A second test flight of the X-51A WaveRider has not gone according to plan. Successfully dropped from the left wing of a B-52H Stratofortress, and then accelerated to over Mach 5 by a solid rocket booster, the unmanned, scramjet-powered, hypersonic vehicle crashed in the Pacific Ocean off Point Mugu NAS, Calif. on June 13

The WaveRider’s first flight on May 26, 2010, also faltered. The aircraft flew for more than 200 seconds powered by its Pratt & Whitney Rocketdyne scramjet motor–the longest scramjet powered flight in history. It reached an altitude of 70,000 feet and approximate speed of Mach 5. “Something then occurred that caused the vehicle to lose acceleration,” the Air Force said at the time. “At that point, the X-51A was terminated as planned.”

The Air Force described the second flight as “less than successful,” but said it produced substantial telemetry data. After release from the B-52 at approximately 50,000 feet, the X-51A was boosted by solid rocket to just over Mach 5.

“The air-breathing scramjet engine lit on ethylene and attempted to transition to JP7 fuel operation, when the vehicle experienced an inlet unstart,” the Air Force said. “The hypersonic vehicle attempted to restart and oriented itself to optimize engine-start conditions, but was unsuccessful. The WaveRider continued in a controlled flight orientation until it flew into the ocean within the test range.”

The Air Force Research Laboratory (AFRL) and contractors Boeing and Pratt & Whitney Rocketdyne were reviewing telemetry data to identify the cause of the “anomaly.” The aircraft was the second of four flight-test vehicles built as part of the technology demonstration, supported by the Defense Advanced Research Projects Agency (DARPA) and AFRL. The next flight is tentatively scheduled for this fall, the Air Force said.