Boeing and Pratt & Whitney Rocketdyne X-51

An X-51A Waverider flight-test vehicle successfully made the longest supersonic combustion ramjet-powered hypersonic flight May 26 off the southern California Pacific coast.

Credit: U.S. Air Force photo/Mike Cassidy

The more than 200 second burn by the X-51's Pratt & Whitney Rocketdyne-built air breathing scramjet engine accelerated the vehicle to Mach 5. The previous longest scramjet burn in a flight test was 12 seconds in a NASA X-43.

Air Force officials called the test, the first of four planned, an unqualified success. The flight is considered the first use of a practical hydrocarbon fueled scramjet in flight.

"We are ecstatic to have accomplished most of our test points on the X-51A's very first hypersonic mission," said Charlie Brink, a X-51A program manager with the Air Force Research Laboratory at Wright-Patterson Air Force Base, Ohio. "We equate this leap in engine technology as equivalent to the post-World War II jump from propeller-driven aircraft to jet engines."

The X-51 launched at about 10 a.m. from here, carried under the left wing of an Air Force Flight Test Center B-52 Stratofortress. Then, flying at 50,000 feet over the Point Mugu Naval Air Warfare Center Sea Range, it was released. Four seconds later an Army Tactical Missile solid rocket booster accelerated the X-51 to about Mach 4.8 mach before it and a connecting interstage were jettisoned. The launch and separation were normal, Mr. Brink said.

Four X-51A cruisers have been built for the Air Force and the DARPA (Defense Advanced Research Projects Agency) by industry partners Pratt & Whitney Rocketdyne and Boeing.

Air Force officials intend to fly the three remaining X-51A flight test vehicles this fall, Mr. Brink said.

Air Force officials currently plan to fly each on virtually identical flight profiles, building knowledge from each successive flight.

Hypersonic flight, normally defined as beginning at Mach 5, five times speed of sound, presents unique technical challenges with heat and pressure, which make conventional turbine engines impractical. Program officials said producing thrust with a scramjet has been compared to lighting a match in a hurricane and keeping it burning.

"This first flight was the culmination of a six-year effort by a small, but very talented AFRL, DARPA and industry development team," Mr. Brink said. "Now we will go back and really scrutinize our data. No test is perfect, and I'm sure we will find anomalies that we will need to address before the next flight. But anyone will tell you that we learn just as much, if not more, when we encounter a glitch."

Mr. Brink noted while development of the X-51A's engine and the test program are complex, controlling costs has been a key objective. The team has incorporated or adapted existing proven technologies and elected from the outset not to build recovery systems in the flight test vehicles, in an effort to control costs and focus funding on the vehicle's fuel-cooled scramjet engine.

Mr. Brink said he believes the X-51A program will provide knowledge required to develop the game changing technologies needed for future access to space and hypersonic weapon applications.