The X-33 program was a government/industry effort to design, build and fly a half-scale prototype that would have demonstrate in flight the new technologies needed for the proposed Lockheed Martin full-scale VentureStar Reusable Launch Vehicle.

The X-33 design was based on a lifting body shape with two revolutionary "linear aerospike" rocket engines and a rugged metallic thermal protection system. The vehicle also was to have used lightweight components and fuel tanks built to conform to the vehicle's outer shape. Time between X-33 flights was planned to normally be seven days, but the program hoped to demonstrate a two-day turnaround between flights during the flight-test phase of the program.

The X-33 was to have been an unpiloted vehicle that took off vertically like a rocket and landed horizontally like an airplane. It was to have reached altitudes of up to 50 miles and high hypersonic speeds. The X-33 Program was managed by the Marshall Space Flight Center and was to have been launched at a special launch site on Edwards Air Force Base. Technical problems with the X-33's composite liquid hydrogen tank resulted in the program being cancelled in February 2001.