A control subsystem failure caused the United States Air Force's X-51A Waverider to crash into the Pacific Ocean during a test flight. The The test was to represent the next step towards hypersonic flight, defined as that exceeding Mach 5 -- five times the speed of sound, or approximately 3,800 mph.
Conducted in May 2010, the first powered flight test of the X-51 Waverider resulted in the aircraft achieving Mach 5, flying for about 200 seconds of the planned 300 second flight time before falling into the ocean. In this third launch (the second test failed after only a few seconds), it was hoped that the Waverider would reach Mach 6 and complete the 300 second flight.
At these speeds the aircraft could traverse the Atlantic in under an hour.
In Wednesday's test, the Waverider, which derives its name from the way it uses the shock waves created by its hypersonic flight to keep itself airborne, was to dropped from a B-52 , then propelled by its booster engine for 30 seconds to Mach 4.5. Finally, its scramjet engine was supposed to have ignited and accelerate the aircraft to Mach 6, as it climbed to 70,000 feet before falling to the Pacific after 5 minutes of flight.
The control subsystem failure occurred 15 seconds after the booster engine had been activated, before the technology’s signature scramjet engine, which uses no moving parts, was ignited. The goal of the launch was to extend the flight time of the aircraft in the scramjet phase, but the error rendered the improvements aerospace engineers had made to the aircraft since the May 2010 flight untested.
Aside from the almost science-ficiton idea of hypersonic commercial travel, the U.S. military has hoped to use the Waverider technology in the development of hypersonic cruise missiles capable of rapidly hitting targets anywhere in the world, as part of its Prompt Global Strike effort. Whereas nuclear-tipped intercontinental ballistic missiles (the only device in the military’s inventory with such rapid-strike capability) can reach any target in the world within an hour, such an analog is absent from conventional warfare.
In an unsuccessful 1998 attack on Osama bin Laden in Sudan, cruise missiles launched from U.S. navy vessels the Arabian sea took 80 minutes to reach their Afghanistan training camp targets, by which time Bin Laden had fled the area. A hypersonic cruise missile could cut this time down to just 12 minutes, illustrating its applicability in counter-terrorism measures.
After Tuesday’s unsuccessful test, however, the future of hypersonic flight seems uncertain. Though plans for another test flight have not been scheduled, the Air Force has one remaining X51-A aircraft. If this last test is similarly unsuccessful, additional funding from DARPA, the research arm of the Pentagon, may not be forthcoming.