The DARPA hopes to develop a reusable spaceplane, the XS-1, that can boost an upper stage and small orbital payload through the first stage ascent profile. The second stage would carry small satellites into low earth orbits. The initial customer for such a system is expected to be U.S. national security agencies.
In effect, the XS-1 is intended to replace the traditional expendable "first stage" of a multistage launch vehicle, but with the capability of flying at hypersonic speeds. In a fashion similar to an expendable first stage, the XS-1 should enable an expendable upper stage to separate and deploy a payload into orbit. This hypersonic stage will be designed to return to Earth and be serviced fast enough to repeat the process at least once every 24 hours. more
