In August 1972 it was proposed to test a subscale version of the shuttle to test the aerodynamics. The 13,750 kg vehicle would be 11 m long and have a wingspan of 7 m. 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 $19.7 million, but opponents said it would cost over $150 million. It was never authorized, and it was only on the first orbital flight that the shuttle aerodynamics were proven at supersonic speeds.

Length: 11.00 m (36.00 ft). Span: 7.00 m (22.90 ft). Mass: 13,750 kg (30,310 lb). Main Engine: XLR-99. Main Engine Propellants: Lox/Ammonia.