Stronger than expected shockwaves on the outer shell of a hypersonic vehicle designed by the Defense Advanced Research Projects Agency (DARPA) caused it to crash during a test flight last year, an independent engineering review board (ERB) has concluded.
DARPA's Hypersonic Technology Vehicle (HTV-2) successfully flew nearly 13,000mph for about three minutes in its second test flight on August 11 before its safety system sent it crashing into the Pacific Ocean nine minutes into the flight. Shockwaves, 100 times more than the vehicle was designed to withstand, peeled off portions of the aircraft's thermal shell, causing the early flight termination, the ERB said after a seven-month analysis of data collected from the flight.
An artist's concept of the flight of DARPA's HTV-2, which crashed into the Pacific last August after greater than expected shockwaves peeled off parts of its thermal shell. (Source: DARPA)
Official findings of the ERB said that the "most probable cause of the HTV-2 Flight 2 premature flight termination was unexpected aeroshell degradation, creating multiple upsets of increasing severity that ultimately activated the Flight Safety System," according to DARPA. The HTV-2 is an unmanned, rocket-launched, maneuverable aircraft capable of going Mach 20, or about 13,000mph. Flying at such a speed would allow an aircraft to travel from New York to Los Angeles in less than 12 minutes.
The agency put a positive spin on feedback gleaned from the investigation, saying the fact that the flight was able to recover from initial shockwaves before the safety system triggered its Pacific plunge showed that HTV-2's engineers learned valuable design lessons from its first flight, which also ended sooner than expected.
The reason is our ability to predict turbulence. Some simulation software has gotten close. But to date we can only predict tested conditions. The facts behind turbulence are still largely guessed and even after a good bit of aviation history we are still working on the kinks. I have been to several meetings with mathematicians that are leaders in this field. It's difficult for them to predict with any great accuracy. Yes 10000% error is outrageous but it's possible in a field we are infants on.
That's a good question, Ann. The fact that it travelled successfully for three minutes might indicate that the shock wave was a sudden anomaly shortly before it failed (I can't imgine any design standing up to 100X loads for three minutes). Still, it's hard to imagine why no one foresaw a shockwave of this magnitude.
I guess what's not clear to me is, why was the aircraft designed to withstand shockwaves 100 times LESS strong than it actually experienced? I'm especially surprised since this was apparently the second flight, not the first. Why didn't engineers do a better job of prediction?
I recall several publications and reporters reveling in the "failure" of the HTV-2 test back in August. But the ability to withstand forces 100x greater than design specifications and still manage to deploy a controlled abort should be a success in everybody's metrics. Controlled flight at Mach 20 for 3 minutes should have provided a wealth of telemetry. And these are the unclassified tests.... exciting.
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