The first hypersonic flight test took place in March 2010, with the X-51 reported to have travelled at Mach - 5 but crashing into the Pacific Ocean when its fuel supply was exhausted.
Lewis is particularly enthusiastic about the X-51 WaveRider, a hypersonicflight - test vehicle funded by the Air Force and the Defense Advanced Research Projects Agency (DARPA), the Pentagon's research and development arm.