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Engine Gases Leaking Into Airframe Led To Early Termination of X-51A Flight

by John Reed on March 17, 2011

A manufacturing flaw led to the early termination of last May’s test flight of the Air Force’s X-51A Waverider hypersonic vehicle, according to Air Force officials.

The historic flight out of the Naval Air Station Point Mugu in California was terminated by safety officials about half way through its mission when the Air Force lost contact with the vehicle due to hot engine gases seeping into the aircraft’s body through a seam separating the engine and airframe.

A significant portion of the X-51A’s actual engine is mounted on the bottom of the aircraft, as seen in the picture above. A weak interface between that and the exhaust nozzle mounted on the back of the vehicle allowed hot engine gases to flow into the actual airframe. This started melting wiring packets and other critical parts, including those that transmit data back to flight controllers. Never a good thing for a flight.

“Because of those hot gases we were starting to have telemetry problems” in receiving flight data a the control center, said Charlie Brink, the Air Force Research Laboratory’s X-51A program manager during a March 15 call with bloggers. “At that 143 second point, we lost it, they did the [five second safety] count, we didn’t hear back from it so the Point Mugu folks hit the button and we terminated it.”

The Waverider was supposed to fly for about 300 seconds, reaching a speed of Mach 6. Instead, it only flew for 143 seconds hitting a max speed of Mach 5 before Point Mugu safety officials killed the flight by ordering the craft’s control surfaces to suddenly pitch, sending the X-51A cartwheeling rapidly toward the sea.

Brink says that a misread of the design team’s intentions by the builders of the X-51A led to a weak seam between the exhaust nozzle on the vehicle and the actual engine.

“There was some discrepancies in what I would call design intent, meaning what that designers wanted to happen and what the folks down in Florida put that installation together,” said Brink. “The interface was rather complex, it was the first time we ever put the vehicle together so there was no indication of sloppy workmanship of things like that. In a demonstrator you learn things and as you start talking to people and you go ‘oh, that’s what you meant, i didn’t understand the nuance of what you were calling out in that drawing’.”

In the months since, the Air Force has made the interface between engine and nozzle “a lot more robust,” added Brink.

The next flight is slated for March 22.

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