WASHINGTON — The U.S. Air Force says it now plans to fly the last in a series of four Boeing-built experimental hypersonic vehicles in late spring or early summer under a troubled testing program that the service weighed canceling following an Aug. 14 failure.

During that flight, the third X-51A Waverider vehicle veered off course soon after separating from its carrier aircraft and crashed into the Pacific Ocean. Investigators have determined that one of the vehicle’s control fins inadvertently came unlocked, and are still trying to determine exactly why that happened, Charlie Brink, X-51A program manager for the Air Force Research Laboratory, said Oct. 24 during a media teleconference.

The failure investigation is expected to be completed by mid December, Brink said.

The WaveRider is designed to achieve speeds in excess of Mach 5 — five times the speed of sound — on the power of a supersonic combustion ramjet engine, also known as a scramjet. Boeing Defense, Space and Security of St. Louis built four X-51A vehicles under contract to the Air Force and Defense Research Projects Agency, but each of the three test flights to date has ended prematurely.

The Waverider vehicles are carried aloft by B-52 aircraft and after being released use solid-rocket boosters to achieve speeds approaching Mach 5. The solid-rocket motor is then jettisoned and the scramjet engine, built by Pratt & Whitney Rocketdyne of Canoga Park, Calif., takes over.

It was only during the first Waverider flight in May 2010 that the vehicle was able to make the full transition to scramjet-powered flight. The scramjet engine ignited and failed shortly thereafter during the second flight, and never got a chance to ignite during the third.

Brink said he expects the Air Force to conduct a follow-on program to “mature scramjet engine technology” after the fourth X-51A flight but declined to discuss specifics.



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U.S. Air Force Set To Begin X-51 Hypersonic Flight Tests