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.
