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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.

The U.S. program started out relatively open to view, a research effort jointly shaped by the Air Force, NASA, the Defense Advanced Research Projects Agency, and Boeing. The Air Force ordered the first prototype, the X-40A, from Boeing in 1996. When it came time to produce the next iteration, the X-37A drop-test vehicle, NASA had the company increase the size by about 20 percent.

But since then, the X-37 has taken a winding and perplexing path among NASA, DARPA, and the Air Force. From 2004 to 2006, DARPA oversaw it. Along the way, both the X-40A and the X-37A have been drop-tested (first over New Mexico in 1998 and California in 2006, respectively), which proved their automated approach and landing abilities. Finally the program was taken over by the Air Force. Today, call up any of these organizations and say "X-37" and it's like spraying a garden hose at housecats.

"I tried to get to the bottom of this program three or four years ago and could not," says John Pike, the director of "All this ‘orbital spaceplane' stuff, ‘space maneuver vehicle,' ‘orbital test vehicle,' ‘X-37,' ‘X-40,' et cetera—six different names for one and a half real programs."

The press-shy Rapid Capabilities Office, established in 2003, is charged with getting special combat support and weapon systems developed and fielded as fast as possible. The office answers directly to the Air Force Chief of Staff, the Secretary of the Air Force, and two high-level procurement officials. Given that short chain of command, it's not unreasonable to imagine that the X-37 could carry classified military payloads like those deployed in 11 shuttle flights made between 1982 and 1992.

We know more about the X-37 itself than about its possible payloads. "Its resemblance to the shuttle is pretty straightforward," says Mark Lewis. Like its shuttle forebear, it has a "cold structure" design: It is made of metals that would melt at reentry and thus need to be shrouded. But while the shuttle's structure is made of conventional aluminum, the X-37's uses lighter composite materials, explained Secretary of the Air Force public affairs officer David Small last March, and "advanced, higher-temperature, more durable thermal protection materials are used to protect the structure during re­entry."

Those materials include silica tiles impregnated with the latest version of Toughened Uni-Piece Fibrous Insulation (TUFI), some of which have flown on the shuttle since the 1994 mission STS-59. The tiles will provide most of the thermal protection for the X-37's underside, and are more durable than earlier shuttle tiles, which have been pocked by debris as light as paint chips. In a TUFI tile, the surface material permeates the underlying insulation, which supports and reinforces the outer surface and renders it more resistant to impacts. In contrast to the shuttle's older, more rigid glass-fiber composite tiles, TUFI tiles have a porous nature that stops cracks from spreading.

The X-37 will also demonstrate a new-generation Conformal Reusable Insulation blanket technology, which provides better protection for top surfaces, along with a hard, smooth finish that produces less drag than the shuttle's 1970s-era thermal blankets.

The X-37's most notable thermal advance is on the wing leading edge. On the shuttle, that vulnerable area was covered with reinforced carbon-carbon; the X-37 uses a different material, called TUFROC, for Toughened Uni-piece Fibrous Reinforced Oxidation-Resistant Composite. TUFROC (pronounced "tough rock") was developed at NASA's Ames Research Center in California by a group led by David Stewart, who has worked on thermal protection systems since the shuttle program.

Stewart explains that during reentry, heat is generated not just by friction of the vehicle against the atmosphere, but also by atoms on the surface recombining. In the shuttle's case, the carbon-carbon oxidizes. As the name implies, the new material resists oxidative damage. The surface of the shuttle's tiles heats up very fast because the insulator's high-density coating is very thin. TUFROC's surface material is thicker, and therefore takes longer to heat up. And the new material will reduce weight, which will enable the spaceplane to carry more payload.

The X-37 embodies other modifications of shuttle technology. All shuttle-era hydraulics have been eliminated; the new spaceplane's flight controls will be operated electromechanically, making the X-37 fly-by-wire. Unlike the shuttle, with its one vertical stabilizer, the X-37 has two short diagonal ones, called ruddervators—surfaces that combine the functions of rudders and elevators. These reduce the amount of propellant needed to handle trim and control during the high-speed, high-angle-of-attack reentry, and provide room for a centerline speed brake that manages the vehicle's glide energy just before landing.

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