A second test of the X-51A Waverider flew June 13, 2011, in the Point Mugu Naval Air Test Range over the Pacific Ocean, bringing significant hypersonic research data in a less than successful flight test. The hypersonic aircraft was successfully boosted to just over Mach 5, and the scramjet engine lit but failed to transition to full power. Air Force Flight Test Center officials said after a flawless flight from Edwards Air Force Base, a U.S. Air Force B-52H Stratofortress released the experimental vehicle from an altitude of approximately 50,000 feet. After release the X-51A was initially accelerated by a solid rocket booster to a speed just over Mach 5.
The problem with scramjets has always been this critical step of transitioning to scramjet mode and initiating liquid fuel combustion.
The experimental aircraft’s air breathing scramjet engine lit on ethylene and attempted to transition to JP7 fuel operation when the vehicle experienced an inlet un-start.
The USAF’s Air Force Research Lab (AFRL) is supported in the X-51A program by Boeing and Pratt & Whitney Rocketdyne. Although the test ended in failure, there is still a lot of data that requires analysis, which may bring valuable insights. The next test is scheduled for Fall 2011.
Perhaps they will publish the results of the data analysis or release more information which will help uncover what caused the inlet to unstart.