ATLANTA — U.S. Air Force officials say the third, and possibly final, attempt to reach or exceed sustained speeds beyond Mach 5 with the X-51A hypersonic demonstrator is set for Aug. 14.

Describing the X-51A as “the key to the next step in hypersonics,” Air Force Research Laboratory Aerospace Systems Director Doug Bowers says that even the mixed success of the initial X-51A flights has proved invaluable to advancing the state-of-the-art. Speaking at the American Institute of Aeronautics and Astronautics Joint Propulsion Conference earlier this week, Bowers said, “The first X-51 was mostly a success, the second flight was a hung store [failed to release from the B-2 mothership] and on the third the inlet started but un-started. Every flight test we’ve had has been a learning opportunity, and until we took it to flight we really didn’t know the unknowns.”

The latest X-51A includes a series of hardware and software changes to counter issues that are thought to have brought the last flight to a premature end after only 9.5 sec. of powered flight at around Mach 5. The second flight, on June 13, 2011, ended when the vehicle’s Pratt & Whitney Rocketdyne SJX61-2 engine failed to transition from ethylene fuel to JP-7. The ethylene is used to start the scramjet, while JP-7 is used for sustained flight.

Designed to demonstrate technologies for sustained, air-breathing hypersonic flight up to Mach 6.5, the first flight of the X-51 on May 25, 2010, reached Mach 4.88. Although the first X-51A did not reach Mach 5, the test was considered a technical success as some 143 sec. of the vehicle’s 210 sec. of total powered flight time was under scramjet power, making the flight 11 times longer than any previous air-breathing flight with a scramjet. A fourth X-51A is close to completion at Boeing’s Palmdale, Calif., facilities, but is currently not funded for flight testing.