DARPA has released the findings from an independent engineering review board on what caused the unmanned Falcon Hypersonic Technology Vehicle (HTV-2) to fail nine minutes into its August 11, 2011 test flight. On that day, the HTV-2 was planned to launch atop a Minotaur IV rocket in California and then be released to glide at up to Mach 20 (about 13,000 mph) before a controlled nosedive into the ocean. Unfortunately, the arrowhead-shaped vehicle experienced an issue nine minutes into the flight and automatically descended into the sea. The review board says that that anomaly was due to the extreme heat of traveling at hypersonic speeds tearing apart the vehicle's skin, disturbing its aerodynamic stability and causing it to roll.
The flight was the second and last scheduled test for the HTV-2 (the first one in 2010 also failed nine minutes into the mission). DARPA is taking the results in stride, however: the agency says that the vehicle maintained controlled Mach 20 flight for three minutes, and was able to right itself after the first disturbances caused by the degradation of the HTV-2's skin — something that was "more than 100 times what the vehicle was designed to withstand." While there aren't any more flights scheduled for the HTV-2, DARPA says that it'll continue to do ground testing to try and improve the heat-stress models for high-speed atmospheric flight that failed to prepare the vehicle for this latest attempt. The original goal of the project was to be able to deliver an attack to anywhere in the world in less than an hour. See the video below for an overview of the vehicle's flight — had it gone to plan.