The X-51 test over the Pacific Ocean last summer failed because unexpectedly large sections of its skin peeled off, the U.S. Defense Advanced Research Projects Agency said.
Most specific details of the program are secret. DARPA has released artist renderings showing a craft that looks something like the tip of a spear. After the 2011 flight the agency released handheld video, taken aboard a monitoring ship, that showed a dot streaking across the sky.
The second flight of the hypersonic Boeing X-51 waverider ended prematurely due to an inlet unstart. The aircraft made a controlled crash into the Pacific Ocean off the California coast on 13 June. The crash is a setback to the revolutionary aircraft program.
After what the US air force described as a 'flawless' flight to the launch point aboard the B-52 mothership, the X-51 was successfully boosted to M5.0 by a rocket booster. The Pratt & Whitney Rocketdyne scramjet engine successfully ignited using its initial fuel, ethylene. During the immediate transition to JP-7, the conventional fuel that makes X-51 unique, an inlet unstart occurred. A subsequent attempt to restart and reorient to optimal conditions was unsuccessful.
An inlet unstart, according to NASA, occurs when the shock wave moves too far out front of the air inlet, causing a momentary lapse in airflow to the engine. Scramjet engines depend on extremely precise shock wave movements and engine airflow to function. No wind tunnel can move air at hypersonic velocities, making hypersonic testing extremely difficult.