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The Air Force hopes its unmanned X-37 (in taxi tests in 2007) will take on some of the functions of the shuttle (USAF)

Space Shuttle Jr.

After 2010, the only spaceplane in the U.S. inventory will be the Air Force's mysterious X-37.

Upon reaching orbit, the craft will deploy a solar array that will power batteries. Those batteries have replaced hydrogen fuel cells, the shuttle's power source in orbit. The vehicle will maneuver in space powered by a combination of nitrogen tetroxide and hydrazine. Theoretically, the X-37 could rendezvous with other satellites of interest to the Air Force, friendly or otherwise.

If the X-37 is to carry out such national security missions, its roots will extend back beyond the space shuttle, to earlier spaceplanes. Says Mark Lewis: "I would draw a heritage not only to the shuttle, but to my very favorite program that never was: the X-20."

A follow-on to the X-15 rocketplane, which didn't have the power to get to orbit, the X-20 Dyna-Soar spaceplane, initiated in 1957, would have ridden a massive Titan III booster all the way to orbit if needed, and carried a pilot. (Neil Armstrong was one NASA test pilot selected to fly it, but in 1962 he transferred to the Apollo program.) Dyna-Soar would have given the Air Force a manned system that could have filled a variety of needs: research, reconnaissance, or even attack. It was designed to reach any target in the world in 45 minutes, deliver a weapon, and glide to a friendly base. Its altitude and hypersonic speed would have made it very difficult to intercept.

While this type of capability sounded like something the Air Force needed, the service had difficulty justifying it. NASA was making progress with blunt-body capsules that reentered the atmosphere without the need for pilot control, and intercontinental ballistic missiles were dominating the nuclear delivery mission. A controlled-reentry spaceplane puzzled Secretary of Defense Robert McNamara; he directed the Air Force to study whether concepts such as NASA's Gemini could handle some of the roles better. In December 1963, shortly after prime contractor Boeing started building the vehicle and after about $660 million had been spent, McNamara killed the X-20.

Still, Lewis admires the X-20 from an engineering standpoint: "When we look at Dyna-Soar, we say: Gosh, that's the program we should have had. Imagine if we had a cheaper way to get to the International Space Station."

Unlike Dyna-Soar, the X-37 will always be unmanned. Without humans who would limit the amount of time it could orbit, the little spaceplane might be able to stay aloft for as long as nine months. Operationally, the X-37 could become a space version of a long-endurance unmanned aerial vehicle—the world's first space UAV.

Even with no astronauts, however, safety will be a big issue—in this case, for the payload. After the 2003 disintegration of the shuttle Columbia, mission planners developed concepts that would protect the X-37 from a similar fate: damage from insulation coming loose during launch. One idea had the spaceplane perched vertically atop a booster, an odd configuration that resembled an aerospace awards trophy. Still, the spaceplane would ride safely above any insulation that broke off.

But the aerodynamics posed a problem. "People learned thousands of years ago that you don't fly arrows with the feathers first," says John Muratore, NASA's former chief engineer for the canceled X-38, an emergency return vehicle for ISS crews. "Feathers in the tail are stabilizing and feathers in the front are destabilizing," he says, referring to the wings of an exposed spaceplane perched vertically atop a tall, cylindrical booster.

So the Air Force covered the new little spaceplane with a launch shroud. For now, two big problems have been solved: The rocket should fly right, and when they roll it out to the launch pad, no one will see that the X-37 is inside. Hidden. In plain view.

Michael Klesius is an Air & Space associate editor.

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