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.
A winged booster rocket with the X-43 itself at the tip, called a "stack", is launched from a carrier plane. After the booster rocket (a modified first stage of the Pegasus rocket) brings the stack to the target speed and altitude, it is discarded, and the X-43 flies free using its own engine, a scramjet.
The first flight in June 2001 failed when the stack spun out of control about 11 seconds after the drop from the B-52 carrier plane. It was destroyed by the range safety officer, and it crashed into the Pacific Ocean. NASA attributed the crash to several inaccuracies in data modeling for this test, which led to an inadequate control system for the particular Pegasus used.
The X-43A's successful second flight made it the fastest free flying air-breathing aircraft in the world, though it was preceded by an Australian HyShot as the first operating scramjet engine flight. While still attached to its launching missile, the HyShot flew in descending powered flight in 2005.
The third flight of the X-43A set a new speed record of 12,144 km/h (7,546 mph), 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.