The X-51A WaveRider is an unmanned U.S. Air Force experimental aircraft capable of reaching hypersonic speeds (specifically, six times the speed of sound, or 4000 miles per hour) using new scramjet engine technology. Test flights began in December 2009 and continue today.
- Design -
The end of the Cold War and the transition from manned spyplanes like the U-2 and the SR-71 Blackbird to satellite surveillance and unmanned, remote-operated spyplanes somewhat reduced the military's long-lived desire to develop a hypersonic high-altitude aircraft. Nevertheless, the air force did begin to fund the studies that led to the X-51 in the 1990s, taking advantage of NASA's own hypersonic research on previous prototypes like the X-43.
To reach its extremely high velocities, the X-51 takes advantage of a new jet propulsion technology known as the scramjet. Previous high-speed aircraft, like the SR-71 Blackbird, used ramjets, which were once the speed kings of the jet world. However, within a ramjet engine, the air actually slows down to subsonic speeds, is then combusted with jet fuel and accelerated back to supersonic speeds, and finally used to propel the aircraft forward. A scramjet design allows the engine to burn fuel without slowing down the air as it enters the engine. Ramjet-powered aircraft never exceeded a few times the speed of sound, but this innovation means that future scramjet aircraft will travel several times as fast, perhaps even Mach 20 or more.
The X-51 WaveRider is not the first aircraft to be built around a scramjet engine, but it is the latest and most promising prototype. At present, the aircraft actually resembles a missile more than it does a conventional airplane - partially because this shape is best for the extremely high speeds, but partially also because this is simply a flying test engine. It will probably be quite some time before larger manned aircraft are designed to fly at such extraordinary speeds.
- Test Flights -
Ground prototypes of the X-51's scramjet engine have been ongoing for several years. In December 2009, however, the program passed a major milestone when the prototype made its first flight, tucked under the wing of a B-52 bomber. In May 2010 it made a free flight on its own, which lasted 200 seconds and attained a speed of Mach 5 (five times the speed of sound).
Test-flying the X-51 is complicated not only because the design is incapable of taking off and landing on its own, but because the scramjet engine itself is simply not capable of such feats. The scramjet can only operate once the aircraft is already in supersonic flight. For this reason, the standard approach to hypersonic flight - at least for now - is to take the prototype to high altitude on the wing of a larger aircraft, the B-52 bomber, and then to let it loose. It then accelerates to high speed using an attached rocket, before the scramjet finally kicks in at about Mach 4.5.