But jet engines can get oxygen from the air. Without oxygen tanks, the craft suddenly gains up to five times more room for payload, it picks up speed, and it costs less to fly. In common jet engines, like the ones on commercial airplanes, rotating blades compress incoming air. Injected fuel mixes with the air and burns. The hot gases turn a turbine to drive a fan and compressor, then expand out the rear nozzle, propelling the airplane.
The trouble with conventional jets is that they have a built-in speed limit. When they approach Mach 4, or four times the speed of sound, the air is arriving faster than they can swallow it. Drag caused by the engine and airplane moving through the air cannot be overcome by the engine’s propulsion. And the friction of the air at that speed heats the engine until it begins to melt.
These dilemmas are compounded by the powerful force of shock waves. An airplane flying faster than sound outruns its own sound waves, which then attach like the wake of a boat to the airplane’s nose and tail. These wakes are the shock waves bystanders on the ground hear as a ba-boom. Closer to the airplane, shock waves are much more intense. NASA learned early on that shock waves can be very fickle—and very dangerous.
A final goal of the X-15, the experimental rocket-powered NASA aircraft that broke speed and altitude records through the 1960s, was to test a new design for a scramjet like the X-43A’s. In 1967 engineers at Dryden fitted beneath the X-15 a mock version of the engine to study the aerodynamics at Mach 7. Neither pilot William “Pete” Knight nor anyone else knew it during his flight, but unexpectedly strong shock waves trailing the dummy engine as Knight reached Mach 6.7 interacted with other streams of air and began cutting into the aircraft like a scalpel heated to 3,000 degrees. Explosive bolts meant to jettison the engine prior to landing detonated early, and the jet fell off. Heat ripped into the X-15’s underbelly, and instruments started going dead. Knight set down just before the fuselage disintegrated.
Test pilot William Dana, serving as mission controller at the time, saw the X-15 riddled with holes, and “my knees started shaking,” he recalls. “It looked like a maniac had gone wild with a cutting torch and had a field day.” NASA cancelled further airborne scramjet tests, and that X-15 never flew again. “We fried the airplane so badly we decided it was better done in a wind tunnel,” Dana says.
The faster a jet goes, the simpler its design must be. To keep heat from building up, the aircraft must have no parts that slow the air and bounce shock waves around. The solution? Get rid of the compressor blades and other rotating pieces. For hypersonic flight, engineers approached airplane design in a whole new way. Instead of fans mechanically compressing air sent to the tunnel-like engine combustion chamber, the bow of the airplane plows into and compresses air, funneling it into the chamber; here, the airplane body forms part of the engine.
“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.