Scramjet power? Simple: Keep a match lit in a 7,000-mph wind.
- By Michael Milstein
- Air & Space magazine, July 2005
(Page 2 of 5)
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.
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.