[2] A winged booster rocket with the X-43 placed on top, called a "stack", was drop launched from a Boeing B-52 Stratofortress.

Each of the other two flew successfully in 2004, setting speed records, with the scramjets operating for approximately 10 seconds followed by 10-minute glides and intentional crashes into the ocean.

One of the primary goals of NASA's Aeronautics Enterprise was the development and demonstration of technologies for air-breathing hypersonic flight.

Traveling at Mach speeds produces significant heat due to the compression shock waves involved in supersonic aerodynamic drag.

Scramjets only operate at speeds in the range of Mach 4.5 or higher, so rockets or other jet engines are required to initially boost scramjet-powered aircraft to this base velocity.

In the case of the X-43A, the aircraft was accelerated to high speed with a Pegasus rocket launched from a converted Boeing B-52 Stratofortress bomber.

[6] The engines in the X-43A test vehicles were specifically designed for a certain speed range, only able to compress and ignite the fuel-air mixture when the incoming airflow is moving as expected.

An investigation into the incident stated that imprecise information about the capabilities of the rocket as well as its flight environment contributed to the accident.

Several inaccuracies in data modeling for this test led to an inadequate control system for the particular Pegasus rocket used, though no single factor could ultimately be blamed for the failure.

[9] It reached Mach 9.68,[10][11] 6,755 mph (10,870 km/h) at 109,440 ft (33,357 m),[12] and further tested the ability of the vehicle to withstand the heat loads involved.

[13] In January 2006 the USAF announced the Force Application and Launch from Continental United States or FALCON scramjet reusable missile.

[14] In March 2006, it was announced that the Air Force Research Laboratory (AFRL) supersonic combustion ramjet "WaveRider" flight test vehicle had been designated as X-51A.

After the X-43 tests in 2004, NASA Dryden engineers said that they expected all of their efforts to culminate in the production of a two-stage-to-orbit crewed vehicle in about 20 years.

The scientists expressed much doubt that there would be a single-stage-to-orbit crewed vehicle like the National Aerospace Plane (NASP) in the foreseeable future.

[15] The linked story reports the project's indefinite suspension and the appearance of Rear Admiral Craig E. Steidle before a House Space and Aeronautics subcommittee hearing on March 18, 2004.

As of September 2007, only a feasibility study had been conducted by Donald B. Johnson of Boeing and Jeffrey S. Robinson of NASA's Langley Research Center.