A space capsule is an often manned spacecraft which has a simple shape for the main section, without any wings or other features to create lift during atmospheric reentry. Capsules have been used in most of the manned space programs to date, including the world's first Vostok and Mercury manned spacecraft, as well as in later Soviet Voskhod, Soyuz, Zond/L1, L3, TKS, US Gemini, Apollo Command Module, Chinese Shenzhou and US, Russian and Indian manned spacecraft currently being developed. A capsule is the specified form for the Crew Exploration Vehicle.
A manned space capsule must have everything necessary for everyday life, including air, water and food. The space capsule must also protect astronauts from the cold and radiation of space. A capsule must be well insulated and have a system that controls the inside temperature and environment. It also must have a way that the astronauts won't be knocked around during launch or reentry. Additionally, since the inside will be weightless, there must be a way for the astronauts to stay in their seats and beds during the flight. For this each seat, bed, table, and chair has a complicated system of straps and buckles. One of the most important things that a space capsule must have is a way to communicate with people back on Earth, or mission control.
Space capsules have typically been smaller than 5 meters in diameter, although there is no engineering limit to larger sizes. As the capsule is both volumetrically efficient and structurally strong, it is typically possible to construct small capsules of performance comparable in all but lift-to-drag ratio to a lifting body or delta wing form for less cost. This has been especially pronounced in the case of the Soyuz manned spacecraft. Most space capsules have used an ablative heat shield for reentry and been non-reusable. The Crew Exploration Vehicle appear likely, as of December 2005, to be a ten-times reusable capsule with a replaceable ablative shield. There is no limit, save for lack of engineering experience, on using high-temperature ceramic tiles or ultra-high temperature ceramic sheets on space capsules.
Materials for the space capsule are designed in different ways, like the Apollo Command Module’s honey-combed structure of aluminum. Aluminum is very light, and the structure gives the space capsule extra strength. The early space craft had a coating of glass embedded with synthetic resin and put in very high temperatures. Carbon fiber, reinforced plastics and ceramic are new materials that are constantly being made better for use in space exploration.
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 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 of 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.
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.
Gravity, drag, and lift
Drag is the space capsule’s resistance to it being pushed though air. Air is a mixture of different molecules, including nitrogen, oxygen and carbon dioxide. Anything falling through air hits these molecules and therefore slows down. The amount of drag on a capsule depends on many things, including the density of the air, and the shape, mass, diameter and roughness of the capsule. The speed of a space craft highly depends on the combined effect of the two forces — gravity, which can speed up a rocket, and drag, which will slow down the rocket. Space capsules entering Earth’s atmosphere will be considerably slowed because our atmosphere is so thick.
When the space capsule comes through the atmosphere the capsule 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 for this is a shooting star. A shooting star, which is usually tiny, creates so much heat coming through the atmosphere that the air around the meteorite glows white hot. So when a huge object like a space capsule comes through, even more heat is created.
As the space 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 1480 °C (2700 F) as it descends through the Earth’s atmosphere. All this heat has to be directed away. Space 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. This controls allows for reduced g-forces for 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.
Before humans went into space, test orbital and suborbital flights of space capsules were made with monkeys, dogs and mice. These were to see what effects a flight in space would have on a living organism. In 1957, Russia sent into orbit the first animal (dog) in Sputnik 2 but without recovery of space capsule to Earth. This was followed by other animal missions of developed as manned Vostok spacecraft (first successful was Sputnik 5 in 1960), until Russian Cosmonaut Yuri Gagarin made a first manned orbital flight of Earth in 108 minutes on April 12, 1961. The first American to orbit Earth was in February 1962 astronaut John Glenn in the Mercury capsule that previously was tested as unmanned and made manned suborbital missions (first of Alan Shepard on May 1961). Later, the Gemini capsule took two astronauts into Earth orbit for longer periods of time. The Soviet Voskhod capsule was a modified Vostok capsule redesigned to cram two or three cosmonauts into orbit per mission. The Apollo capsule was designed to return three astronauts from the Moon.
Not all space capsule missions have been successful. On January 27, 1967, the Apollo 1 capsule was destroyed by cabin fire during a pre-launch simulation, killing all three astronauts by smoke inhalation. Then on April 24 of that year, the Soyuz 11 capsule lost the air inside of it during descent module separation before reentry. Though it landed properly, the crew of three cosmonauts were dead due to depressurization and asphyxiation.