The X-51A Waverider at rest. (Credit: U.S. Air Force)
The X-51A Waverider flew today, and it flew fast.
The scramjet engine in the experimental aircraft burned for a little over three minutes at around 10 a.m. PDT Wednesday in a test range over the Pacific Ocean, pushing the X-51A to the hypersonic speed of Mach 5, or five times the speed of sound. That was the top speed reached by the aircraft in its brief flight, according to Boeing and press reports citing U.S. Air Force officials. (Editors' note: A separate Air Force News Service report had initially cited a higher speed, but was amended to give the Mach 5 figure.)
The 200 seconds of autonomous flight by the U.S. Air Force's X-51A set a duration record for an aircraft powered by a scramjet (short for "supersonic combustion ramjet") engine, though in part that can be chalked up to the rarity of any flights at all at this extreme level. The previous record was set by NASA's X-43A, whose scramjet engine burned for only about 10 to 12 seconds in November 2004; that aircraft zoomed to Mach 9.8.
It had been expected that the X-51A would fly for as long as 300 seconds (that is, five minutes) and would hit Mach 6. During Wednesday morning's flight, the Waverider suddenly lost acceleration shortly after the 200-second mark for an unexplained reason. "At that point," Boeing's press release says tersely, "the X-51A was terminated as planned." (The Air Force had previously said it had no plans to recover the aircraft.)
Still, those involved in the flight were thrilled with the results.
"We are ecstatic to have accomplished most of our test points on the X-51A's very first hypersonic mission," said Charlie Brink, an X-51A program manager with the Air Force Research Laboratory, in a statement. "We equate this leap in engine technology as equivalent to the post-World War II jump from propeller-driven aircraft to jet engines."
The air-breathing scramjet engine does hold promise for advancing the state of aeronautics. Because it can use atmospheric oxygen, for instance, it wouldn't need to carry oxidizers as do today's rockets, or could get by with less fuel. That could mean cheaper rocket launches into orbit for a future generation of space planes. From the Air Force's perspective, scramjets could also mean missiles getting to their targets much faster than current cruise missiles.
Continuing a long tradition of X planes, the X-51A was carried aloft by a B-52 before being released at about 50,000 feet. A solid rocket booster accelerated the Waverider to about Mach 4.5 before its own Pratt & Whitney Rocketdyne scramjet engine was ignited to take it to about 70,000 feet and Mach 5.
The aircraft that flew Wednesday was one of four X-51A cruisers that have been built. The remaining three could fly this fall, the Air Force said Wednesday, and would have nearly identical flight profiles.
Update, Thursday 5:08 a.m. PDT: See video of the launch from the B-52 below:
An artist's rendition of the Boeing X-51A Waverider
The    managed to break a hypersonic flight record Wednesday during a test flight.

The United States Air Force (USAF) said that the scramjet was able to fly for 200 seconds, achieving a top speed of around  5 and setting a new record for what the Air Force called "the longest supersonic combustion ramjet-powered hypersonic flight." The previous record of twelve seconds was set by the  in 2004. This hypersonic flight, also the first to use  fuel, was hailed by US government officials as a success. Despite an unknown failure which caused the X-51 to lose acceleration, an X-51 program manager said that the USAF was "ecstatic" about the event's accomplishments.

The X-51 is 14 feet (4.2 metres) long and has no real wings, allowing it to withstand the  created during flight. The aircraft launched around 10 a.m.  (1700 ) from  in California. It was carried by a  to a height of 50,000 feet (15,240 metres) and then released over the  Naval Air Warfare Center Sea Range. The X-51 was then propelled by a solid rocket booster to Mach 4.8.

After about 200 seconds in flight, Boeing said that "something then occurred that caused the vehicle to lose acceleration. At that point, the X-51A was terminated as planned." Although it was expected to fly for about 300 seconds and reach Mach 6, the scramjet only managed Mach 5 at a height of about 70,000 feet (21,336 metres), possibly due to an engine blowout. The aircraft landed in the ocean as planned, and there are no plans to retrieve it.

This test is the first of four planned flights for the X-51 program, with the other three planned for this coming fall. Previously, the X-51 had flown twice, but was attached to the B-52 both times.