Testing is under way at NASA's Langley Research Center on a jet-fueled, air-breathing engine like the one that will power the U.S. Air Force's X-51 WaveRider vehicle as it sets out in late 2009 to set new records in hypersonic flight.

Aiming for top speeds approaching Mach 7 (around 5,000 miles or 8,050 kilometers per hour), X-51 is not intended to be the fastest air-breathing vehicle the United States has built. But it is the most complicated, designed to achieve five or six minutes of powered flight before doing a controlled glide into the ocean.

Six minutes might not sound like much, but the X-43A, the NASA-led project that set a speed record of 7,545 miles (12,144 kilometers) per hour (Mach 9.8) in November 2004 on the vehicle's third and final flight, fired its supersonic-combustion ramjet for all of about 10 seconds.

James Pittman, NASA principal investigator for the U.S. space agency's $75 million-a-year hypersonics program, called the success of X-43A one of NASA's and the nation's "proudest moments" in the development of hypersonic vehicles.

"But [X-43A] was only for a few seconds," Pittman told reporters during a Sept. 3 media briefing at Langley Research Center, Hampton, Va. "This was a landmark achievement in hypersonics. But in order for us to continue the development of hypersonics, we've got to move along several different paths. One path is to go from seconds of operation to minutes of operation."

Another path, according to Pittman, is to develop a combined-cycle engine that can allow a vehicle to take off from a runway and accelerate to Mach 3 or Mach 4 before transitioning to the scramjet-powered flight necessary to achieve truly hypersonic speeds. Future applications for such a vehicle include next-generation space launch and various fast-reaction military systems.

X-51 goes part of that distance by incorporating the thermal-control systems and other technologies needed to demonstrate sustained hypersonic flight. Its Pratt & Whitney Rocketdyne-built engine also burns fairly conventional jet propellant rather than the comparatively hard-to-store hydrogen fuel that the X-43A burned on its record-breaking flights. Pittman and other program officials said jet propellant is a better fit for the type of combined-cycle engines that are necessary for producing operational vehicles of the sort NASA and the Air Force envision.

NASA is the junior partner in the $246 million X-51 program, with the Air Force putting up roughly three-quarters of the funding. Nearly all of the rest comes from the U.S. Defense Advanced Research Projects Agency (DARPA), which views X-51 as a steppingstone to Blackswift, a flight demonstrator that is slated to take off from a runway around 2012.

The goal of the Blackswift program is to accelerate the vehicle to a speed of more than Mach 6, maneuver it and then return to a runway landing. DARPA currently is said to be in pre-negotiation with a Lockheed Martin Skunk Works-led team that includes Boeing and Alliant Techsystems. A contract award is expected in September, according to sources.

Charlie Brink, the X-51 program manager at the Air Force Research Laboratory's propulsion directorate at Wright Patterson Air Force Base outside Dayton, Ohio, said NASA had made important contributions toward proving the flight readiness of X-51's engine.

Brink told reporters at the Langley briefing that X-51 was in many ways a continuation of X-43C, a planned NASA-Air Force follow-on to X-43A that NASA, he said, was forced to abandon when President George W. Bush refocused the agency in 2004 on returning to the Moon.

"Because of the Bush administration's plan to go to Moon and Mars and other things that went on with NASA's budget, that program did not go through," Brink said of X-43C. But by the time it became clear NASA was not going to go forward with X-43C, Brink said, the engine the Air Force was developing for the program "was effectively 95 percent built."

NASA and the Air Force went ahead with the engine testing, conducting scores of cold-flow and hot-fire tests in the high-speed tunnels at Langley.

Brink said the success of the tests encouraged the Air Force and DARPA to press on with X-51, the overriding goal of which is to take the engines from the test stand and show that they are ready to support military applications.

Testing at Langley is expected to continue through September. Pratt & Whitney Rocketdyne, meanwhile, is getting ready to ship the first of four flight-ready engines to X-51 prime contractor Boeing, which will integrate them into the WaveRider vehicle with an eye toward conducting the first of four planned flights starting in late 2009.

Brink said the flights will take place out of Edwards Air Force Base in California. X-51 will be carried aloft by a B-52 bomber.

The wedge-shaped hypersonic vehicle will be mounted atop a U.S. Army tactical missile system solid-rocket booster that will propel the X-51 to speeds in excess of Mach 4 before its scramjet engine takes over. After five or six minutes of powered flight, Brink said, X-51 will glide into the Pacific Ocean. Brink said the Air Force will coordinate with the Federal Aviation Administration to make sure that flights between Los Angeles and Hawaii and Australia are rerouted during the four-hour window for each X-51 flight. NASA and the Air Force asked the Federal Aviation Administration for similar flight restrictions when X-43A flew, he said.

Also like X-43A, each one of the X-51s are designed as single-use vehicles. "We would love to recover the vehicles," he said, adding that each flight model already is filled with so much avionics and instrumentation that trying to install a parachute or any type of flotation device is prohibitively expensive and "design-wise very tough."