Space capsule
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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 Mercury and Gemini programs, as well as in Apollo and Soyuz. A capsule is the specified form for the Crew Exploration Vehicle.
Manned space capsules must have everything necessary for every day life, including air, water and food. The space capsules must also protect the astronauts from the cold and radiation of space. For this the capsules are well insulated and have a system that controls the inside temperature and environment. They 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.
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[edit] Structure
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’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 imbedded 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.
[edit] 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 from 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, will endure extremely high or low temperatures, and can 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, every thing in the capsule will be crushed. If it doesn’t slow down quickly enough, it will crash into the surface and everything will be destroyed.
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 centre line, this angled the capsule's passage through the air providing some useful lift, and the astronauts and control system could steer the capsule by rotating it using thrusters. If they wished the capsule to go in a straight line the capsule would spin and the lift would essentially cancel out.
At low altitude 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. The Apollo and Gemini capsules would land on water; the Russian Soyuz capsule uses small rockets to touch down on land.
[edit] Gravity and drag
Two of the biggest external forces that a space capsule experiences are gravity and drag.
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 much more 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 goes down through the Earth’s atmosphere. All this heat has to be directed away. Space capsules are typically coated with a material that melts then vaporized ("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.
[edit] History
Early space capsules were based on the designs of the late Maxime Faget and many Russian engineers working under Sergei Korolev.
Before humans went into space, test flights were made with monkeys, dogs and mice. These were to see what effects a flight in a space capsule would have on a living organism. In 1957, Russia sent the first dog into space. This was followed by other animal missions, until Russian Cosmonaut Yuri Gagarin made a successful orbit of Earth in 108 minutes on April 12 1961. The first American to orbit Earth was Astronaut John Glenn in the Mercury capsule. Later, the Gemini capsule took astronauts into space for longer periods of time. The Apollo capsule took astronauts to the moon, and the Lunar Module took them to the surface. The Russian Soyuz has taken many cosmonauts into orbit.
Not all space capsule missions have been successful. Many people have lost their lives in space explorations. One early Soyuz capsule depressurized upon reentry and three cosmonauts died.
[edit] Medical issues
[edit] Weightlessness
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When in space for a long duration there are many medical issues that are run into. One of these things is loss of bone mass.[citation needed] Six months of being weightless in a capsule would greatly reduce the bone mass of the occupants. The bone mass loss would be so great, that if the capsule was traveling to Mars, the space travellers would collapse like a bag of bones upon arrival.[citation needed]
In a weightless environment, astronauts put almost no weight on the back muscles or leg muscles used for standing up. Those muscles then start to weaken and eventually get smaller. If there is an emergency at landing, the loss of muscles, and consequently the loss of strength can be a serious problem. Sometimes, astronauts can lose up to 25% of their muscle mass on long term flights. When they get back to ground, they will be considerably weakened and will be out of action for a while.
Astronauts experiencing weightlessness will often lose their orientation, get motion sickness, and lose their sense of direction as their bodies try to get used to a weightless environment. When they get back to Earth, or any other mass with gravity, they have to readjust to the gravity and may have problems standing up, focusing their gaze, walking and turning. Importantly, those body motor disturbances after changing from different gravities only get worse the longer the exposure to little gravity. These changes will affect operational activities including approach and landing, docking, remote manipulation, and emergencies that happen by landing. This is a big problem for mission success.
[edit] Weakened immune system
Another thing is that extended space flight might slow down the body’s ability to protect itself against diseases.[citation needed] Some of the problems are a weakened immune system and the activation of dormant viruses in the body. Radiation can cause both short and long term consequences to the blood marrow stem cells which create the blood and immune systems. Because a space capsule is so small, a weakened immune system and more active viruses in the body can lead to a fast spread of infection.
[edit] Isolation
When on long missions, astronauts will have to go through the isolation and confinement of a space environment. People isolated for a long time can go into all sorts of kinds of depression that can ruin the mission’s success. Not only do astronauts have to be almost totally isolated from the rest of the world, but they have virtually nowhere to move around. That can also cause some depression.
When on long missions, astronauts will not be able to quickly return to Earth if a medical emergency occurs. For example, a scientist working in the south pole found a lump in her breast and had to wait a two months before a helicopter could come in. In space, even that is not an option. When a medical emergency happens, the astronauts have to rely on the crew and the computers to solve the problem.
[edit] References
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This article needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (June 2006) |
- U.S. Patent 3,093,346 -- Space capsule -- M. A. Faget, et al. (NASA)
