The juvenile stage of spaceplane development involves the cooperation of bigger, more mature rockets. Just as the development of the airplane involved many biwing (and triwing) designs to increase the total lift of the airframe (Fig. 7.1) , so the development of the spaceplane has inevitably led to the piggyback concept, and for related reasons. In the case of early airplanes, one wing simply could not generate the necessary lift. Similarly, for current spaceplanes to have any hope of reaching orbital speed, they must be given a lift by a first stage of some kind.
One way of accomplishing this is by perching a small spaceplane atop a ballistic booster, as we have already seen. But another way is to use a booster stage with a winged orbiter mounted to its side or astride its back. This concept takes several forms, including the familiar Space Shuttle configuration. Other ideas usually involve fully reusable systems with manned fly-back boosters. These may take off either horizontally or vertically.
This is where we are today. The most advanced flying vehicle in the first decade of the twenty-first century, in terms of performance, is a piggyback spaceplane. It is a necessary, and painful, step on the road to eventual maturity, as spaceplanes try to grow up.
There are two major modes within the two-stage-to-orbit spaceplane concept, both of which use airplane-like landings. These are the vertical takeoff horizontal landing (VTHL) and horizontal takeoff horizontal landing (HTHL) plans. The Space Shuttle is a partially reusable VTHL space transportation system with four major components, a compromise forced on the space agency by budget constraints. The preferred plan would have had only two components, a manned flyback booster and winged orbiter. Bristol Spaceplanes have a plan for a TSTO vehicle using an HTHL approach. A huge booster and piggyback orbiter would take off together from a spaceport runway, using the booster's engines to reach as high an altitude and as high a speed as possible. The much smaller orbiter would then separate, ignite its own rocket engines, and continue to accelerate to orbital velocity. This plan is presently only on paper, and it remains to be proven if it can be made practical. As of this writing, the only proven design for a manned piggyback spaceplane is NASA's Space Shuttle.
M.A. Bentley, Spaceplanes: From Airport to Spaceport, 93
doi:10.1007/978-0-387-76510-5_7, © Springer Science + Business Media, LLC 2009
Fig. 7.1 Early airplanes used performance-enhancement devices such as multiple wings to increase lift and aerodynamic cowlings to decrease drag. This is the Curtis Hawk with NACA cowling (courtesy NASA)
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