The X-43 was an unmanned experimental hypersonic aircraft with multiple planned scale variations meant to test various aspects of hypersonic flight. It was part of the X-plane series and specifically of NASA's Hyper-X program. It set several airspeed records for jet-propelled aircraft. The X-43 is the fastest aircraft on record at approximately Mach 9.6 (7,310 mph) (11,850 km/h).[1]

A winged booster rocket with the X-43 placed on top, called a "stack", was drop launched from a Boeing B-52 Stratofortress. After the booster rocket (a modified first stage of the Pegasus rocket) brought the stack to the target speed and altitude, it was discarded, and the X-43 flew free using its own engine, a scramjet.

The first plane in the series, the X-43A, was a single-use vehicle. Three of them were built. The first was destroyed after malfunctioning in flight; the other two have successfully flown, with the scramjet operating for approximately 10 seconds, followed by a 10-minute glide and intentional crash into the ocean.

he X-43 was part of NASA's Hyper-X program, involving the American space agency and contractors such as Boeing, Micro Craft Inc, Orbital Sciences Corporation and General Applied Science Laboratory (GASL). Micro Craft Inc. built the X-43A and GASL built its engine.

One of the primary goals of NASA's Aeronautics Enterprise, as delineated in the NASA Strategic Plan, specified the development and demonstration of technologies for air-breathing hypersonic flight. Following the cancelation of the National Aerospace Plane (NASP) program in November 1994, the United States lacked a cohesive hypersonic technology development program. As one of the "better, faster, cheaper" programs developed by NASA in the late 1990s, Hyper-X used National Aerospace Plane technology, and was to quickly move it forward to the next step, which was demonstration of hypersonic air breathing propulsion in flight.

The Hyper-X Phase I was a NASA Aeronautics and Space Technology Enterprise program being conducted jointly by the Langley Research Center, Hampton, Virginia, and the Dryden Flight Research Center, Edwards, California. Langley was the lead center and is responsible for hypersonic technology development. Dryden was responsible for flight research.

ASA's first X-43A test on June 2, 2001 failed because the Pegasus booster lost control about 13 seconds after it was released from the B-52 carrier. The rocket experienced a control oscillation as it went transonic, eventually leading to the failure of the rocket's starboard elevon. This caused the rocket to deviate significantly from the planned course, so the stack was destroyed by the activation of onboard explosive charges by the Range Safety Officer as a safety precaution. 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.

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