If an oxidizer-propellant cannot be gotten from the atmosphere, are there other methods of getting off the ground without having to lift the heavy oxidizer? One answer may be as simple as refueling in midair (Fig. 8.4). Military aircraft routinely refuel from aerial tankers, greatly extending the range and capability of their forces. The routine and well-practiced nature of these techniques makes it well-suited to serve the spaceplanes of the future. In this scheme, the refueling actually involves a transfer of liquid oxidizer propellant from an aerial tanker to a spaceplane. The technique is called aerial propellant transfer, or APT.
One design that used APT was Blackhorse, a one-man spaceplane studied by the US Air Force in the mid-1990s. It was about the size of an F-16 fighter aircraft, took off from a conventional runway, filled its oxidizer tanks from a KC-135 or KC-17 tanker, and zoomed into space from about 43,000 ft. Blackhorse would burn rocket-grade kerosene and hydrogen peroxide (H2 O2), which is a dense oxidizer similar in many respects to water. It was considered to be a "stage and a half' to orbit vehicle, since it enlisted the help of an aerial tanker. The studies showed that such a vehicle could work feasibly.9
Another technique is to liquefy atmospheric air and store it in a propellant tank just before "lighting off" for orbit. The Gryphon spaceplane uses such a concept in its Air Collection and Enrichment System. This allows a spaceplane to take off from the ground without having to lift any oxidizer at all, relieving the loads on the landing gear
and associated structure during takeoff. The craft then flies subsonically to as high an altitude as practical, and gradually fills its oxidizer tank with liquefied air. During this phase of the flight, the spaceplane is flying as a normal aircraft, generating only enough thrust to overcome drag. A high subsonic lift-to-drag ratio is required, because as the oxidizer tanks are filled, the vehicle becomes heavier and heavier. A good L/D helps to maintain good efficiency. Only when the oxidizer tank is full would acceleration to orbital velocity take place, from an altitude of around 50,000 ft.10
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