How does a suborbital spaceplane launch? There are two obvious methods: air launch and self-launch. Air launch is the method used in the testing of most highspeed rocket planes, including the early rocket-powered X-planes, the NASA lifting bodies, and the X-15. It was also the method used by SpaceShipOne. This technique allows the spaceplane to "light off' from 40,000 or 50,000 ft and some initial non-zero airspeed, giving it an advantage in terms of its flight energy. The total energy of a spaceship, you will recall, depends on the sum of potential and kinetic energies, which in turn depend on altitude and velocity. So the air-launched vehicle can achieve space height much easier than its self-launched counterpart.
On the other hand, if spaceplanes are to someday mature into the capable space transports of tomorrow, then those that launch themselves now may be at a distinct advantage over their overly coddled air-launched cousins. The child who has training wheels may ride his bicycle first, but the one who never has them may ultimately prove to be the better, faster rider. XCOR Aerospace plans to launch its Xerus spaceplane from a runway, under its own power.
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