The unmanned X-51A is expected to fly autonomously for five minutes, after being released from a B-52 Stratofortress off the southern coast of California. The Waverider is powered by a supersonic combustion scramjet engine, and will accelerate to about Mach 6 as it climbs to nearly 70,000 feet. Once flying the X-51 will transmit vast amounts of data to ground stations about the flight, then splash down into the Pacific. There are no plans to recover the flight test vehicle, one of four built, the Air Force stated.
"In those 300 seconds, we hope to learn more about hypersonic flight with a practical scramjet engine than all previous flight tests combined," said Charlie Brink, X-51A program manager with the Air Force Research Laboratory's Propulsion Directorate. Since scramjets are able to burn atmospheric oxygen, they don't need to carry large fuel tanks containing oxidizer like conventional rockets, and are being explored as a way to more efficiently launch payloads into orbit.
The longest previous hypersonic scramjet flight test performed by a NASA X-43 in 2004 was faster, but lasted only about 10 seconds and used less logistically supportable hydrogen fuel, the Air Force stated.
Hypersonic combustion generates intense heat and routing of the engine's own JP-7 fuel will help keep the engine operating properly, the Air Force stated. As the scramjet engine ignites it will initially burn a mix of ethylene and JP-7 jet fuel before switching exclusively to JP-7.
The Air Force describes the X-51 as virtually wingless, designed to ride its own shockwave. The heart of the system is its Pratt & Whitney Rocketdyne SJY61 scramjet engine, but other key technologies will be demonstrated including thermal protection systems materials, airframe and engine integration, and high-speed stability and control.
The Air Force said this will be the only hypersonic flight attempt this fiscal year, a change from the original test plan which was to fly in December 2009 then three more times in 2010.
The X-51A WaveRider program is a joint effort by the Air Force, Defense Advanced Research Projects Agency (DARPA), Pratt & Whitney Rocketdyne, and Boeing.
The X-51 isn't the only hypersonic research going on. Just last week, the Air Force said it was looking to develop what it called the Next Generation Thermal Protection System (TPS). The project looks to develop all manner of advanced thermal protection technology from ceramics to hybrid materials that, when combined with vehicle designs, will enable efficient, supersonic and hypersonic systems, the Air Force stated. Advanced materials and concepts which are highly durable, highly capable, highly supportable/maintainable, structurally efficient, extremely lightweight, and affordable are sought, the Air Force said.
NASA and the Air Force said they would be offering up to $35 million to help fund research that could ultimately develop aircraft that can fly at over five-times the speed of sound or faster. Such hypersonic aircraft face myriad trajectory control, propulsion and heat-related issues akin to what a spacecraft would endure, experts say.
NASA is also looking into developing hypersonic air or spacecraft that could travel in the Earth's atmosphere or between here and other planets. The space agency recently announced a $45 million contract with longtime partner ERC Inc., for just such space vehicle research.
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