness. Brightness measures the abilityof the laser to concentrate power on a distant target. Highbrightness requires high laser power and low angular diver-gence. Low angular divergence in turn requires short wave-length and a large beam diameter. The power density on tar-get is equal to the laser brightness divided by the square ofthe distance to that target.


length, but only 6 kW of average power–whichwould translate into a brightness 100,000,000times less than the level needed for a BMDweapon against hardened Soviet boosters.26

This 6-kW average power was averaged overa 100-microsecond long “macropulse” in agiven second.

ZeThi9 brightne99 calculation assumes that the beam wouldbe expanded to fill a stateof-theart 4-m diameter mirror andwas diffraction-limited.

Figure 5-11 .—FEL Waveforms

Radio frequency LINAC FEL waveform

Duty cycle: Much lower than radio frequency LINAC

Existing laser waveforms from the radio frequency linear ac-celerator (RF Iinac) free electron laser (FEL) and the induc-tion linear accelerator FEL. The laser light is emitted in veryshort pulses. The peak power during these short pulses wouldhave to be extremely high to transmit high average power tothe targets. This peak intensity, particularly for the inductionIinac FEL, would stress mirror coatings and could induceother nonlinear losses such as Raman scattering in theatmosphere. Therefore, a weapon-grade induction-linac FELwould have to have higher repetition rates, perhaps on theorder of 10 kilohertz.


It should be noted, however, that these ex-periments were not designed for maximumaverage power. Low repetition rates were usedprimariliy for economic reasons. SDIO scien-tists say that scaling up the number of macro-pulses from 1/s to 5,000/s is not a serious prob-lem. If correct, this would mean that 30-MWaverage power could be produced with tech-nology not radically different from today ’s. Inaddition, aground-based weapon would use awavelength an order of magnitude smaller. Thebrightness scales as the inverse of the wave-length squared. For a given mirror diameter,then, if a similar power output could beproduced at a smaller wavelength, and the highrepetition rate were achieved, the brightnesswould only need to be increased by a factorof about 200 for 30 MW at 1 µm. Accomplish-

126

Photo credit: U.S. Department of Defense

Model of ALPHA experimental chemical laser.—Thisexperimental chemical laser and its large vacuumchamber have been constructed by TRW at a test sitenear San Juan Capistrano, CA. The cylindrical config-

uration of the laser design may be mostsuitable for basing in space.

ing both modifications would entail significantdevelopment work.

Space-based Chemical Lasers.—Placing high-power lasers directly on satellites would elim-inate the needs for atmospheric compensation,redundant lasers to avoid inclement weather,and relay mirrors in high orbits; it would alsoreduce beam brightness requirements by a fac-tor of 4 to 10 (depending on the wavelengthand atmospheric factors) since the atmospherewould not attenuate the beam.27 These advan-tages are offset by the engineering challengeof operating many tens or hundreds of lasersautonomously in space and by the possiblehigher vulnerability of lasers relative to battle-mirrors.

270ne defense contractor estimated that a space-based chem-ical laser system, including space transportation, would costabout 10 times less than the proposed ground-based free elec-tron laser weapon system.

The laser should operate at short wavelength(to keep the mirror sizes small) and should beenergy efficient (to reduce the weight of fuelneeded in orbit). Although its wavelength band(near 2.8 µm) is rather long, the hydrogen fluo-ride (HF) laser is the most mature and mostefficient laser available today. Table 5-3 com-pares the characteristics of a potential highperformance HF laser BMD system with thecurrent mid-infrared chemical laser (MIRACL)(using deuterium fluoride, or DF) operating atthe White Sands Missile Range in New Mex-ico.28

DEW Technology

How DEW Work.—Directed energy weaponswould change stationary, stored energy froma primary fuel source into a traveling beam ofenergy that could be directed and focused ona target. Several stages of energy conversionmay be necessary. The challenge is to build anaffordable, survivable, and reliable machinethat can generate the necessary beam ofenergy. Lasers can be driven by electricalenergy, chemical energy, or nuclear energy.

Free Electron Lasers.—Through 1987, theSDIO chose the FEL research program to re-ceive the most DEW emphasis (recently, SDIOhas returned to favoring research in space-based chemical lasers). The FEL uses a rela-tivistic29 electron beam from an accelerator toamplify a light beam in a vacuum. The keyadvantage of the FEL is the lack of a physicalgain medium: all other lasers amplify light ina solid, liquid, or gas. This gain medium mustbe stimulated with energy to produce an ex-cited population inversion of atoms or mole-cules. The fundamental limitation with theselasers is the need to remove waste heat beforeit affects the optical transparency of themedium. The FEL achieves its gain while pass-

28The SDIO is also considering lower-performance, “entrylevel, ” space-based chemical lasers for more limited BMDmissions.

29A beam of particles is deemed “relativistic” when it is ac-celerated to speeds comparable to a fraction of the speed of lightand acquires so much energy that its mass begins to increasemeasurably relative to its rest mass.

127

Table 5-3.—Characteristics of an HF Laser Weapons System

Estimated operationalCurrent status requirements a

Number of laser satellites . . . . . . . . . — 50-150Altitude (km) . . . . . . . . . . . . . . . . . . . . . 800-4,000Beam diameter (m) . . . . . . . . . . . . . . . 1.5b 10Power (MW) . . . . . . . . . . . . . . . . . . . . greater than 1 hundreds (single beam)Brightness (W/sr) . . . . . . . . . . . . . . . . . several x 1017C several x 1021

Phased array alternative:Number of beams . . . . . . . . . . . . . . 7Beam diameter (m) . . . . . . . . . . . . . 10Total power (MW): . . . . . . . . . . . . . . 100 (14 MW per beam)

aThese numbers derived from first principles, and from American Physical Society, SCienCe and Technology of Directed-EnergyWeapons. Report of ttre American Physical Society Study Group, April 1987, which contains estimates of booster hardnessfor a fully responsive threat The SDIO neither confirms nor denies these estimates. Current SDIO estimates may be foundIn the classified version of this report, In addition, SDIO has identified earlier entry-level systems with less stressing mls.sions and less stressing requirements with brightnesses on the order of 1010 WLsr.

bTh e LAMP mirror, not yet integrated with a high-power laser, has a diameter of 4 m.CAS~Unling perfect beam quality for a multi-megawatt system with the characteristics of the MIRACL laser

SOURCE Office of Technology Assessment, American Physical Society Study Group, and Strategic Defense Initiative Organ i-zat!on, 1987 and 19fM3

ing through an electron beam plasma, so muchof the “waste heat” exits the active regionalong with the electron beam at nearly thespeed of light.

Two types of electron beam accelerator arecurrently under investigation in the SDIO pro-gram: the radio frequency linear accelerator(RF linac) and the induction linac.30

In the RF linac, electrical energy from theprimary source is fed to radio-frequency gener-ators that produce an RF field inside the ac-celerator cavity. This field in turn accelerateslow energy electrons emitted by a specialsource in the front end of the accelerator. Theaccelerator raises this electron beam to higherand higher energy levels (and hence higher ve-locity) and they eventually reach speeds ap-proaching that of light. Simultaneously, theelectrons bunch into small packets in space,corresponding to the peaks of the RF wave.

This relativistic beam of electron packets isinserted into an optical cavity. There the beampasses through aperiodic magnetic field (calleda “wiggler” magnet) that causes the electrons

‘“Other types of accelerators are possible for a free electronlaser, such as the electrostatic accelerator FEL under investi-gation at the University of California at Santa Barbara, butthe RF linac and induction linac have been singled out as theprimary candidates for initial SDI experiments.

to oscillate in space perpendicular to the beamaxis. As a result of this transverse motion,weak light waves called synchrotrons radiationare generated. Some of this light travels alongwith the electron packets through the wigglermagnets. Under carefully controlled condi-tions, the electron beam gives up some of itsenergy to the light beam. The light beam isthen reflected by mirrors at the end of the op-tical cavity and returns to the wiggler mag-net synchronously with the next batch of elec-trons. The light beam picks up more energyfrom each pass, and eventually reaches highpower levels. This type of FEL is an optical“oscillator”: it produces its own coherent lightbeam starting from the spontaneous emissionfrom the synchrotrons radiation.

As more energy is extracted from the elec-tron beam, the electrons slow down. Theseslower electrons are then no longer syn-chronized with the light wave and the periodicmagnet, so the optical gain (amplification)saturates. To increase extraction efficiency, thewiggler magnet is “tapered”: the spacing ofthe magnets or the magnetic field strength isvaried so that the electrons continue in phasewith the light wave and continue to amplify

the beam as energy is extracted.

For high-power weapon applications, thepower from an oscillator might be too weak:

128

the limit for an RF linac FEL oscillator is near20 MW. In this case additional single-pass am-plifiers can boost the beam energy. This sys-tem is called a master oscillator power ampli-fier (MOPA) laser.

In the second type of FEL, the inductionlinac, large electrical coils accelerate narrowpulses of electrons. The high energy electronsinteract with an optical beam as in the RF linacFEL, but the optical beam as currently plannedwould be too intense to reflect off mirrors andrecirculate to pickup energy in multiple passesas in the RF oscillator. Rather, all of the energytransfer from the electron beam to the opticalbeam would occur on a single pass. This wouldentail very high gain, which demands very highdensity electron beams and very intense laserlight coming into the amplifier. The inductionlinac FEL therefore depends on an auxiliarylaser to initiate the optical gain process; thislimits the tunability of the induction linac FELto the wavelengths of existing conventionallasers of moderately high power.

The process of converting electron energyinto light energy can theoretically approach100 percent efficiency, although it may takevery expensive, heavy, and fragile equipment.3l

Nevertheless, the FEL could achieve very highpower levels, and, unlike other lasers, the RFlinac FEL can be tuned to different wavelengths by changing the physical spacing orfield strength of the wiggler magnets or theenergy of the electron beam.92 Tunability isdesirable for ground-based lasers, which mustavoid atmospheric absorption bands (wave-lengths of light absorbed by the air) if they areto reach into space.

Chemical (EIF) Lasers.–The HF laser de-rives its primary energy from a chemical re-action: deuterium and nitrogen trifluoride

~lTot~ sy9~m efficienW would probably be abOUt 20 Per@nt-

25 percent at best, assuming areasonably optimistic 50 percent-60 percent efficiency to convert chemical to electrical energyusing a rocket-driven turbine, and 40 percent efficiency to gen-erate RF power.

‘The wavelength of the FEL is proportional to the wigglermagnet spacing and inversely proportional to the square of theelectron beam energy. Higher beam energies are necessary forthe short wavelengths needed for BMD.

gases react in a device resembling a rocket en-gine. Hydrogen gas mixes with the combus-tion products. Chemical energy raises the re-sulting HF molecules to an excited state, from ,which they relax later by each emitting a pho-ton of light energy in one of several wavelengthlines near 2.8 µm in the MWIR. A pair of op-posing mirrors causes an intense beam of IRenergy to build up as each pass through theexcited HF gas causes more photons to radi-ate instep with the previously generated lightwave.33 Some additional electrical energy runspumps and control circuits.

Excimer Laser.— In an excimer34 laser, elec-trical energy, usually in the form of an elec-tron beam, excites a rare gas halide35 such askrypton fluoride or xenon chloride.36 Thesegases then emit in the ultraviolet (UV) regionof the spectrum, with wavelengths in the rangefrom.2 to .36pm. This very short wavelengthpermits smaller optical elements for a givenbrightness. However, the optical finish onthose UV optics would have to be of propor-tionately higher quality.

Ultraviolet light is also desirable for spaceapplications, since its high energy generallycauses more damage to the surfaces of targetsthan does that of longer-wavelength visible orIR light. One drawback is that internal mir-rors resistant to UV radiation damage are moredifficult to make. Another is that UV cannotreadily penetrate the atmosphere. These ob-stacles, combined with their relative immatu-rity and low efficiency, have relegated highpower excimers to a back-up role to the FELfor the ground-based BMD laser.

‘This process of repeat43d radiation in step is called “stimu-lated emission”: the traveling wave of light stimulates the ex-cited molecule to radiate with the same phase and direction asthe stimulating energy. The resulting beam of light is “coher-ent”: it can be focused to a very small spot. The term “laser”is derived from the phrase “Light Amplification by StimtiatsdElectromagnetic Radiation.”

~iEXCimer is sho~ for “exci~ state dimer”; the excitationof these rare gas halides produces molecules that only exist inthe excited state, unlike other lasing media which decay to aground state after emitting a photon of light.

S5A “h&de” is a compound of two elements, one of which iSa halogen: fluorine, chlorine, iodine, or bromine.

“Krypton fluoride produces a wavelength too short to pene-trate the atmosphere; for ground-based applications, xenon chlo-ride would be of interest.

Passing a laser beam through a Raman gascell can improve its quality. This cell, typicallyfilled with hydrogen gas, can simultaneouslyshift the laser frequency to longer wavelengths(for better atmospheric propagation), combineseveral beams, lengthen the pulse (to avoidhigh peak power), and smooth out spatial var-iations in the incoming beams. A low-power,high quality “seed” beam is injected into theRaman cell at the desired frequency. One ormore pump beams from excimer lasers supplymost of the power. In the gas cell, Raman scat-tering transfers energy from the pump beamsto the seed. This process has been demon-strated in the laboratory with efficiencies upto 80 percent.

X-ray Laser. —A nuclear explosion generatesthe beam of an x-ray laser weapon. Since thistype of laser self-destructs, it would have togenerate multiple beams to destroy multipletargets at once. It has been proposed that x-ray lasers would be based in the “pop-up”mode; their launch rockets would wait near theSoviet land mass and fire only after a full-scaleICBM launch had been detected. Since the x-rays could not penetrate deeply into the atmos-phere unless self-focused, the earliest applica-tion for the x-ray laser would likely be as anASAT weapon.

Neutral Particle Beam (NPB) Weapon.—TheNPB weapon, like a free electron laser, woulduse a particle accelerator (see figure 5-12). Thisaccelerator, similar to those employed in highenergy physics experiments, would movecharged hydrogen (or deuterium or tritium)ions to high velocities. Magnetic steering coilswould aim the beam of ions toward a target.As the beam left the device, a screen wouldstrip the extra electrons off the ions, result-ing in a neutral or uncharged beam of atoms.37

Unlike laser beams, which deposit theirenergy on the surface of the target, a neutralparticle beam would penetrate most targets,causing internal damage. For example, a 100-MeV particle beam would penetrate up to 4

37A charged beam could not be aimed reliably, since it wouldbe deflected by the Earth’s erratic magnetic field, so the beammust be uncharged or neutral.

129

Figure 5-12.— Schematic of a NeutralParticle Beam Weapon

LH2/Lox- Liquid oxygenturbine

Turbo-generator Liquid hydrogen

I

Beam expandermagnets

Schematic of a neutral particle beam weapon. Primary powermight be generated by firing a rocket engine, similar to theShuttle main engine, coupled to an electrical generator. Al-ternately, the hydrogen and oxygen could be combined in afuel cell to produce electricity. The resulting electrical cur-rent would drive the accelerator that would produce a beamof negatively charged hydrogen ions. This negatively chargedbeam would be expanded and directed toward the target bymagnets. Just before leaving the device, the extra electronon each hydrogen ion would be stripped off, leaving a neu-tral particle beam that could travel unperturbed through theearth’s erratic magnetic field.SOURCE: Office of Technology Assessment, 1988

cm into solid aluminum and a 200-MeV beamwould deposit energy 13 cm deep.38 Thesepenetrating particles could damage sensitivecircuits, trigger the chemical high explosivesin nuclear warheads, and-at high enough in-cident energy levels—melt metal–components.Shielding against neutral particle beams wouldbe difficult; imposing a large weight penalty.

As mentioned in chapter 4, the NPB maybe usable first as an interactive discriminator.The beam of energetic hydrogen atoms woulddislodge neutrons from massive RVs (the dis-

Y3ee W. Barkas and M. Berger, Tables ofEnergy Losses andRanges of Heavy Charged Particles, (Washington, DC: NASA,1964).

130

criminator NPB would presumably dwell oneach RV and decoy for too short a time to dam-age the RV). Separate satellites with neutrondetectors would determine which targets wereRVs and which were light-weight decoys. TheNPB technology development would be thesame for the weapon and the interactive dis-crimination programs, giving it multi-missioncapability.

Current Status of DEW.–Directed energyweapons are at various stages of developmentas discussed below, but none could be consid-ered ready for full-scale engineering develop-ment or deployment in the next decade.39

The characteristics of three potential DEWsystems are summarized in table 5-4. A keyfigure of merit is the brightness of the beam.Precisely what brightness would destroy differ-ent targets is still under investigation: the SD Iresearch program is measuring target lethal-ity for different wavelengths and for differentclasses of targets. The brightness levels of ta-ble 5-4 are derived from physical first princi-ples and assume that the Soviets could con-vert their missiles to hardened, solid-fueledboosters by the time DEWS could be de-ployed.40

WSDIO has recently been considering “entry-level” optionsthat it currently considers feasible for phase-two deployment.

40SD10 is considering “entry level” DEWS that would havemuch lower brightness and might be effective against today’smore vulnerable boosters. A synergistic mix of KEW-DEWboost-phase intercept capability and DEW discrimination is be-ing considered by SDIO as possible parts of a phastwo system.

The Accelerator Test Stand (ATS) neutralparticle beam experimental accelerator at LosAlamos National Laboratory is the weaponcandidate closest to lethal operating condi-tions: its brightness would need to rise byabout a factor of 10,000 to assure destructionof electronics inside an RV at typical battleranges (thousands of km). However, in this killmode, it maybe hard to determine whether theelectronics actually had been destroyed.Another factor of 10 to 100 might be neededto produce visible structural damage.

The MIRACL DF chemical laser operatingat White Sands has greater than 1 megawattoutput power, but its relatively long wave-length, the challenge of unattended space oper-ation, and the uncertainty of scaling this la-ser to the power levels necessary for ballisticmissile defense would make a deployment de-cision now premature. The brightness of an HFor DF laser would have to be increased by afactor of 10,000 to 100,000 over current levelsto be useful against responsively hardened So-viet boosters. However, an “entry-level” sys-tem that might be useful against current boost-ers would entail an increase in brightness ofonly several hundred to several thousandtimes.

To test some aspects of a space-based HFlaser, TRW is installing its “Alpha” laser ina large space-simulation chamber near SanJuan Capistrano, California. The Alpha laseruses a cylindrical geometry (MIRACL uses lin-

Table 5.4.—Characteristics of Directed Energy Weapons Against a Fully Responsive Soviet Threata

FEL—ground-based HF—space-based NPB—space-based

Primary energy source. . . . . . . . Electric Chemical ElectricWavelength or energy . .......0.8-1.3 µm 2.7 µm 100-400 MeVRequired brightness (W/sr) . . . .Several X 1022 Several x 1021 Several x 1019 (for

electronics kill)Current brightness (W/sr) . . . . .Several x 1014 Several x 1017b (potential

(considering for about 1010 1015-1016 (consideringunintegrated if unintegrated unintegratedcomponents) components considered) components)

Minimum penetrationaltitude (km) . . . . . . . . . . . . . . . About 30 About 30 130-170

aTh~ “u~ber~ in this table are ~btai”~d from the American physical Society, science and Technology of Difecfed-EnergY Weapons.’ f?8pOff Of the American ~hySkd

Society Study Group, April 1987, and apply to an advanced BMD system against a responsive threat. The estimates are neither confkmed nor denied by SDIO. SDIOhas identified other BMD missions for which lower “entry-level” systems with lower specifications (on the order of 1010 Wlsr) would be adequate.

bAss uming perfect beam quality for a system with the characteristics of the MIRACL laSer.

SOURCE: Office of Technology Assessment, American Physical Society Study Group, and Strategic Defense Initiative Organization, 1987 and 1988.

131

ear flow) with the supersonic gas flowing out-ward from a central 1.1-m diameter cylinderformed by stacking rings of carefully machinednozzles. The laser beam will take the form ofan annulus passing just outside the radiallydirected nozzles. A complex aspheric mirrorsystem will keep the laser beam within this nar-row ring. The goal of this program is to dem-onstrate multi-megawatt, near-diffractionlimited operation in 1988.

The brightness of a 4-m diameter (the sizeof the Large Aperture Mirror Program mirror),perfect, diffraction-limited beam41 from, for ex-ample, a 1-MW laser, would be over 1018 watts/steradian (W/sr). The Alpha laser was designedto be scaled to significantly higher levels bystacking additional amplifier segments. Itwould take a coherent combination of manysuch lasers to make a weapon able to engagea fully responsive missile threat.

Chemical lasers to meet a responsive Sovietmissile threat would need brightnesses of 1021-1022 W/sr. The level needed would depend onthe target dwell and retarget times. Thesetimes, in turn, depend on the laser constella-tion size and geometry, booster burn time andhardness, and number of targets which mustbe illuminated per unit time. If the Soviets wereto increase the number of ICBMs in a particu-lar launch area or decrease booster burn times,then the laser brightness needed would in-crease.

The brightness of aground-based FEL wouldhave to increase by a factor of 4 to 10 to ac-count for energy losses as the beam passedthrough the atmosphere and travelled to andfrom relay mirrors in space. Several free elec-tron lasers have been built. None has operatedwithin a factor of 100 million (108) of the lethalbrightness levels needed for a fully-responsiveBMD system. Part of the reason is the lowrepetition rate of the pulses in experimentalmachines. For example, one experiment ranwith the accelerator operated at a rate of oneelectron beam pulse every two seconds. Futureaccelerators will probably increase this rate to

“See American Physical Society, op. cit., footnote 1, p. 179.

thousands of pulses per second. This will in-crease average brightnesses accordingly, al-though, as previously discussed, several morefactors of 10 improvement would be needed.

Lawrence Livermore National Laboratory isconducting experiments with an FEL basedon an induction linear accelerator (linac). Boe-ing Aerospace is constructing an RF linacFEL, based on technology developed by Boe-ing and Los Alamos National Laboratory.

Initial experiments on the Livermore FELin 1985 produced microwave beams at 8.6 mmwavelength with peak powers of 100 MW.More recently, the peak power risen to 1.8 GW(1.8x109W),42 although this intensity lasts foronly 15 nanoseconds (15x10-9s); the averagepower at the repetition rates of one shot every2 seconds was only 14 W. Scaling to shorterwavelengths demands higher quality and veryhigh-energy electron beams. Livermore Lab-oratory achieved FEL lasing at 10 µm in thefar IR with its “Paladin” laser experiment inlate 1986. Boeing and a TRW/Stanford Univer-sity collaboration have operated 0.5 µm visi-ble lasers, but at low average power levels.

The Boeing RF linac FEL has the advantageof multiple optical passes through the wigglerof the optical oscillator. This means that highgain is not necessary, as it is with single-passinduction linacs.43 The RF linac also has moretolerance of variations in electron beam qual-ity or emittance. The emittance of the RF linacelectron beam could grow (i.e., deteriorate) byalmost a factor of 10 without deleterious ef-fects. In contrast, the induction linac electronbeam cannot increase in emittance by morethan a factor of two without degrading opti-cal beam brightness.44 However, there has beenmore uncertainty as to whether RF linacs couldbe scaled to the high current levels needed forBMD. Induction linacs, on the other hand,

4ZAn&ew M. Sessler and Douglas Vaughan, ‘‘Free-ElectronLasers, ” Amen”can Scientist, vol. 75, January-February, 1987,p. 34.

4$The RF Iinac fight require single-pass amplifiers in addi-tion to their multi-pass oscillators (MOPA configuration) toachieve weapons-class power levels.

44 Private communication, John M.J. Madey, 1987.

132

have inherently high-current capability. Re-cently, the two FEL concepts have appearedon the whole to compete closely with oneanother.

Excimer lasers have been utilized for lowerpower research and some commercial applica-tions. The UV energy from an excimer laseris generally more damaging than visible or IRenergy. However, UV light can also damagemirrors and other optical components withinthe laser system, making high-power operationmuch more difficult. Scaling to higher poweris possible, but SDIO has judged the excimerprogram less likely to succeed, and has cut itback. The Air Force ASAT program is fund-ing continued excimer laser research jointlywith SDIO.

Los Alamos National Laboratory research-ers have conducted NPB-related experimentson their ATS. They have produced a currentlevel of 0.1 A at 5 MeV. Rocket-borne testsof parts of a NPB system were planned for thelate 1980s. The SDIO had planned a series offull space tests to begin in the early 1990s, in-cluding a NPB accelerator with a target satel-lite and a neutron detector satellite as part ofthe interactive discrimination experimentalprogram. Recently, scheduling of these testshas been delayed due to funding constraints.

Key DEW Issues.–With such a wide gap be-tween operational requirements and the cur-rent status of DEW, many key technical is-sues remain. DEW research over the next 10to 20 years could resolve some issues judgedcrucial today, but could also uncover other, un-foreseen, roadblocks. Some of the current is-sues of concern (large mirrors, pointing andtracking, and lethality measurements) aregeneric to all laser systems, while others arespecific to particular weapon systems.

Large Mirrors. –All laser systems (exceptthe x-ray laser) need very large mirrors to fo-cus the beam to a small spot at the target.45

‘bSpot size k inversely proportional to mirror diameter. La-ser brightness, the primary indicator of weapon lethality, in-creases as the square of mirror diameter. Thus doubling themirror size from 2 meters to 4 meters would increass laser bright-ness by a factor of 4.

This is true for both ground-based lasers withmultiple relay mirrors in space and for space-based lasers with the mirror adjacent to thelaser. In either case, the size of the last mirror(closest to the target) and its distance from thetarget determine the size of the laser spot fo-cused on that target. To achieve the bright-ness levels of 1021 to 1022 W/sr for BMD againsta fully responsive threat, laser mirrors wouldhave to be at least 4 m (assuming mirrors wereganged into coherent arrays), and preferably10 to 20 m, in diameter.

The largest monolithic telescope mirrorstoday are about 5 m in diameter (Mt. Palomar),and the largest mirror built for space applica-tion is the Hubble Space Telescope at 2.4 m.The Hubble or Palomar mirror technologieswould not simply be scaled up for SD I applica-tions. The current trend both in astronomy andin military applications is to divide large mir-rors into smaller segments. Electro-mechan-ical actuators within the mirror segments ad-just their optical surfaces so that they behaveas a single large mirror.

Even for these segments, direct scaling ofold mirror manufacturing techniques usinglarge blocks of glass for the substrate is notappropriate: these mirrors must weigh very lit-tle. They must be polished to their prescribedsurface figure within a small fraction of thewavelengths they are designed to reflect.Brightness and precision make opposite de-mands: usually, a thick and relatively heavysubstrate is necessary to keep good surface fig-ure. SDIO has developed new technologies toreduce substrate weight substantially.

Two segments of a 3-segment, 3-m mirror(HALO) have been built. The 7-segment, 4-mmirror (LAMP) is now assembled and currentlybeing tested. One segment of a l0-m mirroris to be built by 1991, but there are no currentplans to assemble a complete 10-m mirror. Re-cently, the SDIO has begun tests of the light-weight LAMP mirror, designed for space-based lasers.

Durable, high-reflectivity mirror coatings areessential to prevent high laser power fromdamaging the mirrors. The largest mirror that

133

has been coated with a multi-layer dielectriccoating to withstand high energy-densitylevels is 1.8 m in diameter. Multi-layer dielec-tric coatings are generally optimized to pro-duce maximum reflectivity at the operatingwavelength. Their reflectivity at other wave-lengths is low (and transmission is high), mean-ing that off-wavelength radiation from another(enemy) laser could penetrate and damagethem.46 These coatings may also be suscepti-ble to high-energy particle damage in space,either natural or man-made.

Finally, the optical industry must developmanufacturing techniques, infrastructure, andequipment to supply the hundreds of large mir-rors for BMD DEW deployment. The SDI re-search program has targeted mirror fabrica-tion as a key issue, and progress has been goodin the last few years. Techniques have beendeveloped to fabricate light-weight, segmentedmirrors with hollow-cored substrates and ac-tuators to move each segment to correct forsurface figure errors.

These active mirrors could correct both forlarge-scale manufacturing errors and for opera-tional changes such as distortions due to ther-mal warping. They could even correct for broadphase errors in the laser beam. The price wouldbe added complexity. A complex electro-mechan-ical-optical servo system would replace a sim-pler static mirror. And, to make the necessarycorrections, another complex wave-front detec-tion system would measure the phase distor-tions of the laser beam in real time.

With reliable active mirrors, it might be pos-sible to coherently combine the output energyfrom two or more lasers. The brightness of “N”lasers could theoretically be increased to “N2”times that of a single laser with this coherentaddition. (See section below on chemical lasersfor more details.)

‘aDielectric coatings are nominally transmissive off the mainwavelength band, but there are always defects and absorbingcenters that absorb energy passing through, often causing dama-ge and blow-off. At best the transmitted energy would be de-posited in the substrate, which would then have to be designedto handle the high power density of offensive lasers.

Pointing, Tracking and Retargeting Issues.—A DE W beam must rapidly switch from onetarget to the next during a battle. Assumingthat each DEW battle-station within range ofSoviet ICBMs would have to engage 2 ICBMsper second,47 then the beam would have to slewbetween targets in 0.3 s to allow 0.2 s of ac-tual laser dwell time. In addition, the mirrorwould have to move constantly to keep thebeam on the target: the target would move 1.4km during 0.2s exposure, and the beam wouldhave to stay within a 20- to 30-cm diameterspot on the moving target.

Large 10- to 30-meter mirrors could movecontinuously to track the general motion of athreat cloud, but jumping several degrees toaim at a new target in 0.3 s would be ratherdifficult. 48 One solution would be to steer asmaller, lighter-weight secondary mirror in theoptics train, leaving the big primary mirror sta-tionary. This approach would yield only limitedmotion, since the beam would eventually walkoff the primary mirror; in addition, the smallersecondary mirror would be exposed to a higherlaser intensity, making thermal damage moredifficult to avoid.

Alternatively, small-angle adjustmentscould be made with the individual mirror seg-ments that would constitute the primary mir-rors. These mirror segments would probablyhave mechanical actuators to correct for grossbeam distortions and thermal gradient-inducedmirror warpage. Again, moving individual mir-ror segments would produce only limited an-gular motion of the total beam.49

4Tof a laser battle-station fleet of 120, perhaps 10 to 12 wouldbe within range of the missile fields. Assuming that averageSoviet booster bum times were in the range of 130 seconds bythe time a DEW system could be deployed, and allowing 30seconds for cloud break and initial track determination, theneach DEW platform in the battle space would have to engagean average of about 130 ICBMS in 100 seconds. This requiredtargeting rate of about 1.3 per second could be increased byfactors of 2 to 5 or more if the Soviet Union decided to deploymore ICBMS, and if they concentrated those extra ICBMS atone or a few sites. For this discussion, a figure of 2 per secondis taken.

4aSlewing requirements cm be minimized by using appropri-ate algorithms.

4BAl~rnate Concepts are being kveStigated to wow retmget-ing at large angles with steerable secondary mirrors.

134

The mirror servo system would have to ac-complish these rapid steering motions with-out introducing excessive vibration or jitterto the beam. To appreciate the magnitude ofthe steering problem, consider that a vibrationthat displaced one edge of a 10-m mirror by1 micrometer (1 µm—40 millionths of an inchor twice the wavelength of visible light) wouldcause the laser to move one full spot diameteron the target.50 This small vibration would cutthe effective laser brightness in half. Allowa-ble jitter is therefore in the 20 nanoradian, orone part in 50 million, range. Since any servosystem would undoubtedly exceed these jit-ter limits immediately after switching to a newtarget, there would be a resettling time beforeeffective target heating could begin. Thisresettling time would further decrease the al-lowable beam steering time, say from 0.3 s to0.2 or 0.1 s.

Non-inertial methods of steering laser beamsare under investigation. For example, a beamof light passing through a liquid bath whichcontains a periodic acoustical wave is dif-fracted at an angle determined by the acousti-cal frequency. By electronically changing theacoustical frequency in the fluid, the laser beamcould be scanned in one direction without anymoving parts. Two such acousto-optic modu-lators in series could produce a full two-dimensional scanning capability.

Alternatively, the laser beam could bereflected off an optical grating that diffractedit at an angle that depended on its wavelength.If the laser wavelength could be changed withtime, then the beam could be scanned in onedirection. Most of these non-inertial scanningtechniques could not operate at weapons levellaser power without damage. Others place con-straints on the laser, such as limiting tuna-bility.

Approximate beam steering and retargetinglevels are summarized in table 5-5. These pa-rameters would vary with specific weapons de-sign, system architecture, and assumedthreats. In general, demands on beam steer-

‘Whis assumes a 20-cm spot diameter on a target 1,000 kmaway, or an angular motion of 200 nanoradians.

Table 5-5. —Possible Beam Steering and RetargetingRequirements for Boost-Phase Engagement

Retargeting rate . . . . . . . . . . . . . . . . . . . . 2 targets/secondRetarget time. . . . . . . . . . . . . . . . . . . . . ..0.1 to 0.2 secondsJitter resettling time. . ...............0.1 to 0.2 secondsAverage laser dwell time . . . . . . . . . . . . . 0.2 secondsLaser angular beamwidtha. . . . . . . . . . . . 120 nanoradiansAllowable beam jitter . . . . . . . . . . . . . . . . 20 nanoradiansaThe diffraction. [imited beam spread for a 1 ~m laser with a 10 m diameter mir-

ror is 120 nr,


ing speed and precision would increase if theDEW range were extended (by deploying fewerthan the 120 battle stations assumed here, forexample), or if the Soviets increased the offen-sive threat above 1,400 ICBMs with averageburn time of 130 s assumed above.

Beam steering and retargeting needs forpost-boost and midcourse battle phases couldbe more stressing if boost-phase leakage werehigh and discrimination were not reasonablyeffective. In general there would be more timefor midcourse kills, and more DEW platformswould engage targets, but the hard-shelledRVs would withstand much more laser irradi-ation and hence impose longer dwell times.Lasers do not appear likely candidates for mid-course interception of RVs.

A neutral particle beam weapon (NPB) wouldnot have to dwell longer on RVs than on boost-ers or PBVs, since energetic particles wouldpenetrate the RV. Without midcourse discrimi-nation, the NPB system might have to killfrom 50,000 to 1,000,000 objects surviving theboost phase, and a weapon platform wouldhave to kill an average of 3 to 50 targets persecond. At the other extreme, with effectivediscrimination, each NPB platform in the bat-tle might have to engage only one RV or heavydecoy every 20 s.51

Atmospheric Turbulence and Compensationfor Thermal Blooming. -One current DEWcandidate is the ground-based free electron la-

61 A55me–6,()()() RVS, 6,()()0 heavy decoys, and 10 Percent 1e*-age from the boost phase defense. If the discrimination systemreliably eliminated all light decoys and debris, then, with 30of the 120 DEW platforms in the midcourse battle, each plat-form would engage, on the average, one target every 22.5seconds.

135

ser. The beam from this laser would be directedto a mirror in space that would reflect the beamto “fighting mirrors” closer to the targets. Thelaser beam would be distorted in passingthrough the atmosphere, for the same reasonthat stars “twinkle.” If not corrected, atmos-pheric distortion would scramble the beam,making it impossible to focus with sufficientintensity to destroy ICBM boosters.

Techniques have been developed to measurethis distortion of the optical wave front andto modify the phase of low power laser beamsto nearly cancel the effects of the turbulentatmosphere. To correct distortion, the mirroris manufactured with a flexible outer skin orwith separate mirror segments. Mechanical ac-tuators behind the mirror surface move it toproduce phase distortions that complementphase errors introduced by the atmosphere.This “rubber mirror” must continuously ad-just to cancel the effects of atmospheric tur-bulence, which varies with time at frequenciesup to at least 140 hertz (cycles per second).

To measure atmospheric distortion, a testbeam of light must be transmitted through thesame patch of atmosphere as the high powerlaser beam. For the BMD application, this testbeam would be projected from a point near therelay mirror in space, or a reflector near thatrelay mirror would return a test beam from theground to the wave-front sensing system. Sig-nals derived from the wave-front sensor com-puter in response to the test beam would drivethe mirror actuators to correct the high-powerlaser beam.

The wave-front sensor must generate a co-herent reference beam to compare with the dis-torted beam, as in an interferometer. One tech-nique, called shearing interferometry, causestwo slightly displaced versions of the incom-ing distorted image to interfere. A computerthen deduces the character of the distortedwave front by interpreting the resulting inter-ference fringes,

Another wave-ront sensor system under in-vestigation filters part of the incoming refer-ence beam to produce a smooth, undistortedwave front. This clean wave front can then be

combined with the distorted wave front, pro-ducing interference fringes that more clearlyrepresent the atmospheric distortion. Unfor-tunately the energy levels in the filtered wavefront are too low, so an operational systemmight need image intensifiers.

Atmospheric compensation of low powerbeams has been demonstrated in the 1abora-tory and in tests during late 1985 at the AirForce Maui Optical Station (AMOS) in Hawaii.In this test, an argon laser beam was trans-mitted through the atmosphere to a soundingrocket in flight. A reflector on the soundingrocket returned the test signal. Wave-front er-rors generated on Maui drove a “rubber mir-ror” to compensate for the turbulence experi-enced by a second Argon laser beam aimed atthe rocket. A set of detectors spaced along thesounding rocket showed that this laser beamwas corrected to within a factor of two of thediffraction limit.

Successful atmospheric compensation willentail resolution of two key issues: thermalblooming and fabrication of large, multi-element mirrors. As a high-power laser beamheats the air in its path, it will create additionalturbulence, or “thermal blooming,” which willdistort the beam. At some level, this type ofdistributed distortion cannot be corrected. Forexample, if thermal blooming causes the laserbeam to diverge at a large distance from thelast mirror, then the test beam returning fromthe relay satellite would also spread over alarge area and would not all be collected bythe wave-front sensor. Under these conditions,complete compensation would not be possible.

Laboratory tests of thermal blooming wereplanned at MIT’s Lincoln Labs and field-testing was planned for early 1989 using thehigh-power MIRACL laser at the White SandsMissile Range. The latter series of tests is onhold due to lack of funding.

The mirror for a BMD FEL would need 1,000to 10,000 actuators for effective atmosphericcompensation. 52 Experiments to date haveused cooled mirrors with a relatively small

52American Physical Society, op. cit., footnote 1, p. 190.

136

number of elements, and Itek is currentlybuilding a large uncooled mirror with manymore.

Nonlinear optical techniques may offer analternative to the active-mirror correction ofatmospheric turbulence. Laboratory experi-ments at low power have already demonstratedbeam cleanup by stimulated Brillouin scatter-ing, for example. In this technique, a beam oflight with a wave front distorted by the atmos-phere enters a gas cell. The beam passes par-tially through this gas and is reflected backwith complementary phase distortion. Thiscomplementary or “conjugate” phase exactlycancels the phase distortions introduced by theatmosphere. The key is to amplify the phaseconjugate beam without introducing addi-tional phase errors. If perfected, this approachwould eliminate moving mirror elements.

Target Lethality.— One term in the DEW ef-fectiveness equation is the susceptibility of cur-rent and future targets to laser and neutral par-ticle beams. Current U.S. missile bodies havebeen subjected to HF laser beams in ground-based tests, and various materials are beingtested for durability under exposure to high-power laser light.53 Laser damage varies withspot size, wavelength, pulse length, polariza-tion, angle of incidence, and a large range oftarget surface parameters, making lethalitytest programs complex.54 FEL beams with aseries of very short but intense pulses may pro-duce an entirely different effect than continu-ous HF chemical laser beams.

Measuring the lethality of low-power neu-tral particle beam weapons intended to disruptelectronics could be more complicated. Dam-age thresholds would depend on the electronicspackage construction. However, current planscall for particle beam energy density whichwould destroy virtually any electronic sys-

691~ ~~~ highly publicized test at the White S~ds MissileRange, a strapped-down Titan missile casing, pressurized withnitrogen to 60 pounds per square inch pressure to simulate flightconditions, blew apart after exposure to the megawatt-classMIRACL laser.

“Computer models have been developed to help predict tar-get lethality, and these models will be refined and correlatedwith ongoing lethality measurements.

tem.55 The kill assessment issue for NPB weap-ons would then become one of hit assessment:the system would have to verify that the par-ticle beam hit the target.

FEL.-The two types of FEL systems (in-duction linac and RF linac) face different setsof key issues (table 5-6). The induction linacFEL has the potential of very high power, butall of the laser gain must occur on one passthrough the amplifier as currently designed.(Almost all other lasers achieve their amplifi-cation bypassing the beam back and forth be-tween two mirrors, adding up incrementalenergy on each pass.)

To achieve BMD-relevant power levels onone pass, the FEL beam diameter must be verysmall, on the order of a millimeter (mm). Fur-thermore, the beam must be amplified over avery long path, on the order of 100 m. But amillimeter-diameter beam would naturally ex-pand by diffraction over this long path length,56

so the induction linac must utilize the electronbeam to guide and constrain the light beamwhile it is in the wiggler magnet amplifier,much like a fiber optic cable. This optical guid-ing by an electron beam has been demon-

‘What is, the NPB would be designed to deliver 50 J/gin atthe target, whereas 10 J/gin destroys most electronics (see Amer-ican Physical Society, op. cit., footnoti 1, p. 306. This wouldassure electronics kill unless massive shielding were placedaround key components.

66A l.m km of unconstrained l-pm light would expandto 120 mm after traveling 100 m.

Table 5.6.–Key Issues for Free Electron Lasers (FEL)

For induction linear accelerator driven FELs:—Electron beam guiding of the optical beam—Generation of stable, high current, Iow-emittance

e-beams—Scaling to short wavelengths near 1 µm—Raman scattering losses in the atmosphere

For radio frequency accelerator-driven FELs:—Scaling to 100 MW power levels—Efficiency—Mirror damage due to high intercavity power—Cavity alignment

For any FEL:—Long cavity or wiggler path lengths—Sideband instabilities (harmonic generation)—Synchrotron/betatron instabilities (lower efficiency)


strated, but not under weapon-like FEL con-ditions.

Two other disadvantages derive from thisnarrow, intense beam of light produced by theinduction linac FEL. First, the beam is so in-tense that it would damage any realizable mir-ror surface. The current plan is to allow thebeam to expand by diffraction after leavingthe FEL, traveling up to several km in anevacuated tunnel before striking the directormirror which would send the beam to the re-lay mirror in space.

A second disadvantage of such intensepulses of light is that they would react withthe nitrogen in the atmosphere by a processcalled “stimulated Raman scattering. ” Abovea threshold power density, the light would beconverted to a different frequency whichspreads out of the beam, missing the intendedtarget. 57 Again, this effect could be amelioratedby enclosing the beam in an evacuated tube,allowing it to expand until the power densitywere low enough for transmission through theatmosphere to the space relay. On the returnpath to the target, however, the beam wouldhave to be focused down to damage the target.

The experimental induction linac at Liver-more currently uses a (conventional, non-FEL)laser-initiated channel to guide the electronbeam before it is accelerated. This beam hasdrifted several millimeters laterally during theFEL pulse in initial experiments, severelylimiting FEL lasing performance because theelectron beam does not remain collinear withthe FEL laser beam.

The RF linac FEL, as currently configured,has shorter pulse lengths (20 picosecond v.15 nanoseconds for the induction linac FEL)but much higher pulse repetition rates (125MHz v. 0.5 MHz58), giving it higher duty cy-

S~The R~~ t~eshold for stimulated gain in nitrogen gasat one P light is about 1.8 MW/cm2. Above this power den-sity, the atmosphere becomes a singk+pass nitrogen laser: muchof the beam energy is converted to different (Stokes and anti-Stokes) wavelengths which diverge and cannot be focused onthe target.

‘aThe induction linac at Livermore could be operated up to1 kHz for up to 10 pulses. An o~rational linac FEL would havea repetition rate as high as tens of kilohertz.

137

cle and lower peak intensity for a given out-put average power level. It is uncertainwhether the RF linac can be scaled up to pro-duce power levels which seem probable for theinduction linac. By adding a set of power am-plifiers in series, it might be possible to reachthe power needed for a lethal laser weapon withan RF linac FEL.

The RF linac generates very high powerlevels inside the optical cavity. Mirror dam-age is therefore an issue, as is the problem ofextracting energy out of the cavity at thesehigh power levels. Cavity alignment is also crit-ical: the mirrors must be automatically alignedto maintain path-lengths within micrometersover many tens of meters during high-poweroperation.

The RF linac currently has low efficiency.In 1986, Los Alamos National Laboratory anda TRW-Stanford team demonstrated an energyrecovery technique whereby much of the un-used energy in an electron beam was recoveredafter the beam passed through a wiggler-ampli-fier. In principle, this energy could be coupledback to the RF generator to improve efficiencyin an operational system. At the higher opti-cal energy levels envisaged for the amplifiers,the RF linac amplifier should achieve 20 per-cent to 25 percent conversion efficiency, mak-ing energy recovery less advantageous.

An FEL would tend to be fragile. Accelera-tors are notorious for demanding careful align-ment and control, taking hours of manualalignment before operation. Major engineer-ing developments in automatic sensing andcontrol would be necessary before an FELcould become an operational weapon. LosAlamos is working to automate its ATS par-ticle beam accelerator; FEL systems wouldhave to incorporate similar automation, withthe added complexity of optical, as well as ac-celerator, alignment.

An FEL may suffer from electron beam (e-beam) instabilities. For example, unwanted lon-gitudinal e-beam excursions could create “side-band instabilities,” in which part of the opticalenergy would be diverted to sideband frequen-cies. Laser light at these extraneous frequen-

138

cies could damage optical components de-signed to handle high power only at the mainlasing frequency. Such sideband frequencieshave been observed in FEL experiments. Lat-eral motion of the e-beam, called “synchro-tron/betatron instabilities” could reduce FELefficiency, although calculations indicate thatthis should not be a problem.

Chemical Laser Issues.—The chemical HFlaser has some disadvantages relative to theFEL. Its longer wavelength (2.8-pm range)would demand larger mirrors to focus the beamon target. In general, targets would reflect ahigher percentage of IR light than visible or,particularly, UV light. Hence, for a given mir-ror size, an HF laser would have to generate7 to 10 times more power than an FEL laseroperating at one µm, or 80 to 200 times morepower than a UV laser, to produce the samepower density at the target.

Chemical laser experts do not believe thatan individual HF laser could be built at rea-sonable cost to reach the 1021 to 1022 W/srbrightness levels needed for BMD against aresponsive threat, since the optical gain vol-ume is limited in one dimension by gas flowkinetics, and by optical homogeneity in theother directions. However, by combining theoutputs from many HF lasers, it might be pos-sible to produce BMD-capable HF arrays (ta-ble 5-7).

These beams must be added coherently: theoutput from each laser must have the same fre-

Table 5-7.—Key Issues for the HF Chemical Laser

Coherent beam combination: (many HF laser beams wouldhave to be combined to achieve necessary power levels)

Required beam brightnessagainst a responsive threat . . . several x 1021 W/sr a

Reasonable HF Laser brightnessfor a single large unit (10 MWpower and 10-m mirror). . . . . . . 8.6 x l o ”

Coherent Array of seven 10MW/10-m HF lasers . . . . . . . . . . 4.2x10 21

~he American Physical Society, Science and Technology of Directed-EnergyWeapons: Repori of the Arnedcan Physical Society Study Group, April 1967,p. 55, estimated hardnees for a reaporwive threat to be well in excese of 10kJ/cm2. Given a range of 2,000 km and a dwell time of 0.2s, the denoted bright-ness is appropriate.

SOURCE: Office of Technology Asae~wnent, 1966.

quency and the same phase.59 Controlling thephase of a laser beam is conceptually easy, butdifficult in practice–particularly at high powerand over very large apertures. Since an uncon-trolled HF laser generates several different fre-quencies in the 2.6 to 2.9 µm band, the laserarray would have to operate on one spectralline, or one consistent group of lines.

Three coherent coupling techniques havebeen demonstrated in the laboratory:

1. Coupled Resonators—the optical cavities

2

3

of several lasers are optically coupled, sothey all oscillate in phase;Injection Locked Oscillators-one lowpower oscillator output light beam is in-jected into the optical cavity of each laser;Master Oscillator/Power Amplifier (MOPA)—each laser is a singh-pass power amp-lifier fed by the same master oscillator inparallel.

In one experiment, 6 CO, lasers were joinedin the coupled resonator mode. With incoher-ent addition, the output would have been 6times brighter than that of a single laser; withperfect coupling, the output would have been36 times brighter. The experiment actuallyproduced 23.4 times greater brightness. Ex-periments are under way to couple two l-kW,HF/DF lasers (with the coupled resonator ap-proach) and to demonstrate MOPA operationof two HF laser amplifiers.60

Neutral Particle Beam.—Although acceler-ator technology is well established for ground-based physics experiments, much research, de-velopment, and testing are prerequisite to ajudgment of the efficacy of a space-based par-ticle beam weapon system. Key issues are pre-sented in table 5-8.

~If ~d~ ~heren~y, & &am brightness of “N” ISSSrS wollldbe “Nz” times the brightness of one laser. If the “N” laserswere not coherent, then the brightness of the combination wouldbe the sum or “N” times the brightness of one laser.

‘Actually, the MOPA experiment will utilize one amplifierwith three separate optical cavities: one for the master oscilla-tor and two for the amplifiers. (Source: SDI Laser TechnologyOffice, Air Force Weapons Laboratory, unclassified briefing toOTA on Oct. 7, 1986.)

.

139

Photo Credit: U.S. Department of Defense,Strategic Defense Initiative Organization

Artist’s conception of a phased array of lasers.—Sinceit may be impractical to build a single module space-based chemical laser of a size useful for ballisticmissile defense, scientists and engineers are exploringthe possibility of using several smaller laser modulesthat would be phase-locked to provide a singlecoherent beam. This technique could increase the at-

tainable power density on a target by a factorof N* (instead of N for incoherent addition),

where N is the number of modules.

Table 5-8.—Neutral Particle Beam Issues

● Major issues:—Beam divergence: 50 times improvement required–Weight reduction (50 to 100 tonnes projected)—Kill assessment (or hit assessment)

. Other issues:—Beam sensing and pointing—Duty factor: 100 times improvement required—Ion beam neutralization (50°/0 efficient)—Space charge accumulation—ASAT potential

SOURCE Office of Technology Assessment, 1988

The NPB ATS now at Los Alamos gener-ates the necessary current (100 mA) for a NPBweapon, but at 20 to 40 times lower voltage,about 100 times lower duty cycle, and withabout 50 times more beam divergence than

would be needed for a space-based weapon. Acontinuous ion source with the necessary cur-rent levels has been operated at the CulhamLaboratory in the United Kingdom with 30-spulses, but not as yet coupled to an accelerator.

Researchers have planned a series of ground-based and space-based experiments to developbeams meeting NPB weapons specifications.It is possible that these experiments would en-counter unknown phenomena such as beam in-stabilities or unexpected sources of increasedbeam divergence, but there are no known phys-ics limitations that would preclude weaponsapplications.

High energy density at the accelerator wouldnot be sufficient for a weapon. The beam wouldhave to be parallel (or well-collimated, or have“low emittance” in accelerator parlance), tominimize beam spreading and maximize en-ergy transmitted to the target. In general,higher energy beams have lower emittance, butsome of the techniques used to increase beamcurrent might increase emittance, possibly tothe point where increased current would de-crease energy coupled to the distant target.With high emittance, the NPB would be ashort-range weapon, and more NPB weaponswould be necessary to cover the battle space.

The divergence of existing, centimeter-diam-eter particle beams is on the order of tens ofmicroradians; this divergence would have tobe reduced by expanding the particle beam di-ameter up to the meter range.6l This large beamwould have to be steered toward the targetwith meter-size magnets. Full-scale magneticoptics have not been built or tested. However,one-third scale optics have been built by LosAlamos National Laboratory and successfullytested at Argonne National Laboratory on a50 MeV beam line.

The weight of the NPB system would haveto be reduced substantially for space-basedoperation. The RF power supply alone for a

81 In theory, beam divergence decreases as the beam size isincreased. In practice, the magnets needed to increase the beamdiameter might add irregularities in transverse ion motion, whichcould contribute to increased beam divergence; not all of thetheoretical gain in beam divergence would be achieved.

140

weapon-class NPB would weigh 160,000 kg(160 tonnes) if based on existing RF radar tech-nology.62 Using solid-state transistors and re-ducing the weight of other components mightreduce RF weight about 22 tonnes.63 One studyconcluded that a total NPB platform weightof 100 tonnes is “probably achievable. ”64 LosAlamos scientists have estimated that theNPB platform weight for an “entry level,” 100-MeV, NPB system could be 50 tonnes. Someday, if high-temperature, high-current super-conductors became available, NPB weightsmight be reduced substantially.

Thermal management on a NPB satellitewould be challenging. A NPB weapon mightproduce 40 MW of waste heat.65 One proposalis to use liquid hydrogen to dispose of this heat.About 44 tonnes of hydrogen could cool theNPB for 500 s.66 The expulsion of hydrogengas would have to be controlled, since even aminute quantity of gas diffused in front of theweapon could ionize the beam, which wouldthen be diverted by the Earth’s magnetic field.Since the hydrogen gas would presumablyhave to be exhausted out opposing sides of thespacecraft to avoid net thrust, it might be dif-ficult to keep minute quantities of gas out ofthe beam.

A state-of-the-art ion accelerator (theRamped Gradient Drift Tube Linac) can raisebeam energy about 4 MeV per meter of acceler-— -.—. . . . .

‘The vacuum-tube (klystron) RF power supply for the PAVEPAWS radar system weighs approximately 2 g/W of power. ANPB weapon wo~d emit an average power of 20 MW (2x107

watts), assuming 200 MeV beams at a current of 0.1 A. Assum-ing an overall efficiency of 25 percent (50 percent acceleratorefficiency and 50 percent beam neutralization efficiency), thepower supply would have to generate 80 MW average power,and would weigh 160 tonnes.

‘This assumes that the RF power is generatid with l-kW,commercial quality power transistors (80,000 transistors wouldbe required for the hypothetical 80 MW supply). These transis-tors can only be operated at 1 percent duty factor. New coolingtechnology wo~d have to be developed to operati at the 100percent duty factor required for a NPB weapon. (See AmericanPhysical Society, op. cit., footnote 1, pp. 149 and 361.) The over-all efficiency of these power supplies would be 40 percent.

“Ibid., p. 152.66Assuming a zoo.d, loo.Itiev” beam, 50 percent neutr~i-

zation efficiency, and 50 percent power generation efficiency.66Assuming heat of vaporization OIlly (450 J/g)* ~d no ‘te-

mperature rise in the hydrogen. If the gas temperature were al-lowed to rise by 100° K, then the hydrogen mass could be re-duced to about 14 tonnes.

ator length. At this gradient rate, a 200-MeVbeam would have to be over 50 m long. Thisaccelerator could be folded, but extra bendingmagnets would increase weight and could re-duce beam quality. The gradient could be in-creased, but if the ion beam energy were in-creased in a shorter length, then there wouldbe more heating in the accelerator walls. Thisimplies another system trade-off: reducinglength in an attempt to cut weight might even-tually reduce efficiency, which would dictateheavier RF power elements and more coolant.Again, future superconductors might amelio-rate this problem.

The beam would have to be steered to inter-cept the target. A NPB would have two ad-vantages over laser beams: the convenience ofelectronic steering and a lesser need for steer-ing accuracy. Magnetic coils could steer nega-tively charged hydrogen ions before the extraelectrons were stripped off. However, the an-gular motion of electronic steering would belimited: the entire accelerator would have tomaneuver mechanically to aim the beam in thegeneral direction of the target cluster. Like la-ser weapons, a NPB must have an agile opti-cal sensor system to track targets. However,the divergence of the NPB is larger than mostlaser beams (microradians versus 20 to 50nanoradians), so the beam steering need notbe as precise.

On the other hand, a hydrogen beam couldnot be observed directly. The particle beamdirection is detected in the laboratory by plac-ing two wires in the beam. The first wire castsa shadow on the second wire placed down-stream. By measuring the current induced inthis downstream wire as the upstream wire ismoved, the beam direction can be estimatedto something like 6 microradian accuracy.

New techniques would be needed to sensethe beam direction automatically with suffi-cient accuracy. One approach utilizes the factthat about 7 percent of the hydrogen atomspassing through a beam neutralization foilemerge in a “metastable” excited state: theelectrons of these atoms acquire and maintainextra energy. Passing a laser through the beamcan make these excited atoms emit light. The

141

magnitude of this fluorescence depends on theangle between the particle beam and the laserbeam. Thus the NPB direction can be deducedand the beam boresighted to an appropriateoptical tracking system. Laboratory tests havedemonstrated 250 microradian accuracy, com-pared to the l-microradian accuracy neces-sary.67 More recent tests at Argonne at 50 MeVhave yielded better results.

The current technique to neutralize thehydrogen ions is to pass them through a thinfoil or a gas cell. This process strips off, atmost, 50 percent of the electrons, cutting theefficiency of the system in half and thus in-creasing its weight. A gas cell is not practicalfor space applications. A stripping foil mustbe extraordinarily thin (about .03 to.1 µm, orten times less than the wavelength of visiblelight). In the proposed NPB weapon, a thin foil1 m in diameter would have to cover the out-put beam. Clearly such a foil could not be self-supporting, but Los Alamos scientists havetested foils up to 25 cm in diameter that aresupported on a fine wire grid. This grid ob-scures about 10 percent of the beam, but hassurvived initial tests in beams with averagepower close to operational levels.

Another beam neutralization concept is touse a powerful laser to remove the electrons-atechnique that some assert may yield 90 per-cent efficiency. However, the laser strippingprocess would call for a 25 MW Nd:YAG laser(near weapon-level power itself), and it wouldeliminate the excited state hydrogen atomsneeded for the laser beam sensing technique.68

Charged hydrogen ions that escaped neu-tralization might play havoc with an NPB sat-ellite. The accumulation of charge might se-verely degrade weapon system performance inunforeseen ways, although NPB scientists areconfident that this would not bean issue.69 TheBeam Experiment Aboard Rocket (BEAR) ex-periment with an ion source and the planned

67See American Physical Society, op. cit., footnote 1, p, 172.‘8 See American Physical Society, op. cit., footnote 1, p. 148.agone ~ugge9ted that the neutralizing foil be thicker so that

two electrons are stripped from some hydrogen ions, formingpositive hydrogen ions (protons) to help neutralize the chargein the vicinity of the spacecraft.

Integrated Space Experiment (ISE) should an-swer any remaaining doubts about space-chargeaccumulation.

Arcing or electrical breakdown that couldshort out highly charged components may alsobe a problem in space. Dust or metal particlesgenerated in ground-based accelerators fallharmlessly to the ground. In space, floatingparticles could cause arcing by forming a con-ducting path between charged components.

Existing accelerators demand many hoursof careful manual alignment before an experi-ment. Neutral particle beam weapons wouldhave to operate automatically in space. Cur-rent plans call for the ATS accelerator at LosAlamos to be automated soon.

Kill assessment might be difficult for weakparticle beam weapons. Damage deep insidethe target might completely negate its func-tion with no visible sign. The choices wouldbe either to forgo kill confirmation or to in-crease NPB energy levels until observabledamage were caused, possibly the triggeringof the high-energy explosive on the RV. Thecurrent plan is to forgo kill confirmation perse, but to increase the NPB power level to as-sure electronic destruction. Sensors would de-termine that the particle beam had hit eachtarget. Experiments are planned to assesswhether UV light emissions would indicatethat a particle beam had struck the surface ofa target.

The planned (and now indefinitely post-poned) ISE illustrates a point made in chap-ter 11 of this report: many BMD weaponswould have ASAT capabilities long before theycould destroy ballistic missiles or RVs. TheISE accelerator, if successful, would haveASAT lethality at close range, although fora limited duty cycle. Beam divergence mightlimit range, but it could probably destroy theelectronics in existing satellites within 500 to1,000 km.70 Even though not aiming a beam

‘This experiment could have nearly BMD-level lethality, pos-sibly raising issues with respect to the ABM Treaty. However,it would not have the necessary beam sensing and pointing orthe computer software and hardware for a BMD weapon; SDIOconsiders the experiment to be treaty compliant.

142

at other than a target satellite, this experimentconceivably might disrupt nearby satellite elec-tronics. Although this may not be serious (cal-culations indicate that it should not), there isenough uncertainty to cause ISE planners toask whether they should wait until the SpaceShuttle had landed before turning on the ISE.

X-ray Laser. —The nuclear bomb-driven x-ray laser is the least mature DE W technology.

To date this program has consisted of theo-retical and design work at Livermore NationalLaboratory and several feasibility demonstra-tion experiments at the underground Nevadanuclear weapons test site. Actual x-ray gener-ation technology may or may not reach suit-able levels in the years ahead; currently themethods to convert this technology into a via-ble weapons system remain paper concepts.

POWER AND POWER CONDITIONINGThe average electrical power consumed by

some proposed BMD spacecraft during bat-tle might be factors up to 100,000 over cur-rent satellite power levels. Most existing sat-ellites are powered by large solar arrays thatwould be vulnerable to defense suppression at-tack. To provide sufficient survivable powerfor space applications, most BMD satelliteswould require either nuclear reactors, rocketengines coupled to electrical generators, or ad-vanced fuel cells.

In addition to high average power, some pro-posed weapon satellites would demand highpeak power: energy from the prime source, ei-ther a nuclear reactor or a rocket-driven turbo-alternator, would have to be stored and com-pressed into a train of very high current pulses.For example, a railgun might expend 500 MJof energy in a 5-millisecond (ins) pulse, or 100GW of peak power. This is about 1,000 timesmore than current pulse power supplies candeliver.

The following sections outline satellite powerdemands and the technologies that mightsatisfy them. While space systems would callfor the primary advances, ground-based FELswould also depend on advances in pulsed-powersupply technology. Some of the technology de-veloped for space-borne neutral particle beamsystems, such as RF power sources, might beapplicable to FELs.

Space Power Requirements

Estimates of power needs of space-basedBMD systems are summarized in table 5-9.Since most of these systems have not been de-signed, these estimates could change signifi-cantly: the table only indicates a possible rangeof power levels. Power is estimated for threemodes of operation: base-level for general sat-ellite housekeeping and continuous surveil-lance operations lasting many years; alert-levelin response to a crisis, possibly leading to war;

Table 5-9.—Estimated Power Requirements for Space Assets(average power in kilowatts)

Mode of operation Base Alert Burst (battle)BSTS . . . . . . . . . . . . . . . . . . . . . . 4-1o 4-1o 4-1oSSTS (IR) . . . . . . . . . . . . . . . . . . 5-15 5-15 15-50

Ladar. . . . . . . . . . . . . . . . . . . . 15-20 15-20 50-100Ladar imager . . . . . . . . . . . . . 15-20 15-20 100-500Laser illumination . . . . . . . . . 5-1o 5-1o 50-100Doppler Iadar. . . . . . . . . . . . . 15-20 15-20 300-600

SBI carrier . . . . . . . . . . . . . . . . . 2-30 4-50 1o-1ooChemical laser . . . . . . . . . . . . . 50-100 100-150 100-200Fighting mirror . . . . . . . . . . . . . 10-50 10-50 20-100NPB/SBFEL . . . . . . . . . . . . . . . . 20-120 1,000-10,000 100,000-500,000EML (railgun) . . . . . . . . . . . . . . . 20-120 1,000-10,000 200,000-5,000,000

SOURCE: Space Defense Initiative Organization, 1988.

I143

and burst-mode for actual battle, which maylast hundreds of seconds.

In addition to average-power and surviva-bility perquisites, a space-based power systemwould have to be designed to avoid deleteri-ous effects of:

● thrust from power-generating rockets up-setting aiming,

● torque due to rotating components,● rocket effluent disrupting optics and beam

propagation,● vibration on sensors and beam steering,● thermal gradients, and● radiation from nuclear reactors.

Power systems would also have to operatereliably for long periods unattended in space.

Space Power Generation Technology

There are three generic sources of electricalpower in space: solar energy, chemical energy,and nuclear energy.

Solar EnergySolar panels have supplied power for most

satellites. The sun produces about 1.3 kW ofpower on every square meter of solar array sur-face. An array of crystalline silicon cells con-verts the sun’s energy into direct electrical cur-rent through the photovoltaic effect, with anefficiency of about 10 percent. Thus a l-m2

panel of cells would produce about 130 wattsof electricity, assuming that the panel were ori-ented perpendicular to the sun’s rays. A 20-kW array, typical for a BMD sensor, wouldthen have about 150 m2–roughly, a 12-m by12-m array. The Skylab solar array, the largestoperated to date, produced about 8 kW. NASAhas built, but not yet flown in space, a 25-kWexperimental solar array designed to supplyspace station power.

The major disadvantage of solar arrays isthat their large size makes them vulnerable toattack. Crystalline silicon photovoltaic cells arealso vulnerable to natural and man-made ra-diation. One approach to reduce both vulnera-bilities to some degree would be to concentratethe sun’s rays with a focusing optical collec-

tor. The collector would still be vulnerable, butif the system efficiency could be improved, thenthe area of the collector would be smaller thanequivalent ordinary solar cell arrays.

There are two other ways to convert theenergy from solar collectors into electricity.One is to use solar thermal energy to drive aconventional thermodynamic heat engine. Theother is to focus sunlight on more radiation-resistant and higher-efficiency photovoltaiccells such as gallium arsenide. Depending onthe temperature of the working fluid in a ther-modynamic heat engine cycle, efficiencies of20 percent to 30 percent might be achieved.Gallum arsenide cells have shown up to 24 per-cent efficiency in the laboratory, so 20 percentefficiency in space may be reasonable. Thus,either technology could cut the required col-lector area in half compared to conventionalsolar cells, or 75 m2 per 20-kW output. Neitherapproach has been tested in space, but NASAis pursuing both for future space applications.

Nuclear EnergyNuclear energy has also been used in space.

There are two types of nuclear energy sources:radioactive isotope generators that convertheat from radioactive decay to electricity, andnuclear fission reactors. Both have flown inspace, but the radioactive isotope generatoris more common.

Both radioactive decay and a controlled fis-sion reaction produce heat as the intermedi-ate energy form. This heat can be convertedinto electricity by static or dynamic means.A static power source produces electricitydirectly from heat without any moving parts,using either thermoelectric or thermionic con-verters. These converters generate direct cur-rent between two terminals as long as heat issupplied to the device. The efficiency and to-tal practical power levels are low, but for ap-plications of less than 500 W, the advantageof no moving parts makes a radioisotope ther-moelectric generator (RTG) a primary candi-date for small spacecraft.

To produce more than 500 W, a radioisotopesource could be coupled to a dynamic heat en-

144

gine. One dynamic isotope power system(DIPS) with 2 to 5 kW output has been ground-tested. This system weighs 215 kg. However,the U.S. production capacity for radioactiveisotopes would limit the number of satellitesthat could be powered by DIPS.

BMD satellites needing more than 5 to 10kW of power might carry a more powerful nu-clear fission reactor. Static thermoelectric con-verters would still convert the heat to electri-city. This is the approach proposed for theSP-100 space power program, the goal of whichis to develop elements of a system to providepower over the range of 10 to 1,000 kW. TheDepartments of Defense and Energy andNASA are producing a reference design incor-porating these elements to produce a 100-kWtest reactor.

71 This is the major focus for thenext generation of space power systems.

The SP-100 reactor, as currently designed,would use 360 kg of highly enriched uraniumnitride fuel with liquid lithium cooling operat-ing at 1,3500 K. This heat would be conductedto 200,000 to 300,000 individual thermoelec-tric elements which would produce 100 kW ofelectricity. The overall efficiency of the sys-tem would be about 4 percent, which wouldentail the disposal of 2.4 MW of waste heat.Large fins heated to 8000 K would radiate thisheat into space.

The SP-100 program faces numerous chal-lenges. In addition to being the hottest run-ning reactor ever built, the SP-100 would be

The estimated mass of the SP-100 is 3,000kg, or a specific mass of 30 kg/kW. Originalplans called for building a ground-test proto-type SP-100 based on the 100-kW design by1991, with a flight test several years later. Sub-sequently a 300-kW design was consideredwhich would have pushed initial hardwaretoward 1993, but current schedules are fluiddue to uncertain funding.

the first space system to: To produce power levels in excess of a few●

●

●

ž

●

●

use uranium nitride fuel,be cooled by liquid lithium,use strong refractory metals to contain theprimary coolant,have to start up with its coolant frozen,have two independent control mechanisms(for safety), anduse electronic semiconductors under suchintense heat and radiation stress.72

710riginal SDIO plans called for designing a 300 KW system,but as of this writing the goal has been reduced to 100 KW.

‘*See Eliot Marshall, “DOE’s Way-out Reactors,” Science,231:1359, March 21, 1986.

hundred kW, one would have to take the nextstep in the evolution of space nuclear powersystems: a nuclear reactor coupled through adynamic heat engine to an electrical genera-tor. In principle, large reactors in space couldgenerate hundreds of MW, satisfying the moststressing BMD average power demands.

A “multimegawatt,” or MMW, project hasbegun to study some of the fundamental is-sues raised by large reactors, including daunt-ing engineering challenges such as high tem-perature waste heat disposal in space, safetyin launch, operation, and decommissioning.These large nuclear systems might have to be

745

operated “open-cycle,” requiring much “fuel”in the form of cooling gas to dispose of excessheat. At this writing the MMW project is inthe conceptual phase with no well-defined re-search program. Multi-megawatt nuclear re-actors in space would have to be considereda 20-to-30 year project.

In summary, space nuclear power systemswould require extensive development toachieve reliable space operation at the 100-300kW level by the mid-to-late 1990s. Given cur-rent engineering and budget uncertainties, de-velopment of megawatt-class nuclear powersystems for space cannot be projected untilwell into the 21st century.

Chemical EnergySatellites frequently employ chemical energy

in the form of batteries, fuel cells, and tur-bogenerators. Batteries would be too heavy formost BMD applications, except possibly forpop-up systems with very short engagementtimes. Fuel cells, which derive their power bycombining, e.g., hydrogen and oxygen, are un-der active consideration for driving the acceler-ators of NPB weapons.

For the short bursts of MMW power neededby some BMD weapons, an electrical genera-tor driven by a rocket engine (e.g., burning liq-uid hydrogen and liquid oxygen) might be theonly available technology in the foreseeable fu-ture. The Space Shuttle main engine (SSME)develops about 10 GW of flow power, whichcould generate 5 GW of electrical energy if itcould be coupled to a turboalternator. Alter-natively, rocket exhaust could, in principle, beconverted to electricity by magnetohydro-dynamics (MHD).

The engineering challenges of using rocketengines to produce electrical power on boarda BMD satellite are posed not only by thegenerator itself, but by its effects on other com-ponents such as sensors, electronics, and weap-ons. Two counter-rotating and counter-thrust-ing rockets would probably be essential tocancel torque and thrust. Even then, sensorsand weapons-aiming devices would have to beisolated from vibration. Similarly, the effluent

from the rocket engines must not interfere withsensors or weapon beam propagation, and elec-trical noise must not interfere with communi-cation or data processing electronics.

It might be necessary to place rocket enginesand power generators on separate platformshundreds or a few thousands of meters awayto achieve the necessary isolation, transmit-ting power by cable or microwaves. Thismethod, however, would raise vulnerability is-sues, presenting to the adversary an additionaltarget and a vulnerable umbilical cord.

Power Conditioning

Power conditioning is matching the electri-cal characteristics of a power source with thoserequired by the load. A generator might pro-duce a continuous flow of electrical current,but a load, such as railgun firing, would requirea series of very high-current pulses. Power con-ditioning equipment would convert the contin-uous flow into pulses.

In some cases the projected power condition-ing device requirements exceed existing capa-bilities by two or three orders of magnitude,even for ground-based experiments. In manyareas, no space-qualified hardware exists atany power level. Pulsed power technology de-velopment efforts are underway in capacitiveand inductive energy storage, closing and open-ing switches, transformers, RF sources, AC-DC converters, and ultra high-voltage tech-niques and components.

Particle accelerators that drive the FEL andthe NPB use RF power. Railgun requirementswould present the greatest challenge: veryshort (millisecond) pulses of current severaltimes a second. Many electrical componentswould have to be developed to produce theproper current pulses for a railgun.

A homopolar generator combined with an in-ductor and opening switch is now the primarycandidate for the generation of very shortpulses. A homopolar generator is a rotatingmachine that stores kinetic energy in a rotat-ing armature. At the time of railgun firing,brushes would fall unto the armature, extract-

146

ing much of its energy in a fraction of a sec-ond. This would result in a sudden jerk in thetorque of the generator, which would disturba spacecraft unless compensated by a balancedhomopolar generator rotating in the oppositedirection.

The brushes would also wear out, whichraises questions about the durability of a rail-gun with high repetition rates. Very fastswitches would be essential. These switcheswould have to be light enough to move rap-idly, but heavy enough to handle the extraor-dinarily high currents.

Researchers at the University of Texas haveinvestigated one advanced modification to thehomopolar generator. They have replacedbrushes and switches with inductive switchesin a “compulsator,” a generator which pro-duces a string of pulses. By replacing non-current carrying iron with graphite-epoxy com-posites, these compulsators could be muchlighter than the homopolar generators.

While space applications drive power devel-opment requirements, emerging ground-baseddefensive systems would also stress existingpower sources. Ground-based BMD elementsmight require diesel and turbine driven elec-tric generators and MHD generators for mo-bile applications. A fixed-site system such asthe FEL might draw on the commercial util-ity grid, dedicated power plants, or supercon-ducting magnetic energy storage (SMES). Theelectrical utility grid could meet peacetimehousekeeping power needs and could keep astorage system charged, but, due to its extremevulnerability to precursor attack, could not be

relied on to supply power during a battle.Therefore, a site-secure MMW power systemwould probably be necessary.

Superconducting magnetic energy storageis a prime candidate for ground-based energystorage; an SMES system would be a large,underground superconducting coil with con-tinuous current flow. The science of SMES iswell established, but engineering developmentremains.

Recent discoveries of high-temperature su-perconductors could have an impact on futurepower supplies and pulse conditioning sys-tems. Given the likely initial cost of manufac-turing exotic superconducting materials andthe probable limits on total current, their firstapplications will probably be in smaller devicessuch as electronics, computers, and sensor sys-tems. But if:

●

●

scientists could synthesize high temper-ature superconducting materials able tocarry very large currents; andengineers could develop techniques tomanufacture those materials on a largescale suitable for large magnetic coils, RFpower generators, accelerator cavity walls,the rails of electromagnetic launchers, etc.;

then superconductors could substantially re-duce the power demand. Efficiency of thepower source and power conditioning networkscould also be improved. High temperature su-perconductors would be particularly attractivein space, where relatively cold temperaturescan be maintained by radiation cooling.

COMMUNICATION TECHNOLOGY

Communication would be the nervous sys- dreds to several thousands of weapons plat-tem of any BMD system. A phase-one defense forms in low-Earth orbits. “would include hundreds of space-based com-ponents, separated by thousands of kilometers, Three fundamental communication pathsfor boost and post-boost interception. A would link these space assets: ground to space,second-phase BMD system would include space to space, and space to ground. Groundmany tens of sensors in high orbits and hun- command centers would at least initiate the

147

battle; they would also receive updates onequipment status and sensor data in peacetimeand as the battle developed.

The attributes of an effective communica-tion system would include:

● adequate bandwidth and range,● reliability,● tolerance of component damage,● security from interception or take-over,● tolerance of nuclear effects, and● jam- or spoof-resistance.The bandwidths, or frequency space avail-

able, from links in the millimeter-wave bandswould be adequate for most near-term BMDfunctions. The most demanding element wouldbe the boost surveillance and tracking system(BSTS) satellite, with perhaps a l-million-bit-per-second data rate. Second-phase elementssuch as a space surveillance and tracking sys-tem (SSTS) sensor satellite might operate atmuch higher rates, up to 20 million bits persecond, while battle management might take50 or more million bits per second of informa-tion flow. Various additional data for syn-chronization signals would have to be commu-nicated. Transmission bandwidths might haveto be very large-perhaps 1-10 gigahertz (GHz)–to reduce the chances of jamming.

The communication system must be dura-ble and survivable even if some nodes fail dueto natural or enemy action. Redundant linksin a coupled network might assure that mes-sages and data got through even if some sat-ellites were destroyed. Tying together a vastBMD space network would be challenging,especially given that the satellites in low-Earthorbit would constantly change relative po-sitions.

One key issue for BMD communications isjamming by a determined adversary. Success-fully disrupting communications would com-pletely negate a BMD system that relied onsensors and command and control nodes sep-arated from weapons platforms by tens of thou-sands of km. Jammers could be developed, de-ployed, and even operated in peacetime withlittle risk of stimulating hostile counteraction.

Space-to-ground communication links wouldbe particularly vulnerable. Ground-based, ship-based or airborne high-power jammers mightblock the flow of information to satellites. Inwartime, nuclear explosions could disrupt thepropagation of RF waves. Ground-basedreceivers would also be susceptible to directattack. Even space-based communicationswould be susceptible to jamming.

Recently there have been two primary SDIcandidates for BMD communication links inspace: laser links and 60-GHz links. A 60-GHzsystem once seemed to promise a more jam-resistant channel for space-to-space commu-nications, since the atmosphere would absorbenough 60-GHz energy to reduce the threat ofground-based jammers. Recent analyses, how-ever, indicate that space and air-based jam-mers may limit the effectiveness of 60-GHzlinks.

60-GHz Communication Links

The operating frequencies of space commu-nication systems have been steadily increas-ing. For example, the Milstar communicationssatellite will use the extremely high frequency(EHF) band with a 44-GHz ground-to-space up-link and a 20-GHz downlink. These high fre-quencies allow very wide bandwidth (1 GHzin the case of Milstar) for high data transmis-sion rates, but also for more secure communi-cations through wide-band-modulation andfrequency-hopping anti-jamming techniques.

For space-to-space links, BMD designers areconsidering even higher frequencies-around60 GHz. This band includes many oxygen ab-sorption lines. It would be very difficult forground-based jammers to interfere with 60-GHz communications between, for example,a BSTS early warning satellite and SBI CVs:oxygen in the atmosphere would absorb thejamming energy.

Pre-positioned jammer satellites, or possi-bly rocket-borne jammers launched with an at-tack, might still interfere with 60-GHz chan-nels. The main beam of radiation from a60-GHz transmitter is relatively narrow, mak-

148

ing it difficult for an adversary to blind thesystem from the main lobe: the enemy jam-mer transmitter would have to be located veryclose to the BMD satellite broadcasting itsmessage. But a 60-GHz receiver would alsopick up some energy from the “sidelobes” andeven some from the opposite side of the receiver(the “backlobes”). While a receiver may be10,000 to 100,000 times less sensitive to energyfrom these sidelobes than from the main lobe,it must be extremely sensitive to pickup sig-nals from a low-Earth orbit satellite tens ofthousands of km away.

At high-Earth orbit, a near-by jammer withonly a few hundred watts of power could over-whelm a much more powerful 60-GHz systemon a sensor satellite. This neighbor mightmasquerade as an ordinary communicationssatellite in peacetime. In wartime, it could aimits antenna at the BSTS and jam the channel.The countermeasure would be to station theBSTS out of standard communications satel-lite orbits.

Laser Communication Links

The low-power diode laser offers the possi-bility of extremely wideband, highly direc-tional, and, therefore, very jam-proof commu-nications. The MIT Lincoln Laboratory hasdesigned a 220 megabit-per-second (Mbs) com-munication link that would need just 30 mil-liwatts of laser power from a gallium alumi-num arsenide (GaAIAs) light emitting diode(LED) to reach across the diameter of the ge-osynchronous orbit (about 84,000 km). The re-ceiver, using heterodyne detection,73 could pull

in a signal of just 10 picowatts (10-11 W)power. A 20-cm mirror on the transmitterwould direct the laser beam to an intended re-ceiver.

The high directionality of narrow laserbeams also complicates operation. A wide-angle antenna could flood the receiver area withsignal, even sending the same message to manyreceivers in the area at one time. A narrow la-ser beam must be carefully aimed at each sat-ellite. This would require mechanical mirrorsor other beam-steering optics, as well as soft-ware to keep track of all friendly satellites andto guide the optical beam to the right satel-lite. The lifetime of a laser source and an agileoptical system may be relatively short for thefirst few generations of laser communicationsystems.

A laser communication system, as presentlydesigned, would require up to eight minutesto establish a heterodyne link between a trans-mitter and a receiver. Plans call for reducingthis acquisition time to one minute. With avery narrow laser beam, even minute motionsof the transmitter platform could cause amomentary loss of coupling, forcing a delayto reacquire the signal.

While laser links might provide jam-proofcommunications between space-based assetsof a BMD system, laser communications to theground would have to overcome weather limi-tations. One approach would use multiplereceivers dispersed to assure one or more clearweather sites at all times. Alternatively, onecould envisage an airborne relay station, par-ticularly in time of crisis.

“The common “heterodyne” radio receiver includes a localoscillator which generates a frequency that is combined or“mixed” with the incoming radio signaL This process of “mix-ing” the local oscillator signal with the received signal improvesthe ability to detect a weak signal buried in noise, and reduces

interference. A laser heterodyne receiver would include its ownlaser source, which would be “mixed” at the surface of a lightdetector with the weak light signal from a distant laser trans-mitter.

SPACE TRANSPORTATION

Reasonable extensions of current U.S. space launch capabilities would be necessary to lifttransportation capability might launch the several hundred to over one thousand carriertens of sensor satellites envisaged by some vehicles and their cargoes of thousands to tensBMD architectures, but entirely new space of thousands of kinetic kill missiles into space

149

in a reasonable period of time. Therefore spacelaunch capability would have to evolve alongwith phase-one and phase-two weapon systemsto assure the United States—and to persuadethe Soviet Union-that a defense-dominatedworld would be feasible and enduring.

Space Transportation Requirements

Space-based interceptors and their carriersatellites would dominate initial space deploy-ment weights. Assuming that a phase-one de-ployment would include a few hundred CV\sSand a few thousand SBIs based on the "state-of-the-art” rockets described above, then to-tal launch weight requirements might be in therange of 1 million to 2 million kg.

The range of weights estimated by SD I sys-tem architects for a more advanced phase var-ied from 7.2 to 18.6 million kg. The large rangeof weight estimates reflects differences in ar-chitectures, and particularly differences in sur-vivability measures. Several contractors indi-cated that survivability measures—such asshielding, decoys, proliferation, and fuel formaneuvering-would increase weight by a fac-tor of about three. One could infer that theheavier designs might be more survivable.

Additional space transportation would be re-quired over time for servicing, refueling, orreplacement of failed components. One un-resolved issue is how best to maintain this fleetof orbiting battle stations: by originally includ-ing redundant components such as intercep-tor missiles on each satellite, by completereplacement of defective satellites, by on-orbitservicing, or by some combination of the above.One contractor estimated, for example, thatit would take 35 interceptor missiles on eachbattle station to assure 20 live missiles after10 years, with the attrition due entirely to nat-ural component failures.

Soviet countermeasures might drive upweight requirements substantially in lateryears. Increased Soviet ICBM deploymentsmight be countered with more SB I platforms.Defense suppression threats such as direct-ascent ASATs might be countered in part byproliferation of SBI battle stations or by other

heavy countermeasures. Advanced decoys dis-persed during the post-boost phase of missileflight might require some type of interactivediscrimination system in space. Reduced So-viet booster burn times would eventually im-pel a shift to DE W. Deploying these counter-measures would necessitate additional spacetransportation capability. Directed-energyweapon components in particular would prob-ably be very heavy. The range of SDI systemarchitects’ estimates for some far-term sys-tems was from 40 million to 80 million kg.

Space Transportation Alternatives

There seem to be two fundamental optionsfor lifting the postulated BMD hardware intospace: use derivatives of existing space trans-portation systems; or design, test, and buildanew generation space transportation system.The first option might be very costly; the sec-ond might postpone substantial space-basedBMD deployment into the 21st century.

Some BMD advocates outside the SDIOhave suggested that existing United Statesspace launch systems might be adequate foran initial spacebased BMD deployment in theearly 1990s. But the existing United Statesspace launch capability is limited in vehicle in-ventory, payload capacity per launch, cost,launch rate, and launching facilities. As shownin table 5-10, today’s total inventory of U.S.rockets could lift about 0.27 million kg intolow-Earth orbit (180 km) at the inclination an-gle of the launch site (28.50 for the KennedySpace Center in Florida) .74

The bulk of early SBI deployments wouldhave to be launched into near-polar orbits fromVandenberg AFB, which would now only bepossible for the 6 remaining Titan 34D vehi-cles with a combined lift capacity of 75,000 kg.

“Missile launch capacity is usually specified in terms of thepayload which can be lifted into direct East-West flight at analtitude of 180 km, which produces an orbit inclined at the lati-tude of the launch point. Extra propellant is required to lift thepayload to higher inclinations or to higher altitudes. ProposedBMD weapons systems would require higher inclinations (700to850, and higher altitudes (600 to 1,000 km), which translatesinto lower payload capacity.

150

This would correspond to about 6 percent ofthe initial phase-one BMD space deploymentrequirements. Some have suggested refurbish-ing Titan-IIs, which have been retired from theICBM fleet. If all 69 Titan-IIs were refur-bished, then the United States could liftanother 130,000 kg into polar orbit, or another11 percent of the near-term BMD needs.

The rate of missile launch might also belimited by the existing space transportationinfrastructure. Launching one Shuttle nowtakes a minimum of 580 hours at the KennedySpace Center (and might take about 800 hoursat Vandenberg AFB75), limiting potentiallaunches to one per month or less from eachcomplex. After the Shuttle accident, NASAestimated that 12 to 16 flights per year wouldbe reasonable. Clearly 16 launches per yearwould not be sufficient for BMD deployment.76

Several aerospace companies have proposedbuilding launch vehicles with increased lift ca-pacity to meet SDI, DoD, and civilian spacetransportation demands. Many of these vehi-cles would be derived from various Shuttle or

75Completion of the Vandenberg Shuttle launch site SLC-6has been postponed until 1992.

‘“Assuming 16 Shuttle launches per year with 9,000 kg pay-load to low polar orbit, it would take between 8 to 12 years todeploy a phase-one BMD system and 48 to 125 years to deploya phase-two system weighing 7 to 18 million kilograms.

Titan predecessors, such as the Titan-4, in-cluded in table 5-10. Twenty-three Titan-4s willbe built by 1988, but these have only margin-ally increased lift capacity. A major increasein lift capacity to the 40,000 to 50,000 kg rangewould be required for an effective space-basedBMD system. Even for a phase-one system,far more would be needed by the mid-1990s.Both SDIO and Air Force officials have calledfor anew space transportation system that isnot a derivative of existing technology.

Four aerospace companies analyzed variousspace transportation options under joint AirForce/NASA/SDIO direction. The Space Trans-portation Architecture Study (STAS) com-pared manned v. unmanned vehicles, horizon-tal v. vertical takeoff, single v. 2-stage rockets,and various combinations of reusable v. ex-pendable components.77 The &r Force, afterreviewing the initial STAS work, appears tobe leaning toward a decision that the BMD de-ployment should use an unmanned, expenda-ble, 2-stage heavy-lift launch vehicle (nowcalled the ALS or advanced launch system) .78

TTThe spice Trmsportation Architecture Study (S’I’AS) wasa joint Air Force/NASA/SDIO study on future space transpor-tation systems. The Air Force Systems Division contracted withRockwell and Boeing, while NASA employed General Dynamicsand Martin Marietta to analyze U.S. civilian and military spacerequirements and possible alternatives to satisfy them.

7sThe name HLLV (heavy lift launch vehicle) was changedto ALS in April 1987.

Table 5-10.—Current U.S. Space Launch Inventorya

Payload per vehicle (thousands of kg)

Inventory LEO Polarquantity (180 km) (180 km) Geo

Shuttle . . . . . . . . . . . . . . . 3 25 15 Centaur-G:4.5IUS: 2.3

Titan 34D . . . . . . . . . . . . . 15,3 12.5 IUS: 1.8Titan-4 b . . . . . . . . . . . . . . (23) 17.7 14.5 Centaur-G:4.6

IUS: 2.4Titan 11-SLV . . . . . . . . . . . (13)C 3.6 1.9Delta. . . . . . . . . . . . . . . . . 2.9Delta (MLV) . . . . . . . . . . . (7) 4 1.5Atlas. . . . . . . . . . . . . . . . . 13 6scout . . . . . . . . . . . . . . . . 21 .26(ALS) d . . . . . . . . . . . . . . . . ? (50-70) (40-55)aparentheses indicate future SyStemS.bThe Titan.4 or the complementary Expendable Launch Vehicle (CELV) iS the latest in the line Of Titan miSSile configurations;23 have been ordered.

CThe Titan 11.sLvs are Ming refurbished from the ICBM inventory. The first Titan-n may be available by 1989. An additional56 Titan II could be refurbished from the retired ICBM fleet.

dTh e Advanced Launch System is proposed to deploy the bulk of the BMCI SpaCe CO171pOIlWlk.


151

An interim STAS study suggested that sucha vehicle would have to evolve to a partiallyreusable system to meet SDIO cost reductiongoals. The STAS contractors projected thatdevelopment of a heavy-lift unmanned vehi-cle would require about 12 years, although atleast one aerospace company estimates thatan ALS could be developed in 6 years. If theoriginal 12-year estimate is correct, significantspace deployment of a BMD system could notbegin until the turn of the century even if theweapon systems were ready earlier. If the 6-year estimate were correct, then initial deploy-ment could begin by 1994.

To deploy space-based assets earlier, SDIOhas suggested a two-tier level program: buildpart of an ALS by the mid-1990s, but designthis system to evolve into the long-range sys-tem by the year 2000. The initial system wouldinclude some of the advanced features of theheavy-lift launch vehicle concepts outlined bySTAS, but would not have a fly-back boosterand would not meet the SDIO cost goals of$300 to $600 per kilogram. The interim goalwould be to reduce the current costs of $3,000-$6,000 per kilogram to $1,000-$2,000. Build-ing a space transportation system while try-ing to meet these two goals simultaneouslycould be risky. Compromises might be requiredeither to meet the early deployment date orto meet the long-term cost and launch rategoals.

The estimated launch rate for a fully devel-oped ALS vehicle is about once per month perlaunch complex.79 Assuming a 40,000-kg pay-load to useful BMD orbits, then between 30and 45 successful flights would be required fora phase-one BMD deployment and from 180to 460 flights for a much larger second phase.Allocating 5 years to deploy the latter system,the United States would need to build threeto eight new launch facilities.80

—. -—‘YFhe current maximum launch rate for Titans is three per

year from each pad, which might be increased to five per year.Further increases are unlikely because the Titans are assem-bled on-site. This is one of the reasons an entirely new spacelaunch system would be needed to meet the SDI launch rates.

*’The United States now has four launch pads for Titan-classboosters, two on the east coast and two on the west coast. Onewest coast pad is being modified to handle the CELV. Since

Photo credit: U.S. Department of Defense,Strategic Defense Initiative Organization

Advanced launch system (ALS).—Large-scale deploy-ment of space-based interceptors (SBI) or otherweapons in space will require a dramatic expansionof US. space-launch capabilities. Various proposals,including a Shuttle-derived, unmanned launch vehicle

such as this have been under consideration bythe Air Force, NASA, and the SDIO.

Figure 5-13 presents one very optimistic scenario which might lead to space launch facil-ities adequate for proposed second-phase BMD

SBIs would have to be launched from Vandenberg to reach near-polar orbits, all early deployments would have to be from onepad. The estimated time to build a new launch pad complexis 7 to 10 years,

152

Figure 5-13a. —Annual Space Launch Capacity(near polar orbits at 800 km)

1985 1990 1995 2000 2010Year

This is one possible scenario to achieve the 2 million kg peryear space launch capability into near-polar orbits requiredfor an intermediate ballistic missile defense system. This sys-tem could conceivably reach this goal by the year 2003, as-suming that three new launch pads were built at VandenbergAFB, and the proposed Heavy Lift Launch Vehicle (HLLV)/Ad-vanced Launch System (ALS) could be developed, flighttested, and ready for initial service with 30,000 kg lift capac-ity by 1994. This would be 5 years ahead of the schedule ini-tially suggested by the Space Transportation ArchitectureStudy (STAS). The HLLV is further assumed to evolve intoa 44,000 kg capability by the year 2000, without any engineer-ing delays. The SDIO launch goals as of early 1987 are shownfor comparison.


systems by 2000-2005. This scenario assumesthat the SDIO two-level space transportationdevelopment approach would be successful: aninterim ALS vehicle, with a 30,000 kg capa-bility to near-polar orbits, would be availableby 1994; a more advanced ALS would comeonline in 2000 with 44,000 kg capacity. Threenew launch pads would be built (although thereis no room for three new pads at VandenbergAFB, the only existing site in the contiguousUnited States with near-polar orbit capability).

Assuming approval to proceed with the newlaunch system in 1988, the first flights of thenew ALS would begin in 1994, using the refur-bished SLC-6 launchpad at Vandenberg, builtoriginally for the Space Shuttle. The three newpads would become operational in 1997,1998,and 1999. Flights would be phased in at eachsite, increasing up to 12 flights per year perpad. With these assumptions, the SDIO goal

Table 5-il.—Space Transportation

Vehicle capacity to 600 km, high inclination:(thousands of kilograms)

CELV (Titan-4) 115Titan 34DEarty HLLV 30 (1995-2000)Final HLLV 44 (2000+)

TotalNumber of launches per year annual

launchLaunch pads: 4-East: SLC-6

(34D/CEL) (HLLV) (HLLV) (HLLV) (HLLV)Capacity(M kg)

Year1965 3 0031986 3 0031987 3 0031966 3 0031989 4 0051990 4 0051991 5 0.061992 5 0.061993 6 0071994 6 1 0101995 7 2 0141996 7 6 0261977 6 6 1 0361998 6 10 2 2 0511999 6 12 4 4 2 0752000 6 12 6 6 4 1322001 6 12 8 8 6 1.59

6 12 10 10 6 1652000 6 12 12 12 10 2122004 6 12 12 12 12 220

6 12 12 12 12 2.202006 8 12 12 12 12 2202007 12 13 12 12 225

82008 12 13 13 12 2292009 6 12 14 13 13 2362010 6 12 14 14 13 242

Tabular data for figure 3-13a.


of 2 to 2.5 million kg per year could be achievedby 2003.

If the United States were to operate 10launch facilities, each with one ALS launch permonth, then it would take about 10 years toorbit the 50 million kg estimated for a far-term,third-phase system.81 If political or strategicconsiderations (such as transition stability)would not allow as long as 10 years to deploy,then the United States would have perhapsthree choices:

1. develop another new vehicle with lift ca-pacity above 50,000 kg to 800-km, highinclination orbits;

2. build and operate more than ten ALSlaunch facilities simultaneously; or

ElThe 50 fion kg assumes the low end of the 40 to 80 ~-iion kg estimated above for phase three with spacebased lasers.A successful ground-based laser system could reduce this esti-mate by about 15 million kg, or 25 to 65 million kg for a totalphase-three constellation.

153

3. improve launch operations to reduce turn-around time below 30 days per pad.

The country would have to expand boostermanufacturing capacity to meet this demandfor up to 120 launches per year. Historically,Titan production lines completed up to 20 mis-siles per year, and Martin Marietta has esti-mated that it could easily produce 14 of theTitan class per year with existing facilities.82

Space Transportation Cost Reduction

Identifying 42 technologies related to spacetransportation, the STAS listed several whereresearch might lead to reduced operating costs(it emphasized the first three as offering espe-cially high leverage for cost reduction):

●

●

●

●

●

●

●

●

●

lightweight materials,expert systems and automated program-ming to cut software costs,better organization,reducing dry weights substantially,better ground facilities,higher performance engines,fault-tolerant avionics,reusability of major components, andbetter mating of spacecraft to launch ve-hicle for reduced ground costs.

— ..———sZThiS would inClu& 5 CELVS, 6 Titan 11s, and 3 Titan 34Ds.

The operating (as opposed to life-cycle) costsof space transportation are currently estimatedat $3,300 to $6,000 per kilogram of payloadto low-Earth orbit, and $22,000 to $60,000 perkilogram to geosynchronous orbit. At thatrate, it would cost $24 billion to $200 billionto launch a phase-two BMD system, and $140billion to $450 billion for a responsive phase-three deployment, based on the constellationweights estimated by various SDIO systemarchitects. The SDIO has set a goal of reduc-ing launch operating costs by a factor of 10.

Operating costs are estimated at about onethird of the total life-cycle costs of a spacetransportation system. Based on current oper-ating costs, total life-cycle costs for transport-ing a phase-two BMD system into space mightbe $72 billion to $600 billion; for phase three,the costs might range from $420 billion to$1.35 trillion. Reaching the goal of reducingoperating costs by a factor of 10 would reducelife-cycle costs for space transportation by only30 percent. Assuming that this percentagewould be valid for a new space transportationsystem, and assuming a 10 to 1 reduction inoperating costs only, then the total life-cyclecosts for space transportation might be $50billion to $420 billion for a phase-two deploy-ment and $290 billion to $900 billion for aphase-three deployment. Clearly the other kindsof costs for space transportation would have tobe reduced along with the operating costs.

CONCLUSIONS

Weapon Technology Conclusions

Phase One

Kinetic Energy Weapons.—KEWs (or else thekinds of nuclear-armed missiles developed forBMD in the 1960s) would most likely be the onlyBMD weapons available for deployment in thiscentury and possibly the first decade of the 21stcentury. Several varieties of non-nuclear, hit-to-kill KEW form the backbone of most near-and intermediate-term SDI architecture pro-posals. Considering the steady evolution ofrockets and “smart weapon” homing sensors

used in previous military systems, it seemslikely that these KEWs could have a high prob-ability of being able to destroy individual tar-gets typical of the current Soviet ICBM forceby the early to mid-1990s. The key unresolvedissue is whether a robust, survivable, in-tegrated system could be designed, built,tested, and deployed to intercept—in the faceof likely countermeasures—a sizeable fractionof evolving Soviet nuclear weapons.

Space-Based Interceptors.—SBIs deployed inthe mid to late 1990s could probably destroy

75-9220 - 88 - 6

154

some Soviet ICBMs in their boost phase. Thekey issue is whether the weight of the SBIprojectiles could be reduced before Sovietbooster burn times could be shortened, giventhat existing SS-24 and SS-25 boosters wouldalready stress projected SBI constellations.The probability of post-boost vehicle (PBV) killsis lower due to the smaller PBV size and IR sig-nal, but SBIs might still achieve some successagainst current PBVs by the mid to late 1990s.

Exe-atmospheric Reentry Interceptor Sys-tem.—The ERIS, which has evolved from pre-vious missiles, could probably be built by theearly to mid-1990s to attack objects in late mid-course. The key unknown is the method oftracking and discriminating RVs from decoys.Existing radar sensors are highly vulnerable,the SSTS space-based IR sensor probablywould not be available until the late 1990s toearly 2000s, and the AOS airborne sensorwould have limited endurance and range. Thiswould leave either new radars or some type ofpop-up, rocket-borne IR probe, which haveapparently received little development effortuntil recently. Given the uncertainty in sen-sors suitable for the ERIS system, its rolewould probably be confined to very late mid-course interceptions and it might have limitedBMD effectiveness until the late 1990s.

Phase TwoHigh Endo-atmospheric Defense Interceptor.

—The HEDI could probably be brought to oper-ational status as soon as the mid-1990s. To over-come the unique HEDI window heating prob-lem, the HEDI on-board homing IR sensorneeds more development than its ERIS cousin.But the HEDI system does not depend on long-range sensors to achieve its mission within theatmosphere. The HEDI could probably pro-vide some local area defense of hardened tar-gets by the mid-1990s against non-MaRVedRVS.83 HEDI performance against MaRVedRVs appears questionable.

‘S’’ MaRV” refers to maneuvering reentry vehicles, or RVswhich can change their course after reentering the atmosphereto improve accuracy or to avoid defensive interceptors.

SBIs against Reentry Vehicles.-The probabil-ity that SBIs would kill RVs in the mid-courseis low until the next century, given the difficultyin detecting and tracking many small, cool RVsin the presence of decoys, and given uncertain-ties in the SSTS sensor and battle managementprograms.

Phase Three

Directed-Energy Weapons. -It is unlikely thatany DEW system could be highly effective be-fore 2010 to 2015 at the earliest. No directedenergy weapon is within a factor of 10,000 ofthe brightness necessary to destroy respon-sively designed Soviet nuclear weapons. (OTAhas not had the opportunity to review recentSDIO suggestions for “entry level” DEWS ofmore modest capability. SDIO contends thateffective space-based lasers of one to twoorders of magnitude less than that needed fora responsive threat could be developed muchsooner.) At least another decade of researchwould likely be needed to support a decisionwhether any DEW could form the basis for anaffordable and highly effective ballistic mis-sile defense. Further, it is likely to take at leastanother decade to manufacture, test, andlaunch the large number of satellite battle sta-tions necessary for highly effective BMD.Thus, barring dramatic changes in weapon andspace launch development and procurementpractices, a highly effective DEW system isunlikely before 2010 to 2015 at the earliest.

Neutral Particle Beam.-The NPB, under de-velopment initially as an interactive discrimina-tor, is the most promising mid-course DEW.84

Shielding RVs against penetrating particlebeams, as opposed to lasers, appears prohibi-tive for energies above 200-MeV. Although lab-oratory neutral particle beams are still about10,000 times less bright than that needed forsure electronics kills of RVs in space, the nec-essary scaling in power and reduction in beamdivergence appears feasible, if challenging.

8iThe NPB would have virtually no boost phase capabilityagainst advanced “responsive” boosters since particle beamscannot penetrate below about 150 kilometers altitude.

155

However, as discussed in chapter 4 under thetopic of NPB interactive discrimination, it isunlikely that engineering issues could be re-solved before the late 1990s, which would mostlikely postpone deployment and effective sys-tem operation to at least 2010-2015.

Free Electron Laser.—The free electron laser(FEL) is one of the more promising BMD DEWweapon candidates. The FEL is in the researchphase, with several outstanding physics issuesand many engineering issues to be resolved.Even if powerful lasers could be built, the highpower optics to rapidly and accurately steerlaser beams from one target to the next couldlimit system performance. Although the basicsystem concept for an FEL weapon is well de-veloped, it is too early to predict BMD per-formance with any certainty.

Chemical Laser.—There are too many uncer-tainties to project BMD performance for thechemically pumped hydrogen fluoride (HF) la-ser. The HF laser has been demonstrated atrelatively high power levels on the ground, al-though still 100,000 times less bright than thatneeded for BMD against a responsive threat.Scaling to weapons-level brightness would re-quire coherent combination of large laserbeams, which remains a fundamental issue.This, coupled with the relatively long wave-length (2.8 micron region), make the HF laserless attractive for advanced BMD than theFEL.

Electromagnetic Launcher.—There are toomany uncertainties in the EML or railgun pro-gram to project any significant BMD capabil-ities at this time.

Space Power Conclusions

Phase OnePower Requirements.—Nuclear power would

be required for most BMD spacecraft, both toprovide the necessary power levels for station-keeping, and to avoid the vulnerability of largesolar panels or solar collectors.

Dynamic Isotope Power System.–The DIPS,which has been ground-tested in the 2 to 5 kW

range, should be adequate and available by themid to late 1990s, in time for early BSTS-typesensors.

Phase TwoNuclear Reactors.—Adequate space power

may not be available for SSTS or weapon plat-forms with ladars before the year 2000. ForBMD satellites that require much more than10 kW of power the SP-100 nuclear reactor/thermoelectric technology would have to be de-veloped. This is a high-risk technology, withspace-qualified hardware not expected beforethe late 1990s to early 2000s.

Phase ThreeChemical Power.—Chemically driven energy

sources (liquid oxygen and liquid hydrogen driv-ing turbogenerators or fuel cells) could proba-bly be available for burst powers of MW up toGW to drive weapons for hundreds of secondsby 2000-2005.

Power for Electromagnetic Launchers. -High-current pulse generators for electromagneticlaunchers (EML) would require extensive de-velopment and engineering, and would mostlikely delay any EML deployments well into the21st century.

High-Temperature Superconductors.–Re-search on high-temperature superconductorssuggests exciting possibilities in terms of reduc-ing the space power requirements and improv-ing power generation and conditioning efficien-cies. At this stage of laboratory discovery,however, it is too early to predict whether orwhen practical, high current superconductorscould affect BMD systems.

Space Communications Conclusion

Laser communications may be needed forspace-to-space and ground-to-space links toovercome the vulnerability of 60-GHz links tojamming from nearby satellites. Wide-bandlaser communications should be feasible by themid-1990s, but the engineering for an agilebeam steering system would be challenging.

156

Space Transportation Conclusions

Phase OneMid-1990s Deployments.—Extrapolating

reasonable extensions of existing space trans-portation facilities suggests that a limited-effectiveness, phase-one BMD system begunin the mid-1990s could not be fully deployedin fewer than 8 years.85 Assuming that thehardware could be built to start deploymentin 1994, the system would not be fully deployeduntil 2002. A more ambitious launcher-devel-opment program and a high degree of successin bringing payload weights down mightshorten that period.

Phase TwoNew Space Transportation System.—A fully

new space transportation system would be re-quired to lift the space assets of a “phase-two”BMD system. This system would have to in-clude a vehicle with heavier lift capability(40,000 to 50,000 kg v. 5,000 kg for the Tita.n-4), faster launch rates (12 per year v. 3 per yearper pad), and more launch pads (4 v. 1).

‘J6’l”& ~Sumes that two launch pads at Vandenberg AFB,4-East and the SLC-6 pad intended for the Shuttle, are modi-fied to handle the new Titam4 complementary expendable launchvehicle (CELV), and the launch rates are increased from threeTitans per year per pad up to six per year.

Optimistic Assumptions.—Even under veryoptimistic assumptions,86 the new space trans-portation system would be unlikely to reach thenecessary annual lift requirements for a large-scale, second-phase BMD until 2000-2005, withfull phase-two deployment completed in the 2008-2014 period.

Phase ThreeUltimate DEW Systems.–It might take 20 to

35 years of continuous launches to fully deployfar-term, phase-three BMD space assets designedto counter with very high effectiveness an ad-vanced, “responsive,” Soviet missile threat. Thisestimate assumes deployment of the proposedALS space transportation system and the kindof advanced space-based laser constellationsuggested by SD I system architects. A set ofground-based laser installations could reducethe space launch deployment time estimate to12-25 years.

~his assumes that the SDIO bifurcated goal is met: a revolu-tionary space transportation system with 10 times lower costis developed in 12 years, while a near-term component of thatsystem yields a working vehicle of reduced capability by 1994.

Documents

Chapter 5 Ballistic Missile Defense Technology: Weapons