Debrief: Hyper-X

Scramjet power? Simple: Keep a match lit in a 7,000-mph wind.

Flying doorstop: The wedge shape of the X-43 compresses air entering the engine. This computational fluid dynamics image shows the vehicle's pressure gradients at Mach 7. (NASA Dryden)
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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.

Instead, it became the X-43A. The razor-like X-43A is the spitting image of the National Aerospace Plane, but less than a tenth the original size. A 1995 NASA competition made it next in the line of experimental X-planes. The details of its engine chamber remain highly guarded by the military—so much so that while it’s on the ground, padlocked covers on both ends of the chamber shroud it from anyone without clearance. Its scaled-down size was matched to a mission that was narrowed to focus on one basic goal: Prove a scramjet can power an airplane, and do it quickly and cheaply.

That meant keeping it simple. The X-30 was to have a human crew; the X-43A would be automated. It would fly only as long as it took to burn the hydrogen fuel it could carry. Engineers thought about trying to land it on the Navy’s San Nicolas Island, off the coast of California, but gritted their teeth and decided it would be simpler for it to transmit data and ditch in the ocean.

The SR-71 lifts off with a turbojet before accelerating to ramjet mode. The National Aerospace Plane was also intended to shift gears in flight. The X-43A had to rely on a Pegasus rocket to get up to speed.

That raised a question: How do you push the 3,000-pound X-43 off a rocket going thousands of miles an hour? Earlier scramjet experiments in Australia and Russia had attached scramjets to rockets to prove the engine would function at high speeds. They did not try to power an airplane with it. The X-43A had to leave the rocket and fly on its own. Finally engineers found a technology to push the two apart fast enough so that they would not collide: the piston system that B-1 bombers use to eject bombs.

The X-43A was packed to the gills with instruments, fuel lines, and controls. Its budget did not allow for miniaturizing, and its size wouldn’t allow for backup systems. Technicians shaved its battery to the hundredths of an inch. “There was no room to drop a marble inside,” says Castrogiovanni.

Everything was enclosed within a body that would endure the highest temperatures and pressures an airplane had ever faced. Stainless steel and carbon fibers formed the wings’ leading edges, sharpened like razor blades. The aircraft’s nose was made from an enormous piece of tungsten to take the heat and balance the heavy back end of the aircraft. Its engine chamber was crafted from copper and cooled by water lines to control temperatures not much lower than those on the surface of the sun. Its fuselage was covered with the same sort of thermal tiles that cover the space shuttle. But instead of crafting each tile to precise dimensions, as NASA does for the shuttle, X-43A designers kept costs down by first cementing the tiles in place and then machining them to a smooth surface.

About Michael Milstein

Michael Milstein is a freelance writer who specializes in science. He lives in Portland, Oregon.

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