- Air launch to orbit
Air launch to orbit is the method of launching
booster rockets at altitude from a horizontal-takeoff turbojetaircraft, either subsonic or supersonic. This method, when employed for orbital payload insertion, presents significant advantages over conventional vertical rocketlaunches, particularly because of the reduced mass, thrust and cost of the rocket booster.
The most significant advantage of air launching is the considerable amount of
propellantconserved. This is because the carrier aircraftis able lift the rocket to altitude much more efficiently with use of turbojet engines, which do not require onboard storage of an oxidizer. This allows the launch system to conserve a significant amount of mass that would otherwise be reserved for fuel, reducing overall size. A larger fraction of the rocket mass can then include payload, thus reducing payload launch costs. Launching at altitude also presents significant performance benefits to the rocket. The high horizontal speed provided by the aircraft gives the rocket a large initial velocity and reduces the delta Vrequired to reach orbit. Figure 1 below indicates that high speeds can reduce delta V requirements up to 15% over the vertical launch case.
Cost calculations show that a supersonic air launch system has the potential to reduce launch costs over conventional vertical takeoff vehicles by an
order of magnitude. Launch cost can be shown to be proportional to the peak power of the rocket engines, as given by the empirical relation:
Peak power at maximum thrust is expressed as:
where horizontal is related to the initial launch acceleration, is the mass placed in low earth orbit, and
space shuttleand Saturn VApollo moon rocket are 1.49 and 1.43 respectively. For an air launch with a climb angle of 30 degrees this number can be as low as 1.00. Figures 2 and 3 provide a comparison of launch costs versus thrust power for several launch vehicles.
Air launch to
orbitalso offers the potential for aircraft-like operations such as launch on demand, and is also less subject to launch-constraining weather. This allows the aircraft to fly around weather conditions as well as fly to more optimal launch points. Other advantages include reduced national range scheduling constraints, minimum launch site requirements, and reduced range, safety concerns, and equatorial launch from the U.S.
upersonic Air Launch
By providing an initial supersonic speed to the rocket, the delta V required to reach orbit can be significantly reduced over the subsonic case. Referring again to Figure 3 shows that for a horizontal air launch at 30 degrees inclination, the delta V reduction can be as much as 25% for an air launch at Mach 4.0. Figure 4 shows the cost per kilogram in orbit versus system specific impulse and delta V. The figure indicates that launch cost to
low Earth orbitper kilogramis far more dependent on delta V requirements than on system specific impulse.
Air Launch to Orbit is being explored on subscale turbojet-powered aircraft. Currently, a 1/10th scale subsonic model, with a 1/10th scale Falcon 1 rocket mounted on the upper side of the fuselage is being used to investigate the feasibility of an air launch to orbit system.
* [http://mae.ucdavis.edu/faculty/sarigul/aiaa2001-4619.pdf A Study of Air Launch Methods for RLVs] (AIAA 2001)
* [http://www.ae.uiuc.edu/ISJ/Reports/BurtonJSRPaper.pdf Low Cost Launch of Payloads to Low Earth Orbit ]
* [http://www.ae.uiuc.edu/ISJ Illini Space Jet ]
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