The term "heat shield" refers primarily to a special structure that protects a re-entry vehicle from the intense heat generated by friction with a planet's atmosphere. Less commonly, it can refer to the insulating material that surrounds the entire spacecraft, protecting the interior from the extremes in temperature encountered during the course of the mission. Heat shields are a vital part of every vehicle designed to return its crew and/or instruments safely to Earth, as the heat of re-entry would easily incinerate a spacecraft without this form of protection.
The cone-shaped capsules of the early U.S. space program had heat shields attached to their base. These shields were designed to vaporize slowly during re-entry. The materials used in the heat shield, as they vaporized, would carry excess heat away from the spacecraft and its crew. For example, the Mercury and Gemini capsules of the early 1960s were protected by heat shields made of silica-fiber resin, while the later Apollo capsules had shields made of phenolic epoxy resin, a form of plastic. Apollo heat shields were nearly 7 centimeters (2.7 inches) thick and weighed 1,360 kilograms (3,000 pounds).
The Space Shuttle
With the development of the space shuttle in the late 1970s came the need for lighter materials that could protect the orbiter on multiple re-entries. The surface of each orbiter is covered by the Thermal Protection System (TPS), an outer layer primarily consisting of more than 24,000 heat-resistant ceramic tiles. These tiles dissipate heat so efficiently that it is safe to touch one by its corners only a few seconds after it is removed from a 1,260°C (2,300°F) oven, the temperature most of the heat shield reaches during re-entry. Most of the orbiter's underside is covered by one of the three types of tile, known as high-temperature reusable surface insulation (HRSI) and distinguished by its black color. These 99.8-percent silica tiles are approximately 15 centimeters (6 inches) square and between 2.5 and 12.5 centimeters (1 and 5 inches) thick. The rest of its underside, primarily the leading edges of the orbiter's nose and wings, reaches temperatures exceeding 1,260°C (2,300°F) during re-entry and must be protected by an all-carbon composite known as reinforced carbon-carbon.
The remainder of the shuttle is covered by low-temperature reusable surface insulation (LRSI) tiles. LRSI tiles have the same basic characteristics as HRSI tiles but are cast thinner (0.2 to 1.4 inches) and in larger, 20 centimeter by 20 centimeter (8 inch by 8 inch), sections. LRSI tiles have a white optical-and moisture-resistant coating made of silica compounds and shiny aluminum oxide, which helps the orbiter control its temperature while in orbit.
Looking to the Future
Despite the advantages of ceramics, the tiles still require heavy maintenance, which adds to the cost of each shuttle flight. Several tiles are shaken loose during each shuttle mission and must be replaced. The National Aeronautics and Space Administration (NASA) is already developing heat shield technology for the next generation of re-entry vehicles. One promising material is a nickel-chromium alloy known as Inconel 617, which was proposed to form the surface panels for the heat shield on the X-33 (an experimental space plane designed to test single-stage-to-orbit technologies; the project was canceled in 2001). Inconel panels for the X-33 were crafted to be highly resistant to corrosion, require only a single waterproofing (unlike shuttle tiles which must be waterproofed frequently), and be more easily removed than ceramic tiles because of a simpler mounting system.
see also Re-entry Vehicles (volume 3); Solar Particle Radiation (volume 2).
Angelo, Joseph J., Jr. The Dictionary of Space Technology, 2nd ed. New York: Facts on File, 1999.
Jenkins, Dennis R. Space Shuttle: The History of the National Space Transportation System: The First 100 Missions, 3rd ed. Cape Canaveral, FL: Author, 2001.
Lee, Wayne. To Rise from Earth. New York: Facts on File, 1995.