The United States Air Force (USAF) has released the results of last August’s third test of the X-51a Waverider, which resulted in the crash of the unmanned scramjet demonstrator. At a press teleconference featuring the Program Manager for Air Force Research Laboratory, Charles Brink, it was confirmed that a malfunctioning fin was the cause of the crash. However, engineers are confident of correcting the fault in time for the fourth test flight scheduled for (Northern Hemisphere) late spring or early summer of next year.

The X-51a is designed to test the technology needed to build a hypersonic missile or aircraft. The 25-foot (7.62 m) long vehicle is a combination of of a wingless cruise vehicle powered by a SJY61 supersonic combustion ramjet or scramjet engine built by Pratt & Whitney and a modified Army Tactical Missile used to boost it to near-hypersonic speeds 26 seconds after being dropped from a B-52 bomber. It’s called Waverider because it rides its own shockwave at hypersonic speeds in excess of Mach 5 (3,800 mph, 3,300 knots, 6,125 km/hr).

X-51a Waverider on wing of a B-52 (Inage: U.S. Air Force)

According to Brink, the third test went as planned with the X-51a “stack” dropping from the B-52 and the booster igniting on schedule. However, about 15.5 seconds into the flight the upper right-hand fin unlocked and deployed while the booster was still firing. The stack began to slowly corkscrew, but the booster’s guidance system managed to maintain the proper angle.

After the other three fins on the cruiser deployed and powered up, the booster fell away, but the cruiser’s onboard computer couldn't maintain control because the electrically-driven actuator of the fourth fin was damaged or locked in place. Before the scramjet engine could be ignited, the cruiser went out of control, resulting in the loss of the vehicle.

Brink said that investigations are still ongoing and aren’t expected to be completed until near the end of the year, but that at this stage software and electronics failures have been eliminated as a cause. Indications are that the fin deployed because a random vibration issue caused the assembly to vibrate harmonically while in boost phase, so that the actuator responded and sprang open.

Until the exact cause of the fault is determined it won’t be possible to correct it with certainty, but Brink says that the simplest fix will be to deploy the fins on the cruiser about one or two seconds after being dropped from the B-52 instead of later when the vibration problem occurred so that the fins are powered up and protected from damage.

Test firing of the SJY61 scramjet (Image: NASA)

Brink went on to say that the US$300 million project will continue and that follow-on plans are in place, though details are not available. As to the third test, he said that even though the cruiser was lost before it could start its engine, valuable data was recovered that will help in modifying the Waverider’s air intakes prior to the fourth test flight.

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