Space Capsule - Reentry

Reentry

Space capsules are well-suited to high-temperature and dynamic loading reentries. Whereas delta-wing gliders such as the Space Shuttle can reenter from Low Earth Orbit and lifting bodies are capable of entry from as far away as the Moon, it is rare to find designs for reentry vehicles from Mars that are not capsules. The current RKK Energia design for the Kliper, being capable of flights to Mars, is an exception.

Engineers building a space capsule must take forces such as gravity and drag into consideration. The space capsule must be strong enough to slow down quickly, must endure extremely high or low temperatures, and must survive the landing. When the space capsule comes close to a planet’s or moon’s surface, it has to slow down at a very exact rate. If it slows down too quickly, everything in the capsule will be crushed. If it doesn’t slow down quickly enough, it will crash into the surface and be destroyed. There are additional requirements for atmospheric reentry. If the angle of attack is too shallow, the capsule may skip off the surface of the atmosphere. If the angle of attack is too steep, the deceleration forces may be too high or the heat of reentry may exceed the tolerances of the heat shield.

Capsules are formed in a rounded shape called a blunt body instead of a pointed one, as this forms a shock wave that doesn't touch the capsule, and the heat is deflected away rather than melting the vehicle.

The Apollo capsules were guided through the atmosphere — the center of mass of the capsule was offset from the center line. This angled the capsule's passage through the air, providing a sideways lift. Rotational thrusters were used to change the lift vector, allowing the capsule to be steered under either automatic or manual control.

At lower altitudes and speeds parachutes are used to slow the capsule down by making more drag.

The space capsules also have to be able to withstand the impact when they reach the Earth’s surface. All US manned capsules (Mercury, Gemini, Apollo) would land on water; the Soviet/Russian Soyuz and Chinese Shenzhou (and planned US, Russian, Indian) manned capsules uses small rockets to touch down on land. In the lighter gravity of Mars, airbags were sufficient to land some of the robotic missions safely.