By Larry Shaughnessy

A test flight of an aircraft designed to whip around the world at Mach 20 failed when the high speeds peeled the skin off the unmanned plane, Pentagon researchers conclude in a long-awaited report.

For nine minutes in August, the Defense Advanced Research Projects Agency flew its Hypersonic Technology Vehicle at speeds reaching 20 times the speed of sound - fast enough to fly from New York to Los Angeles in less than 12 minutes. But then, something went wrong, and DARPA is finally explaining what happened.

"The most probable cause of the HTV-2 Flight 2 premature flight termination was unexpected aeroshell degradation," the research group explained in a new statement on the test flight.

Translation: The unmanned glider was streaking through the atmosphere when its outer skin started to rip off the airframe.

DARPA knew friction from the glider passing through the air so fast would cause it to heat up. It also expected "a gradual wearing away of the vehicle's skin as it reached stress tolerance limits," based on ground tests.

"However, larger-than-anticipated portions of the vehicle's skin peeled from the aerostructure," the statement said. "The resulting gaps created strong, impulsive shock waves around the vehicle as it traveled nearly 13,000 miles per hour, causing the vehicle to roll abruptly."

When that "anomaly" occurred, a safety computer slowed the glider down and it splashed down in the Pacific Ocean, where it's now probably an artificial reef with a moon roof.

The goal of the project is to create an aircraft that can fly anywhere in the world in under an hour to strike a target. The test plane was launched by a Minotaur IV rocket from Vandenberg Air Force Base in California, then separated from the booster and re-entered the atmosphere over the Pacific.

At Mach 20, friction from the surrounding air subjects the vehicle to temperatures of more than 3,500 degrees Fahrenheit (1,925 C).

DARPA said a group of independent experts decided they couldn't have predicted the problem. "The data from second flight revealed that extrapolating from known flight regimes and relying solely on advanced thermal modeling and ground testing could not successfully predict the harsh realities of Mach 20 atmospheric flight," the agency said.

A 2010 test also ended with the craft plunging into the Pacific. In spite of the problems, DARPA said it did learn a lot from the August test.

"We successfully incorporated aerodynamic knowledge gained from the first flight into the second flight," Maj. Chris Schultz, the program manager, said. And future flights will incorporate what was learned in the second test, he said.