The Defense Advanced Research Projects Agency's experimental Hypersonic Technology Vehicle (HTV-2), lost significant portions of its outer skin and became uncontrollable after three minutes of sustained Mach 20 speed last August.

That was the conclusion of an independent engineering review board (ERB) investigating the cause of what DARPA calls a "flight anomaly" in the second test flight of the HTV-2.

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From the ERB report:

The flight successfully demonstrated stable aerodynamically-controlled flight at speeds up to Mach 20 (twenty times the speed of sound) for nearly three minutes. Approximately nine minutes into the test flight, the vehicle experienced a series of shocks culminating in an anomaly, which prompted the autonomous flight safety system to use the vehicle's aerodynamic systems to make a controlled descent and splashdown into the ocean.

Based on state-of-the-art models, ground testing of high-temperature materials and understanding of thermal effects in other more well-known flight regimes, a gradual wearing away of the vehicle's skin as it reached stress tolerance limits was expected. However, larger than anticipated portions of the vehicle's skin peeled from the aerostructure. The resulting gaps created strong, impulsive shock waves around the vehicle as it travelled nearly 13,000 miles per hour, causing the vehicle to roll abruptly. Based on knowledge gained from the first flight in 2010 and incorporated into the second flight, the vehicle's aerodynamic stability allowed it to right itself successfully after several shockwave-induced rolls. Eventually, however, the severity of the continued disturbances finally exceeded the vehicle's ability to recover.

"The initial shockwave disturbances experienced during second flight, from which the vehicle was able to recover and continue controlled flight, exceeded by more than 100 times what the vehicle was designed to withstand," said DARPA Acting Director, Kaigham Gabriel in a statement. "That's a major validation that we're advancing our understanding of aerodynamic control for hypersonic flight."

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Prior to the Aug. 11, 2011 flight DARPA said its technical team completed the most sophisticated simulations and extensive wind tunnel tests possible. But these ground tests have not yielded the necessary knowledge. "Filling the gaps in our understanding of hypersonic flight in this demanding regime requires that we be willing to fly. In the HTV-2's first test in April 2010, we obtained four times the amount of data previously available at these speeds. Today more than 20 air, land, sea and space data collection systems were operational. We'll learn. We'll try again. That's what it takes," said then DARPA director Regina Dugan.

The HTV-2 could fly anywhere in the world in less than 60 minutes. This capability requires an aircraft that can fly at 13,000 mph, while experiencing temperatures in excess of 3,500F. With that information as a backdrop, DARPA describes the Falcon as a "data truck" with numerous sensors that collect data in an uncertain operating envelope. For its second test flight, engineers adjusted the HTV-2's center of gravity, decreased the angle of attack flown, and will use the onboard reaction control system to augment the vehicle flaps to maintain stability during flight operations, the agency stated.

The first flight of the Falcon in 2010 "collected data that demonstrated advances in high lift-to-drag aerodynamics; high temperature materials; thermal protection systems; autonomous flight safety systems; and advanced guidance, navigation, and control for long-duration hypersonic flight."

Moving forward, DARPA said the HTV-2 program will incorporate new knowledge gained to improve thermal uncertainties and heat-stress allowances for the vehicle's outer shell. The remediation phase will involve further analysis and ground testing using flight data to validate new tools for this type of flying.

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