X-51A on a B-52 wing

Tuesday's test, weather permitting, will be the first free flight for the X-51A. The scramjet aircraft has been aloft twice before, both times attached to the wing of a B-52. And that's how Tuesday's flight will begin, too. The B-52 will take off from Edwards Air Force Base in California, then head out to a Pacific ocean test range where the Waverider will be released at approximately 50,000 feet. A solid rocket booster on the X-51A will accelerate the test vehicle to about Mach 4.5, at which point the scramjet engine will kick in to push the vehicle toward 70,000 feet and Mach 6, according to the Air Force. At the end of its five-minute autonomous flight, the X-51A will head for a planned splashdown and crackup--the Air Force does not plan to recover the vehicle.

"In those 300 seconds, we hope to learn more about hypersonic flight with a practical scramjet engine than all previous flight tests combined," said Charlie Brink, X-51A program manager with the Air Force Research Laboratory's Propulsion Directorate, in a statement last week.

The Air Force says this will be the only hypersonic flight this year, largely for budgetary reasons but also to allow for maintenance to the Air Force Flight Test Center's B-52.