Aerospace

Engineeringthe space shuttle

New development and design solutions byEEs will help to launch a winged space vehicle

Howard Falk Associate Editor

NASA is scheduled to spend about $5.2 billion dol- a 184-km due-east launch orbit is 65 000 pounds, andlars during the next six years to develop a space shut- the orbiter can return and land with 40 000 pounds oftle system.* Approval of the program by President payload. Clamshell cargo doors swing open to exposeNixon and the Congress was a momentous decision the payload completely and thereby to allow its directfor the United States, and, in particular, for the aero- removal.space and electronics industries. Aside from long- The expendable liquid-oxygen/liquid-hydrogen ex-term benefits, the program is expected to provide ternal tank is 7.7 meters in diameter and 57.8 meters50 000 new jobs for people who have been displaced by long. It contains 1.65 m4illion pounds of propellants atGovernment spending cuts. Although it is not yet lift-off. A major design and development problem ofclear how many engineers' jobs are involved, the tech- the space shuttle concept is to reduce the cost of thisnical requirements and plans for the shuttle system rather complex expendable element to an absoluteare now well-defined. minimum while retaining an acceptable low mass

This article describes development and design ef- with satisfactory safety margin.forts planned for the shuttle in areas of electrical and Two solid-rocket motors (SRMs) mounted on oppo-electronics engineering. site sides of the expendable tank provide a parallel

burn boost at lift-off and for a duration of 115 secondsSpace shuttle system thereafter to an altitude of about 25 statute miles.Employing a reusable orbiter vehicle and rocket The SRMs each burn 415 000 pounds of propellant re-

motors, the shuttle will be 64.8 meters high? and 25.6 gressively to produce 4 540 000 pounds of thrust at lift-meters wide, and will lift a gross mass of 5 246 000 off, which reduces to about 2 600 000 pounds at bur-pounds. nout. Immediately after SRM thrust termination, theThe vehicle is attached "piggyback" to a large ex- casings will be jettisoned, parachuted into the ocean,

ternal tank that provides liquid-oxygen and liquid- and recovered. Although large solid-propellant rocketshydrogen propellants to the orbiter's main propulsion with aluminized PBAN (polybutadiene-acrylic acid-system from lift-off to insertion into orbit. Two large acrylonitrite) propellant are a mature technology,solid-rocket motors attached to the tank each produce questions surrounding the costs of recovery and refur-4.5 million pounds of thrust at lift-off. These motors, bishment, and the final selection of diameter, mass,in combination with the three main engines of the or- and other characteristics are still under study at thebiter, provide a total thrust at lift-off of over 10.4 mit- present time.lion pounds. Payloads. Space shuttle missions will involve di-The orbiter is basically a hypersonic delta-wing air- rect delivery of payloads to specified low earth orbits;

craft with a payload bay 4.57 meters in diameter by placement of payloads and their transfer stages in18.28 meters long, mounting for rocket engines and parking orbits for subsequent transfer to other orbits;power systems, and crew/passenger accommodations rendezvous and station-keeping with detached pay-for up to ten persons with staytime in earth orbit of ~loads for on-orbit checkout; and return of payloads totwo weeks or more. It is about the size of a DC-9- the earth from a specified orbit. The shuttle will alsotransport aircraft. Maximum payload capability into provide routine and special support to space activi-

ties, such as sortie missions, rescue, repair, mainte-nance, servicing, assembly, and disassembly, as well as

* Background of the program, and some criticism, was discussed in docking"Space Shuttle: Spirit of '76" in the August 1972 issue of Spectrum Alhog a wievreygfpyod.ixetd(pp. 61-62). Atog ievreyo alasi xetdt Numbers cited in this article are from preliminary design data. these are to be made up using a small inventory of

50 IEEE spectrum MAY 1973

standard modules. For instance, the research applica- tion, transponder, UHF/AM voice transceivers, startions module system will provide practical, economi- tracker, and range subsystem elements. This functioncal space laboratories suitable for several scientific will provide for sending and receiving data from thedisciplines, whereas modules for a standard space- ground, for voice communication inside and outsidecraft payload will allow both earth observation and the orbiter, for aircraft identification and altitude in-astronomical observation functions to be economically formation to the air traffic control ground system, forassembled. manual spacecraft position determination on-orbit

using star reference, and for determining distance andAvionics subsystems velocity for landing approach.To keep the avionics cost for the space shuttle from Flight instrumentation will consist of data sensors,

rising above its $600 million budget, off-the-shelf data acquisition, built-in testing, systems monitoringhardware will be used wherever it is cost-effective. and checkout, and wide-band recording elements forDevelopment of new equipment will be kept at a subsystem maintenance. It will provide the capabilityminimum, thus reducing the requirement for compo- to acquire measurements, to determine the subsystemnent development and qualification testing. Aeroflight status, to isolate failures, and to record all necessaryfunctions will generally be kept separate from space- data.craft functions so as to reduce the amount of new equip- Displays and controls will consist of standard flightment development and to minimize integration prob- displays such as altimeters; pressure, temperature,lems. and propellant-level gages; three CRTs for alphanum-The avionics equipment to be onboard the orbiter eric information; circuit-breaker switches; system on-

includes several different subsystems: off switches; and keyboards. These will be positionedAeroflight control will consist of the autopilot, sta- at each of the crew stations.

bility augmentation, speed brake control, and auto- Engine control will include temperature, pressure,matic landing subsystem elements. It will provide the propellant-flow and vibration sensors, data processor,capability to steer the orbiter as an aircraft, to control and thrust level monitoring elements. It will providespeed, to maintain stabilization, and to fly and land for closed-loop engine control and will adjust valvesmanually or automatically. in the propellant lines to maintain thrust at the re-

Spacecraft guidance, navigation, and control con- quired levels.sist of an inertial measurement unit, horizon sensor, Propellant gaging will consist of level, pressure,attitude hold, and propulsion controls. They will pro- temperature, vibration, and cut-off sensors for the ex-vide the capability to determine position and attitude ternal tanks. It will provide the capability for propel-manually or automatically with reference to the lant loading on the ground, propellant depletion mea-earth, to hold position on-orbit, to change orbits, and surements, and engine cut-off.to reenter the atmosphere. The avionics for the solid-rocket motors will in-Communications and tracking will include S-band clude controls for igniting the motors and instrumenta-

receivers and transmitters, audio intercommunica- tion sensors for failure detection.

Falk-Engineering the space shuttle 51

Electric power systems well as reactor-powered dynamic and static powerPower supplies for the shuttle will be designed to plants. Payload power levels could range from less

use well-proven existing technology, drawing heavily than 1 kW for small unmanned earth satellites toon the experience of Apollo spacecraft and launch ve- more than 100 kW for nuclear electric propulsionhicle systems. Power sources now planned include units. Intermediate power levels will be required forbatteries, generators, fuel cells, and reactant tanks for research and applications modules and manned spaceelectric power, with hydraulic systems and auxiliary station applications.power units (APUs) used for mechanical power.Three-phase, 115/200-volt ac electric power would be Navigation, guidance, and controlprovided by APU-driven generators during ascent, de- Several interesting engineering problems are posedscent, and landing, and 28-volt dc power would come by the navigation, guidance, and control require-from two of three fuel cells during the orbital mission ments of the space shuttle. Most of these problemsphases. This dual approach is designed to save the arise because a winged vehicle must reenter the at-mass that would be needed to remove fuel-cell waste mosphere and land safely, but the problems beginheat during atmospheric flight. Rechargeable bat- with lift-off.teries would provide control power to initiate or restart From launch to orbit. Use of a very large wingedthe APUs, fuel cells, and reactant tanks, and to ener- vehicle means coping with heavier aerodynamic loadsgize various special-purpose devices such as explosive than were experienced with previous space vehiclesbolts. and the interaction of structural dynamics with con-

Fuel cells will probably be versions of an advanced trol system dynamics is likely to be complex. Digitalcapillary-matrix cell, which has already demonstrated flight-control systems will be used to meet these load-a 3000-hour lifetime at an operating temperature of alleviation and -interaction problems.366°K (200°F). This cell uses stored hydrogen and During orbital operations, docking will probablyoxygen and generates pure, potable water as a present the main challenge for control system design-"waste" product. No major critical problem areas ers. Because of the orbiter shape and the probablehave yet appeared in the development of capillary- location of the docking hatch, control operations arematrix fuel cells and it seems reasonable to expect likely to be complex. New analyses and simulation3000- to 5000-hour operating lifetimes in the up-to- techniques will be needed to design manipulators to10-kW range that is probably needed for minimal rou- move payloads and dock the orbiter.tine space transportation system operations. Experience with Gemini and Apollo seems to offerPower requirements. Electric power energy alloca- an adequate technique for rendezvous navigation, but

tion needed for on-orbit payload support is 50 kWh selection of appropriate sensors and of techniques for(minimum) from a total capability of 1600 kWh. Av- completely automated docking open promising areaserage payload support power in orbit would be 3 kW, for new engineering work.with a peak power capability of 6 kW. Support to the A major problem for orbital navigation design is thepayload could be increased to 1000 kWh at a mass selection and implementation of proper sensors, suchpenalty of about 1300 pounds and to 5900 kWh for a as star trackers, horizon seekers, and landmark track-penalty of about 7800 pounds, including tankage and ers, to meet mission requirements. And one of thereactants. These capabilities should be adequate for a toughest requirements is to handle a mission abort,majority of the expected delivery, recovery, and ser- where there is a once-around orbit that offers little orvice missions, as well as for short-duration sortie op- no opportunity for navigation update sensing and sub-erations. Extension of space operations to 30 days or sequent control action.longer may require power subsystems with greater Reentry and landing. During reentry, the orbitercapabilities. will first be a blunt-body spacecraft, then a hyperson-Power systems for the kinds of payloads that are ic aircraft, and later a transonic and subsonic air-

expected may include solar arrays with batteries and craft. The ranges of such parameters as Mach num-isotope-powered dynamic and static subsystems as bers and angle of attack, therefore, will be greater

52 IEEE spectrLum MAY 1973

than those that have been experienced with any exist- ing such trajectories properly will involve the formu-ing aerodynamic vehicle. The design of the control lation and solution of guidance equations involvingsystem must be flexible enough to accommodate all of targeted state vectors such as those for position andthe flight regimes as well as a wide range of aerody- velocity, and on-board digital computers will carrynamic stability variations. For any given portion of out these calculations.the entry, control system design does not appear to be Atmospheric reentry will require long entry times,a new problem, but the design of a single system that particularly when there is a large cross range to theis adaptable to the whole flight regime presents a desired landing site. The resulting inertial navigationsubstantial challenge. drift errors will create a need for navigation sensing

It is expected that the control system will use both after the orbiter emerges from communications black-reaction control thrusters and aerodynamic control out. This added navigation will allow landing sites tosurfaces, and at times a combination of the two. Al- be reached with an energy reserve consistent withthough maneuver-rate requirements are not expected landing approach requirements.

Although existing navigation systems, such asground-based radar, TACAN, VOR, and DME, maybe adequate for this postblackout energy managementfunction, precision range measurement (onboard in-terrogators together with ground-based transponders)is being carefully studied. This approach can providean all-purpose navigation system covering all missionphases from lift-off to landing. Existing hardware hasproven out the technology of this type of equipment,but the possible application in shuttle missions doesinvolve its use at much greater ranges than in the past.Large mass penalties for carrying air-breathing en-

gines and fuel into orbit and back dictate that therewill be no such engines for the operational missions.The orbiter will have to perform terminal area ma-neuvering, approach, and landing without power.Steep approach angles associated with such power-offoperation will necessitate extremely accurate landingcontrol.

Furthermore, an unpowered orbiter will have no go-around capability, so the landing must be accom-plished at first try. Existing navigation aids appear tobe adequate for this mission phase unless an auto-matic landing in zero visibility conditions becomes arequirement. If so, greater accuracy will be required,and new developments, such as the precision rangingsystem, described previously, and the microwavescanning beam system, now under development bythe Department of Transportation for commercial air-craft application, will be used.

Communications and trackingExisting Apollo systems for communications and

tracking, with state-of-art improvements in systemreliability, mass, and precision, seem suitable forfirst-generation orbiter requirements. However, sys-tem complexity will be considerably greater. For ex-ample, not only will Apollo-type spacecraft uplinksand downlinks be needed in telemetry and voice, butnormal aircraft functions such as two-way voice com-munications, navigation and landing aids, ferry navi-gation aids, air-traffic-control transponders, and radaraltimeters also will be required. In addition, for spaceoperations, two-way voice communication will be

to be large, possible wind-shear environment will needed between crew members; between the space-have to be accounted for in control power and stabili- craft and ground stations, other satellites, or otherty margin analysis. Control systems design will prob- spacecraft; and between the spacecraft and crewmenably be a reiterative process that could affect the or- involved in extravehicular activity, as well as betweenbiter's aerodynamic configuration. such crewmen and ground stations, other satellites, orThe entry trajectory must be controlled carefully to other spacecraft.

avoid excessive heating, which would increase the re- Tracking requirements, in addition to those neededquired mass of the thermal protection system. Shap- for precise orbiter locations and landing guidance,

Falk-Engineering the space shuttle 53

also include those needed to pinpoint booster recov- As in previous space systems, cost-effectiveness andery, in the first-generation systems, for rapid and eco- safety will determine the answers to such questions asnomic retrieval. Existing Apollo tracking methods the relative usage of digital, analog, or hybrid com-seem adequate for this task, although some upgrading puters, the extent of redundancy used, the extent toin automation will be advisable to reduce the cost of which floating-point hardware will be used, and thethese operations while allowing routine recovery of optimum word size. In any case, the following compu-new space transportation system solid-rocket motor tational functions will need to be satisfied: (1) execu-boosters. tive; (2) navigation, guidance, and flight control; (3)Needed sensors, displays, and recording systems monitoring of propellants and other consumables; (4)

should be directly derivable from existing Apollo computation for mission planning; (5) software forhardware and current or projected operational com- input/output and mass memory control; (6) pre-mercial and military aircraft technology. However, launch and in-flight checkout; (7) software for crew/the overall information system will be more extensive, computer interface; (8) instrument and controls mon-and the integration task, therefore, will be substan- itoring and processing; and (9) all communicationtially more complex than in the past. processing.

Displays and information will be needed by theCentralized computer control pilot and copilot to perform each of the followingNew computer systems can provide the more com- functions: (1) operation of the vehicle, including air

plex computational capability required to accommo- traffic control, landing aids, navigation aids, etc.; (2)date these vehicle and mission requirements, and monitoring of all vehicle subsystems; (3) monitoringmultipurpose display systems can make the required of the condition and functioning capability of theinformation available to the crew in a timely, usable payload; (4) facilitating of all communications func-manner. Probably the most demanding task will be to tions; (5) operation of the payload hardware systems,provide enough system redundancy to insure that whether of the teleoperator or manually controllednone of the essential functions are lost, especially in type; and (6) identification of any "danger signals"time-critical situations. The magnitude of this task- and locating the subsystem or compartment thatespecially the detail design integration and the digital needs attention.software development-is likely to be greater than insoftware development-is likely to be greater than in

This article is based on the report New Space Transportation Sys-previous programs. tems: An AIAA Assessment (January 1973), prepared by the AmericanAdvances in computer technology will make it pos- Institute of Aeronautics and Astronautics Ad Hoc Committee on the

Assessment of New Space Transportation Systems.sible to share a single computer system for manyfunctions. Furthermore, a high degree of automationin the management of various subsystems has becomefeasible. Onboard operational data management willbe a departure from that of the Apollo program, and Reprints of this article (No. X73-052) are available atthe extent to which it will be applied to the shuttle $1.50 for the first copy and $0.50 for each additional copy.

Please send remittance and request, stating article num-will depend on the trade-off between hardware advan- ber, to IEEE, 345 E. 47 St., New York, N. Y. 10017, Att:tages gained by an integrated design vs. system soft- SPSU. (Reprints are available up to 12 months from date ofware complexity and hence development risk. publication.)

54 Falk-Engineering the space shuttle