The second hypersonic test flight of Boeing's X-51A Waverider has ended prematurely with the craft failing to transition to full power. The X-51A Waverider broke the record for the longest ever supersonic combustion scramjet-powered flight on its first autonomous flight in May 2010, flying under scramjet power for just under three and a half minutes at a top speed of Mach 5. For the second hypersonic flight test, the craft was expected to accelerate to about Mach 6, but only managed to accelerate to Mach 5 under solid rocket booster propulsion before the flight had to be terminated.

The second test flight was originally slated for March, 2011, but was cancelled at the last minute because, "all required test conditions could not be met." The X-51A finally took to the air for the second time on June 13, 2011, in the Point Mugu Naval Air Test Range over the Pacific Ocean. After what U.S. Air Force Flight Test Center (AFFTC) officials called a flawless flight from Edwards Air Force Base, the experimental vehicle was released from a U.S. Air Force B-52H Stratofortress at an altitude of approximately 50,000 feet.

A B-52H Stratofortress taxis to the runway carrying the X-51A Waverider on its second flight test (Image: Boeing)

The X-51A then accelerated to a speed just over Mach 5 powered by a solid rocket booster, but the vehicle experienced an inlet un-start when the aircraft's air breathing scramjet engine lit on ethylene and attempted to transition to JP7 fuel operation. The X-51A oriented itself to optimize engine start conditions but restart attempts proved unsuccessful and the craft continued a controlled flight orientation until it flew into the ocean within the test range.

Despite the less than successful flight, U.S. Air Force Flight Test officials were trying to put a positive light on things, saying the flight will provide significant hypersonic research data. The Air Force Research Laboratory, Boeing and Pratt-Whitney Rocketdyne engineers are now reviewing the telemetry data collected during the test flight to identify the cause of the problem.

"Obviously we're disappointed and expected better results," said Charlie Brink, the Air Force Research Laboratory's X-51A program manager, "but we are very pleased with the data collected on this flight. I am extremely pleased with the AFFTC and Point Mugu's support and execution of this complex flight test mission, as they provided us every opportunity for success in this endeavor. We have attempted two scramjet experiments now where one successfully lit, and one did not.

The X-51A Waverider affixed to an Edwards B-52H Stratofortress before the second flight test (Image: Boeing)

"We will continue to examine the data to learn even more about this new technology," said Brink. "Every time we test this new and exciting technology, we get that much closer to success."

Although the first test flight in May, 2010, saw the X-51A achieve scramjet-powered flight for just under three and a half minutes at a top speed of Mach 5, that flight was also cut short when communications with flight controllers was cut off after hot engine gases seeped into the airframe.

Boeing and Pratt-Whitney Rocketdyne have built four X-51A flight test vehicles with the program goal of reaching Mach 6 in hypersonic test flight. The next test flight has been tentatively scheduled for fall 2011.

Source: Air Force Materiel Command