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[i]NASA used two categories of Thermal Protection System tiles on the Orbiter — low and high temperature tiles. The surface coating was the primary difference between these two categories. High-temperature tiles used a black borosilicate glass coating and covered areas of the vehicle in which temperatures reached up to 2,300F. Low-temperature tiles contained a white coating with the proper optical properties needed in areas of the vehicle where temperatures reached up to 1,200F. ...the HRSI tiles are made of a low-density, high-purity silica 99.8-percent amorphous fiber (fibers derived from common sand, 1 to 2 mils thick) insulation that is made rigid by ceramic bonding. ...there are two different densities of HRSI tiles. The first weighs 22 pounds per cubic foot and is used in all areas around the nose and main landing gears, nose cap interface, wing leading edge, RCC/HRSI interface, external tank/orbiter umbilical doors, vent doors and vertical stabilizer leading edge. The remaining areas use tiles that weigh 9 pounds per cubic foot. ...the LRSI tiles are of the same construction and have the same basic functions as the 99.8% pure silica HRSI tiles, but they are thinner (0.2 to 1.4 inches) than HRSI tiles. The thickness of the tile is determined by the amount of heat the tile encounters. The LRSI tiles are manufactured the same way as the 99.8% pure silica HRSI tiles, except the tiles are coated to be optically and water resistant (the coating is about .010 in. thick). The coating is made of silica compounds with shiny aluminum oxide to enhance and obtain the best optical properties possible.[/i]
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