While they have interests in developing much small winged reusable launch vehicles via its ALASA (Airborne Launch Assist Space Access) programme, the US government organisation DARPA (Defence Advanced Research Project Agency), is reported by Space News to be about to start a programme to build a much larger reusable launch vehicle with a first stage called XS-1 (Experimental Spaceplane 1). According the report, the first stage would release an expendable upper stage at Mach 10 and, despite its name, the first stage may or may not have wings.
Comment by David Todd: Reusability is now the in thing in space launch research. whether it is Blue Origin, SpaceX, Reaction Engines, and now DARPA doing it. Separating bodies at Mach 10 will however be problematic if it is done in the atmosphere.