X-43
The Hyper-X was so called because it flew at over five times the speed of sound. The 43rd X-plane in the government’s catalogues, the aircraft was the first to demonstrate scramjet propulsion, in which airflow through the vehicle’s engine remains at supersonic speeds. This allows the craft to keep moving at extremely fast speeds, as the X-43 exhibited twice; once in March 2004 when it flew at Mach 6.83, outpacing the record set by the rocket plane X-15, and again in November 2004 when it flew at an astonishing Mach 9.6—almost 7,000 miles per hour. Now, the X-43’s engine ran for only about 10 seconds, so sustaining flight at such speeds is another matter.
