The U.S. Air Force is gearing up for the first of four planned test flights of a hypersonic aircraft designed to operate for much longer durations and cover far greater distances than previous platforms of its type.
The maiden flight of the X-51 Waverider 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 on behalf of the Air Force Research Laboratory and Defense Advanced Research Projects Agency.
The missile-shaped X-51 will be carried aloft under the wing of 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 drop for four seconds until its rocket motor ignites and accelerates it to about 5,800 kilometers per hour, just shy of the widely accepted start of hypersonic flight at Mach 5, or about 6,100 kilometers per hour. At that point, its air-breathing scramjet — or supersonic combustion ramjet — engine, built by Pratt & Whitney Rocketdyne of Canoga Park, Calif., will kick in, shooting the craft to Mach 6, or more than 7,400 kilometers per hour.
Grand plans for hypersonic vehicles have been around for decades, but their goals were often unrealistic and not matched by budgets, resulting in failure. The approach on X-51 has been to demonstrate the technologies that could one day enable things like single-stage-to-orbit vehicles.
"Theoretically you can probably get there someday, but trying to do it all at once with not enough money is very, very challenging," Vogel said.
Potential applications for hypersonic technology are superfast airplanes, missiles and reusable space launch vehicles, Vogel said. While the technology is not ready to ferry passengers from New York to Los Angeles in under an hour, such a scenario is not all that far-fetched, Vogel said. The upcoming demonstrations should show that the technology could be used in a next-generation missile program, he said.
Boeing has 42 people working on the X-51 program, down from a peak of about 90 people in 2007. Pratt & Whitney Rocketdyne's team peaked around 60 people and is now down to nine people, Vogel said.
Boeing also built the United States' previous hypersonic flight demonstrator, the X-43A, on behalf of NASA. The X-43A program made two successful flights in 2004: an 11-second flight that reached Mach 7, and a 10-second flight that approached Mach 10 and set a new record for fastest flight by a jet-powered aircraft. Both vehicles were designed to plummet into the ocean and be destroyed.
Scramjet engines like those on the X-43A and X-51 must be accelerated to very high speeds to deliver compressed air to their combustion chambers. Both crafts rely on rocket propulsion to create this initial speed.
While the X-51 will not reach the top speed of its predecessor, it is intended to demonstrate more operationally realistic technologies, Vogel said. Whereas the X-43A used a highly energetic hydrogen fuel, the X-51 uses the same JP-7 fuel that powered the SR-71 surveillance aircraft, 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 in about five minutes, 30 times longer in duration than the X-43A flights.
Boeing has built four X-51 aircraft for the upcoming test campaign. Though none will be recovered after its test flight, their liquid-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 for reuse.
"This [the X-51] engine has been tested extensively in the laboratory, and it's come out and been reused multiple times," Vogel said. "In theory, if we had more time and more money and more space in the vehicle, we probably would have put a recovery system into it. Future vehicles could have a recovery system, and we have started looking at ways to recover the engine."
The government does not currently plan to support the X-51 program beyond the four identical flight tests, which should be complete by the fall, Vogel said. Boeing has proposed a next phase of the program to the government, but he declined to be specific.
Since 2003 the government has spent about $250 million on the X-51 program, Vogel said. Air Force Research Laboratory spokesman Derek Kaufman was unable to provide funding details by press time.
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