Then the X-51’s SJY61 engine ignited, initially on a mix of ethylene, similar to lighter fluid, and JP-7 jet fuel then exclusively on JP-7 jet fuel, the same fuel once carried by the SR-71 Blackbird before its retirement. The flight reached an altitude of about 70,000 feet and a peak speed of Mach 5.
Onboard sensors transmitted data to an airborne U.S. Navy P-3, as well was ground systems at Point Mugu, Vandenberg and Edwards Air Force Bases. After about 200 seconds of engine operation a vehicle anomaly occurred and the flight was terminated. Engineers are busily examining the data to identify the cause of the problem.
The heart of this aircraft is its SJY61Pratt & Whitney Rocketdyne scramjet engine, which is capable of producing between 400 to 1000 lbs of thrust. Like a conventional jet engine, the SJY61 is capable of adjusting thrust throughout the X-51’s flight envelope.
Hypersonic flight, normally defined as beginning at Mach 5, five times speed of sound, presents unique technical challenges with heat and pressure, which make conventional turbine engines impractical. Program officials said producing thrust with a scramjet has been compared to lighting a match in a hurricane and keeping it burning.
George Thum, Pratt & Whitney Rocketdyne X-51 program manager, said the key technical challenge for X-51 has been integrating a fuel cooled scramjet, where the JP-7 fuel runs through the walls of the engine, cooling it in flight, into a compact flight vehicle capable of hypersonic flight. The X-51’s fuel-cooled engine design serves to both heat the JP-7 to an optimum combustion temperature and help the engine itself endure extremely high operating temperatures during the long burn.