The last of four unmanned experimental military aircraft built by Boeing flew for at a top speed of Mach 5.1 over the Pacific Ocean on May 1, the Air Force said. The total flight covered 230 nautical miles in just over six minutes before the hypersonic cruiser plunged into the ocean.
"It was a full mission success," said Charlie Brink, who runs the X-51A program for the Air Force Research Laboratory Aerospace Systems Directorate.
The Air Force said it was the longest of the four X-51A test flights and the longest air-breathing hypersonic flight ever. The technology opens the door to future practical uses for hypersonic jet-fueled aircraft.
A hypersonic aircraft developed by NASA used hydrogen as a fuel to fly briefly at even higher speeds in 2004, but it would take a giant fuel tank to fly for longer periods.
"All we have learned from the X-51A Waverider will serve as the bedrock for future hypersonics research and ultimately the practical application of hypersonic flight," Brink said.
A video released by the Air Force showed the Waverider dropping down from under the left wing of a B-52 bomber at an altitude of about 50,000 feet and then accelerating away at great speed, leaving behind a long vapor trail.
The cruiser accelerated to March 4.8 in about 26 seconds, powered by a solid rocket booster built by Pratt & Whitney Rocketdyne, a unit of United Technologies. After separating from the booster, the cruiser's scramjet engine lit and accelerated the vehicle to Mach 5.1 at 60,000 feet.
The vehicle continued to send back data to the control station at Edwards Air Force Base in California until it made a controlled dive into the Pacific Ocean.
"This demonstration of a practical hypersonic scramjet engine is a historic achievement that has been years in the making," said Darryl Davis, president of Boeing Phantom Works, the company's advanced research and prototyping arm.
"This test proves the technology has matured to the point that it opens the door to practical applications, such as advanced defense systems and more cost-effective access to space," Davis said.
The first of the four X-51A vehicles flew in May 2010, hitting nearly Mach 5 for nearly two and a half minutes.
The nearly wingless X-51 was made using mostly standard aerospace materials such as aluminum, steel and titanium, although some carbon composites were used in the fins. For heat protection, the vehicle used insulation tiles similar to those used on board the NASA Space Shuttle orbiters.
The Air Force said the four X-51As were built to demonstrate the new technology, not as a prototype for a new weapon system. The program is aimed at paving the way to future hypersonic weapons, hypersonic intelligence, surveillance and reconnaissance, and future access to space, it said.
Since scramjets are able to burn atmospheric oxygen, they can be made lighter than conventional rockets, which may allow satellites to be launched into orbit more efficiently and cheaply.