Large chunks of material wearing away from the airframe caused the US Defense Advanced Research Projects Agency's (DARPA) hypersonic technology vehicle (HTV-2) test flight to go awry on 11 August, 2011.
Using computer models and ground tests of the craft's high-temperature materials, the DARPA's Engineering Review Board (ERB) discovered that larger than expected pieces of the vehicle's skin had peeled off. The agency had expected a more gradual wearing away of the vehicle's skin.
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As chunks of the machine's skin peeled away, shock waves caused the unmanned test aircraft to abruptly roll. But while the vehicle was initially able to correct for those, the aerodynamic forces eventually exceeded its ability to overcome the rolling moment.
The aircraft's safety system then took over and guided the vehicle into a "controlled decent" into the ocean.
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," the ERB says.
For future tests, the HTV-2 programme will incorporate the new data gained from the experience.
The HTV-2 programme is being used to inform the US Department of Defense's conventional prompt global strike (CPGS) effort, which calls for a capability to hit targets anywhere in the world in less than one hour.
Source: Flight International