The Air Force says an unmanned experimental aircraft suffered a control failure during an attempt to fly at six times the speed of sound.
The X-51A Waverider was designed to reach Mach 6, or 3,600 mph, after being dropped by a B-52 bomber off the California coast on Tuesday.
But the Air Force said Wednesday that a faulty control fin prevented it from starting its exotic scramjet engine and it was lost.
The first Boeing-designed X-51A achieved Mach 5 for about 140 seconds in 2010. A second Waverider flight last year ended prematurely with the craft trying to restart its engine until it plunged into the Pacific.
The Pentagon has been testing hypersonic technologies in hopes of being able to deliver strikes around the globe within minutes.
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