U.S. Air Force

The U.S. Air Force's sleek, light-colored X-51A Waverider hypersonic vehicle can be seen tucked under the wing of a B-52H Stratofortress for this week's test launch.

The U.S. Air Force's $300 million, nine-year test program for a hypersonic plane ended on a high note this week, when the last of its X-51A Waverider vehicles made the longest flight of its kind. The success was made sweeter by the fact that it followed last year's high-profile failure.

"I believe 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," Charlie Brink, X-51A program manager for the Air Force Research Laboratory Aerospace Systems Directorate, said in a news release.

The 14-foot-long (4.3-meter-long), scramjet-powered vehicle hit a top speed of Mach 5.1 during just over six minutes of flight on May 1, the Air Force said. That's the longest of the Boeing-built X-51A's four test flights, and the longest air-breathing hypersonic flight ever.

Hypersonic scramjet propulsion has been widely touted as eventually opening up the way for flights between London and New York in less than an hour. But in reality, the first application is more likely to come in the form of super-fast cruise missiles.

Scramjet is a short way of saying "supersonic combustion ramjet." There have been many efforts through the years to perfect hypersonic aircraft — that is, vehicles that travel at speeds beyond Mach 5. But the Air Force says the X-51A is unique primarily because it used hydrocarbon fuel rather than hydrogen fuel. Without any moving parts, the fuel is injected into the scramjet's combustion chamber, where it mixes with the air rushing through the chamber. The fuel is ignited in a process that's been likened to lighting a match in a hurricane.      

This week's experiment followed the flight profile used for the X-51A's earlier tests: A B-52H Stratofortress took off from California's Edwards Air Force Base, flew 50,000 feet over a Pacific test range, and then released a solid rocket booster with the plane attached. When the cruiser reached Mach 4.8, the X-51A separated from the booster and lit up its scramjet engine. The scramjet exhausted its fuel in 240 seconds. The sleek vehicle coasted for another couple of minutes and splashed down into the ocean as planned. The X-51 traveled more than 230 nautical miles and yielded 370 seconds of data, the Air Force said.

"This success is the result of a lot of hard work by an incredible team.  The contributions of Boeing, Pratt and Whitney Rocketdyne, the 412th Test Wing at Edwards AFB, NASA Dryden and DARPA were all vital," Brink said.  

All this is a huge improvement over the previous test, which ended in failure last August. During that flight, the X-51A veered off course less than a minute after launch and crashed, due to a problem with one of its control fins. The issue was resolved after a months-long investigation. The first X-51 test was successful in May 2010, resulting in a 200-second flight, but the second test in June 2011 was a disappointment. 

There's no immediate successor to the X-51A, but the Air Force has pledged to continue with hypersonic research. It says the lessons learned during the X-51A program "will pay dividends to the High Speed Strike Weapon program" at the Air Force Research Laboratory.

More about supersonic flight:

• Video: What the X-51 was designed to do
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• Supersonic biplane puts an end to sonic booms

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the dwarf planet and the search for new worlds.

The U.S. Air Force says its most ambitious test of its X-51 WaveRider hypersonic aircraft ended in failure less than a minute after launch on Tuesday, due to a flaw in one of the craft's control fins. The X-51 broke apart after it was dropped from a B-52 bomber, with pieces falling into the Pacific Ocean, a spokesman for the project told me today.

If the test had proceeded as planned, the Boeing-built X-51 would have shot through the sky for a five-minute flight at a speed of up to 3,600 mph (5,800 kilometers per hour), or six times the speed of sound. Instead, the Air Force is going back to the drawing board.

Hypersonic scramjet propulsion has been widely touted as eventually opening up the way for flights between London and New York in less than an hour. But in reality, the first application is more likely to come in the form of super-fast cruise missiles. (Scramjet is a short term for "supersonic combustion ramjet," and there have been many efforts through the years to perfect scramjet-powered aircraft.)

In a statement, the Air Force said the unmanned craft was successfully launched from the B-52 over Point Mugu Naval Air Warfare Center Sea Range, in the Pacific near California's coast, at about 11:36 a.m. PT (2:36 p.m. ET) on Tuesday. The X-51's rocket booster fired as planned — but 16 seconds later, a fault was identified with the cruiser control fin, the Air Force said. When the X-51 separated from the booster, about 15 seconds later, the cruiser couldn't maintain control and was lost.

"'Came apart' is the term that they used," said Daryl Mayer, a spokesman for the Air Force's 88th Air Base Wing at Wright-Patterson Air Force Base in Ohio.

The WaveRider never had a chance to reach supersonic speed.

"It is unfortunate that a problem with this subsystem caused a termination before we could light the scramjet engine," Charlie Brink, X-51A program manager for the Air Force Research Laboratory, said in today's statement. "All our data showed we had created the right conditions for engine ignition, and we were very hopeful to meet our test objectives."

The Air Force said the control system had proven reliable during the X-51A's two previous flights — including a successful test in May 2010 and a not-so-successful test in June 2011.

Today's statement said program officials will conduct a "rigorous evaluation" of this week's test to assess all the factors behind the failure. One of the four X-51A vehicles remains, but officials have not decided when or if that vehicle will fly, the Air Force said. The X-51 project's cost has been estimated at $140 million.

More about supersonic flight:

• Video: What the X-51 was designed to do
• Futuristic space plane closer to reality
• Supersonic biplane puts an end to sonic booms
• Supersonic drone set to fly in 2013, if ...

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the dwarf planet and the search for new worlds.