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.