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USAF will attempt hypersonic flight for the fourth time

The U.S. military is hoping that the fourth time will be the charm when it comes to its experimental hypersonic flight program, following a third failed attempt to achieve Mach 6, according to a report from The Associated Press.

Once a year since 2010, the Air Force has conducted a test of the unmanned, experimental X-51A Waverider aircraft, the most recent attempt coming two months ago. During the test, the X-51A is carried up and then dropped from a B-52 bomber, after which it is supposed to ignite its rocket booster. It then jettisons the booster, and activates its exotic scramjet engine.

During each test a different problem with the scramjet engine, the key feature of the test, has prevented the X-51A from reaching the target goal of flying at six times the speed of sound, or 3,600 mph, and sustaining that speed for five minutes. In a successful test, the X-51A would travel 300 miles during its 5 minutes at Mach 6.

During the first test flight in 2010, an X-51A flew at Mach 5 for three minutes. While Mach 5 is generally considered the lower level of the hypersonic range, the aircraft was unable to reach the target of Mach 6, or even the target duration of 300 seconds of flight.

The second attempt came in 2011, when the test flight had to be ended early after an X-51A was unable to restart its scramjet engine after having again reached Mach 5. In August of this year, the aircraft again successfully completed the first two stages of the test, but after detaching from the B-52 and accelerating with the rocket booster up to Mach 4.8, the plane became unstable and never activated its scramjet engine.

Charlie Brink at the Air Force Research Laboratory at Wright-Patterson Air Force Base in Ohio says signs point to a “random vibration issue” as the cause of the aircraft’s problem with a control fin that led to the aircraft’s loss of balance and subsequent crash into the Pacific off the coast of Southern California. However, he said more review of the data from the flight is needed before they can exactly identify the cause.

The U.S. military announced Wednesday that it is planning a fourth test of the X-51A, scheduled for the spring or summer of 2013. Brink said he expected that the test would not mark the end of hypersonic flight research, although he did not go into specifics. The test is ultimately a study of hypersonic technologies, which the Air Force hopes can be used to deploy fast strikes around the globe.

Eventually, the U.S. military hopes to achieve extreme hypersonic flight. In a July 2012 statement, the Defense Advanced Research Projects Agency (DARPA) argued that extreme hypersonic flight has the potential to replace stealth technology as the dominant technology on the battlefield.

Extreme hypersonic flight, which is defined as 20 times the speed of sound, would allow the U.S. military to reach anywhere in the world in less than an hour. Recent programs by DARPA, the Army and the Air Force have helped the agency learn more about the technology needed for achieving extreme hypersonic flight.

“DoD’s hypersonic technology efforts have made significant advancements in our technical understanding of several critical areas including aerodynamics; aerothermal effects; and guidance, navigation and control,” said Acting DARPA Director, Kaigham J. Gabriel. “But additional unknowns exist.”

The U.S. military’s test of hypersonic flight is the first step on the path toward extreme hypersonic flight.