[American Institute of Aeronautics and Astronautics AIAA SPACE 2012 Conference & Exposition - Pasadena, California ()] AIAA SPACE 2012 Conference & Exposition - A New Air Launch Concept:

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American Institute of Aeronautics and Astronautics

A New Air Launch Concept: Vertical Air Launch Sled (VALS)

Marti M. Sarigul-Klijn1 Ph.D. and Nesrin Sarigul-Klijn

2 Ph.D.

Mechanical and Aerospace Engineering Dept., SpaceED, University of California, Davis, CA 95616-5294

Gary C. Hudson3

AirLaunch LLC, Renton, Washington, 98059

and

Charlie Brown4

Space Vector Corp., Chatsworth, California, 91311

This paper describes a new patent pending air launch concept that combines a proven tow technique with a novel Vertical Air Launch Sled (VALS) to produce a low-cost, near-term method of air launching large diameter launch vehicles (LV) using unmodified commercial or military aircraft. Key advantages of the VALS concept include: (1) high

system safety since the LV is far from the tow aircraft at all times, (2) no objects such as expensive wings or pallets falling off, (3) near vertical attitude for the LV at ignition, (4)

capability to ignite engines while attached to VALS and to shutdown & return LV for safe landing if health is not good, and (5) capability to carry cryogenic propellants in insulated heavy duty Dewars located in the VALS strong back, which can greatly reduce propellant

boil-off and increase LV safety on the ground. This paper also describes methods to attach the tow line to existing aircraft without any modifications to the aircraft. These study results show that VALS can greatly improve the simplicity, cost and reliability of the booster.

Copyright by all the authors. All rights reserved. 1 Lecturer, UC Davis. Senior Member AIAA.

2 Contact Author: Professor & Director, [email protected], 530.752.0682. Associate Fellow of AIAA.

3 CEO, 2210 Ilwaco Ave. Associate Fellow of AIAA.

4 Senior Designer, 9174 Deering Avenue.

AIAA SPACE 2012 Conference & Exposition11 - 13 September 2012, Pasadena, California

AIAA 2012-5156

Copyright © 2012 by all the authors. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

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mailto:[email protected]

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I. Introduction

here have been many studies of air launch concepts during the past fifty years. In all that time, only one air launched low earth orbit (LEO) capable system has been fielded – Orbital Science’s Pegasus. It was originally

funded by the Defense Advanced Research Projects Agency (DARPA) in 1987 in the hopes that it would lead to a

low-cost small spacecraft capability. Since that time several other programs have attempted to achieve the dual breakthroughs of inexpensive air launching coupled to a responsive Concept of Operations (CONOPS) free from the constraints of launch ranges. None have been put into practice yet.

A significant concern has been launch vehicle (LV) safety. The fear of a LV exploding near the carrier aircraft has been a serious issue to aircraft flight clearance authorities. For example, one of the primary reasons that

AirLaunch LLC’s QuickReach LV was cancelled in 2007 was the flight clearance authorities’ fear of carrying a liquid fueled LV inside a C-17A carrier aircraft. Such fear is not unfounded. Six months prior to the QuickReach project cancellation, a liquid fueled Sea Launch Zenit-3SL exploded on its floating launch pad seconds after engine

ignition. Since the launch pad platform was vacated by all engineers during the automated launch process, there were no injuries. In 1960, the second ground test of the liquid fueled X-15 rocketplane with the XLR-99 engine resulted in a major explosion that shattered the tail of the aircraft and hurled the remaining fuselage forward about

twenty feet. The pilot was not injured in the explosion or fire, but if this explosion had occurred in-flight when attached to the B-52 carrier aircraft, both aircraft and all of the crew would have likely been lost. The X-15 program

continued because at the time the United States was in race with the Soviet Union to build ever more capable flight vehicles, and the research data from the X-15 program was deemed important enough to outweigh the safety risks. No liquid fueled LV have been air launched since the X-15 program ended in 1968.

Solid rockets can also be dangerous. In 2003, a Brazilian VLS-1 solid fueled rocket explosion caused twenty-one fatalities and leveled the launch pad. In 1993, a Swedish technician was killed when a solid fueled sounding rocket ignited at the European Sounding Rocket Range. Earlier in 1964, the solid fuel in the third stage of a Delta

rocket ignited while in the spin test facility building at Cape Kennedy, killing eleven workers. Although hybrid rockets are considered safer than either liquids or solids there still have been accidents. For example, in 2007 there

was an explosion in Mojave California during a test of a nitrous oxide injector that killed three technicians. This paper describes a new patent-pending air

launch concept called Vertical Air Launch Sled

(VALS) that offers many advantages over other air launch methods that have been tried before. In particular it solves the safety concerns outlined in the

previous paragraphs. The first section of this paper compares air launch with ground launch and describes

the advantages and disadvantages of each method. The second section describes the various means to air launch an earth-to-orbit LV and gives their advantages

and disadvantages. Then the new VALS concept is described together with its CONOPS, weight and cost estimates. Additional sections describe (a) several means of attaching a tow line to existing aircraft that involve no modifications to the aircraft, (b) evaluation of various candidates tow aircraft, (c) a new aft-crossing trajectory that is

used by VALS and has several advantages over a traditional forward crossing trajectory used in all previous air launches, and (d) several alternate release methods from VALS including the trapeze-lanyard air drop (t/LAD)

1

method. The last section is the conclusion.

II. Ground versus Air Launching of Space Vehicles

Earth to orbit missions are typically launched from ground-based installations such as Cape Canaveral. Ground

launches, however, present problems of launch delays due to inclement weather and the necessity to clear the vicinity of air traffic. Current range policies do not permit a launch if there is maritime shipping traffic within the

potential impact zone of lower stages, regardless of the fact that official notices were given. Ground launch ties the vehicle to a range and their associated high costs and the costs of major range and launch pad infrastructure investments. Typically only one launch is conducted at a time, with weeks required to refurbish a given launch pad

for the next launch. In addition to weather delays, the ranges are also vulnerable to natural disasters such as hurricanes. Finally, orbital inclinations are limited. For example, from Cape Canaveral only east to northeast bound trajectories are permitted due to the concern of overflight of the eastern most Caribbean islands.

These problems can be overcome by launching vehicles from an airborne flight vehicle.2 An air launch offers

several advantages over a ground launch, such as the avoidance of weather related delays, the simplification of

T

Fig. 1 Vertical Air Launch Sled (VALS) on the ground

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operations, and increases in public safety - owing to the ability to avoid the over flight of populated areas. Air launch can occur over the

open ocean, sufficiently far from populations or crowded sea-lanes near the shore. There are large off-shore areas in which there is no ocean maritime or air traffic. An air launch carrier aircraft can fly

around or over launch constraining weather. Air launch allows positioning of the launch point to intercept any desired orbital plane.

It is also possible to intercept another spacecraft in orbit on the first orbital pass, since the launch point can be positioned for the desired orbital phasing. Any runway of suitable length can serve as a launch

site, missions are recallable, and the carrier aircraft can serve as the LV transporter. Air launch also eliminates the acoustic reflection from the ground, which tends to size the nozzle end of LVs. Finally, there

are no major infrastructure expenditures for new launch pads. In addition, air launching presents design options that simplify the

operation of the LV engine. The low outside atmospheric pressure at higher altitude allows a LV to use high area ratio nozzles while operating at relatively low engine pressures. Furthermore, nozzle

expansion ratio can be set closer to ideal since atmospheric back pressure varies less throughout the ascent trajectory with an air launch. The performance gain of launching at 25,000 to 35,000 feet (ft)

altitude are approximately 1,100 feet per second (fps) to 1,800 fps delta V improvement - depending on carrier aircraft flight path angle

at launch.3 or 4

On the other hand air launching does have some limitations . Some air launch methods require a specially modified carrier aircraft capable of carrying the LV. The carrier aircraft will typically be heavier than the LV itself;

for example, the 480,000 pound (lb) B-52 carrier aircraft carrying a 33,000 lb X-15. This limits the size of the LV and means that there is limited growth potential for air launching.

Propellant boil off can be a major problem for those concepts that use cryogenic propellants. Propellants are

heated by radiation heating from the sun and convective heating from the air stream. For example, the X-15 boiled off 60% to 80% of its liquid oxygen (LOX) during its 45 minute to 1 hour climb and ferry while attached to its B-52

carrier aircraft. The LOX was replenished from an internal insulated tank carried in the B-52’s bomb bay. Finally air launched LV engines are typically started after they are dropped from the carrier aircraft for the safety

of the carrier aircraft. If the engine fails to start, then the LV may be lost. This happened several times during the

X-15 rocketplane’s 199 test flights. Fortunately it had the capability to dump propellant and complete an emergency landing.

III. Air Launch Methods

Several air launched methods have been proposed.2 None of these methods solve the safety concerns outlined in

Section I, Introduction. They all have flight crew near the LV during carriage and ignition.

A. Captive on Top Air Launch No examples of captive-on-top LV have been actually built, but

the Space Shuttle's approach and landing demonstrator, the Enterprise, used this method to test its landing. The advantage of this method is the capability to carry a large volume vehicle on top

of the carrier aircraft. Disadvantages include extensive modifications (high cost) to the carrier aircraft. Also the LV must have wings large enough to support the LV at separation from the

carrier aircraft, and it must have active controls at release to prevent the LV from hitting the carrier aircraft. Aircraft flight clearance

authorities might not permit such a release because of the need for active controls. Also the wings have to be discarded after an aerodynamic pull-up into a vertical trajectory, thus increasing

launch costs. Further, placing a LV on top of the carrier aircraft

Fig. 2 VALS in Flight

Fig. 3 Shuttle Launched from 747

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destroys the lift produced by the aircraft fuselage and causes a large amount of drag that in turn limits launch altitude. For example, during its approach and landing test flights, the Space Shuttle orbiter was launched at

altitudes between 19,000 to 26,000 feet from its carrier Boeing 747 because of the high drag from the attached Shuttle, even though a clean 747 normally cruises at 38,000 to 45,000 feet.

B. Internally Carried Air Launch

Internal air launch has been demonstrated as early as 1974 when a C-5A Galaxy dropped a 78,000 lb LGM-30A Minuteman

I missile using drogue chutes to extract the missile and its 8,000 lb launch sled. The missile was then successfully fired. In 2005 to 2007, AirLaunch LLC successfully dropped three Drop Test

Articles that simulated the size and mass properties of their QuickReach LV from a C-17 aircraft.

5 Advantages of internally

carried concepts include little or no modification to the carrier

aircraft. Propellant boil-off concerns are minimized since the LV is not subject to either solar radiation heating or airstream

convective heating. The LV is in a benign environment inside the carrier aircraft that allows maintenance and safety problems to be detected prior to launch. Release altitude can be at a higher

altitude because the LV does not increase the carrier aircraft’s drag. The main disadvantage is that the LV must be sized to fit inside the carrier aircraft. Also operations must be conducted over the water since many parts such as a launch sled or a parachute fall to the ground.

C. Captive on Bottom Air Launch Many examples of captive on the bottom air launch exist

including Pegasus, X-15, and Scaled Composites’ SpaceShipOne and SpaceShipTwo. Advantages include proven and easy separation from the carrier aircraft and the option of not using a

wing or sizing the wing smaller than required for level flight at the release altitude and airspeed. Disadvantages include limits to LV size due to under the carrier aircraft clearance limitations and

the cost of carrier aircraft modifications. A new design carrier aircraft such as the WhiteKnightOne or WhiteKnightTwo can

eliminate the clearance limitations.

D. Tow Air Launch One of the first occurrences of towing a rocket-powered aircraft was during the summer months of 1942 at

Peenemünde, Germany. Twin engined Bf-110C fighters were used to tow prototypes of the Me-163 rocket fighter for flight tests, typically to altitudes of 16,400 ft. Also extremely heavy tow operations were accomplished with the 75,000 lb German ME-321 transport glider. The United States

developed a successful glider version of the C-47 (DC-3) that weighed 26,000 lb. In addition, the U.S. Air Force demonstrated

long distance cross-country towing of the F-80 fighter plane with a B-29 bomber. In 1998, Kelly Space conducted a towed flight demonstration with a Lockheed C-141A cargo aircraft towing a

30,300 lb QF-106 delta wing fighter.6 The advantages of this

approach are large safety distances between the LV and the towing aircraft and low cost or no modifications to the towing

aircraft. The principal disadvantage of towing is that up to now, the LV needed to be attached to a glider with a pilot onboard.

Although the crew of the towing aircraft is safe from a LV explosion, the pilot of the glider that is carrying the LV is not. Also, a broken tow line could be fatal to the glider pilot. Fig. 6 QF-106 towed by C-141A

Fig. 4 QuickReach Dropped from C-17

Fig. 5 X-15 launched from B-52

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IV. Vertical Air Launch Sled (VALS) Description

VALS is a sled which carries a LV and is towed aloft by an aircraft. The aircraft does most of the lifting and a variety of aircraft can be used as discussed in Section V with no permanent modifications to the aircraft. Unlike a glider, VALS does not have wings, hence when airborne it does not need active fligh t controls or a pilot in order to

maintain position behind the tow aircraft – position keeping happens naturally and automatically. VALS can be sized to carry a LV from a few thousand pounds (minimum to reach LEO) up to 200,000 lb with currently available tow aircraft. However with a large custom aircraft such as the one being built for Stratolaunch Systems, a much

larger LV is possible. During the last century, aircraft have towed a variety of objects and towing is still being done today. Currently the US Army’s Towed Aerial Targets Management Office (PM ITTS) located in Orlando, Florida

provides live towed-target prototypes and production hardware that closely emulates the signature level (radar or infrared) and performance of typical threat aircraft or cruise missiles. They have 11 different towed targets that can be towed by either droned or human piloted aircraft including the F-16, T-38, or Lear 36 aircraft

Although a glider could carry a larger LV than VALS, the glider would need to have human p ilots onboard to provide the level of control system reliability that can not be obtained at reasonable cost in both time and dollars with today’s technology. The concern is that a glider capable of lifting itself has sufficient control authority to cause

a mishap to the tow aircraft. For example, the glider could lift off before the tow aircraft and then prevent the tow aircraft from taking off due to the tension from the tow line. In contrast, a sled without wings simply does not have

this control authority.

Top

Side

Bottom

Front

Fig. 7 Vertical Air Launch Sled (VALS)

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A. VALS Advantages The new VALS concept has several advantages. First, there is excellent separation distance between the crewed

tow aircraft and the LV. The separation distance can be any distance desired simply by changing the length of the tow line. The flight crew of the towing aircraft are completely safe from the LV, even if the LV explodes at any time. If the LV blows up, the VALS sled is lost, but aircraft and crew are safe. Note that the riskiest time is when

the towing aircraft and VALS are stationary on the ground. As soon as the tow aircraft is rolling, then the aircraft is moving away and above any explosion. Furthermore at anytime, the flight crew can cut the tow line, and a broken

tow line poses no risk to the crewed tow aircraft. VALS is very much like Sea Launch - the LV is on one platform and the people are on another. Sea Launch did have a Zenit LV explode on the pad in 2007 and no one was killed.

LV propellant loading is also safer. There is no loading of propellant over, in, under, or near the tow aircraft.

Also cryogenic propellant such as liquid oxygen (LOX) can be stored in an insulated heavy duty Dewar in the VALS strongback (note that none of the VALS images in this paper have the volume for such Dewar). Then jus t prior to launch, the LOX can be transferred into the LV. This not only greatly reduces LOX boil-off, but also

increases safety on the ground since the fuel and LOX are completely separated. Note that propellant loading is always done with no one near a ground launched LV and then in the case of a manned LV such as Shuttle or Soyuz

only astronauts and close out crews are allowed near the vehicle after p ropellant loading is completed. VALS means the LV size and form factor is unconstrained by the tow aircraft. LV length and diameter can be of

any size and as previously mentioned weigh up to 200,000 lb using currently available tow aircraft, and much larger

using the proposed Stratolaunch carrier aircraft. VALS can be adapted to any type of LV, whether they are solid, hybrid, or liquid fueled. Also the LV can be essentially be an unmodified ground launch vehicle since ignition occurs vertically with the LV experiencing 1 G along the direction it normally would be during a ground launch.

The LV designer does not need to worry about the location of the propellant ullage or whether the propellant feed lines will be uncovered. The VALS’s strongback supports the LV in bending so there is no significant increase in

LV empty mass to deal with sideways bending loads during taxi, takeoff and in-flight carriage. VALS simplifies LV integration. VALS can be the ground support equipment (GSE) for horizontal integration.

The LV and the payload are both at ground level allowing for easy access. Compared to other air launch methods

VALS is also much safer and easier to attach to the aircraft – all that is required is to connect the tow line. In contrast, the captive on top method such as used with the Space Shuttle and 747 required large mate / demate facilities located at two sites.

A key advantage is that engine(s) start and ignition can be confirmed prior to release. This is equivalent to a ground launch’s pad hold-down, except that the acoustic environment is better since there are no reflections from the

ground. In the event that the LV’s health is not satisfactory, then the engine(s) can be shut down, propellant dumped, and the VALS and LV can be returned for landing. The recent SpaceX Falcon 9 launch to the ISS is an example where this capability would have been useful. With its engines throttling up to full thrust, launch was

aborted at the last instant when telemetry showed higher-than-allowable pressure readings in one of the LV's nine first-stage powerplants. A post-scrub inspection revealed the problem was caused by a failed check valve in an engine turbopump. With all other air launch methods, the engines are started after the launch vehicle is released,

meaning that a similar engine fault would result in the loss of the LV. VALS’s hold-down feature allows the engines to be shut down and the launch vehicle returned in case of such faults.

Also unlike other air launch methods, the booster is released at the tow aircraft’s airspeed and with no downward velocity relative to the tow aircraft. VALS allows an aft crossing trajectory which will be described later. This trajectory provides for lower peak aerodynamic pressure, lower product of dynamic pressure and angle of attack (Q-

alpha), and lower thrust vector control (TVC) requirements. Larger payload to orbit is possible since flight path angle and pitch attitude at ignition are close of optimum. Depending on tow aircraft and length of tow line, up to about 30,000 feet release altitude is possible.

Finally, launch costs can be lower. There are no parts falling into the ocean such as in some proposed 747 top launch concepts, or pallets and parachutes as used in some internal air launch methods. In fact the VALS is entirely

reusable and, as will be described later, is not damaged at all by the LV during ignition and launch. The tow aircraft fleet can be rapidly and inexpensively expanded. Furthermore, the tow aircraft require no permanent modifications.

E. VALS Hardware Description

Figure 7 presents a 3-view of VALS. The LV shown in the figure has a diameter of 7 ft, is 63 ft long, and weighs 100,000 lb. However VALS can be sized for LV’s of any size, depending on carrier aircraft capability .

The LV is attached to a strongback of sufficient strength and stiffness to protect the LV from bending loads

during taxi, takeoff, in-flight, and landing. It also houses both the GSE and Airborne Support Equipment (ASE). For LVs that use cryogenic propellants, the strongback can be enlarged and contain insulated heavy duty Dewars.

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Then, about 30 minutes prior to drop, propellants such LOX can flow by gravity from the Dewar into the LV, first at a low rate to chill the tank, and then at a fast fill rate. The original Thor ballistic missiles were routinely filled in 15

minutes with LOX in this way. This strategy should minimize propellant boil-off losses and also greatly increase safety on the ground since the fuel and LOX are held completely separate. The VALS stub wings produce enough lift to support about half its empty weight - not enough so that it can climb level with the tow aircraft or require a

pilot. The landing gear would be re-purposed from an existing aircraft. In Figure 7, we are using the main and nose

gear from the Boeing 737 airliner. The nose wheels are free to swivel with one wheel having nose wheel steering. VALS also has controllable rudders. Both are used during takeoff and landing to help maintain VALS on the runway centerline and VALS does not have active pitch or roll controls. VALS is very resistant to turnover when on

the ground. It must be tilted by 59 degrees before it will turn over on its side. This compares to 44 degrees for a Boeing 737 airliner and only 31 degrees for a small airplane like the Piper Super Cub.

The tow line is in a Y-bridle configuration that attaches near the leading edge of the stub wings . The tow line

would be made from AmSteel®-Blue Ship Mooring Lines which are made from a synthetic fiber called Dyneema® SK-75, an ultra high molecular weight polyethylene material. The tow line is a torque-free, 12-strand braided rope

which, size-for-size, has comparable strength to wire rope but only 1/7th the weight – the rope actually floats in water. As an example, a 3 inch diameter rope has a minimum strength of 750,000 lbf. Streamlined short sections of

tubing slipped over the rope (as in beads on a necklace) can greatly reduce the tow line aerodynamic drag. The forces from the tow line Y-bridle are sufficient to stabilize VALS in roll.

The tail surfaces reconfigure in a matter similar to Scaled Composites’ SpaceShipOne to pitch the VALS into a launch attitude of about 60 degrees,

see Figure 10. Note that SpaceShipOne is not the first aircraft that could reconfigure its tail. The idea was first invented by Grumman Aircraft’s Bob Kress in 1976 for its Nutcracker Vertical Takeoff and Landing (VTOL)

concept for the US Navy.7

An alternate VALS configuration would place the LV on-top of the strongback. This may be required for pump-fed liquid LVs those tanks are too

fragile to hang from a strongback. In this case, a VALS launch reconfiguration would pitch VALS pass the vertical. Note that launch

reconfiguration decribed above could also work for a glider concept which has wings that support the LV.

F. VALS Concept of Operations (CONOPS)

For the purposes of this paper to illustrate the concept, we will describe a case study using the C-17A as the tow aircraft. The main reasons for this choice are that we have experience with this aircraft.

5 To provide a numerical

example, we will assume operations from a 15,000 ft runway at a 2,000 ft field elevation (Edwards Air Force Base)

with no winds on 30 degree Celsius (86 deg F) day and 2,100 ft of towline

1. Takeoff CONOPS Below is a step by step description of takeoff with a C-17A towing a VALS:

a. The VALS would be positioned at the approach end of the runway while the C-17 would be positioned 2,100 ft upwind from approach end.

b. The C-17 takes off in about 2,200 ft of ground roll. At this point there is about 10,700 ft runway remaining.

At this scale, only 400 feet of tow line shown

Approach

End

Fig. 9 VALS Takeoff CONOPS

Fig. 8 Nutcracker

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c. The C-17 climbs-out at best angle of climb airspeed of 133 knots (225 fps). Its climb capability is a 20% climb gradient. Its configuration during the climb would be flaps at 8 degrees down, slats fully down, landing gear

up, and 340,000 lb takeoff weight. d. A remote pilot steers the VALS along the runway centerline initially using nose wheel steering and then

transitions to the aerodynamic rudder at about 80 knots.

e. It takes 48 seconds for C-17 to fly down the remaining 10,700 ft of the runway. The C-17 climbs 2,100 ft by the time it reaches the end of the runway. As the C-17 passes the departure end of the runway the VALS is lifted off

the runway by the C-17. f. VALS’ peak ground speed will be 159 knots as it moves forward and under the C-17 as the C-17 climbs. g. As the VALS passes 500 ft above the ground, the C-17 levels off, accelerates to flap retraction speed (about

170 knots), retracts flaps and slats, and then continues to climb.

Note that VALS is three-fault tolerant to aircraft takeoff aborts. The 1st fault is the tow aircraft conducting a

takeoff abort. In the event that happens the VALS brake system would slow the VALS and prevent it from hitting

the tow aircraft. In the event that the VALS brakes fail (the 2nd

fault), then the VALS nose wheel steering and rudder system would steer the VALS around the tow aircraft. Typically when a tow aircraft conducts a takeoff abort, it would steer for the left side of the runway, while the VALS would steer for the right side of the runway.

Only if a nose wheel steering and VALS rudder failure (the 3rd

fault) combined with a brake failure would there be a possible collision between the VALS and the tow aircraft. This fault logic is the same used in commercial sailplane and tow plane operations and has proven to be very safe.

Allowable tow line length varies depending on runway length and aircraft type. In our current example using the C-17 aircraft, up to 2,100 ft of towline can be used on 15,000 ft runway. If the runway is 10,000 ft long, then the

tow line has to be shorten to 1,300 ft. For a 5,000 ft runway, then 450 ft of towline is allowed. These numbers all assume a C-17 tow aircraft. The reason the tow line length varies with runway length is that the tow aircraft lifts the VALS from the runway. So by the time VALS is at the end of the runway, the tow aircraft needs to have climbed to

an altitude that exceeds the tow line length.

2. Launch CONOPS

Typically in a clean configuration, the C-17 cruises at about Mach 0.74 and above 30,000 ft. While towing VALS the aircraft cabin will be unpressurized because the tow line is attached to a reinforced pallet on the cargo

compartment floor. The flight crew exposure above 18,000 ft is limited to 2 hours by US Air Force regulations (AFI 11-2C-17V3, Chapter 19 - Airdrop Procedures). Hence the C-17 would cruise to the launch point at about 17,500 ft and at 230 knots indicated airspeed (KIAS), equal to Mach 0.5 and 300 knots true airspeed. Because of the added

drag of the VALS and the tow rope, the payload range is about 80% of the clean range at this altitude. Straight line distance of 2,000 nautical miles with MIL-C-5011A fuel reserves is still available. These fuel reserves allow the aircraft to reach a divert field in

case the primary field is weathered in. Here is a step by step description of the launch CONOPS:

a. The C-17 tows VALS and the LV to the launch point.

b. When the C-17 reaches the launch point, it climbs to its service ceiling which would be about 25,000 ft with the added drag of VALS factored in. Airspeed remains at 230 KIAS which is now equal to Mach

0.57 and 345 knots true airspeed at this higher altitude. c. VALS reconfigures into the launch configuration by deploying its

tail, see Figure 10. Drag increases greatly in this configuration which causes VALS and the LV to swing backward and upward relative to the C-17. Five seconds after the reconfiguration, the VALS and the LV are

moving upward at about 25 feet per second (fps) and aft 80 fps relative to the C-17 (again this assumes a 2,100 ft tow line).

d. The LV engine(s) ignites at 5 second after VALS reconfigure. If

engine(s) health is good, VALS releases the LV. Since the VALS is now much lighter it continues upward and aft faster than the LV does. Relative

acceleration is more than 1 G between the LV and VALS. The C-17 immediately starts a clearing turn to the left, which moves the VALS out of the LV’s flight path. The LV’s engine exhaust does not touch VALS and

VALS is undamaged by the launch. The LV continues on toward orbit .

Fig. 10 VALS in Launch Configuration

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e. If LV engine(s) health is unsatisfactory, then the engine(s) is shut down. VALS returns back into the flight configuration, LV propellant is dumped if possible, and the C-17 returns the LV and VALS back to the airfield.

3. Landing CONOPS Landing the VALS and the C-17 requires three passes to the airport’s runway:

a. On the first pass to the airport, the C-17 flies a GPS approach to a waypoint above the approach end of the

runway. If tow line is 2,100 ft long, then the waypoint is slightly less than 2,100 ft above runway. b. If required, a landing signal officer (LSO) stationed next to runway can assist the tow pilot via radio

communications and a TV camera on VALS can provide a remote image to the tow pilot. c. VALS executes a no flare type landing on its landing gear. Vertical descent at touchdown is about 10 fps,

same as a Navy carrier landing.

d. At touchdown the tow rope is remote released at the VALS end. e. The remote pilot steers VALS initially using aerodynamic rudders and then transitions to nose wheel steering

as the VALS slows.

f. Remote control brakes are used to slow the VALS to stop. g. The C-17 circles airport and drops the tow line besides the runway.

h. On third pass to airport, the C-17 lands.

G. VALS Weight There are many methods for estimating a vehicle's

weight. The simplest is past history. For example, if we need an estimate of the weight of an airplane to replace the C-17 cargo aircraft, use the C-17 weight as

an initial estimate. Our problem is that no one has ever built anything like VALS, so there are no

examples that we can use. An alternate method is to start with an existing aircraft and start removing components while leaving the takeoff gross weight

(GW) fixed. As components are removed the operating empty weight (OEW) would decrease while the Payload would increase. This is approach we use

here. We first start with a military cargo aircraft since they are designed to carry cargo from point to point.

We picked the Lockheed C-141, mainly because we have its detailed weight statement from Reference 8. Also its ratio of Payload to OEW of 1.21 is

comparable to modern cargo aircraft; the C-17 ratio is 1.08 while the C-5 ratio is 1.19. In other words, the C-141 can carry 1.21 times its empty weight in fuel

and cargo. Next we start removing component weights. If we

wish to convert the C-141 into a glider we remove the engines, the engine nacelles, the fuel system, and the trapped fuel and oil. The Payload to OEW ratio

becomes 1.85. If we want to remove the pilot, then we can remove the oxygen system, cabin pressurization

At this scale, only 400 feet of tow line shown

Fig. 11 VALS Landing CONOPS

Table 1. VALS Weight Estimate (lb)

Manned Unmanned

C-141 Glider Glider VALS Wing Group 35,272 35,272 35,272

Empennage Group 5,907 5,907 5,907 5,907 Fuselage Group 36,857 36,857 36,857 36,857

Nacelle Group 5,168

Nose Gear 1,234 1,234 1,234 1,234

Main Gear 9,616 9,616 9,616 9,616

Engines 23,665

Fuel System 1,802

Avionics 3,078 3,078 3,078 3,078 Surface Controls 3,701 3,701 3,701 3,701

Hydraulic System 1,604 1,604 1,604 1,604 Electrical System 2,826 2,826 2,826 2,826

Electronics 1,163 1,163 1,163 1,163 Auxiliary Power Unit (APU) 534 534 534 534

Oxygen System 479 479

Cabin Pressure & AC 2,283 2,283

Anti-icing 453 453 453

Furnishings 5,210 5,210

Auxiliary Gear 103 103 103 103

Oil & Trapped Fuel 1,327 Operating Empty Weight (OEW) 142,282 110,320 102,348 66,623 Takeoff Gross Weight (GW) 314,000 314,000 314,000 314,000

Payload = GW - OEW 171,718 203,680 211,652 247,377

Payload / OEW 1.21 1.85 2.07 3.71

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and air conditioning, and furnishings for a ratio of 2.07. In other words, we can conclude that a glider can carry about 2 times its empty weight in payload. This ratio matches historical trends for actual gliders, thus giving us

some confidence in this weight estimation method. Finally for VALS, we remove the weight of the wings and anti-icing. The Payload to OEW ratio is 3.71. We

can conclude that the actual ratio might be higher than this because VALS does not have the same functionally in its

avionics, electrical, and electronics systems as the C-141 aircraft has (for example, it does not need weather radar or navigation systems). Hence it might be reasonable to assume a ratio of 4 for VALS, prior to installation of any LV’s

ASE and GSE. The installation of ASE and GSE might lower the ratio to 3, especially for LV’s that use cryogenic propellants and require insulated heavy duty Dewars to carry and provide the propellants just prior to launch.

For the concept illustrated in this paper, then VALS might weigh about 25,000 lb based on the fact that the LV

weighs 100,000 lb. This paper’s concept uses the landing gear from the Boeing 737 and that gear weighs 4,353 lb, so our preliminary VALS weight estimate appears to be conservative.

H. VALS Relative Cost

Cost estimation is largely statistical in which we predict cost based on the actual cost of prior vehicles. Unfortunately, we are in the same situation as in weight estimation – there are no prior vehicles like VALS.

Regardless we can make some relative comparisons. Compared to either a custom aircraft or a custom glider, VALS should be lower in cost to design, develop, and test. For a given LV weight, it is the lightest vehicle and has the fewest subsystems. If we ratio cost on the basis of OEW, then VALS should be 1/2 the cost of a new custom

glider, and 1/4 the cost of of new custom aircraft.

V. Tow Aircraft Considerations

A. Tow Line Attachment The obvious way to attach a tow line is to specially modify an aircraft with a custom tow line attachment. This

would be costly and limit VALS to that specific aircraft. Table 2 presents three other methods of attaching a tow line that do not require permanent modifications to the aircraft. Items in green and preceded by a ‘+‘ are advantages of the method, while items in red and preceded by a ‘-‘ are disadvantages.

Table 2. Three Methods to Attach Tow Line that do not require Permanent Aircraft Modifications

Tow Rope Attached to Pallet on Cargo Ramp

Similar to pallet used in Dryden Ellipse test program (C-141 towing a QF-106). Pallet sits on open cargo ramp. Pallet is assumed to weigh about 5% to 10% of VALS+LV’s weight + No modif ications to aircraft + Very quick installation of pallet onto tow aircraft + VALS & LV weight up to 60,000 lb if C-17A used - Cabin is unpressurized, which limits flight above 18,000 feet to 2 hours, and abov e 25,000 feet to 1/2 hour

Tow Rope Attached to an Adapter

Adapter attaches to existing hard points on Orbital’s L-1011 or 10 Tanker (DC-10 f ire fighting aircraft). + No modif ications to aircraft

+ Tow loads pass near aircraft center of gravity + VALS+LV weight up to 100,000 lb if 10 Tanker used - Need sev eral hours to attach adapter to aircraft

Tow Rope Attached to Internal Truss

Truss sits on cabin floor and distributes loads onto floor. Y -bridle tow line attaches to hard points passing through passenger or paratrooper doors. Truss assumed to weigh 10% of VALS+LV weight + No modif ications to basic aircraft + Can use NASA’s 747’s, C-5, C-17, C-130, An-124 or any freighter conv erted airliner + VALS+LV weight up 225,000 lb if Boeing 747-400F freighter used and 125,000 lb if C-17A is used - Need sev eral hours to install internal truss

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B. Available Aircraft Characteristics Table 3 presents the characteristics of nine different aircraft types that can be used as tow aircraft. Tow lines can

be attached to these aircraft without any permanent modifications to the aircraft. The Stargazer L-1011, White Knight, White Knight 2, and DC-10 Air Tanker can use the Tow Rope attached to an Adapter method. The C-17, C-5, and C-130 can use the Tow Rope attached to Pallet method. These three cargo aircraft, the 747 variants , and

any airliner can use the Tow Rope attached to an Internal Truss method. No permanent aircraft modifications means the aircraft can be rented when they are n eeded. There is no

requirement to buy an aircraft and maintain it annually.

Table 3. Tow Aircraft Characteristics

Stargazer L-1011 - 1 aircraf t - 52,700 lb airdrop weight - 466,000 lb max takeoff weight - 38,000 f t launch altitude -155.3 f t wingspan - 7.3 f t clearance under fuselage

White Knight - 1 aircraf t - 8,000 lb airdrop weight - 18,000 lb max takeoff weight - 53,000 f t launch altitude - 82 f t wingspan - 6 f t clearance under fuselage

White Knight 2 - Under dev elopment, 5 planned - 30K lb airdrop weight - ?? lb max takeoff weight - 50,000 f t launch altitude - 141 f t wingspan - 10 f t clearance under fuselage

DC-10 Air Tanker - 1 aircraf t (2nd under mod.) - 94,800 lb pay load weight - 433,000 lb max takeoff weight - 34,800 f t service ceiling - 155 f t wingspan - 7.3 f t clearance under fuselage - Water tanks removable in 4 hrs

747 Variants - 2 NASA aircraft available, many inexpensive used aircraft - 150,000 - 240,000+ lb pay load - 710,000+ lb max takeoff weight - 26,000 f t launch altitude - 195.7 f t wingspan - 6.1 f t clearance under fuselage

- No bottom f uselage hard points

C-5 Galaxy - 108 USAF C-5 aircraft - 85,300 lb airdrop weight (currently not used in airdrop) - 769,000 lb max takeoff weight - 35,750 f t service ceiling - 228 f t wingspan - 120 f t long x 19 f t wide x 13.5 ft

high cargo compartment

C-17 Globemaster III - 223 USAF aircraft - 60,000 lb unit airdrop weight - 585,000 lb max takeoff weight - 45,000 f t service ceiling

- 169.8 f t wingspan - 68 f t long x 18 ft wide x 13.5 ft high cargo compartment

C-130 Hercules - 428 USAF aircraft - 35,000 lb unit airdrop weight - 155,000 lb max takeoff weight - 33,000 f t service ceiling

- 132.6 f t wingspan - 41 f t long x 9 ft wide x 8.3 ft high cargo compartment

VI. Aft Crossing versus Forward Crossing Trajectories

While existing air launched, captive-on-bottom vehicles use different types of carrier aircraft, each of these LVs

employs a forward trajectory that carries it in front of its carrier aircraft. Typically the LV will drop below the carrier aircraft and then re-cross the carrier aircraft’s altitude in front of it. Vehicles such as the X-15, the Pegasus rocket, and SpaceShipOne have used this forward crossing trajectory. These vehicles use wings in order to

transition from the horizontal to vertical orientation when using this forward crossing trajectory .

Fig. 12 Comparison of Aft Crossing (Behind) and Forward Crossing (Ahead) Trajectories

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Using wings subjects these vehicles to large longitudinal bending stress during the 2–3 g pull-up maneuver they must do as they transition from horizontal to vertical flight. This high sideways acceleration requires a stronger and

heavier booster fuselage structure. In addition to the inert weight of the wings, the added weight of control surface actuators, auxiliary power units (APUs) to power the actuators, and wing thermal protection has historically limited the propellant mass fraction of winged vehicles to no more than 63 percent – an amount insufficient to reach orbit.

An alternate method is to fly a forward crossing trajectory without wings. This requires LV flight at large angles of attack at high dynamic pressure during the transition from horizontal to vertical flight. This imposes high

loads due to the very large products of aerodynamic pressure and angle of attack and requires a stronger fuselage structure, thereby increasing the weight of the LV and offsetting the weight savings of eliminating the wings.

Another disadvantage of flying the forward crossing trajectory without wings is the need for greater peak first

stage engine thrust vectoring control (TVC). The engine thrust vectoring assists in the change of orientation from horizontal to vertical of the LV, and helps to maintain stability during this orientation transition.

Lastly, there is the safety concern during a forward crossing air launch, though minimized through careful

planning, of the possibility of falling debris from the LV hitting the carrier craft, either accidentally or as a result of the launch system’s operation.

In contrast, an aft-crossing trajectory eliminates flying at high angle of attack throughout the high dynamic pressure segment of the trajectory, reduces peak aerodynamic pressure by more than 65% as compared to a forward crossing trajectory to less than 500 psf, reduces peak 1

st stage engine TVC from about 6 degrees for a forward

crossing trajectory to less than 1.5 degrees for an aft crossing trajectory, and completely eliminates the possibility of the carrier aircraft from being struck by debris from the LV. In addition, VALS in conjunction with an aft crossing trajectory eliminates the need for heavy wings, control surface actuators, APUs, and wing thermal protection and

subjects the LV to very low (less than 0.5 G) sideways accelerations. Figure 12 compares the TVC angles, dynamic

pressure (Q), and the product of Q and angle of attack. This data is a comparison developed during the DARPA Falcon program.

VI. VALS Alternate Release Methods

There are two alternate release methods available to release

the LV from VALS. Both of these alternates eliminate the need to reconfigure VALS into the Launch configuration by feathering its tail. The first alternate is what we call the

“Pegasus” method of launching. This method is used in the Orbital Sciences Pegasus small launch vehicle as well as by the

X-15 and SpaceShipOne. Its main advantage is that it is well understood and tested. Its disadvantages include requiring a wing and tail surfaces to turn the launch vehicle upward from

essentially horizontal flight toward an earth-to-orbit trajectory. This launch method subjects the LV to high sideways accelerations and bending moments, high aerodynamic

pressures (the Pegasus's maximum was 1,250 psf), and requires a large amount of thrust vectoring control (TVC).

A second alternate method is to use Trapeze Lanyard Air Drop (t/LAD).

1 t/LAD causes a LV to transition from a

horizontal orientation under VALS to a substantially vertical

orientation behind and below the VALS. Reference 1 describes the series of three flight tests over Mojave, CA in June 2005 using Scaled Composites’ Proteus aircraft. A scaled mockup of

the LV was dropped using t/LAD, and the tests proved the viability of this new method for air launching spacecraft.

The t/LAD components are shown in Figure 14. A trapeze controls the motion of the LV as it falls away from either a carrier aircraft or from VALS. The arrangement of the trapeze

ensures that the LV cannot impact VALS as it clears the near-field aerodynamic effects of VALS. It also nulls out any LV

Trapeze stabilizes rocket during drop

Lanyard pitches rocket toward vertical

Drogue chute stabilizes rocket in flight

Rocket approaches ideal launch position

Trapeze stabilizes rocket during drop

Lanyard pitches rocket toward vertical

Drogue chute stabilizes rocket in flight

Rocket approaches ideal launch position

Fig. 13 t/LAD Launch

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yaw or roll motions at separation release. A flexible line or lanyard with one end

attached to a braking mechanism mounted on VALS and the other end connected to the LV causes the vehicle to rotate. The brake has a

spool about which the lanyard is wound. As the LV falls clear of the trapeze, the lanyard

unwinds from the brake spool under a preselected tension. The tension causes the LV to rotate nose up. The rate of rotation can

be adjusted by setting either the length of the lanyard or the amount of the brake tension. At the desired rotation rate (approximately 5

rpm) the lanyard separates from both the LV and VALS. A drogue parachute stabilizes the LV in yaw and roll as it drops down and rearward. A Y bridle attachment with

the parachute risers attached to the outer edges of the engine nozzles at the 9:00 and 3:00 o’clock positions provided the best combination of yaw and roll stability. The load from the parachute is small, less than 10% of the engine thrust.

The drogue chute also progressively reduces the rate of rotation and when the LV nears its maximum vertical attitude, the rotation stops for a moment and at that moment (T = 0 seconds) the vehicle’s engines are ignited, burning through the chute riser lines and releasing the drogue chute. This method of chute release is not only

simple, but improves overall system reliability and eliminates pyrotechnic cutters. The engine then comes up to full thrust. Because of the relatively low thrust to weight (compared to most air

launched missiles) of its engines, the vehicle arrests its descent at T + 6 seconds, about 750 ft below VALS. At T + 12 seconds the LV crosses the altitude of VALS behind and separated from VALS by more than 1,000 ft. Four seconds later, the LV transitions to a standard gravity turn trajectory to low-earth orbit. The LV retains a substantial

portion of the VALS’s velocity with the t/LAD launch method, and this velocity allows greater payload to orbit.

VII. Conclusions

This paper has presented a new patent pending air launch concept that combines a proven tow technique with a novel Vertical Air Launch Sled (VALS). This method can produce a low-cost, near-term method of air launching large diameter launch vehicles (LV) using commercial or military aircraft. Advantages include:

- High system safety since the LV is far from the tow aircraft at all times - Capability to carry cryogenic propellants in insulated heavy duty Dewars located in the VALS strong back,

which can greatly reduce propellant boil-off and increase LV safety on the ground - LV length and diameter can be of any size and weight up to 200,000 lb using currently available tow aircraft,

and much larger using the proposed Stratolaunch Systems carrier aircraft

- VALS costs are expected to be less than 1/4 of a new custom aircraft and 1/2 of a custom glider - Nothing falls from VALS such as expensive wings, strong backs, pallets, or parachutes. - Near vertical attitude for the LV at ignition with the LV experiencing 1 G along the direction it normally

would be during a ground launch - No downward vertical velocity at release

- Pad hold down like capability at engine start, i.e., the capability to ignite engines while attached to VALS and if LV engine(s) health is unsatisfactory, then shut down engine(s) and return LV to airfield

- VALS can be the ground support equipment (GSE) for horizontal integration

- Easy to attach the VALS to the tow aircraft - VALS supports an aft crossing trajectory that lowers LV’s peak aerodynamic pressure, lowers product of

dynamic pressure and angle of attack, and lowers LV’s thrust vector control requirements

- Up to about 30,000 feet release altitude is possible - Three methods of attaching a tow line are available that do not require permanent aircraft modifications

VIII. Acknowledgments

Tom Brosz was the artist for the Launch Vehicle used in this paper.

Trapeze

ParachuteLanyard

Trapeze

ParachuteLanyard

Fig. 14 t/LAD System Components

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IX. References

1. Sarigul-Klijn, M., Sarigul-Klijn, N., Morgan, B., Tighe, J., Leon, A., Hudson, G., McKinney, B., and Gump, D., “Flight Testing of a New Earth-to-Orbit Air-Launch Method,” AIAA Journal of Aircraft, Vol. 43, No.3, 2006.

2. Sarigul-Klijn, N. and Sarigul-Klijn, M., “Comparative Analysis of Methods for Air Launching Vehicles from

Earth to Sub-orbit or Orbit,” Journal of Aerospace Engineering, vol. 220, no. 5, pp. 439-452, 2006. 3. Sarigul-Klijn, N., Sarigul-Klijn, M., and Noel, C., “Air-Launching Earth-to-Orbit: Effects of Launch

Conditions and Vehicle Parameters,” AIAA Journal of Spacecraft and Rockets, Vol. 42, No. 3, 2005.

4. Sarigul-Klijn, N., Sarigul-Klijn, M., and Noel, C., "Air Launching Earth-to-Orbit Vehicles: Delta V gains from Launch Conditions and Vehicle Aerodynamics," AIAA 2004-872, 42nd AIAA Aerospace Sciences Meeting,

Reno, NV, January 2004. 5. Sarigul-Klijn, M., Sarigul-Klijn, N., Hudson, G.C., Holder, L., Fritz, D. Webber, C. Liesman, G., Shell, D.,

and Gionfriddo, M., “Flight Testing of a Gravity Air Launch Method to Enable Responsive Space Access,” AIAA

2007-6146, AIAA Space 2007, Long Beach, CA, 2007. 6. Murray, J.E., Bowers , A.H., Lokos, W.A., Peters, T.L., and Gera, J., “An Overview of an Experimental

Demonstration Aerotow Program,” NASA/TM-1998-206566, September 1998.

7. Kocivar, B., ”Nutcracker VTOL folds in Flight,” Popular Science, pp. 68-70, Sept. 1976. 8. Roskam, J., “Airplane Design, Part V: Component Weight Estimation,” The University of Kansas, Lawrence,

KS, 1989.

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Documents

[American Institute of Aeronautics and Astronautics AIAA SPACE 2012 Conference & Exposition - Pasadena, California ()] AIAA SPACE 2012 Conference & Exposition - A New Air Launch Concept: