Scramjet power? Simple: Keep a match lit in a 7,000-mph wind.
- By Michael Milstein
- Air & Space magazine, July 2005
(Page 3 of 5)
“What you have is a flying engine,” says McClinton, technology manager for the Hyper-X. When the supersonic speed of the airplane rams oncoming air into the combustion chamber, the engine is called a ramjet. But that works only at very high speeds. The SR-71 Blackbird, for example, has turbojets with afterburners to accelerate and cruise at supersonic speed. When the aircraft is flying fast enough to compress the air on its own, its engines remake themselves. Ducts direct part of the airstream around the rotating compressors, and the engines become ramjets that propel the airplane to Mach 3 or higher.
But ramjets, like turbojets, slow the air to subsonic speed before combining it with fuel. This minimizes formation of troublesome shock waves, but it works only up to about Mach 6. Any faster and the engine begins to melt from the effort to slow all the air down. That’s when a scramjet becomes the only option. As the air races through the engine, it is moving at supersonic speeds and generating shock waves. But a scramjet uses the shock waves to advantage.
Picture a river with banks lined with concrete. The banks reflect waves and boat wakes in the water. A scramjet combustion chamber does much the same with shock waves. On the ground, you could peer into it and it would look like an empty pipe. But once moving beyond the speed of sound, its internal design and shape orient shock waves into a precise pattern. It directs, compresses, and focuses the airflow, creating the right temperatures and pressures for combustion. A scramjet fashions the essential components of a jet engine from the air currents racing through it. Joel Sitz, NASA’s X-43A project manager at Dryden, shrugs and says: “You’re training the air to do tricks.”
The X-43A’s combustion chamber measures about three feet in length, so at a top speed of 7,000 mph, air whips through it in roughly .001 second. That’s not much time for fuel to mix with oxygen, ignite, and burn. Engineers liken the challenge to keeping a match lit in a tornado—but more difficult. If fuel burns too fast, the airflow inside the engine reverses, causing power loss—an “unstart,” in jet jargon. If it burns too slowly, it’s as if your car’s gasoline were igniting a block behind you. All its energy goes to waste.
But in the mad rush of air, the shock waves form a kind of shelter, as you would with your hands to light a candle in the wind. Into the eye of this storm, the X-43A’s engine injects hydrogen fuel and ignites it with silane, a silicon gas that instantly burns on contact with air.
For the better part of 50 years, scramjets were more theory than reality. It’s impossible to thoroughly test them in laboratory wind tunnels; even the fastest tunnels can produce scramjet airspeeds only in brief pulses. They provided split-second snapshots, but never a moving picture of a scramjet in continuous flight. Some doubted a scramjet could ever produce enough power to overcome the drag of air hitting any airplane at over Mach 7, no matter how sleek and aerodynamic.
But if it could, it would enable production of a jet fast and powerful enough to do much of the hard work of reaching space. Because a scramjet must move at Mach 6 or 7 to ignite, it needs a booster to take off and accelerate, but the rocket could be much smaller. A scramjet-rocket combo could carry the same payload as the space shuttle, but weigh only a quarter as much. That reduction could cut the cost of putting a pound in orbit by perhaps 80 percent, although some engineers say the research cost to develop the scramjet could cancel any gains.
President Ronald Reagan believed. In his 1986 State of the Union speech, he announced “a new Orient Express that could, by the end of the next decade, take off from [Virginia’s] Dulles Airport, accelerate up to 25 times the speed of sound, attaining low Earth orbit or flying to Tokyo within two hours.” He was invoking the National Aerospace Plane, a NASA outgrowth of a highly classified military project to develop a reusable, scramjet-driven aircraft that could take off from the ground and accelerate into space or travel between continents. Though NASA called it the X-30 and a successor to the space shuttle, it got its billions in funding from the military. Invoking the image of an airplane that could race in and out of enemy airspace before it could be detected and shot down may have been mostly cold war intimidation, though, because by the 1990s the military lost interest and Reagan’s vision never flew.