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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