A U.S. military drone that hit 13,000 mph before crashing in the Pacific was damaged by the excessive heat of hypersonic flight, military officials say.

The tremendous heat generated as the Defense Advanced Research Projects Agency's Hypersonic Technology Vehicle 2 reached a speed of Mach 20 in its August 2011 test stripped the vehicle of its metal skin, DARPA officials said.

"A gradual wearing away of the vehicle's skin as it reached stress tolerance limits was expected," DARPA said. "However, larger than anticipated portions of the vehicle's skin peeled from the aerostructure."

The HTV-2 was launched atop a rocket from California's Vandenberg Air Force Base, then separated and came hurtling back to Earth at hypersonic speeds, experiencing temperatures as high as 3,500 degrees Fahrenheit, SPACE.com reported.

An onboard safety system, sensing the HTV-2 was in an unrecoverable situation, destroyed the vehicle by guiding it into the ocean, DARPA said.

"The result of these findings is a profound advancement in understanding the areas we need to focus on to advance aerothermal structures for future hypersonic vehicles. Only actual flight data could have revealed this to us," Air Force Maj. Chris Schulz, DARPA program manager, said.