PROGRAM DETAILS CLASSIFIED

The aircraft is known as the Waverider because it stays airborne, in part, with lift generated by the shock waves of its own flight. The Boeing Co’s Phantom Works division performed design and assembly on the aircraft, according to the military.

Four X-51A aircraft were built for the military, one of which flew for more than three minutes at nearly five times the speed of sound during a 2010 test flight, the Air Force said.

The experimental aircraft are expected to crash at the end of test flights in any case, and are not considered retrievable.

Pratt & Whitney Rocketdyne designed the X-51A’s “scramjet” engine, which uses the forward motion of the craft to compress air for fuel combustion, according to a description of the project from the military.

After being dropped from a B-52 bomber, a solid-rocket booster is used in the initial phase of the plane’s flight to bring it up to speeds that can allow its engine to take over, by drawing in air through the craft’s forward momentum.

The cost of the experimental aircraft has not been disclosed because many details of the program are classified.

In 2004, NASA reached a speed of Mach 9.6, or nearly 7,000 miles per hour, with a jet-powered aircraft. But that vehicle, known as X-43, only flew for a few seconds and its copper-based engine was not designed to survive the flight.

Engineers have hoped to see the hypersonic X-51A travel for five minutes of powered flight. For protection from extreme heat, it uses insulation tiles, similar to those on the NASA space shuttle orbiters, according to a 2011 military description of the project.