The X-43 is an unmanned experimental hypersonic aircraft design with multiple planned scale variations meant to test different aspects of highly supersonic flight. It is part of NASA's Hyper-X program.
The third flight of a Boeing X-43A set a new speed record of 7,000 mph (11,200 km/h), or Mach 9.8, on November 16, 2004. It was boosted by a modified Pegasus rocket which was launched from a Boeing B-52 at 13,157 meters (43,166 feet). After a free flight where the scramjet operated for about ten seconds, the craft made a planned crash into the Pacific ocean off the coast of southern California.
The craft was created to develop and test an exotic type of engine called a supersonic-combustion ramjet, or "scramjet," an engine variation where external combustion takes place within air that is flowing at supersonic speeds. The X-43A's developers designed the aircraft's airframe to positively affect propulsion, just as it affects aerodynamics: in this design, the forebody is a part of the intake airflow, while the aft section functions as a nozzle.
Scramjets only operate at hypersonic speeds in the range of Mach 6 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 B-52 Stratofortress bomber.
(This text was adapted from http://www.wikipedia.org/ )(GFDL)
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