The X-51 WaveRider is designed to fly longer hypersonically than all of its predecessors combined.

WASHINGTON ? The U.S. Air Force is gearing up for the firstof four planned test flights of a hypersonic aircraft designed to operate formuch longer durations and cover far greater distances than previous platformsof its type.

The maiden flight of the X-51Waverider aircraft ? the first U.S. hypersonic vehicle to fly in six years? is scheduled to take place later in March. Boeing Defense, Space &Security Systems of St. Louis has been developing the aircraft since 2003 onbehalf of the Air Force Research Laboratory and Defense Advanced ResearchProjects Agency.

The missile-shaped X-51 will be carried aloft under the wingof a B-52 bomber, Joe Vogel, Boeing?s director of hypersonics, said in a Feb.22 interview. It will be released from the jet over the Pacific Ocean and dropfor four seconds until its rocket motor ignites and accelerates it to about5,800 kilometers per hour, just shy of the widely accepted start of hypersonicflight at Mach 5, or about 6,100 kilometers per hour. At that point, itsair-breathing scramjet ? or supersonic combustion ramjet ? engine, built byPratt & Whitney Rocketdyne of Canoga Park, Calif., will kick in, shootingthe craft to Mach 6, or more than 7,400 kilometers per hour.

Grand plans for hypersonic vehicles have been around fordecades, but their goals were often unrealistic and not matched by budgets,resulting in failure. The approach on X-51 has been to demonstrate thetechnologies that could one day enable things like single-stage-to-orbitvehicles.

?Theoretically you can probably get there someday, buttrying to do it all at once with not enough money is very, very challenging,?Vogel said.

Potential applications for hypersonic technology aresuperfast airplanes, missiles and reusable space launch vehicles, Vogel said.While the technology is not ready to ferry passengers from New York to LosAngeles in under an hour, such a scenario is not all that far-fetched, Vogelsaid. The upcoming demonstrations should show that the technology could be usedin a next-generation missile program, he said.

Boeing has 42 people working on the X-51program, down from a peak of about 90 people in 2007. Pratt & WhitneyRocketdyne?s team peaked around 60 people and is now down to nine people, Vogelsaid.

Boeing also built the United States? previous hypersonicflight demonstrator, the X-43A, on behalf of NASA. The X-43A program madetwo successful flights in 2004: an 11-second flight that reached Mach 7, and a10-second flight that approached Mach 10 and set a new record for fastestflight by a jet-powered aircraft. Both vehicles were designed to plummet intothe ocean and be destroyed.

Scramjet engines like those on the X-43A and X-51 must beaccelerated to very high speeds to deliver compressed air to their combustionchambers. Both craft rely on rocket propulsion to create this initial speed.

While the X-51 will not reach the top speed of itspredecessor, it is intended to demonstrate more operationally realistictechnologies, Vogel said. Whereas the X-43A used a highly energetic hydrogenfuel, the X-51 uses the same JP-7 fuel that powered the SR-71 surveillanceaircraft, and its engine could be adapted to use other hydrocarbon-based fuels,he said. The X-51 is expected to fly about 900 kilometers under jet power inabout five minutes, 30 times longer in duration than the X-43A flights.

Boeing has built four X-51 aircraft for the upcoming testcampaign. Though none will be recovered after its test flight, theirliquid-cooled scramjet engines have shown in ground testing to be very durable,Vogel said. The X-43A engine was not actively cooled and was not intended forreuse.

?This [the X-51] engine has been tested extensively in thelaboratory, and it?s come out and been reused multiple times,? Vogel said. ?Intheory, if we had more time and more money and more space in the vehicle, weprobably would have put a recovery system into it. Future vehicles could have arecovery system, and we have started looking at ways to recover the engine.?

The government does not currently plan to support the X-51program beyond the four identical flight tests, which should be complete by thefall, Vogel said. Boeing has proposed a next phase of the program to thegovernment, but he declined to be specific.

Since 2003 the government has spent about $250 million onthe X-51 program, Vogel said. Air Force Research Laboratory spokesman DerekKaufman was unable to provide funding details by press time.