Why Did The Hypersonic 'Waverider' Crash?

The U.S. Air Force is investigating what happened to the X-51 Waverider aircraft that fell into the Pacific last week. The X-51 is part of an Air Force/DARPA research project trying to build a "scramjet" engine that will work at hypersonic speeds. NPR's Larry Abramson reports.

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The Air Force is trying to figure out what happened to an experimental jet that fell into the Pacific Ocean last week. The X-51 Waverider is meant to fly at least six times the speed of sound and beyond.

As NPR's Larry Abramson reports, trying to achieve that feat has tempted and frustrated engineers for decades.

LARRY ABRAMSON, BYLINE: Rockets like the Space Shuttle already travel more than 20 times the speed of sound to get into orbit, so why do we need a plane that can get up to Mach 6? One reason is practical. Richard Hallion, former historian for the Air Force, points out that rockets have to carry oxygen with them. The engine in the X-51 uses oxygen from the atmosphere.

RICHARD HALLION: You don't have this burden of carrying your oxidizer with you. You have more capacity for payload. You have more flexibility in your operations.

ABRAMSON: Unlike a typical jet engine, the power behind an X-51 is called a scramjet. It has no moving parts and burns its fuel while air is rushing in at thousands of miles per hour. Richard Hallion says that task has been compared to lighting a match in a hurricane and keeping it lit.

HALLION: So you have to mix your fuel with that. You have to get a proper fuel mix, you have to ignite it, and then you have to sustain that combustion so it doesn't blow out. That's a very, very difficult challenge.

ABRAMSON: That's once it's reached hypersonic speeds. First, the X-51 has to get airborne. It is dropped from the belly of a B-52 bomber and uses a booster rocket to get past the speed of sound. Then that scramjet is supposed to kick in. In past tests, the jet has worked for a few minutes and provided valuable data. Last week's test never got to the scramjet phase. Another part failed, and the X-51 fell into the ocean. John Pike of globalsecurity.org says these craft are not designed to be recovered. It's a high-risk strategy to get information that cannot be obtained from a wind tunnel.

JOHN PIKE: The only way that they can get the data to validate their computer models to tell them how to build operational systems is by actually doing flight tests.

ABRAMSON: The Air Force would not talk about exactly what happened during last week's test, not until an investigation is completed. Four Waveriders were built by Boeing and Pratt & Whitney. Now, there's only one left. At one time, there was interest in hypersonic flight for down-to-earth reasons. President Reagan wanted to build an airborne Orient Express that could go from Washington to Tokyo in two hours. Now, the demand is strategic. John Pike says the Air Force needs a way to deliver weapons quickly over long distances without using intercontinental ballistic missiles.

PIKE: Normally, they've got big hydrogen bombs on the front end of them. And so if the Russians or the Chinese saw a bunch of these coming at them, they would assume it was a nuclear attack and quite possibly launch a nuclear retaliation.

ABRAMSON: The X-51 gets its Waverider moniker from the way it handles the aerodynamics of hypersonic flight. It's essentially wingless and is designed to ride the shock wave it creates. It's hard to say how much last week's failed test cost, but most estimates put the military's investment into hypersonic technology in the hundreds of millions of dollars. Larry Abramson, NPR News.

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