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.

"Obviously we're disappointed and expected better results," said air force program manager Charlie Brink, "but we are very pleased with the data collected on this flight."

"We will continue to examine the data to learn even more about this new technology," he added. "Every time we test this new and exciting technology we get that much closer to success."

In its first flight, on 26 May, 2010, the X-51 experienced a similar inlet unstart about 110s after the scramjet ignited; the engine recovered successfully, and the flight continued until 143s, when an unrelated seal in the engine failed.

Source: Flight International