The problem with the fin on the craft known as the Waverider or X-51A was identified in a test flight on Tuesday, 16 seconds after a rocket booster on the remotely monitored craft was ignited to propel it forward, the Air Force said in a statement.

Fifteen seconds later, when the X-51A separated from the rocket booster, it lost control due to a "faulty control fin," the statement said. The 31 seconds of flight fell far short of the military's goal for the X-51A to fly for five minutes.

The aircraft broke apart immediately and fell into the Pacific Ocean near Point Mugu northwest of Los Angeles, said Daryl Mayer, a spokesman for the 88th Air Base Wing at Wright-Patterson Air Force Base in Ohio.

Even if the test had been a success, the aircraft would have crashed at the end of the flight in any case and was not considered retrievable.

The Waverider was designed to reach speeds of Mach 6 or above, six times the speed of sound and fast enough to zoom from New York to London in less than an hour. The military has its eye on using the Waverider program to develop missiles with non-nuclear explosives that could strike anywhere in the world within an hour, analysts said.

The cost of the experimental aircraft, which military officials said was dropped from a B-52 bomber before its rocket booster was ignited, has not been disclosed because many details of the program are classified.

The aircraft is known as the Waverider because it stays airborne, in part, with lift generated by the shock waves of its own flight. The Boeing Co's Phantom Works division performed design and assembly on the craft, the military said.

The fins on the rocket booster kept the aircraft on-course during the initial phase of the flight, despite the problem with the control fin on the X-51A itself, Mayer said.

A Boeing spokeswoman declined to comment on the test flight, citing an Air Force request to have all public communication come from the military.

FUTURE X-51A FLIGHT

This was the third of four X-51A aircraft built for the military, one of which flew for over three minutes at nearly five times the speed of sound during a 2010 test flight, the Air Force said in a statement.

The Air Force, which is analyzing data from Tuesday's test flight, said one X-51A aircraft remains and that a decision has not been made "when or if that vehicle will fly at this time."

The Waverider is part of efforts by the U.S. military to develop a prompt global strike capability to hit targets anywhere in the world within an hour, said Guy Ben-Ari, senior fellow with the Center for Strategic and International Studies.

Over the years, the global strike program will likely eat up billions of dollars in development costs, Ben-Ari said. If the program becomes operational, targets could include conventional military sites or militants, he added.

A missile would likely not be fired from a vehicle like the X-51A, but the craft itself would be the missile, he said.

"The differences between what's an aircraft and what's a missile and what you would call a drone or a remotely piloted vehicle are becoming very fuzzy," Ben-Ari said.

That the test flight crashed early due to a problem with a fin would likely be frustrating for the military because that part was relatively easy to build, unlike the largely untested Scramjet engine, said John Pike, director of GlobalSecurity.org, a website for military policy research.

"The only way that you can develop this stuff is to actually take it out and fire it, and so the problem is they've got computer models but not much data," Pike said.

Pratt & Whitney Rocketdyne designed the X-51A's "Scramjet" engine, which uses the forward motion of the craft to compress air for fuel combustion, according to a description of the project from the military.

"It is unfortunate that a problem with this subsystem caused a termination before we could light the Scramjet engine," Charlie Brink, X-51A Program Manager for the Air Force Research Laboratory, said in a statement.

"All our data showed we had created the right conditions for engine ignition and we were very hopeful to meet our test objectives," Brink said.

(Reporting By Alex Dobuzinskis; Editing by Cynthia Johnston and Cynthia Osterman)

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