Fire in the sky: the Air Launched Sortie Vehicle of the early 1980s (part 1)
by Dwayne Day
|Air-launching a small spaceplane from the back of a jumbo jet is an idea that was considered by both superpowers during the Cold War.|
In the early 1980s, the United States Air Force sponsored a study of what was initially designated a Space Sortie Vehicle, then renamed the Air Launched Sortie Vehicle, or ALSV. The ALSV does not appear to have resulted from a military requirement established at the senior levels of the Air Force. Instead, the limited available documentation about the ALSV indicates that it was essentially a technology trade study sponsored among at least three separate contractors who may have contributed their own funds, at least one or two of them receiving government funding for their study.
The purpose of the studies was apparently to determine whether new technologies, some of them developed for NASA’s Space Shuttle program, would enable the development of a small reusable space vehicle capable of relatively quick launch from virtually any location to virtually any orbit and able to deploy and recover small satellites. The answer produced by these initial studies was that it was feasible. However, the studies did not progress. Until more data is gathered it is impossible to determine why the ALSV concept did not advance any further, but the performance of the ALSV was not particularly impressive, and the technical challenges were also daunting.
In late 1980, Don Hart of the Air Force Rocket Propulsion Laboratory outlined what was described as “an Air Force Sortie Space System” (hereafter AFSSS) in a seven-page overview document. This was apparently a revision of the concept, although when it had originally started remains unknown. The AFSSS had three major parts: a launch platform, drop tanks, and a space vehicle.
The launch platform was defined as a 747 which not only carried the space vehicle and drop tanks on its back, but also had liquid oxygen and liquid hydrogen storage tanks inside its fuselage. The tanks would be low boil-off dewars. The propellants would be pumped into the drop tanks just before separation of the vehicle and drop tanks.
This would be no ordinary 747. Not only would it have the internal tanks, but the hydrogen would also be pumped into afterburners on the 747’s large turbofan engines, providing up to 400 percent thrust augmentation. The 747 would fly a zoom parabola, with vehicle separation at 15,200–16,800 meters (50,000–55,000 feet) altitude. As jumbo jets go, it would have been a real hot rod.
The space vehicle would have two attached drop tanks based upon design, materials, tooling, and fabrication techniques for the Atlas launch vehicle. They would use the Space Shuttle’s spray-on insulation if necessary. The tanks would form an aerodynamically-shaped nose for a lifting ascent trajectory. The tanks would be released from the space vehicle shortly before it reached orbit, and they would burn up on reentry. If necessary, they would be explosively blown into pieces. Unlike more traditional launch vehicles, like the Space Shuttle, each tank would have three inner tanks, with hydrogen in the front, oxygen in the middle, and hydrogen at the rear to control the center of gravity for a lifting ascent. Overall, the vehicle would be approximately 10.7 meters (35 feet) wide and 15.2 meters (50 feet) long.
|Until more data is gathered it is impossible to determine why the ALSV concept did not advance any further, but the performance of the ALSV was not particularly impressive, and the technical challenges were also daunting.|
The space vehicle would be reusable, making a runway landing after return from orbit. The vehicle’s shape would be derived from several flown or heavily-studied lifting body type vehicles such as the FDL-5, FDL-8, or X-24C. The vehicle would be powered by ten modified RL-10 engines arranged in a 2 x 5 array. The turbomachinery and injector would be the same as the standard RL-10, but the thrust chamber would be slightly longer, the expansion ratio slightly less, and there would be upper and lower expansion plates for additional vacuum performance. The backside of the expansion plates would also provide vehicle pitch control. During reentry, the plates would close completely, forming a boattail over the rear of the vehicle.
The vehicle itself would have a large crossrange, the ability to come down a significant distance from its orbital track. It could be either manned or unmanned, but could probably carry no more than one person if manned. The 747’s payload capability dictated the gross weight of the space vehicle and its drop tanks of no more than 100,000 kilograms (220,000 pounds).
As Hart explained it, the spacecraft could fly several kinds of missions. The basic mission would be to fly to low Earth orbit, stay there for the mission, possibly leaving a small payload in orbit, and then return to a runway landing. Another mission could involve delivering a medium weight payload to low Earth obit by releasing the payload at vehicle apogee and immediately returning to Earth. A mission described as “low altitude penetration of target area” would involve flying a shallow trajectory, skipping into the target area, and then reigniting the engines to power out and land. Finally, there was a maximum weight payload to low Earth orbit mission. This would not involve the space vehicle itself, only a cargo carrier and the ten RL-10 rocket engines which would fly into orbit and would not be returned, although they could be retrieved by a Space Shuttle at a later date.
Hart also noted that another option would involve a very different vehicle sized to fit inside the shuttle bay. Three shuttle flights would bring up the vehicle and separate fueled drop tanks. The vehicle would be assembled in orbit and could fly up to geosynchronous or another high energy orbit. After the mission it would return directly to a runway landing.
|What prompted the Air Force Rocket Propulsion Laboratory to propose such a vehicle? Most importantly, what kinds of missions could it fly?|
According to Hart, a major goal for the Space Sortie System was rapid response. The aircraft and spacecraft should be supported from any Air Force base as long as there was a supply of liquid hydrogen and oxygen available. The vehicle could stand at “alert status,” with propellants inside the launch platform. It could take off on warning and fly to any azimuth because it would not produce debris. The goal would be to enable flyover of any point on Earth within 75 minutes, including 747 taxi, takeoff and climbout. It could fly by any low Earth orbit satellite or space station within 75 minutes and rendezvous with any satellite or station within five hours.
The vehicle would not only be reusable, but would have rapid turnaround of hours, not days. “All phases of sortie are under Air Force control. All ground operations by Air Force crews at Air Force bases,” Hart wrote. Nobody would have to worry about working with NASA.
According to Hart, there were few technological risks and he outlined various aspects of the technology that already existed. The ability of a 747 to carry a piggyback payload had already been demonstrated by the shuttle program. An afterburner modification for the 747’s engines was already available and would not require changes to the fan, compressor, burners, or turbine. The drop tanks would be based on the Atlas tooling and fabrication techniques. The RL-10 rocket engine had a long history and there was data on how it operated in multiple engine configurations. The “lifting ascent trajectory” had already been “examined in other programs,” according to Hart. The one unknown that he identified was whether or not the drop tanks would burn up entirely. This would require further study.
However, despite the optimistic assessment of the technology maturity, Hart’s document overlooked several important technology issues. What kind of thermal protection would the vehicle use? Although the Space Shuttle had pioneered a new thermal protection system, would it be applicable to a differently-shaped vehicle? What kind of structure would the spacecraft use? Would separating a large vehicle from a 747 at high altitude pose any problems? And finally, would pumping large amounts of liquid oxygen and liquid hydrogen from one vehicle to another while in flight be relatively straightforward?
The system would be a near-term solution whose existence “would certainly challenge USSR strategic defense structure,” Hart wrote. It was “not a traditional launch vehicle, nor a traditional satellite, nor a traditional aircraft.” As a result, “non-traditional thinking” was needed to determine what it could accomplish.
Much remains unknown about the Air Force Space Sortie System proposal. What prompted the Air Force Rocket Propulsion Laboratory to propose such a vehicle? Considering that Hart’s December 1, 1980 outline was apparently a revision, who initiated the idea?
Most importantly, what kinds of missions could it fly? Considering that it could carry either a small deployable payload, or a single crewman, probably without much room for equipment, what would it actually do? Were there any useful missions that a small satellite or a single astronaut in a small spacecraft with minimal payload could actually accomplish?
If this December 1980 document was intended to prompt aerospace contractors into action, it certainly had an effect. Within a short period of time, several contractors began considering such a spacecraft, and trying to come up with a way of flying a space vehicle off the back of a 747 that didn’t end up looking like the opening scene of Moonraker.