A second test of the X-51A Waverider flew on Monday in the Point Mugu Naval Air Test Range over the Pacific Ocean but was not fully successful, Air Force Flight Test Center officials said.

Similar experiment last year set a record for hypersonic flight, flying more than 3 minutes at Mach 6 – six times the speed of sound.

This time, the hypersonic aircraft was able to gain over Mach 5 but the scramjet engine lit failed to transition to full power, they said.

After a flawless flight from Edwards Air Force Base, a U.S. Air Force B-52H Stratofortress released the experimental vehicle from 50,000 feet above. The X-51A was able to accelerate to a speed just over Mach 5, they said.

Later, when the experimental aircraft's air breathing scramjet engine was lit on ethylene, it attempted to transition to JP7 fuel operation but the vehicle experienced an inlet un-start and failed to reach its full capacity, explained the officials. So far, two scramjet experiments have been conducted and the last year's experiment was successfully lit, and this year it failed.

The hypersonic vehicle fell into the ocean within the test range on Monday, officials said.

Obviously we're disappointed and expected better results, said Charlie Brink, the Air Force Research Laboratory's X-51A program manager, but we are very pleased with the data collected on this flight. I am extremely pleased with the AFFTC and Point Mugu's support and execution of this complex flight test mission, as they provided us every opportunity for success in this endeavor. We have attempted two scramjet experiments now where one successfully lit, and one did not.

We will continue to examine the data to learn even more about this new technology, he said. Every time we test this new and exciting technology, we get that much closer to success.

He said AFRL, Boeing and Pratt-Whitney Rocketdyne engineers are reviewing the large amount of telemetry data collected during the test flight to identify the cause of the anomaly.

Boeing and Pratt-Whitney Rocketdyne built four X-51A flight test vehicles with the program goal of reaching Mach 6 in hypersonic flight. The next flight is tentatively schedule for fall 2011.