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Friction and Space Shuttles

Every August, meteors fill the night sky, racing toward Earth. As the meteors get closer to Earth, they leave a streak of light as they heat up and burn. The Perseid meteor shower, for example, is a spectacular show. The meteor’s streak of light is created by tiny dust particles in space colliding with air particles as they approach Earth. These collisions produce friction between the particles, causing the dust particles to heat until they start to glow. Thermal energy burns up the dust particles before they strike the Earth’s surface. Space shuttles are reusable aircraft that allow human beings to travel in space. They are designed to withstand the special stresses caused by launch, space travel, and reentry into Earth’s atmosphere. When a space shuttle returns from a mission, it reenters the atmosphere about 400,000 feet above Earth’s surface. Its speed is about 25 times the speed of sound. When the shuttle hits the atmosphere, the friction of the air against it slows it down. The friction from reentry makes temperatures outside the shuttle climb to over 2,000° F. If the shuttle were made of materials that go into most aircraft, it would burn up as it reentered Earth’s atmosphere. Fortunately, technology has provided a solution for keeping the shuttle safe when it reenters Earth’s atmosphere. Special ceramic tiles are used to give the shuttle thermal protection. A shuttle uses over 24,000 tiles—each one a unique shape, cut specifically to cover a certain spot on the craft. They are made of a porous silicon material that is very light. In some tiles, 90% of the space inside is empty. A cubic foot of the material can weigh less than ten pounds. There are different types of tiles for various parts of the shuttle. One of them is called a High-Temperature Reusable Surface Insulation (HRSI) tile. These tiles can withstand temperatures as high as 2,300° F. Shuttle technicians place them on the

A space shuttle entering and leaving the Earth’s atmosphere must protect itself from

thermal energy caused by friction.

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Friction and Space Shuttles

bottom of the shuttle and around the front windows, where temperatures are the highest. HRSI tiles are colored black because dark objects radiate more thermal energy. This allows the shuttle to cool down more quickly. Another type of tile is the Low-Temperature Reusable Surface Insulation (LRSI) tile. They are white, larger, and thinner and can tolerate temperatures up to 1,200° F. All tiles are manufactured from pure silica, which comes from refined sand. The silica is formed into fibers, mixed with water and chemicals, and poured into a mold. The extra water is removed. Then, a giant microwave oven heats the fibers to 2,350° F—enough to fuse them together without melting them. For doors and places on the shuttle that do not get very hot, technicians use Felt Reusable Surface Insulation. This is a synthetic material similar to what firefighters use for thermal protection. It is usable in places where the temperature will not reach over 700° F. Space shuttles travel through the atmosphere twice during each mission—at launch and reentry. But astronauts do not need to worry about thermal energy danger during launch. There are two reasons for this. One factor is the speed of the shuttle. Although a shuttle must go fast to break the pull of Earth’s gravity, it is still going slower than it does when it returns to Earth. The slower speed results in less friction. Another factor is the density of the air. A shuttle going into orbit travels through air that is lower in density as the shuttle rises. There are fewer air molecules in low-density air, so that also produces less friction. Upon reentry, these variables are reversed. The shuttle is at its highest speed when it returns to the Earth’s atmosphere. The air with which it collides becomes increasingly dense as it gets nearer to Earth. This is when thermal energy danger is at its greatest.

Discovery Education Science © Discovery Communications, LLC

Friction and Space Shuttles

Friction from reentry presents serious challenges to shuttle designers. But careful engineering and the creative use of exotic materials help keep astronauts safe from harm.