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X-51A scramjet readied for flight tests

Hypersonic wind tunnel tests concluding this month at NASA's Langley Research Centre will put the US Air Force Research Lab (AFRL) and its partners one step closer to the goal of flying a hydrocarbon-fuelled scramjet to hypersonic speeds for a full 5min in September or October 2009, the first of four flight tests aimed at demonstrating the viability an actively cooled scramjet-powered missile or vehicle that can fly to speeds beyond Mach 6.0.

As of 1 September, engineers had completed 11 tests of the Pratt & Whitney Rocketdyne SJX61-2 "flight clearance" combined cycle scramjet engine, the latest iteration of a family of X-51A (formerly called the HySET programme) engines that have been in test since 2001. The engine features an active closed-loop heat exchanger cooling system using the vehicle's JP-7 fuel. Included in the heat exchanger is a catalyst that "cracks" the fuel into a supercritical state, a more reactive mixture that increases thrust potential and heat absorption. The engine is considered "dual mode" because it results in supersonic combustion with regions of subsonic combustion, says NASA.

X-51 engine test

The broader M4.6-M5.0 wind tunnel test programme, to be completed by the end of September, is designed to bound the possible ignition conditions for the Boeing-built airframe, called the cruiser, that will be mounted to a modified US Army tactical missile solid-motor booster and dropped from a US Air Force B-52 at 15,088m (48,500ft) over the ocean off of Pt Mugu, California. Boeing says the booster will accelerate the X-51A to M4.7-4.8 before burnout at approximately 24,384m (80,000ft), where after a short coast phase, the scramjet will be ignited with an injection of approximately 2.72kg (6lb) of ethylene. All 124kg of JP-7 will then burn with the vehicle guidance system holding roughly a 4 degree angle-of-attack for constant aerodynamic pressure for approximately 5min. AFRL does not plan to recover the first test vehicle, though Boeing says studies are under way to address recovery for later tests. Mission termination systems will be included in both the solid-rocket booster and the X-51A, says AFRL.

The work is part of $246 million AFRL and US Defence Advanced Research Projects Agency programme, $200 million of which is funding Boeing for four flight vehicles and Pratt & Whitney for the flight and test engines. NASA is supplying its 8ft high-temperature hypersonic wind tunnel for the tests. Pratt & Whitney says it is 95-97% finished with four engines for the flight tests, and Boeing says the first two 4.27m-long cruisers, which will be guided by electrically actuated fins, are being assembled.

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