Reentry capsule
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A reentry capsule is the portion of a
Structure
Reentry capsules have typically been smaller than 5 meters (16 feet) in diameter due to
Materials for the capsule are designed in different ways, like the
Reentry
Most reentry capsules have used an
Reentry capsules are well-suited to high energy reentries. Capsules reenter aft-end first with the occupants lying down, as this is the optimum position for the human body to withstand the g-forces induced as the capsule impacts the atmosphere. The rounded shape (blunt body) of a capsule forms a shock wave that keeps most of the heat away from the heat shield, but a
When the reentry capsule comes through the atmosphere the capsule compresses the air in front of it, which heats up to very high temperatures. The surface temperature of a capsule can reach 1,480 °C (2,700 °F) as it descends through the Earth's atmosphere.[
The Apollo command module reentered with the center of mass offset from the center line; this caused the capsule to assume an angled attitude through the air, providing lift that could be used for directional control. Reaction control system thrusters were used to steer the capsule by rotating the lift vector.
Parachutes are used for the final descent, sometimes augmented by braking rockets if the capsule is designed to land on the Earth's surface. Examples of land landing capsules include Vostok, Voskhod, Soyuz, Shenzhou and the
Aerodynamic heating
Capsules are well-suited to high-temperature and dynamic loading reentries. Whereas delta-wing gliders such as the
Engineers building a reentry capsule must take forces such as gravity and drag into consideration. The capsule must be strong enough to slow down quickly, must endure extremely high or low temperatures, and must survive the landing. When the 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 does not 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 reenter aft-end first with the occupants lying down, as this is the optimum position for the human body to withstand the decelerative g-force. The aft end is formed in a rounded shape (blunt body), 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 Command Module reentered with the center of mass offset from the center line; this caused the capsule to assume an angled attitude through the air, providing a sideways lift to be used for directional control. Rotational thrusters were used to steer the capsule under either automatic or manual control by changing the lift vector.
At lower altitudes and speeds parachutes are used to slow the capsule down by making more drag.
Capsules also have to be able to withstand the impact when they reach the Earth's surface. All US crewed capsules (Mercury, Gemini, Apollo) landed on water; the Soviet/Russian Soyuz and Chinese Shenzhou (and planned US, Russian, and Indian) crewed capsules use small
Gravity, drag, and lift
Two of the biggest external forces that a reentry capsule experiences are gravity and drag.
Drag is the capsule's resistance to it moving through
When the capsule comes through the atmosphere, it compresses the air in front of it which heats up to very high temperatures (contrary to popular belief friction is not significant).
A good example of this is a
As the capsule slows down, the compression of the air molecules hitting the capsules surface creates a lot of heat. The surface of a capsule can get to 1,480 °C (2,700 °F) as it descends through the Earth's atmosphere. All this heat has to be directed away. Reentry capsules are typically coated with a material that melts and then vaporizes ("ablation"). It may seem counterproductive, but the vaporization takes heat away from the capsule. This keeps the reentry heat from getting inside the capsule. Capsules see a more intense heating regime than spaceplanes and ceramics such as used on the Space Shuttle are usually less suitable, and all capsules have used ablation.
In practice, capsules do create a significant and useful amount of lift. This lift is used to control the trajectory of the capsule, allowing reduced g-forces on the crew, as well as reducing the peak heat transfer into the capsule. The longer the vehicle spends at high altitude, the thinner the air is and the less heat is conducted. For example, the Apollo CM had a lift to drag ratio of about 0.35. In the absence of any lift the Apollo capsule would have been subjected to about 20g deceleration (8g for low-Earth-orbiting spacecraft), but by using lift the trajectory was kept to around 4g.[citation needed]
Current designs
Shenzhou
The reentry capsule is the "middle" module of the three-part
Few details are known about the Shenzhou reentry capsule, except that it uses some technology from the Soyuz TM design. The new Soyuz TMA spacecraft, now used solely for International Space Station flights, had its couches modified to allow for taller crewmembers to fly, and features "glass cockpit" technology similar to that found on the Space Shuttle and newer commercial and military aircraft.
Soyuz
The former Soviet Union suffered two disasters, and one near-disaster, all three involving the capsule during the de-orbit and reentry.
List of reentry capsules
Flight-proven:
- KH-1 to KH-4 CORONA
- Vostok reentry module
- KH-5 Argon
- McDonnell Mercury capsule
- KH-6 Lanyard
- Zenit (Vostok-derived)
- KH-7 Gambit (Two capsules per launch)
- McDonnell Gemini capsule
- Voskhod reentry modules
- Apollo Command Module
- KH-8 Gambit 3
- Soyuz reentry module
- Luna 16
- KH-9 Hexagon (4 capsules per launch)
- Luna 20
- Yantar
- Fanhui Shi Weixing
- Luna 24
- VA TKS reentry capsule
- Orbital Re-entry Experiment
- Stardust Sample Return Capsule
- Genesis Sample Return Capsule
- Atmospheric Reentry Demonstrator
- Shenzhou reentry module
- Space Capsule Recovery Experiment
- SpaceX Dragon
- Hayabusa Reentry Capsule
- VBK-Raduga
- Orion MPCV
- OSIRIS-REx return capsule
- SpaceX Dragon 2
- Boeing CST-100
- IXV (lifting body)
- Next-generation crewed spacecraft
- Chang'e 5's re-entry module
- Varda W-1
In development:
- SpaceX Starship
- Gaganyaan
- Oryol/Orel
- Chang'e 6's re-entry capsule.
- Martian Moons eXploration (MMX)
- Earth Entry vehicle (Mars sample-return)
- Kavoshgar E
References
- "Soyuz Landing". NASA. – Describes Soyuz-TMA descent.