31-08-2012, 01:11 PM
MOMENTUM EXCHANGE TETHER
[attachment=33002]
ABSTRACT
A space tether is a conducting cable attached to a spacecraft that can be used for a variety of purposes such as supplying power to the spacecraft, raising or lowering its orbit, or transferring payload from Earth to high altitude orbits at a significantly lower cost than with conventional methods. This project examines several different types of space tethers with the object of trying to identify the ones with the greatest promise for future applications. It identifies the Momentum Exchange Electrodynamic Reboost (MXER) tether as a concept with a rich variety of applications that is worth studying further for the impact it is likely to have on future space missions
The concept of the space tether has been around for over a century, and was conceived by a Russian scientist named Konstantin Tsiolkovsky in 1895.2 Much later, in 1979, the science-fiction writer Arthur C. Clark described Space Elevators in his novel, The Fountains of Paradise.3 Tsiolkovsky, and a few others after him, such as Giuseppe Colombo, theorized that a large structure could be built that would extend from the surface of the Earth to height above geostationary orbit and could be used as a convenient means of transporting objects from Earth into space. These seemingly far-fetched theories were the basis of the concept of the modern Space Tether. The two most common types of Space Tethers are the Momentum-Exchange Tether (MXT) and the Electrodynamic Tether (EDT).
INTRODUCTION
Tether is a rope or chain which is used to fastening the animals . The concept of tether is modified as space tether here . And we are concerning only about space tethers. A Space Tether is a long cable that connects two masses and uses fundamental laws of physics to generate electric power, artificial gravity, and thrust or drag, among other things1, all without using propellant. The concept of the space tether has been around for over a century, and was conceived by a Russian scientist named Konstantin Tsiolkovsky in 1895.2 Much later, in 1979, the science-fiction writer Arthur C. Clark described Space Elevators in his novel, The Fountains of Paradise.3 Tsiolkovsky, and a few others after him, such as Giuseppe Colombo, theorized that a large structure could be built that would extend from the surface of the Earth to height above geostationary orbit and could be used as a convenient means of transporting objects from Earth into space. These seemingly far-fetched theories were the basis of the concept of the modern Space Tether. The two most common types of Space Tethers are the Momentum-Exchange Tether (MXT) and the Electrodynamic Tether (EDT).
STRUCTURE OF A SPACE TETHER
The space tether is also rotating around the earth as a satellite with a specified orbital velocity. To describe the structure of a tether , 100 km tether structure to be divided into ten sections, each 10 km in length. Further, each 10 km section is composed of several parallel pairs of 5 km tether modules that are attached back-to-back to form the total length. This is illustrated in Figure on the right most side.
The truss structure serves as a support for each of the tether deployer modules, sitting in between each section of the tether. Eight of the junctions will contain power modules which will supply the high voltage needed for the electrodynamic reboost. An important feature of the 10 km segments, comprised of two 5 km tethers, is that it affords the tether deployment or retraction mechanism to be placed at both ends of the tether. With this system, in the case that one end become damaged or severed, both halves of the tether can be retracted which will prevent the generation of orbital debris and allow stowing of the broken pieces to facilitate replacement. In this modular architecture, each of the 5 km lengths of high-strength tether and conductive Electrodynamic Tether will be deployed by its own deployer module.
MOMENTUM EXCHANGE TETHER
Primary function of a Momentum-Exchange Tether (MXT) is to transfer a given amount of momentum and orbital energy from the MXT to a spacecraft, launching it into a higher orbit. This is accomplished by using the principle of conservation of angular momentum.
The MXT would be orbiting the Earth in LEO, with a perigee of 300-500 km and an apogee of 5000-8000 km. A typical payload would be launched into a circular orbit around Earth at an altitude of 300-500 km, enabling it to rendezvous with the MXT when it is at perigee. This is the fundamental idea that makes the momentum-exchange possible, since the specific energy (energy per unit mass) of an orbiting object is proportional to the distance between its perigee and apogee. Thus, a MXT in a 400 x 8000 km orbit has much more orbital energy per unit mass than a payload in a 400 x 400 km circular orbit and can transfer energy to it without having its own orbit severely modified.
As the tether passes through perigee, it will be at its greatest orbital velocity and will be moving faster than a spacecraft in a circular orbit at the same altitude. For example, if the tether is moving at a speed of 8.9 km/s at perigee, a spacecraft in circular orbit would be moving at a speed of 7.7 km/s at the same altitude. In order for the tether to “catch” the payload effectively, it needs to be rotating about its center-of-mass at the rate that would force its tip velocity equal to the difference between its center-of-mass velocity and the velocity of the payload; since the rotational motion of the tip is opposite to the motion of the payload, the relative velocity of the tip and payload is then zero and a smooth catch can be effected. It is required that both the relative position and relative velocity of the tip of the tether and the payload be zero at the moment of the catch.
CONSTRUCTION
With the current construction concepts, the tether facility would consist of a rotating, 100-120 km long tether with a variety of masses and other mechanisms in place along its length. The tether tip would have a mechanism that would assist in hooking up the tether to a suitable payload aimed at it. The other end of the tether would consist of a ballast mass, which would probably be the spent stage of the launch vehicle that would put the MXT into orbit before any use of it can be made.
The 100-120 km tether would be a composite material with an extremely high tensile strength that is then coated or treated in some manner to protect the tether from ultraviolet radiation and atomic oxygen. The material used would most likely be one of two materials: Dyneema or carbon nanotubes. Carbon nanotubes are one of the strongest and most rigid materials known to mankind in terms of both tensile strength and elastic modulus. In 2000, a multi-walled carbon nanotube was tested to have a tensile strength of 63 GPa. This can be compared to high-carbon steel, which has a tensile strength of only about 1.2 GPa. Carbon nanotubes also have a very high elastic modulus, up to 1 TPa, and their specific strength (tensile strength divided by density) is 1.3-1.4 g/cm3, giving them the best specific strength of any known material. Dyneema, comparatively, is a readily-available synthetic fiber with a high tensile strength that has been in use since 1979, when it was invented. Dyneema, formally known as Ultra-High Molecular Weight Polyethylene (UHMWPE), has some defects that may limit its use in atmospheric or near-atmospheric temperatures as its melting point is only about 144 degrees Centigrade, while its use is not recommended above 100 degrees Centigrade. Carbon nanotubes seem to be the safest and strongest choice, but the cost of a tether of such size constructed out of a new and untested material make some engineers uneasy.
LIMITATIONS
While carbon nanotubes provide a high-strength material from which the MXT can be constructed, space debris and other hazards can compromise the security and integrity of a tether structure that is permanently in orbit above the Earth.
Micrometeoroids currently pose the greatest threat to space tether structures and the space tether itself. These small particles of rock, which usually weigh only a gram or less, pose a major threat to not only the tether, but to space exploration in general. This is already evident from the care that NASA expends in organizing spacewalks from the ISS. Small particles are extremely common in space and their velocities relative to spacecraft in orbit are on the order of kilometers per second
According to the Space Tethers Handbook, typical micrometeoroid velocities are about 20 km/s, with an average density of about 0.5 kg/m3. At impact speeds above the speed of sound, solids become compressible and the shockwave due to impact produces effects analogous to those of an explosion.20 To put the large number of micrometeoroids around the Earth into perspective, it is estimated that micrometeoroids comprise most of the 30,000 tons of space debris that impact and deposits itself upon Earth every year.The strength of the materials used in the space tether’s construction is generally tested as the length of the tether is made longer and longer
[attachment=33002]
ABSTRACT
A space tether is a conducting cable attached to a spacecraft that can be used for a variety of purposes such as supplying power to the spacecraft, raising or lowering its orbit, or transferring payload from Earth to high altitude orbits at a significantly lower cost than with conventional methods. This project examines several different types of space tethers with the object of trying to identify the ones with the greatest promise for future applications. It identifies the Momentum Exchange Electrodynamic Reboost (MXER) tether as a concept with a rich variety of applications that is worth studying further for the impact it is likely to have on future space missions
The concept of the space tether has been around for over a century, and was conceived by a Russian scientist named Konstantin Tsiolkovsky in 1895.2 Much later, in 1979, the science-fiction writer Arthur C. Clark described Space Elevators in his novel, The Fountains of Paradise.3 Tsiolkovsky, and a few others after him, such as Giuseppe Colombo, theorized that a large structure could be built that would extend from the surface of the Earth to height above geostationary orbit and could be used as a convenient means of transporting objects from Earth into space. These seemingly far-fetched theories were the basis of the concept of the modern Space Tether. The two most common types of Space Tethers are the Momentum-Exchange Tether (MXT) and the Electrodynamic Tether (EDT).
INTRODUCTION
Tether is a rope or chain which is used to fastening the animals . The concept of tether is modified as space tether here . And we are concerning only about space tethers. A Space Tether is a long cable that connects two masses and uses fundamental laws of physics to generate electric power, artificial gravity, and thrust or drag, among other things1, all without using propellant. The concept of the space tether has been around for over a century, and was conceived by a Russian scientist named Konstantin Tsiolkovsky in 1895.2 Much later, in 1979, the science-fiction writer Arthur C. Clark described Space Elevators in his novel, The Fountains of Paradise.3 Tsiolkovsky, and a few others after him, such as Giuseppe Colombo, theorized that a large structure could be built that would extend from the surface of the Earth to height above geostationary orbit and could be used as a convenient means of transporting objects from Earth into space. These seemingly far-fetched theories were the basis of the concept of the modern Space Tether. The two most common types of Space Tethers are the Momentum-Exchange Tether (MXT) and the Electrodynamic Tether (EDT).
STRUCTURE OF A SPACE TETHER
The space tether is also rotating around the earth as a satellite with a specified orbital velocity. To describe the structure of a tether , 100 km tether structure to be divided into ten sections, each 10 km in length. Further, each 10 km section is composed of several parallel pairs of 5 km tether modules that are attached back-to-back to form the total length. This is illustrated in Figure on the right most side.
The truss structure serves as a support for each of the tether deployer modules, sitting in between each section of the tether. Eight of the junctions will contain power modules which will supply the high voltage needed for the electrodynamic reboost. An important feature of the 10 km segments, comprised of two 5 km tethers, is that it affords the tether deployment or retraction mechanism to be placed at both ends of the tether. With this system, in the case that one end become damaged or severed, both halves of the tether can be retracted which will prevent the generation of orbital debris and allow stowing of the broken pieces to facilitate replacement. In this modular architecture, each of the 5 km lengths of high-strength tether and conductive Electrodynamic Tether will be deployed by its own deployer module.
MOMENTUM EXCHANGE TETHER
Primary function of a Momentum-Exchange Tether (MXT) is to transfer a given amount of momentum and orbital energy from the MXT to a spacecraft, launching it into a higher orbit. This is accomplished by using the principle of conservation of angular momentum.
The MXT would be orbiting the Earth in LEO, with a perigee of 300-500 km and an apogee of 5000-8000 km. A typical payload would be launched into a circular orbit around Earth at an altitude of 300-500 km, enabling it to rendezvous with the MXT when it is at perigee. This is the fundamental idea that makes the momentum-exchange possible, since the specific energy (energy per unit mass) of an orbiting object is proportional to the distance between its perigee and apogee. Thus, a MXT in a 400 x 8000 km orbit has much more orbital energy per unit mass than a payload in a 400 x 400 km circular orbit and can transfer energy to it without having its own orbit severely modified.
As the tether passes through perigee, it will be at its greatest orbital velocity and will be moving faster than a spacecraft in a circular orbit at the same altitude. For example, if the tether is moving at a speed of 8.9 km/s at perigee, a spacecraft in circular orbit would be moving at a speed of 7.7 km/s at the same altitude. In order for the tether to “catch” the payload effectively, it needs to be rotating about its center-of-mass at the rate that would force its tip velocity equal to the difference between its center-of-mass velocity and the velocity of the payload; since the rotational motion of the tip is opposite to the motion of the payload, the relative velocity of the tip and payload is then zero and a smooth catch can be effected. It is required that both the relative position and relative velocity of the tip of the tether and the payload be zero at the moment of the catch.
CONSTRUCTION
With the current construction concepts, the tether facility would consist of a rotating, 100-120 km long tether with a variety of masses and other mechanisms in place along its length. The tether tip would have a mechanism that would assist in hooking up the tether to a suitable payload aimed at it. The other end of the tether would consist of a ballast mass, which would probably be the spent stage of the launch vehicle that would put the MXT into orbit before any use of it can be made.
The 100-120 km tether would be a composite material with an extremely high tensile strength that is then coated or treated in some manner to protect the tether from ultraviolet radiation and atomic oxygen. The material used would most likely be one of two materials: Dyneema or carbon nanotubes. Carbon nanotubes are one of the strongest and most rigid materials known to mankind in terms of both tensile strength and elastic modulus. In 2000, a multi-walled carbon nanotube was tested to have a tensile strength of 63 GPa. This can be compared to high-carbon steel, which has a tensile strength of only about 1.2 GPa. Carbon nanotubes also have a very high elastic modulus, up to 1 TPa, and their specific strength (tensile strength divided by density) is 1.3-1.4 g/cm3, giving them the best specific strength of any known material. Dyneema, comparatively, is a readily-available synthetic fiber with a high tensile strength that has been in use since 1979, when it was invented. Dyneema, formally known as Ultra-High Molecular Weight Polyethylene (UHMWPE), has some defects that may limit its use in atmospheric or near-atmospheric temperatures as its melting point is only about 144 degrees Centigrade, while its use is not recommended above 100 degrees Centigrade. Carbon nanotubes seem to be the safest and strongest choice, but the cost of a tether of such size constructed out of a new and untested material make some engineers uneasy.
LIMITATIONS
While carbon nanotubes provide a high-strength material from which the MXT can be constructed, space debris and other hazards can compromise the security and integrity of a tether structure that is permanently in orbit above the Earth.
Micrometeoroids currently pose the greatest threat to space tether structures and the space tether itself. These small particles of rock, which usually weigh only a gram or less, pose a major threat to not only the tether, but to space exploration in general. This is already evident from the care that NASA expends in organizing spacewalks from the ISS. Small particles are extremely common in space and their velocities relative to spacecraft in orbit are on the order of kilometers per second
According to the Space Tethers Handbook, typical micrometeoroid velocities are about 20 km/s, with an average density of about 0.5 kg/m3. At impact speeds above the speed of sound, solids become compressible and the shockwave due to impact produces effects analogous to those of an explosion.20 To put the large number of micrometeoroids around the Earth into perspective, it is estimated that micrometeoroids comprise most of the 30,000 tons of space debris that impact and deposits itself upon Earth every year.The strength of the materials used in the space tether’s construction is generally tested as the length of the tether is made longer and longer