Space Shuttle Jr.
After 2010, the only spaceplane in the U.S. inventory will be the Air Force's mysterious X-37.
- By Michael Klesius
- Air & Space magazine, January 2010
(Page 3 of 4)
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.
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.