artificial gravity

ARTIFICIAL GRAVITY IN SPACE CRAFT

Artificial gravity For artificial gravity in fiction, see Artificial gravity (fiction).This article includes a list of references, but its sources remain unclear because it has insufficient inline citations. Artificial gravity is the emulation in outer space or free-fall of the effects of gravity felt on a planetary surface.[citation surface.[citation needed] It can be achieved by designing for the use of some different force, such as the centrifugal force that is felt within a rotating space habitat. Artificial gravity is desirable for long-term space travel or habitation, for ease of mobility, for in-space fluid management, and to avoid the adverse health effects of weightlessness.

Requirement for gravity Without g-force, space adaptation syndrome occurs in some humans and animals. Many adaptations occur over a few days, but over a long period of time bone density decreases, and some of this decrease may be permanent. The minimum gforce required to avoid bone loss is not known—all current experience is with gforces of 1g (on the surface of the Earth) or 0g in orbit. There has been insufficient insufficient time spent on the moon to determine whether lunar gravity is sufficient.A similar effect occurs with people who are bedridden or wheelchair-bound.A wheelchair-bound.A limited amount

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of experimentation has been done with chickens experiencing high g-force over long periods in centrifuges on the Earth.

Methods Artificial gravity could be created in numerous ways: Rotation Artist's conception of the interior of a Stanford torus with a diameter of 1.8 kilometers (1.1 miles) and that revolves at 1 rpm to produce 1 g.A rotating spacecraft will produce the feeling of gravity on its inside hull. The rotation drives any object inside the spacecraft toward the hull, thereby giving the appearance of a gravitational pull directed outward. Often referred to as a centrifugal force, the "pull" is actually a manifestation of the objects inside the spacecraft attempting to travel in a straight line due to inertia. The spacecraft's hull provides the centripetal force required for the objects to travel in a circle (if they continued in a straight line, they would leave the spacecraft's confines). Thus, the gravity felt by the objects is simply the reaction force of the object on the hull reacting to the centripetal force of the hull on the object, in accordance with Newton's Third Law.From the point of view of people rotating with the habitat, artificial gravity by rotation behaves in some ways similarly to normal gravity but has the following effects:

Centrifugal force: Unlike real gravity which pulls towards a center, this pseudo-force that appears in rotating reference frames gives a rotational 'gravity' that pushes away from the axis of rotation. Artificial gravity levels vary proportionately with the distance from the centre of rotation. With a small radius of rotation, the amount of gravity felt at one's head would be significantly different from the amount felt at one's feet. This could make movement and changing body position awkward. In accordance with the physics involved, slower rotations or larger rotational radii would reduce or eliminate this problem.

The Coriolis effect gives an apparent force that acts on objects that move. This force tends to curve the motion in the opposite sense to the habitat's spin. Effects produced by the Coriolis effect act on the inner ear and can cause dizziness, nausea and disorientation. Experiments have shown that slower rates of rotation reduce the Coriolis forces and its effects. It is generally believed that at 2 rpm or less no adverse effects from the Coriolis forces will occur, at higher rates some people can become accustomed to it and some do not, but at rates above 7 rpm few if any can become accustomed.[citation needed] It is not yet known if very M.PRAKASH B.E EEE

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long exposures to high levels of Coriolis forces can increase the likelihood of becoming accustomed. The nausea-inducing effects of Coriolis forces can also be mitigated by restraining movement of the head.

This form of artificial gravity gives additional system issues: Kinetic energy: Spinning up parts or all of the habitat requires energy. This would require a propulsion system and propellant of some kind to spin up (or spin down) or a motor and counterweight of some kind (possibly in the form of another living area) to spin in the opposite direction.Extra strength is needed in the structure to avoid it flying apart due to the rotation. However, the amount of structure needed over and above that to hold a breathable atmosphere (10 tonnes force per square metre at 1 atmosphere) is relatively modest for most structures.If parts of the structure are intentionally not spinning, friction and similar torques will cause the rates of spin to converge (as well as causing the otherwise-stationary parts to spin), requiring motors and power to be used to compensate for the losses due to friction.Angular inertia can complicate spacecraft propulsion and altitude control particularly when no counterweight is employed.

Where: 

g = Decimal fraction of Earth gravity

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R = Radius from center of rotation in meters

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3.14159

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rpm = revolutions per minute

The size and speeds and period of different radii of space stationThe engineering challenges of creating a rotating spacecraft are comparatively modest to any other proposed approach. Theoretical spacecraft designs using artificial gravity have a great number of variants with intrinsic problems and advantages. To reduce Coriolis forces to livable levels, a rate of spin of 2 rpm or less would be needed. To produce 1g, the radius of rotation would have to be 224 m (735 ft) or greater, which would make for a very large spaceship. To reduce mass, the support along the diameter could consist of nothing but a cable connecting two sections of the spaceship, possibly a habitat module and a counterweight consisting of every other part of the spacecraft. It is not yet known if exposure to high gravity for short periods of time is as beneficial to health as continuous exposure to normal gravity. It is also not known how effective low levels of gravity would be to countering the adverse effects on health of weightlessness. Artificial gravity at 0.1g would require a radius M.PRAKASH B.E EEE

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of only 22 m (74 ft). Likewise, at a radius of 10 m, about 10 rpm would be required to produce Earth gravity (at the hips; gravity would be 11% higher at the feet), or 14 rpm to produce 2g. If brief exposure to high gravity can negate the health effects of weightlessness, then a small centrifuge could be used as an exercise area. The Gemini 11 mission attempted to produce artificial gravity by rotating the capsule around the Agena Target Vehicle which it was attached to by a 36-meter tether. They were able to generate a small amount of artificial gravity, about 0.00015 g, by firing their side thrusters to slowly rotate the combined craft like a slow-motion pair of bolas.[2] The resultant force was too small to be felt by either astronaut, but objects were observed moving towards the "floor" of the capsule. The Mars Gravity Biosatellite was a proposed mission meant to study the effect of artificial gravity on mammals. An artificial gravity field of 0.38g (Mars gravity) was to be produced by rotation (32 rpm, radius of ca. 30 cm). Fifteen mice would have orbited Earth (Low Earth orbit) for five weeks and then land alive. However the program was canceled on June 24, 2009 due to lack of funding and shifting priorities at NASA.

Linear acceleration Linear acceleration, even at a low level, can provide sufficient g-force to provide useful benefits. Any spacecraft could, in theory, continuously accelerate in a straight line, forcing objects inside the spacecraft in the opposite direction of the direction of acceleration.Most chemical reaction rockets already accelerate at a sufficient rate to produce several times Earth's g-force but can only maintain these accelerations for several minutes because of a limited supply of fuel. A propulsion system with a very high specific impulse (that is, good efficiency in the use of reaction mass that must be carried along and used for propulsion on the journey) could accelerate more slowly producing useful levels of artificial gravity for long periods of time. A variety of electric propulsion systems provide examples. Two examples of this long-duration, low-thrust, high-impulse propulsion that have either been practically used on spacecraft or are planned in for near-term in-space use are Hall effect thrusters and Variable specific impulse magnetoplasma rockets (VASIMR). Both provide very high specific impulse but relatively low thrust, compared to the more typical chemical reaction rockets. They are thus ideally suited for long-duration firings which would provide limited amounts of, but longterm, milligee levels of artificial gravity in spacecraft.Low-impulse but long-term linear acceleration has been proposed for various interplanetary missions. For example, even heavy (100 tonne) cargo payloads to Mars could be transported to Mars in 27 months and retain approximately 55 percent of the LEO vehicle mass upon arrival into a Mars orbit, providing a low-gravity gradient to the spacecraft during the entire journey.

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Constant linear acceleration could theoretically provide relatively short flight times around the solar system. If a propulsion technique able to support 1g of acceleration continuously were available, a spaceship accelerating (and then decelerating for the second half of the journey) at 1g would reach Mars in 2–5 days, depending on the starting point in the synodic period.[citation needed] In a number of science fiction plots, acceleration is used to produce artificial gravity for interstellar spacecraft, propelled by as yet theoretical or hypothetical means.This effect of linear acceleration is very well understood,[citation needed] and is routinely used for 0g cryogenic fluid management for post-launch (subsequent) inspace firings of upper stage rockets

Mass Another way artificial gravity may be achieved is by installing an ultra-high density mass in a spacecraft so that it would generate its own gravitational field and pull everything inside towards it. Technically this is not artificial gravity—it is natural gravity, gravity in its original sense. An extremely large amount of mass would be needed to produce even a tiny amount of noticeable gravity. A large asteroid could exert several thousandths of a g and, by attaching a propulsion system of some kind, would qualify as a space ship, though gravity at such a low level might not have any practical value. In addition, the mass would obviously need to move with the spacecraft; if the spacecraft is to be accelerated significantly, this would greatly increase fuel consumption. Because gravitational force is proportional to the square of the distance from the center of mass, it would be possible to have significant levels of gravity with much less mass than such an asteroid if this mass could be made much denser than current materials (see neutronium). In principle, small charged black holes could be used and held in position with electromagnetic forces. However, carrying a sufficient quantity of mass to form significant gravity fields in a spacecraft is well beyond current technology.

Tidal forces In a planetary orbit, artificial gravity can be obtained from the tidal force by two spacecraft above each other (or one spacecraft and another mass) connected by a tether.[citation needed] See also tidal stabilization. Examples of rotating tethered approaches to the production of artificial gravity, for human and animal research, are discussed here.

Magnetism A similar effect to gravity has been created through diamagnetism. It requires magnets with extremely powerful magnetic fields. Such devices have been made that were able to levitate at most a small mouse and thus produced a 1 g field to M.PRAKASH B.E EEE

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cancel the Earth's; yet it required a magnet and system that weighed thousands of kilograms, was kept superconductive with expensive cryogenics, and required 6 megawatts of power. Such extremely strong magnetic fields are far above the permitted levels[specify], and safety for use with humans is at best unclear. In addition, it would involve avoiding any ferromagnetic or paramagnetic materials near the strong magnetic field required for diamagnetism to be evident. Some other disadvantages of using magnetism on a spaceship are found here: However, facilities using diamagnetism may prove excellent laboratories for simulating low gravity conditions here on Earth. The mouse was levitated against Earth's gravity, simulating a condition similar to microgravity. Lower forces may also be generated to simulate a condition similar to lunar or Martian gravity with small model organisms.

Gravity generator/gravitomagnetism In science fiction, artificial gravity (or cancellation of gravity) or "paragravity" is sometimes present in spacecraft that are neither rotating nor accelerating. At present, there is no confirmed technique that can simulate gravity other than actual mass or acceleration. There have been many claims over the years of such a device. Eugene Podkletnov, a Russian engineer, has claimed since the early 1990s to have made such a device consisting of a spinning superconductor producing a powerful gravitomagnetic field, but there has been no verification or even negative results from third parties. In 2006, a research group funded by ESA claimed to have created a similar device that demonstrated positive results for the production of gravitomagnetism, although it produced only 100 millionths of a g.

Current proposals A number of current proposals for producing artificial gravity are under consideration.Multi-Mission Space Exploration Vehicle (MMSEV): this 2011 NASA proposal for a long-duration crewed space transport vehicle includes a rotational artificial gravity space habitat intended to promote crew-health for a crew of up to six persons on missions of up to two years duration. The partial-g torus-ring centrifuge would utilize both standard metal-frame and inflatable spacecraft structures and would provide 0.11 to 0.69g if built with the 40 feet (12 m) diameter option.] ISS Centrifuge Demo: Also proposed in 2011 as a demonstration project preparatory to the final design of the larger torus centrifuge space habitat for the Multi-Mission Space Exploration Vehicle. The structure would have an outside diameter of 30 feet (9.1 m) with a 30 inches (760 mm) ring interior cross-section diameter and would M.PRAKASH B.E EEE

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provide 0.08 to 0.51g partial gravity. This test and evaluation centrifuge would have the capability to become a Sleep Module for ISS crew.

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artificial gravity

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