Scramjet-powered hypersonic aircraft artwork by Nick Kaloterakis

Scramjet-powered hypersonic aircraft artwork for Popular Science by Nick Kaloterakis

Recent breakthroughs in scramjet engines could mean two-hour flights from New York to Tokyo. They could also mean missiles capable of striking any continent in a moment’s notice. No wonder the race to develop them is as fierce as ever.

By Michael Belfiore.

Popular Science, December 10, 2007

Last March, engineers from Pratt & Whitney Rocketdyne (PWR) gathered in the control room of a high-temperature tunnel at NASA’s Langley Research Center in Virginia. After a countdown, a jet of blue flame fueled by methane gas roared down the 12-foot length of the tunnel. A low rumble crept into the control room. It sounded like a rocket firing, which actually wasn’t far from the truth.

“Okay to inject,” a test director announced when the flame had reached full force. An angular pedestal covered in bolted copper plates rose from the floor of the chamber, placing an experimental scramjet engine called the X-1 into the inferno. “AOA modulating,” called the test director as the engine tilted slightly. “Model on centerline.” Then, “We are in ignition.” And with that, an exhaust flame even hotter than the 2,000°F-plus methane jet around it began to dance behind the activated engine, growing brighter as it ramped up to full thrust. After one minute, the engine shut down and descended through the floor.

The test was part of the X-51A Flight Test Program, a research project funded by the Air Force Research Laboratory and the Defense Advanced Research Projects Agency (Darpa), the Pentagon’s research arm. The X-51A project is, in turn, one piece of a global effort—part collaboration, part race—to build jet-powered aircraft that fly as fast as rocket ships. And the technology that will make this breakthrough possible is the scramjet, an engine that inhales air at tremendous speeds, squeezes the air until it’s thousands of degrees hot, and then mixes that air with fuel to generate massive thrust at higher speeds than any other jet-engine design.

The X-1 scramjet engine, which will eventually power the X-51A aircraft, is the most advanced scramjet engine ever built. The blowtorch blasting through the chamber was meant to simulate the extreme heat generated by flying faster than Mach 6. In all, the team at Langley would repeat this test 44 times. “We tested it at Mach 4.6, 5.0 and 6.5,” says Curtis Berger, the X-51A program manager at PWR. “The amount of time that this thing was actually running and creating thrust was just about 17.8 minutes.” He pauses to let that sink in. “Over 17 minutes of time on this engine. That’s a lot of time for a scramjet engine.”

To put things in context, the world’s fastest jet, the Air Force’s SR-71 Blackbird spy plane, set a speed record of Mach 3.3 in 1990 when it flew from Los Angeles to Washington, D.C., in just over an hour. That’s about the limit for jet engines; the fastest fighter planes barely crack Mach 1.6. Scramjets, on the other hand, can theoretically fly as fast as Mach 15—nearly 10,000 mph.

This could mean two-hour flights from New York to Sydney. It could also mean missiles capable of hitting targets on another continent at a moment’s notice, and when you put it that way, it’s not surprising that militaries around the world—the U.S., Australia, China and perhaps others—are trying to build them. After decades on the drawing board, it seems scramjet technology is finally about to arrive.

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