A control fin that had functioned correctly on two previous flights malfunctioned on the Aug. 14 third flight of the U.S. Air Force Research Laboratory’s (AFRL) Boeing X-51A Waverider, causing the hypersonic demonstrator to lose control before its scramjet engine could be ignited.

The first flight of the X-51 in May 2010 was shorter than planned but largely a success, reaching Mach 4.88 under scramjet power. But the second flight in March 2011 ended after a few seconds when the scramjet did not transition from the ethylene used for startup to the JP-7 fuel used for sustained operation.

Boeing has built a fourth X-51, but its flight is unfunded. “AFRL officials have not decided when or if that vehicle will fly at this time,” the Air Force says in a statement.

The third X-51 was planned to fly for 300 sec. under the power of its Pratt & Whitney Rocketdyne SJX61 dual-mode ramjet/scramjet engine, reaching Mach 5.

The cruiser vehicle, attached to a modified Atacms missile booster, was launched from the Boeing B-52 mother ship over the Pacific at 11:36 a.m. local time. The stack separated from the B-52 and the booster fired as planned.

After 16 sec., while still under boost, a fault was identified with one of the hypersonic cruiser’s control fins, the Air Force says.

Once the X-51 separated from the booster, about 15 sec. later, “the cruiser was not able to maintain control due to the faulty control fin and was lost.”

“It is unfortunate that a problem with this subsystem caused a termination before we could light the scramjet engine,” says AFRL X-51A Program Manager Charlie Brink in the statement. “All our data showed we had created the right conditions for engine ignition and we were very hopeful to meet our test objectives.”

The third X-51A included a series of hardware and software changes to counter issues thought to have brought the second flight to a premature end after only 9.5 sec. of powered flight at around Mach 5.