August X-51A Test Flight Crash Likely Due To Unlocked Actuator

Lawrence LeBlond for redOrbit.com – Your Universe Online

On August 14, 2012 the US Air Force conducted a scheduled test flight of its experimental X-51A Waverider aircraft. But the test ended in failure, with the plane crashing into the Pacific Ocean. Details of the failed flight were not provided at the time, and it was unclear if experiments would continue.

Now, more than 2 months after that failed test, the Air Force has come forward and explained why the experimental hypersonic plane went down.

X-51A program manager Charlie Brink said during a conference call yesterday (Oct. 24) that an actuator inadvertently unlocked during the flight sending the aircraft into a deadly spiral, causing it to crash into the ocean waters below. He said it took a few months worth of investigation for he and his colleagues to make an official determination of why the experiment failed.

The August flight was the third of four planned tests of its $260 million experimental program. The goals of the program are to examine the feasibility of scramjet engine technology, which the military hopes will one day result in hypersonic weaponry and more efficient ways to get into space.

Brink said the first two tests produced useful data, but neither of those tests were fully successful either. He said now that the military group has a better understanding of what went wrong during the third test, they are readying the fourth and final test.

Many experts were skeptical that a fourth test would be implemented after the failure of previous experiments. But Brink assured there would be another test flight.

“I see that flight happening sometime late spring, early summer,” he said.

The X-51A Waverider is a unique aircraft, labeled Waverider for a reason. Once the aircraft hit’s the right velocity, it is meant to surf along its own shockwave. In the Air Force’s previous experiment (May 26, 2010), its Waverider briefly made hypersonic flight, hitting “approximately Mach 5, nearly 3,400 miles per hour,” according to military facts on the flight. It flew at those speeds for roughly 200 seconds before losing acceleration and thrust. Military ground control had to remotely destroy the craft due to an “anomaly.”

For these experiments to work, the X-51A is tucked under the wing of a B-52 Stratofortress and then dropped at about 50,000 feet, after which a rocket booster kicks in, sending the aircraft to about Mach 4.5. Once the booster is dropped, the scramjet engine–developed by Pratt & Whitney Rocketdyne–kicks in. The scramjet engine is designed to burn oxygen from the atmosphere mixed with a small amount of jet fuel.

Brink said the third flight started off as planned. Yet, after the rocket booster fired and got the craft up to about Mach 4.8, the craft’s upper right actuator, which was supposed to be stowed, locked and un-powered like the other three actuators, came loose. Though the actuator remained un-powered, it caused the craft to go into a “corkscrew,” bringing the plane down.

This all took place before the craft’s scramjet engine was powered on, meaning the Air Force was not able to garner any useful data from the experiment. While it is not known exactly what caused the actuator to become unlocked, Brink explained that investigators are fairly certain the problem did not stem from a software glitch or power problem.

Brink said “there are indications that it looked like it could have been caused by a random vibration issue” affecting the mechanism that locks the actuator.”

During the conference call, he said in order to really understand what happened to the X-51A, it might make sense to imagine the aerodynamic forces at work that destroyed the Tacoma Narrows Bridge in 1940.

He added that the team is currently conducting vibration tests. Although the team’s suspicions are not yet “conclusive,” Brink said it is “looking more and more like that’s the cause” of the accident.

For next year’s fourth test, Brink said it is likely engineers will power up the actuators within one or two seconds after dropping the X-51A from its carrier rather than waiting longer. This would unlock the fins earlier, allowing the aircraft to fly freely without unexpected aerodynamic pressures. “That would be the most simple solution we could implement,” he said during the call.

He said the investigation of Augusts’ crash should be completely wrapped up by Christmas.

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