Space Shuttle

Some of the most difficult design problems faced by shuttle engineers were those involving the reentry process. When the spacecraft has completed its mission in space and is ready to leave orbit, its OMS fires just long enough to slow the shuttle by 200 MPH (320 km/h). This modest change in speed is enough to cause the shuttle to drop out of its orbit and begin its descent to Earth.

When the shuttle reaches the upper atmosphere, significant amounts of atmospheric gases are first encountered. Friction between the shuttle—now traveling at 17,500 MPH (28,000 km/h)—and air molecules causes the spacecraft's outer surface to heat. Eventually, portions of the shuttle's surface reach 3,000°F (1,650°C).

Most materials normally used in aircraft construction would melt or vaporize at these temperatures. It was necessary, therefore, to find a way of protecting the shuttle's interior from this searing heat. NASA decided to use a variety of insulating materials on the shuttle's outer skin. Parts less severely heated during reentry are covered with 2,300 flexible quilts of a silica-glass composite. The more sensitive belly of the shuttle is covered with 25,000 porous insulating tiles, each approximately 6 in (15 cm) square and 5 in (12 cm) thick, made of a silica-borosilicate glass composite.

The portions of the shuttle most severely stressed by heat—the nose and the leading edges of the wings—are coated with an even more resistant material termed carbon-carbon. Carbon-carbon is made by attaching a carbon-fiber cloth to the body of the shuttle and then baking it to convert it to a pure carbon substance. The carbon-carbon is then coated to prevent oxidation (combustion) of the material during descent.