A vivid X-51A

For more than 65 years, NASA (and its predecessor, NACA), the Pentagon, and various defense contractors have collaborated on the experimental X-Plane program, testing innovative technologies and looking at cutting-edge engineering concepts that push the boundaries of flight.

This week, at a test range over the Pacific Ocean, aerospace engineers had hoped that a new generation of X-marked aircraft, the unmanned X-51A Waverider, would be able to fly at Mach 6 -- six times the speed of sound, or roughly 4,000 miles per hour -- for a sustained 300 seconds.

That was not to be. The Air Force said today that yesterday's bid by the X-51A ended abruptly about 31 seconds into free flight after "a fault was identified with one of the cruiser control fins."