The loss of the Falcon HTV-2 hypersonic aircraft last summer was caused by the vehicle's skin peeling away, DARPA has revealed.
The vehicle - designed to reach anywhere in the world in under an hour - was lost just minutes after takeoff when HTV-2 separated from the Minotaur rocket and started a 13,000 mph flight above the Pacific.
But DARPA's remaining determinedly upbeat about the test, which it opints out resulted in three minutes' successful flight.
"The greatest achievement from Flight Two, which the ERB’s findings underscored, was that we successfully incorporated aerodynamic knowledge gained from the first flight into the second flight," says Air Force Major and DARPA program manager Chris Schulz.
The trouble started around nine minutes into the test flight, the vehicle experienced a series of shocks. The vehicle was able to make a controlled descent and splashdown into the ocean.
"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," says DARPA acting director Kaigham J Gabriel.
"That’s a major validation that we’re advancing our understanding of aerodynamic control for hypersonic flight."
DARPA had been expecting the vehicle's skin to gradually wear away as it reached stress tolerance limits. However, much more skin than expected peeled right off. The resulting gaps created strong, impulsive shock waves around the vehicle as it travelled nearly 13,000 miles per hour, causing it to roll abruptly.
While the vehicle was initially able to right itself, the shock waves proved too much in the end.
DARPA concedes that extrapolating from known flight regimes and relying solely on advanced thermal modeling and ground testing simply wasn't enough to successfully predict the harsh realities of Mach 20 atmospheric flight.
"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," says Schultz. "Only actual flight data could have revealed this to us."
There's no word on when DARPA might make another attempt - and this could be difficult, given that only two Falcon prototypes were built. The first was lost two years ago. DARPA says it plans to carry out more tests.