A closely watched test flight of an unmanned experimental aircraft designed to travel up to 3,600 miles an hour ended in disappointment for a third time when a part failed, causing it to plummet into the Pacific Ocean, the Air Force said. The Los Angeles Times reports that the X-51A WaveRider, which includes components from Boeing Co.'s research center in Huntington Beach and Pratt & Whitney Rocketdyne in Canoga Park, was designed to hit six times the speed of sound and fly for five minutes on Tuesday. But the engine never lit.