The Hypersonic Age is Near

A Match in a Hurricane

Ordinary jets have a major limitation: They can't go faster than Mach 3 without their turbine blades melting. Rocket ships can reach Mach 25, but they have to carry tremendous amounts of liquid oxygen to burn their fuel. The space shuttle, for example, weighs only 165,000 pounds empty, but it must carry 226,000 pounds of liquid hydrogen and 1.4 million pounds of liquid oxygen to reach orbit.

An air-breathing jet engine with no moving, meltable parts, such as a scramjet, can solve these problems. A scramjet is an advanced form of a "ramjet," an engine that takes the air rushing into the engine and "rams" it into the combustion chamber, creating intense pressures that can sustain combustion at the furious rate that Mach-3-plus speeds demand. But ramjets have limits too. The air entering the engine has to be slowed to subsonic speeds for it to run efficiently. And that air is so hot that no matter what measures are taken to cool it, a ramjet-powered craft must stay under Mach 5 to keep from disintegrating.

But a scramjet—a "supersonic combustion ramjet"—changes things. A scramjet does away with the diffuser that a ramjet uses to slow down incoming air, allowing the air to move through the engine at supersonic speeds so it can fly above Mach 5. The tradeoff: A scramjet engine in flight is a delicate system. Achieving balanced combustion at those speeds is an engineering challenge often compared to keeping a match lit in a hurricane.

So far, the most public scramjet project has been the National Aerospace Plane, or NASP. Unfortunately, it was a spectacular failure. Announcing the project in his 1986 State of the Union address, President Reagan called it "a new Orient Express" that would be able to reach Tokyo from Dulles Airport in two hours; the goal was to have it running by the late 1990s. NASP was meant to be all things to all customers—America's next space shuttle as well as the Air Force's next bomber and the next big thing in passenger travel. But by 1994, it appeared that research had stalled, and President Clinton canceled NASP. That might have been a good thing. "We didn't stop our research," says Charlie Brink, a scramjet program manager at the Propulsion Directorate at the Air Force Research Laboratory. "We reevaluated it and said: Now that we're not trying to make a Mach-0-to-25 vehicle take off from a runway, let's take the technical problem and break it down into more manageable chunks."

"What you're seeing now is a transition of the technology out of the laboratories into the flight-test domain," says David Van Wie, a scramjet research scientist at the Johns Hopkins University Applied Physics Laboratory. Armed with a new understanding of hypersonic aerodynamics and air-breathing propulsion, Van Wie says, "it's really to the point that people who work in the field feel they're ready to take the steps into flight test, experimentation and demonstration."

by Nick Kaloterakis: A scramjet engine stays lit at hypersonic speeds by using that speed to compress its fuel-air mixture. Air rushing in gets squeezed down as the intake narrows to a thin tunnel; it gets so hot that the fuel ignites in the combustion chamber, producing thrust.  Nick Kaloterakis

Escape from the Lab

In 2002, Australian researchers with the HyShot program at the University of Queensland's Centre for Hypersonics made history by conducting the world's first scramjet "flight." They strapped a small scramjet engine into the nose cone of a solid-fuel rocket and launched it to the edge of space. Then, some 200 miles up, the rocket dropped off, the scramjet shed its protective fairing and, as planned, nosed over and plummeted back toward Earth at thousands of miles an hour. At an altitude of 20 miles, the scramjet engine kicked in, firing for five seconds and reaching Mach 7.6, or more than 5,000 mph, before slamming into the ground. It wasn't graceful, but it was a historic achievement and a scientific success—a low-cost way to gather data from a scramjet while subjecting it to brutal heat and incredible velocity outside of a wind tunnel.