Ground crew members make the final checks to the X-51A Waverider scramjet, which is affixed to an Edwards B-52H Stratofortress, before the Waverider's June 13 flight test.CREDIT: U.S. Air Force/Boeing
Hypersonic X-51A Scramjet Failure Perplexes Air Force -- Space.com
Air Force engineers are scratching their ends after a test flight of an experimental hypersonic X-plane ended last month prematurely when the vehicle failed to reach full power.
A rocket successfully boosted the hypersonic X-51A Waverider vehicle to just over five times the speed of sound (Mach 5) on June 13. The experimental craft's air-breathing scramjet engine then lit briefly on ethylene but failed to transition to its primary JP7 fuel, Air Force officials said.
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My Comment: Wikipedia has a good summary on the X-51A, and the Air Force page dedicated to the X-51A is a bit more detailed.