Debrief: Hyper-X

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

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)
Air & Space Magazine

IT WILL NEVER HANG IN THE SMITHSONIAN, because it ended up on the bottom of the Pacific Ocean. Its pilots will not enjoy ticker tape parades; they were all on the ground, watching the computer-operated craft on video screens. But in about 10 seconds, a combination of scientific persistence and untested technology constructed of leftovers from secret military projects changed how we define “fast.” The little black wedge called the X-43A, smaller than a Ferrari, flew faster than any air-breathing airplane ever—nearly 10 times the speed of sound, and at about two miles per second, swift enough to cross the country in 20 minutes. It owes its tremendous speed to a jet engine that has no moving parts; in fact, it seems nothing like an engine, and some scientists doubted it could ever work.

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For proponents of the engine known as a scramjet—short for supersonic combustion ramjet—the brief flight offered the same sort of vindication the Wright brothers earned in 1903 at Kitty Hawk: The project rebounded from a catastrophic failure of the first X-43A flight in 2001, when a booster carrying the airplane careened out of control. And the flight finally turned the experimentalists’ concept into reality, a result of five decades of research aimed at producing a hypersonic jet engine that could propel airplanes nearly into space and back again more cheaply and safely than rockets.

“It was the Holy Grail, if you will,” says Anthony Castrogiovanni, the X-43A propulsion team leader at Alliant Techsystems, the NASA contractor that engineered it. “A successful flight was going to change the way the world thinks about hypersonics.”

There’s fast, there’s supersonic, and then there’s hypersonic—more than five times the speed of sound, a realm rarely probed by airplanes because few can survive it. Scramjets are the only engines other than rockets that can reach hypersonic speeds. Their first use would likely be by military forces, as light and adaptable engines for ultra-fast and maneuverable missiles. But they are such a temperamental blend of speed, flame, and fuel that any vision of passenger-carrying scramjet-powered airliners crisscrossing the skies remains a distant one. “It’s a very simple concept that’s very complicated to pull off,” says Vince Rausch, manager of NASA’s Hyper-X program, parent of the X-43A.

The first of three X-43A flights was set for June 2001. A winged Pegasus rocket modified to boost the 12-foot-long, five-foot-wide research craft to its planned altitude and speed dropped like clockwork from the wing of a NASA B-52 mothership over the Pacific Ocean northwest of Los Angeles. After five seconds the Pegasus’ solid rocket motor ignited in a blaze. But eight seconds later, as the rocket neared the speed of sound, its right fin tore off, then the left fin, rudder, and wing. The 49-foot rocket tumbled wildly in a smoky blur until a Western Test Range safety officer triggered onboard explosives to destroy it. The X-43A aircraft attached to its nose never had a chance.

For Griff Corpening, an engineer at NASA’s Dryden Flight Research Center in California, and others who had shepherded the project for some five years, the failure felt like a kick in the gut. “People look at science and technology as dry stuff, but I wish they could understand all of the rush of emotions that you feel,” he says. At least the failure had been the rocket’s, not the scramjet’s. An investigation determined that the Pegasus rocket was too far out of its element. Orbital Sciences had designed the launch vehicle to be released from an airplane at 40,000 feet and to loft satellites into orbit. But this time it was released 20,000 feet lower, where the air is denser. Computer models based on data from earlier Pegasus flights predicted the craft’s fins and rudders could handle the denser air.

The project team members were buoyed by one batch of data, though: The little X-43A held together even as the out-of-control rocket whipped it every which way. “It told us we had one heckuva solid vehicle,” says Ted Rothaupt, an engineer at Boeing Phantom Works, Boeing’s advanced research and development branch, which helped develop the X-43A. The X-43 looked tough enough for hypersonic flight—if only it could get there.

As long as humans have flown, they have longed to fly faster. Most members of the X-43 team are all about speed. When Paul Reukauf, deputy project manager for the X-43A at Dryden and a pilot himself, heard that fellow scientist Chuck McClinton had bought a boat, he had only one question: “Why would anyone spend that amount of money to go six knots?”

Rockets have always outpaced airplanes. They accelerate by rapidly combining fuel with oxygen they carry on board. The drawbacks are that liquid oxygen is heavy and hard to handle.

Because the oxygen tanks are so weighty and take up so much space, they reduce the rocket’s payload-carrying capacity. And that’s one reason why the cost of firing something into space is about $10,000 per pound. Speed is even pricier. The faster a rocket must go, the more fuel and oxygen it needs—in other words, still more tankage. The craft gets heavier and heavier, eventually reaching a point where it simply cannot carry enough fuel and oxygen to go any quicker.

About Michael Milstein

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

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