The last of four unmanned experimental U.S. military aircraft designed to fly at six times the speed of sound is expected to be tested next year, the program manager said on Wednesday, months after its predecessor broke up during a trial.

The third test flight of the craft, known as the Waverider or X-51A, broke apart over the Pacific Ocean seconds into a test flight in August. U.S. Air Force officials said at the time they did not know if or when their fourth aircraft would fly.

Preliminary results from an investigation into what went wrong during the August flight indicate that a “random vibration issue” caused one of the control fins to deploy early, the X-51 program manager at the Air Force Research Laboratory, Charlie Brink, told reporters on a conference call.

“I can’t say conclusively that’s it, but it’s looking more and more like the cause,” Brink said, adding that investigators quickly ruled out a software or power malfunction as a cause of the aircraft’s break up.

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.

Analysts say the military has its eye on using the Waverider program to develop missiles with non-nuclear warheads that could strike anywhere in the world within an hour.

Results from the investigation into the third aircraft’s failed test flight are expected to be complete in mid-December, Brink said.

“I’m fairly confident that in the next couple of months we’ll have the investigation complete and we’ll move on. We’re already preparing the fourth flight vehicle. We’re doing those things in parallel,” he said.

Engineers are already modifying the final test X-51A to be ready in late spring or early summer of 2013, he said.