Status: Study 1972. Gross mass: 13,750 kg (30,310 lb). Height: 11.00 m (36.00 ft). Span: 7.00 m (22.90 ft).
Air dropped from an NB-52 carrier aircraft, it would use the XLR-99 engine developed for the X-15 to boost it to over Mach 5.
It would be equipped with off-the-shelf on-board systems, but use the thermal protection materials planned for the shuttle. Proponents said they could accomplish the program for 19ドル.7 million, but opponents said it would cost over 150ドル million. It was never authorized, and it was only on the first orbital flight that the shuttle aerodynamics were proven at supersonic speeds.