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 X-43A aircraft was a small unpiloted test vehicle measuring just over 3.7 m in length. The vehicle was a lifting body design, where the body of the aircraft provides a significant amount of lift for flight, rather than relying on wings. The aircraft weighed roughly 3,000 pounds (about 1,300 kilograms). The X-43A was designed to be fully controllable in high-speed flight, even when gliding without propulsion. However, the aircraft was not designed to land and be recovered. Test vehicles crashed into the Pacific Ocean when the test was over.
Traveling at Mach speeds produces a lot of heat due to the compression shock waves involved in supersonic drag. At high Mach speeds, heat can become so intense that metal portions of the airframe melt. The X-43A compensated for this by cycling water behind the engine cowl and sidewall leading edges, cooling those surfaces. In tests, the water circulation was activated at about Mach 3. In the future, fuel may be cycled through such areas instead, much like what is currently done in many liquid-fuel rocket nozzles and high speed planes such as the SR-71.