The X-43A was a small experimental research aircraft designed to flight-demonstrate the technology of airframe-integrated supersonic ramjet or "scramjet" propulsion at hypersonic speeds above Mach 5, or five times the speed of sound. Its scramjet engine is an air-breathing engine in which the airflow through the engine remains supersonic. The X-43A sustained a record speed of Mach 6.83 or about 4,900 mph on March 27, 2004, and a follow-on flight on Nov. 16, 2004, sustained speeds of Mach 9.6, or almost 7,000 mph, after being boosted to test altitudes by modified first-stage Pegasus rockets.