X-51A Makes Longest Scramjet Flight

An engine first validated in a NASA wind tunnel successfully made the longest supersonic combustion ramjet-powered hypersonic flight to date off the southern California coast on May 26.

The air-breathing scramjet engine, built by Pratt & Whitney Rocketdyne, burned for more than 200 seconds to accelerate the U.S. Air Force's X-51A vehicle to Mach 5, or five times the speed of sound. It broke the previous record for the longest scramjet burn in a flight test, set by NASA's X-43 vehicle.

"This is great news for the hypersonics community," said Jim Pittman, principal investigator for the Hypersonics Project of NASA's Fundamental Aeronautics Program. "It's also good for NASA's research into flight at Mach 5 or faster. We will receive the X-51 flight data for analysis and comparison to the data we obtained during ground tests at NASA Langley's 8-Foot High Temperature Tunnel and to predictions from our propulsion codes."

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 program manager Charlie Brink of the Air Force Research Laboratory at Wright-Patterson Air Force Base in Dayton, 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-51A launched from Edwards Air Force Base in California, carried aloft under the left wing of an Air Force Flight Test Center B-52 Stratofortress. It was released while the B-52 flew at 50,000 feet over the Pacific Ocean Point Mugu Naval Air Warfare Center Sea Range. After release, an Army Tactical Missile solid rocket booster accelerated the X-51A to about Mach 4.8 before it and a connecting interstage were jettisoned. The launch and separation were normal, according to Brink.

Once the X-51A was free of its booster and interstage, its SJY61 engine ignited, initially on a mix of ethylene, similar to lighter fluid, and JP-7 jet fuel then exclusively on JP-7 jet fuel. The flight reached an altitude of about 70,000 feet and a peak speed of Mach 5.

Onboard sensors transmitted data to an airborne U.S. Navy P-3, as well was ground systems at Point Mugu, Vandenberg and Edwards Air Force bases in California. The flight was terminated after about 200 seconds of engine operation because of a technical issue. The X-51A was not designed to be recovered for examination, so engineers are busily examining the data to identify the cause of the problem.

Four X-51A cruisers have been built for the Air Force and the Defense Advanced Research Projects Agency by industry partners Pratt & Whitney Rocketdyne, West Palm Beach, Fla., and The Boeing Company, Palmdale, Calif. Brink said the Air Force intends to fly the three remaining X-51A flight test vehicles this fall on virtually identical flight profiles, building knowledge from each successive flight.

"This first flight was the culmination of a six-year effort by a small, but very talented AFRL, DARPA, NASA and industry development team," 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."

The engine can produce between 400 and 1,000 pounds of thrust. Like a conventional jet engine, the SJY61 is capable of adjusting thrust throughout the X-51's flight envelope.

Hypersonic flight 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.