Look closely at a space shuttle, and you might notice that much of its surface is made up of thousands of black or white rectangular blocks. These blocks, or tiles—more than 30,000 in all—performed an essential safety function when the shuttle program was active. They were part of the shuttle’s thermal protection system (TPS). The TPS consisted of seven different materials. Each material, which was fabricated from simpler materials, provided a different measure of heat protection according to where it was positioned on the spacecraft.
Like many commercial aircraft, space shuttles were constructed of aluminum. While strong and relatively light, an aluminum structure is prone to expand as temperature rises and may fail if temperatures reach 350°F (177°C) or more. During a space shuttle flight, the surface of the shuttle was exposed to temperatures well above that—especially during reentry into Earth’s atmosphere, when a speed of 17,000 miles per hour (27,359 kilometers per hour) generates lots of heat.
As heat increases through a material, it moves from warmer to cooler areas. Materials that are able to slow the flow of heat are called thermal insulators. The insulating tiles on the shuttle acted in this way to shield its aluminum structure from outside heat. Once fully heated, as by a kiln, the tile material also takes a long time to cool to room temperature. The edges and especially the corners cool faster than the sides and center area because they share more surfaces with the air. They are protected from the center heat by the insulating material. As shown in the video, a person could hold a tile by the corners just seconds after it left a high-temperature kiln.
Made from very pure quartz sand, or silica, these tiles—called high-temperature, reusable surface insulation (HRSI) tiles—were installed along most of the fuselage. That’s the bottom part of the spacecraft that bears the most heat during reentry. HRSI tiles could withstand the transition from the extremely low temperatures of space (-518°F/-270°C) to the extremely high temperatures of reentry. While the leading edge of the wings and the nose of the shuttle may have reached well over 2500°F (1,371°C), the fuselage heated up far less: the outside of the black tiles almost never exceeded 1700°F (927°C). As long as these tiles had been properly made and securely mounted, the shuttle’s aluminum structure would rarely exceed 160°F (71°C).
Because the aluminum structure of the shuttle expanded and contracted with temperature changes, HRSI tiles had to be indirectly adhered to the shuttle to prevent cracking. They were glued to a felt pad before both tile and pad were bonded together to the structure. Despite this precaution, shuttles occasionally lost tiles. Liftoff is a particularly dangerous moment during any space mission, as the sound waves and energy waves from the engines can actually shake tiles like these loose. In 2003, the shuttle Columbia lost control and broke up during reentry. An accident investigation revealed that during liftoff, pieces of the foam insulation that surround the large external fuel tank fell off and damaged the heat protection tiles on the left wing. When Columbia reentered the atmosphere, hot gases penetrated the damaged area and melted the airframe.