The
            X-24 was a joint NASA/U.S. Air Force project to investigate
            the use of lifting bodies for returning from space.  Lifting
            bodies are wingless aircraft that derive lift from their shape.
            The X-24A's first flight was in 1970.  The
            typical flight profile was to be carried aloft and released from a
            B-52.  The rocket engine then boosted the X-24 to altitude and the
            pilot glided the craft back to a landing.
            In 1972, the X-24A was rebuilt as the X-24B with
            twice the lifting surface and more stable flight characteristics. 
            The X-24 flight program ended in 1975.