A re-entry vehicle is the part of a spacecraft that is designed to return through Earth's atmosphere. It is built to survive intense heating during high-velocity flight through the atmosphere and to protect the crew and/or instruments until it brings them safely to Earth. Although the technology has changed over time, re-entry vehicles since the early Mercury program have used the same basic design concept: a blunt shape protected by a heat shield.
Early Re-entry Vehicles
Early re-entry vehicle design benefited primarily from ballistic missile research. Designers initially thought that a re-entry vehicle should have a sleek aerodynamic shape, but launch and wind tunnel tests demonstrated that no known material with that shape could withstand the heat of re-entry. National Aeronautics and Space Administration (NASA) engineer Harvey Allen decided that a blunt-shaped vehicle should be used. The increased air resistance of that type of vehicle would, like the bow of a ship in the water, produce a "shock wave" that would absorb much of the vehicle's kinetic energy that was transformed into heat as it entered the atmosphere. Blunt re-entry vehicles were used successfully as intercontinental ballistic missile (ICBM) warheads and later as piloted and unpiloted spacecraft.
The blunt-body concept furnished only part of the solution to the heating problem; a form of heat shield was also necessary. Extensive testing in arc jet heated wind tunnels showed that the most effective thermal protection method for single re-entry vehicles was ablation . An ablative heat shield is made of a resinous composite material that slowly vaporizes during descent, allowing the heat to dissipate along with the ashes. Ablative heat shields were used on all early NASA missions.
Lifting Body Research
Although NASA used ballistic capsules for its earliest re-entry vehicles, another vehicle type had been proposed—the lifting body—a shape that combined the blunt-body concept with the aerodynamics of a glider. Designers continued to do research on the shape of the lifting body. Between 1963 and 1975 NASA built and tested eight different lifting body designs. These craft varied tremendously. The M2-F1, for example, was an unpowered plywood glider, whereas the X-24 was a rocket-powered metal aircraft capable of supersonic flight. Data on aerodynamic performance during re-entry obtained from lifting bodies was crucial in the design of the space shuttle orbiter and the X-38.
Modern Re-entry Vehicles
The successful launch of the space shuttle in 1981 provided a significant demonstration of several new technologies, one of which was its thermal protection system. Because the shuttle was designed for repeated reentry, an ablative heat shield was not an option. The thermal blankets and the silica and reinforced carbon-carbon tiles that make up the shuttle's heat shield were tested extensively on the ground before the shuttle's first launch.
The X-38 resembles the lifting bodies of the 1960s and 1970s more than it does the shuttle. The X-38 could provide an emergency lifeboat for the crew of the International Space Station. Its heat shield will employ the same kind of tiles and blankets used on the shuttle, but they will be easier to attach and maintain because of the advanced composite materials that form the X-38's hull.
see also Heat Shields (volume 3); Hypersonic Programs (volume 3); Launch Vehicles, Reusable (volume 1).
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"Re-entry Vehicles." Space Sciences. . Encyclopedia.com. (June 20, 2019). https://www.encyclopedia.com/science/news-wires-white-papers-and-books/re-entry-vehicles
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