The U.S. Air Force Research Laboratory (AFRL) plans to fly the fourth and final Boeing X-51A WaveRider hypersonic engine demonstrator in mid-2013.

Tests of the component suspected to have caused the early end to the third flight in August are to begin on Friday, Oct. 26. If confirmed, the fix should be “pretty simple,” says Charlie Brink, AFRL X-51A program manager.

The fourth flight, planned between mid-spring and mid-summer next year, will be the last chance for the X-51A to prove that a hydrocarbon-fueled, fuel-cooled scramjet engine can accelerate a missile-class vehicle to speeds approaching Mach 6.

But despite two failures following a partially successful first flight in May 2010, the Air Force remains convinced enough of the potential for a Mach 5-plus, 600-nm-range missile to direct AFRL to move ahead with the follow-on High-Speed Strike Weapon (HSSW) demonstration program.

Vibration in the boost phase is suspected to have caused a control fin on the X-51A to malfunction, causing the hypersonic cruiser to lose control after separating from its booster and break up before the scramjet engine could ignite.

On the Aug. 14 third flight the X-51A “stack” — the cruiser attached its booster — separated from the B-52 and the booster ignited 4 sec. later as planned. At this stage all four control fins on the cruiser were unpowered and locked.

At 15.5 sec. after release, the upper right fin unlocked and aerodynamic forces caused the control surface to pitch up from zero angle of incidence to fully trailing-edge down, then stay in that position, Brink says.

Despite the change in aerodynamic forces on the stack, the booster continued to work well, he says, delivering the cruiser to the planned Mach 4.9 airspeed at 4-deg. angle-of-attack for separation.

But then a “gentle rolling corkscrew” motion started. The booster separated normally, but instead of rolling upside down after separating, ready for engine start, the cruiser rolled rightside up.

Meanwhile, 2 sec. before separation as programmed, the cruiser’s control fins were powered up and unlocked. “Three actuators unlocked as directed. The fourth was powered up and commanded back into place, but was unresponsive.”

After booster separation “the three active fins worked hard to overcome the fourth, stuck maximum trailing-edge down. But the vehicle could not control itself with three good fins and one bad, and it tumbled and broke apart in 1.2 sec,” Brink says.

The investigation suggests vibration of the stack during boost caused the spring in solenoid of the locking mechanism to vibrate and unlock the fin. “The second bending moment of the stack is close to the second natural frequency of the spring,” says Brink.

A week of tests on a flight actuator assembly will begin on Friday in a bid to confirm the vibration theory. If confirmed, Brink says, the fix to be implemented for flight four will be to power up and unlock the control fins 2-3 sec. into the flight, after the stack is released from the B-52 and before the booster ignites.

Under power, the actuators will then actively hold the fins at zero angle of attack throughout the boost “so we will not have an unlock,” he says. “It’s a pretty simple software change” and there is enough battery capacity and control authority for it to work.

AFRL, meanwhile, plans to award a contract in 2013 for the 78-month HSSW demo program to mature propulsion, airframe, guidance, navigation and control, and warhead technologies for a hypersonic air-launched cruise missile.

The program is planned to include ground and flight tests of a demonstrator weapon to achieve technology readiness level 6, ready to enter engineering and manufacturing development of an operational weapon around 2020.