The X-43 is an unmanned experimental hypersonic aircraft with multiple planned scale variations meant to test various aspects of hypersonic flight. It was part of NASA‘s Hyper-X program. It has set several airspeed records for jet-propelled aircraft. 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 scramjet concept is simple: Accelerate the vehicle to about Mach 4 by a conventional jet engine, then start the scramjet engine (which has few or no moving parts) by introducing fuel and mixing it with oxygen obtained from the air and compressed for combustion. The air is naturally compressed by the forward speed of the vehicle and the shape of the inlet, similar to what turbines or pistons do in slower-moving airplanes and cars.
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. The X-43A’s successful second flight made it the fastest free flying air-breathing aircraft in the world. 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.