How Thermal Covers Control Space Shuttle Skin Temperature

A SPACE shuttle wouldn't be a space shuttle without the 2,400 silica tiles and the 2,300 fiberglass blankets that insulate it from temperature extremes. What turns the shuttle's airplane-like orbiter into a reusable space truck is its thermal protection system. Throughout the orbiter's 100-mission life, the thermal protection system keeps temperatures on the spacecraft's aluminum skin within design limits - 350 degrees Fahrenheit.

Without the tiles and blankets, the orbiter's fuselage would contract or expand as it encountered temperatures as cold as minus 250 degrees in space or as hot as 3,000 degrees as it returned to earth. Parts of the surface can heat up past the melting point of steel because of friction the orbiter encounters 20 minutes before it lands.

Nearly three-fourths of each orbiter has a heat-resistant covering of tiles and blankets to shield against the searing temperatures. Reinforced carbon-carbon, the gray material on the orbiter's nose cap and leading edges, protects against the hottest temperatures. Black tiles are placed where temperatures can reach 2,300 degrees - on the orbiter's belly, around its forward windows, and along the edges of its tail and flaps. White tiles cover the orbiter's steering jets and areas above the windows, and protect against temperatures up to 1,200 degrees.

The blankets not only cover large areas of the fuselage, but also fill gaps between the white and black silica tiles. The slightest gap can increase the friction and direct hot plasma gas like a blowtorch to melt the skin beneath the thermal barrier.

A tile's ability to shield the shuttle from heat depends on its purity. Tiles are made from 99.7 percent pure silica that has been refined from common sand. The silica fibers are mixed with de-ionized water and other chemicals to make a slurry that is molded into blocks. The blocks are dried in the nation's largest microwave oven at the Sunnyvale, Calif., plant at Lockheed Space Operations Company. Then they are sintered in a 2,350-degree oven, cut to shape, and coated with a reflective, glasslike material.

The tiles work so efficiently that bare hands can hold a piece of the material by the edges only seconds after it is removed from a hot oven - even while the center is still glowing red.

No two tiles are alike, because each one is matched to the contour of the orbiter at the exact spot where it will be bonded to the fuselage. As a result, fitting the tiles together is as difficult as assembling a jigsaw puzzle twice the size of a basketball court. They are an average of 6 inches square. A single coated tile can cost as much as $2,000.

The blankets are more durable than the tiles, and they cost less to make and install.