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.