LOS ANGELES — A closely watched test flight of an experimental aircraft designed to travel up to 3,600 mph ended in disappointment when a part failed, causing it to plummet into the Pacific, the Air Force revealed.

The unmanned X-51A WaveRider was launched Tuesday over the Pacific from above the Point Mugu Naval Air Test Range in a key test to fine-tune its hypersonic scramjet engine.

The aircraft was designed to hit Mach 6, or six times the speed of sound, or about 3,600 mph, and fly for five minutes. But that didn't happen. The engine never lit. About 15 seconds into the flight, a fault was identified in one of the WaveRider's control fins, and the aircraft was unable to maintain control and was lost.

"It is unfortunate that a problem with this subsystem caused a termination before we could light the scramjet engine," said Charlie Brink, program manager for the Air Force Research Laboratory at Wright-Patterson Air Force Base in Ohio. "All our data showed we had created the right conditions for engine ignition, and we were very hopeful to meet our test objectives."

The hypersonic vehicle is being developed by the Air Force, Pratt & Whitney Rocketdyne and Boeing.

This is the third time a WaveRider has flown. Not one flight has gone the distance.

Engineers thought they were on the right track with the WaveRider program in May 2010, when the high-speed aircraft made its first flight. In that test, the WaveRider sped westward for about 143 seconds at 3,500 mph before plunging into the ocean, as planned.

But in another WaveRider flight in June 2011, a lapse in airflow to the jet engine caused a premature shutdown.

In the test Tuesday, a B-52 took off from Edwards Air Force Base and flew to 50,000 feet near Point Mugu. The B-52 then dropped the aircraft, and it fell like a bomb for about four seconds before its booster rocket engine ignited and propelled the aircraft.

It was supposed to separate from the rocket and speed across the sky, powered by the air-breathing hypersonic engine, but the control-fin flaw caused the craft to fail.

The Air Force said program officials would investigate the malfunction.

The Pentagon believes hypersonic missiles are the best way to hit a target anywhere in the world in an hour or less. The military's only vehicle currently with that kind of capability is the massive, nuclear-tipped intercontinental ballistic missile.

Other means of hitting a distant target, such as cruise missiles and long-range bomber planes, can take hours to reach their destination.

The Pentagon is funding six major hypersonic-technology programs. In the past 10 years, the Pentagon said it spent up to $2 billion on hypersonic technologies and supporting engineering. The cost of the WaveRider program is estimated at $140 million, according to GlobalSecurity.org, a website for military-policy research. One of the four WaveRider aircraft remains. Officials have not decided when, or if, that vehicle will fly.

Aerospace engineers say that harnessing technology capable of sustaining hypersonic speeds — going five times the speed of sound or more — is crucial to the next generation of missiles, military aircraft, spacecraft and passenger planes. Indeed, a passenger aircraft traveling at the WaveRider speed could fly from Los Angeles to New York in 46 minutes.

But engineers have been trying since the 1960s to sustain hypersonic flight, with few positive results.

Material from The Associated Press and Seattle Times archives is included in this report.