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Flying doorstop: The wedge shape of the X-43 compresses air entering the engine. This computational fluid dynamics image shows the vehicle's pressure gradients at Mach 7. (NASA Dryden)

Debrief: Hyper-X

Scramjet power? Simple: Keep a match lit in a 7,000-mph wind.

Even the airplane’s fins were designed with a small gap so that when they expanded from the heat of high-speed flight, they would not bind with adjacent parts. Designers were careful not to make the gap too large, though, or the airflow would become turbulent and slow the vehicle down.

To avoid another catastrophe, engineers beefed up the Pegasus’ fins and rudders and removed 3,300 pounds of the rocket’s propellant. By eliminating some fuel, the engineers could launch at the higher, thinner altitudes the rocket was designed for without worrying that it would loft the X-43A too high.

On March 27, 2004, the second X-43A took to the sky. This time it stayed on course. When it hit 95,000 feet, the bomb pistons fired with 10,000 pounds of force. The aircraft was finally on its own and flying. Within 2.5 seconds, the engine door opened, the train of shock waves instantaneously lined up inside, the silane ignited the hydrogen, and, for the first time, a scramjet was accelerating on its own. The engine burned for 11 seconds, propelling the X-43 to Mach 6.86. It bested the previous air-breathing speed record—just over Mach 5, set by a ramjet-powered missile—and outdid the airplane speed record, Mach 6.7, set by the X-15.

Eight months later, an X-43A fine-tuned for even higher speeds ran its engine for 10 seconds, shooting to a record-breaking Mach 9.8 on November 16, 2004. About 500 sensors transmitted temperature, pressure, strain, and other readings back to NASA, providing hundreds of times more information than the most elaborate wind tunnel test ever could.

After its engine stopped, the X-43 coasted through a sequence of maneuvers to reveal more about its aerodynamics. One signal suggested it may have started melting on its way down.

Researchers will be crunching the data for years, but they already know a few things. One is that they have the workings of scramjets well figured out: The X-43A’s performance came very close to their predictions. Another is that flying a scramjet for much longer than 10 seconds will demand a far more sophisticated cooling system and revolutionary new materials to handle the intense heat.

The X-43A’s fleeting glimpse into the future may also be the last for a long time. NASA has shifted focus to President Bush’s moon-Mars exploration initiative, carving aeronautics research down to just five percent of its newest budget. There is now almost no money for hypersonics. Engineers have transferred to other projects, and Chuck McClinton is sending wind tunnel models and other stepping stones in scramjet science to the Smithsonian.

“We’re going to be [part of] history,” he says. “It’s like many technology demonstrations. You’ve proven it works. Now you’re done.”

Contractors are lobbying Congress to continue work on the X-43C, a sibling of the original but powered by more easily handled hydrocarbon fuel, and the military continues modest hypersonic research under such programs as the Falcon project, co-sponsored by the Defense Advanced Research Projects Agency and the U.S. Air Force. But without a national effort, it’s unlikely to get very far, McClinton says. The scramjet, not quite rocket or jet, may end up retreating to the shadows.

For 10 seconds, though, it was way out in front.

About Michael Milstein

Michael Milstein is a freelance writer who specializes in science. He lives in Portland, Oregon.

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