Last year, the Pentagon's Falcon HTV-2 glider -- which the US Air Force says can fly from New York to LA in under 12 minutes, disappeared. Nobody knew why! Now we know why: it went so fast it ripped itself apart.
DARPA has a clean little mea culpa on what was certainly a very expensive accident:
Larger than anticipated portions of the vehicle's skin peeled from the aerostructure. The resulting gaps created strong, impulsive shock waves around the vehicle as it travelled nearly 13,000 miles per hour, causing the vehicle to roll abruptly. Based on knowledge gained from the first flight in 2010 and incorporated into the second flight, the vehicle's aerodynamic stability allowed it to right itself successfully after several shockwave-induced rolls. Eventually, however, the severity of the continued disturbances finally exceeded the vehicle's ability to recover.
In other words, it crumbled and crashed. But holy hell: at 20,921km/h, is this really surprising? The unmanned arrowhead was barreling through the air at Mach 20 (20 times the speed of sound). By humble comparison, the Air Force's state of the art manned fighter can barely top Mach 2. Which is still very fast! DARPA expects part of the thing to burn off mid-flight -- but not quite this much. The whole vehicle itself is essentially disposable, intended to "have the capability to reach anywhere in the world in less than one hour" (and drop a nuke on that part of the world). In other words, let it burn. [DARPA via CNN]