One of the primary goals of NASA's Aeronautics Enterprise was the development and demonstration of technologies for air-breathing hypersonic flight. Phase I was a seven-year, approximately $230 million, program to flight-validate scramjet propulsion, hypersonic aerodynamics and design methods.
Unlike rockets, scramjet-powered vehicles do not carry oxygen on board for fueling the engine. Removing the need to carry oxygen significantly reduces the vehicle's size and weight. In the future, such lighter vehicles could take heavier payloads into space or carry payloads of the same weight much more efficiently.
The first plane in the X-plane series, the X-43, was a small experimental hypersonic unpiloted test vehicle measuring just over 12 ft (3.7 m) in length able to launch multiple planned scale variations meant to test various aspects of hypersonic flight.
The X-43, a winged booster rocket with the X-43 placed on top (called a "stack"), was drop launched from a Boeing B-52 Stratofortress. Scramjets only operate at speeds in the range of Mach 4.5 or higher, so rockets or other jet engines are required to initially boost scramjet-powered aircraft to this base velocity. After the booster rocket (a modified first stage of the Pegasus rocket) brought stack to the target speed and altitude, it was discarded and the X-43 flew free using its own scramjet engine.
Designed to be fully controllable in high-speed flight, even when gliding without propulsion, the X-43 was a lifting body design, where the body of the aircraft provides a significant amount of lift for flight, rather than relying on wings. However, the aircraft was not designed to land and be recovered. Test vehicles crashed into the Pacific Ocean when the test was over.
The first two X-43A aircraft were intended for flight at approximately Mach 7, while the third was designed to operate at speeds greater than Mach 9.8 at altitudes of 98,000 ft (30,000 m) or more.
NASA's first X-43A test on 2 June 2001 failed because the Pegasus booster lost control about 13 seconds after it was released from the B-52 carrier. In the second test in March 2004, the Pegasus fired successfully and released from the test vehicle. After separation, the engine's air intake was opened, the engine ignited and the aircraft then accelerated away from the rocket reaching Mach 6.83. Fuel flowed to the engine for 11 seconds in which time the aircraft had traveled more than 15 miles (24 km).
On 16 November 2004, NASA flew a third version of the X-43A. The craft was was launched from a B-52 mother ship at an altitude of 43,000 ft (13,000 m) and the X-43A set a new speed record of Mach 9.6 at about 110,000 feet (33,500 m) altitude
Other X-43 vehicles were planned, but as of June 2013 subsequent phases were not continued and the X-43 series of aircraft was replaced by the X-51.