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Photo of the Space Shuttle
 with some of its characteristics

As an aircraft moves through the air, the air molecules near the aircraft are disturbed and move around the aircraft. Exactly how the air re-acts to the aircraft depends upon the ratio of the speed of the aircraft to the speed of sound through the air. Because of the importance of this speed ratio, aerodynamicists have designated it with a special parameter called the Mach number in honor of Ernst Mach, a late 19th century physicist who studied gas dynamics.

As a spacecraft re-enters the earth's atmosphere, it is traveling very much faster than the speed of sound. The aircraft is said to be hypersonic. Typical low earth orbit re-entry speeds are near 17,500 mph and the Mach number M is nearly twenty five, M < 25. The chief characteristic of re-entry aerodynamics is that the temperature of the flow is so great that the chemical bonds of the diatomic molecules of the air are broken. The molecules break apart producing an electrically charged plasma around the aircraft. The air density is very low because re-entry occurs many miles above the earth's surface. Strong shock waves are generated on the lower surface of the spacecraft.

The only manned aircraft to currently fly in this regime are the American Space Shuttle, the Russian Soyuz spacecraft, and the Chinese Shenzhou spacecraft. The figure shows the Shuttle after it has passed through the re-entry regime. The Shuttle uses a rocket propulsion system to get into orbit, but during re-entry the aircraft is actually an un-powered glider. Small steering rockets are used for maneuvering early in the re-entry because the low density of the air at altitudes above 50 miles makes aerodynamic surfaces ineffective. The heat is so great during re-entry that a special thermal protection system is used to keep the spacecraft intact. On the Shuttle, special silicon tiles are placed on the aluminum skin to insulate the skin. On the leading edge of the wings, carbon-cabon composite material is used to withstand the heat. The high forces and high heat dictate that the Shuttle has short, blunt wings. The Shuttle flies at a high angle of attack during re-entry to generate drag to dissipate speed. It executes hypersonic "S-turn" maneuvers to kill off speed during re-entry. The lift of the wings is only important in the final flare maneuver at touchdown.

The Soyuz, Shenzhou, and all of the early Apollo, Gemini, and Mercury spacecraft used a thermal protection system that is different than the Space Shuttle. Each of these spacecraft use an ablative, or "burning", heat shield. The heat shield is made of special ceramic materials and is designed to slowly burn away as it encounters the high temperature plasma flow aft of the bow shock wave. The change of phase from solid to liquid to gas and the convection of the flow away from the spacecraft help to protect the astronauts from the heat of re-entry.


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Editor: Tom Benson
NASA Official: Tom Benson
Last Updated: May 07 2021

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