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Unlocked fin likely brought down X-51A in August crash

The flight failed due to aerodynamic problems caused when a fin became unlocked, sending the experimental aircraft into a corkscrew. A fourth X-51A flight will likely take place next year.

The X-51A Waverider is meant to demonstrate hypersonic flight. Powered by a Pratt Whitney Rocketdyne SJY61 scramjet engine, it is designed to ride on its own shockwave and accelerate to about Mach 6. The U.S. Air Force today revealed the likely cause of the August crash of the third X-51A test (U.S. Air Force graphic) NASA

The U.S. Air Force said today that an experimental test in August of its hypersonic X-51A Waverider failed due to a fin inadvertently unlocking and sending the aircraft into a corkscrew that ended in a crash into the Pacific Ocean.

At the time, the Air Force said only that the August flight had ended with the crash, but didn't reveal what had been at fault. But in a conference call today, X-51A program manager Charlie Brink explained what he and his colleagues have learned over a couple of months of investigation.

The August flight was the third of four tests of the $260 million experimental aircraft program -- which is meant to examine the viability of so-called scramjet engine technology, which the military hopes may one day result in hypersonic weaponry such as ultra-fast cruise missiles, reconnaissance vehicles, or even more efficient ways to get into space than ever before.

It had conducted two previous tests, both of which produced useful data, Brink said, but neither of which were fully successful. Now, with a better -- albeit not yet complete -- understanding of what went wrong during the third test, the Air Force and its partners at Boeing and Pratt & Whitney Rocketdyne are readying the fourth and final X-51A test. Brink said that flight would likely take place in late spring or early summer of next year.

The X-51A is called the Waverider because of an element of the plane's expected flight performance: once it hits the right velocity, it essentially surfs its own shockwave. To the Air Force, the aircraft isn't a prototype for any individual weapons system, but rather is a technology demonstrator.

Two years ago, the Air Force sent up an X-51A which briefly made it to the edge of hypersonic flight, hitting "approximately Mach 5, nearly 3,400 miles per hour," according to an Air Force fact sheet. The X-51A flew at that speed for about 200 seconds, although it was only able to send back normal telemetry data for 140 seconds before losing acceleration and thrust and having to be destroyed by ground controllers after some sort of "anomaly." Afterwards, the military said the X-51A actually hit Mach 4.88

In order to make a hypersonic flight, the X-51 is tucked under a B-52's wing. Then, the total assembly "stack," which is about 25 feet long and which weighs about 4,000 pounds, is dropped at 50,000 feet. At that point, a rocket booster kicks in, sending the aircraft to about Mach 4.5. Then the booster is dropped, and the X-51-A's Pratt & Whitney Rocketdyne scramjet engine takes over. That engine is made to burn oxygen from the atmosphere mixed with a small amount of jet fuel.

Third flight

According to Brink, August's third flight of the X-51A started as planned. It was dropped from the B-52 and after about four seconds, its booster lit, taking the aircraft up to a speed of about Mach 4.8.

At about 15.5 seconds after being dropped from the mother ship, however, the assembly's upper right actuator, which like its three other actuators, was supposed to be stowed, locked, and unpowered, came loose. Though it was still unpowered, over the course of about two-tenths of a second, the assembly pitched up and moved from a zero-angle of attack to having its full trailing edge pointing down. With the stack's aerodynamics now off-kilter, the entire assembly "started to do a whole corkscrew," Brink said, causing it to crash.

Unfortunately, because the crash happened prior to putting fuel into and lighting the scramjet engine, the Air Force was not able to conduct the planned propulsion experiment, Brink said.

Although it is not known for certain what caused the actuator to become unlocked, Brink explained that investigators are fairly certain that neither a software nor a power problem were to blame. "What we're honing in on," Brink said, "is that there are indications that it looked like it could have been caused by a random vibration issue" affecting the mechanism that locks the actuator.

He suggested that in order to understand what happened, it could be informative to imagine the aerodynamic forces that destroyed the Tacoma Narrows Bridge -- known as "Galloping Gertie" -- in 1940. "It looks like the second bending mode of the stack, the frequency that occurs," Brink explained, was "very close to the response of the spring-loaded mechanism that holds the lock on the second" actuator.

Brink added that he and his team are currently conducting vibration testing, and that while their suspicions are not yet "conclusive," it is "looking more and more like that's the cause" of the accident.

As a result, Brink said that during the fourth test next year, it's likely that engineers will power up the actuators within one or two seconds after dropping the X-51A from the B-52 rather than waiting longer. That would unlock those fins earlier, he said, and could allow the aircraft to fly freely without unexpected aerodynamic pressures. "That would be the most simple solution we could implement," Brink said.

He added that he expects the investigation into the cause of the August accident to be concluded by Christmas.