20-11-2012, 02:38 PM
ELECTRO DYNAMIC TERTHE SEMINAR REPORT
[attachment=40360]
INTRODUCTION
Satellites have a major part to play in the present communication system. These satellites are launched with the help of rockets. Typically a payload will placed by a rocket in to Low Earth Orbit or LEO (around 400 km) and then boosted higher by rocket thrusters. But just transporting a satellite from the lower orbit to its eventual destination can to several thousand dollars per kilogram of payload. To cut expenses space experts are reconsidering the technology used to place payload in their final orbits.
There are over eight thousand satellites and other large objects in orbit around the Earth, and there are countless smaller pieces of debris generated by spacecraft explosions between satellites. Until recently it has been standard practices to put a satellite in to and leave it there. However the number of satellites has grown quickly, and as a result, the amount of orbital debris is growing rapidly. Because this debris is traveling at orbital speed (78km/s), it poses a significant threat to the space shuttle, the International Space Station and the many satellites in Earth orbit.
One method of removing a satellite from orbit would be to carry extra propellant so that the satellite can bring itself down out of orbit. However this method requires a large mass of propellant and every kilo of propellant that must be carried up reduces the weight available for revenue-producing transponders. Moreover this requires that the rocket and satellites guidance systems must be functional after sitting in orbit for ten years or more.
HISTORY OF SPACE TETHERS
While space-based tethers have been studied theoretically since in the 20th century, it wasn’t until 1947 that Giuseppe Colombo came up with the idea of using a long tether to support a satellite System (TSS) to investigate plasma physics and the generation of electricity in the upper atmosphere.Up until the TSS the use of tethers in space has been limited. The best-known applications are the tethers that connect spacewalking astronauts to their spacecraft. Astronauts can work and fly free of the Space Shuttle using the Manned Maneuvering Unit (MMU), but for most work activities in the Shuttle payload bay (and during the assembly of the International Space Station) astronauts still use a safety tether.
However, spacewalk tethers are very short and are not stabilized by gravitational forces. The TSS-IR mission and rocket-launched experiments, such as the SMALL expendable Deployer System (SEDS) and the Plasma Motor Generator (PMG), have increased our understanding of the way tethers behave in space. Each used different types of tether to deploy satellites and conduct research, demonstrating the usefulness of tether technology.
PRINCIPLE
The basic principle of an electro dynamic tether is Lorentz force. It is the force that a magnetic field exerts on a current carrying wire in a direction perpendicular to both the direction of current flow and the magnetic field vector.
The Dutch physicist Hendrik Androon Lorentz showed that a moving electric charge experiences a force in a magnetic field. (if the charge is at rest, there will not be any force on it due to magnetic field ) Hence it is clear that the force experienced by a current conductor in a magnetic field is due to the drifting of electrons in it.
WORKING
An electro dynamic tether is essentially a long conducting wire extended from a space craft. The electro dynamic tether is made from aluminum alloy and typically between 5 and 20 kilometers long[1]. It extends ‘downwards’ from an orbiting platform. Aluminum alloy is used since it is strong, lightweight, inexpensive and easily machined.
The gravity gradient field (also known as “tidal force”) will tend to orient the tether in a vertical position. If the tether is orbiting around the Earth, it will be crossing the earth’s magnetic field lines orbital velocity (7-8 km/s). The motion of the conductor across the magnetic field induces a voltage along the length of the tether. This voltage can up to several hundred volts per kilometer.
STABILIZATION OF ELECTRODYNAMIC TETHERS
Electro dynamic tethers have strong potential for providing propellant less propulsion to spacecraft in low-earth orbit for application such as satellite deorbit, orbit boosting and station keeping. However electro dynamic tethers are inherently unstable. When a tether in an orbit carries a current along its length, the interaction of the tether with the geometric field creates a force on the tether that is directed perpendicular to the tether. The summation of these force along the length of the tether can produce a net propulsive force on the tether system, raising or lowering its orbit. The tether however is not a rigid rod held above or below the spacecraft it is a very long thin cable and has little or no flexural rigidity. The transverse electro dynamic forces therefore cause the tether to bow and to swing away from the local vertical. Gravity gradient forces produces a restoring force that pulls the tether back towards the local vertical but this results in a pendulum-like motion. Because the direction of the geomagnetic field varies as the tether orbits the Earth the direction and magnitude of the electro dynamic forces also varies and so this pendulum motion develops in to complex librations in both the in-plane and out-of-plane direction. Due to coupling between the in-plane motion and longitudinal elastic oscillations as well as coupling between in-plane and out-cf-plane motions an electro dynamic tether operated at a constant current will continually add energy to the libration motions, causing the libration amplitudes to build until the tether begins rotating or oscillating wildly In addition orbital variations in the strength and magnitude of the electro dynamic force will drive transverse higher order oscillations in the tether which can lead to the unstable growth of “Skip-rope” modes[2].
EDT APPLICATIONS,MERITS AND DEMERITS
APPLICATIONS
Propellant less propulsion for LEO spacecraft:
ED tether system can provide propellant less propulsion for spacecraft operating in low Earth orbit. Because the tether system does not consume propellant, it can provide very large delta-V’s with a very small total mass dramatically reduce the cost for missions that involve delta-V hungry maneuvers such as formation flying low-altitude station keeping orbit raising and end-of-mission deorbit. TUI is developing several ED tether products including the µPET Propulsion System and Terminator Tether Satellite Deorbit Device[3].
[attachment=40360]
INTRODUCTION
Satellites have a major part to play in the present communication system. These satellites are launched with the help of rockets. Typically a payload will placed by a rocket in to Low Earth Orbit or LEO (around 400 km) and then boosted higher by rocket thrusters. But just transporting a satellite from the lower orbit to its eventual destination can to several thousand dollars per kilogram of payload. To cut expenses space experts are reconsidering the technology used to place payload in their final orbits.
There are over eight thousand satellites and other large objects in orbit around the Earth, and there are countless smaller pieces of debris generated by spacecraft explosions between satellites. Until recently it has been standard practices to put a satellite in to and leave it there. However the number of satellites has grown quickly, and as a result, the amount of orbital debris is growing rapidly. Because this debris is traveling at orbital speed (78km/s), it poses a significant threat to the space shuttle, the International Space Station and the many satellites in Earth orbit.
One method of removing a satellite from orbit would be to carry extra propellant so that the satellite can bring itself down out of orbit. However this method requires a large mass of propellant and every kilo of propellant that must be carried up reduces the weight available for revenue-producing transponders. Moreover this requires that the rocket and satellites guidance systems must be functional after sitting in orbit for ten years or more.
HISTORY OF SPACE TETHERS
While space-based tethers have been studied theoretically since in the 20th century, it wasn’t until 1947 that Giuseppe Colombo came up with the idea of using a long tether to support a satellite System (TSS) to investigate plasma physics and the generation of electricity in the upper atmosphere.Up until the TSS the use of tethers in space has been limited. The best-known applications are the tethers that connect spacewalking astronauts to their spacecraft. Astronauts can work and fly free of the Space Shuttle using the Manned Maneuvering Unit (MMU), but for most work activities in the Shuttle payload bay (and during the assembly of the International Space Station) astronauts still use a safety tether.
However, spacewalk tethers are very short and are not stabilized by gravitational forces. The TSS-IR mission and rocket-launched experiments, such as the SMALL expendable Deployer System (SEDS) and the Plasma Motor Generator (PMG), have increased our understanding of the way tethers behave in space. Each used different types of tether to deploy satellites and conduct research, demonstrating the usefulness of tether technology.
PRINCIPLE
The basic principle of an electro dynamic tether is Lorentz force. It is the force that a magnetic field exerts on a current carrying wire in a direction perpendicular to both the direction of current flow and the magnetic field vector.
The Dutch physicist Hendrik Androon Lorentz showed that a moving electric charge experiences a force in a magnetic field. (if the charge is at rest, there will not be any force on it due to magnetic field ) Hence it is clear that the force experienced by a current conductor in a magnetic field is due to the drifting of electrons in it.
WORKING
An electro dynamic tether is essentially a long conducting wire extended from a space craft. The electro dynamic tether is made from aluminum alloy and typically between 5 and 20 kilometers long[1]. It extends ‘downwards’ from an orbiting platform. Aluminum alloy is used since it is strong, lightweight, inexpensive and easily machined.
The gravity gradient field (also known as “tidal force”) will tend to orient the tether in a vertical position. If the tether is orbiting around the Earth, it will be crossing the earth’s magnetic field lines orbital velocity (7-8 km/s). The motion of the conductor across the magnetic field induces a voltage along the length of the tether. This voltage can up to several hundred volts per kilometer.
STABILIZATION OF ELECTRODYNAMIC TETHERS
Electro dynamic tethers have strong potential for providing propellant less propulsion to spacecraft in low-earth orbit for application such as satellite deorbit, orbit boosting and station keeping. However electro dynamic tethers are inherently unstable. When a tether in an orbit carries a current along its length, the interaction of the tether with the geometric field creates a force on the tether that is directed perpendicular to the tether. The summation of these force along the length of the tether can produce a net propulsive force on the tether system, raising or lowering its orbit. The tether however is not a rigid rod held above or below the spacecraft it is a very long thin cable and has little or no flexural rigidity. The transverse electro dynamic forces therefore cause the tether to bow and to swing away from the local vertical. Gravity gradient forces produces a restoring force that pulls the tether back towards the local vertical but this results in a pendulum-like motion. Because the direction of the geomagnetic field varies as the tether orbits the Earth the direction and magnitude of the electro dynamic forces also varies and so this pendulum motion develops in to complex librations in both the in-plane and out-of-plane direction. Due to coupling between the in-plane motion and longitudinal elastic oscillations as well as coupling between in-plane and out-cf-plane motions an electro dynamic tether operated at a constant current will continually add energy to the libration motions, causing the libration amplitudes to build until the tether begins rotating or oscillating wildly In addition orbital variations in the strength and magnitude of the electro dynamic force will drive transverse higher order oscillations in the tether which can lead to the unstable growth of “Skip-rope” modes[2].
EDT APPLICATIONS,MERITS AND DEMERITS
APPLICATIONS
Propellant less propulsion for LEO spacecraft:
ED tether system can provide propellant less propulsion for spacecraft operating in low Earth orbit. Because the tether system does not consume propellant, it can provide very large delta-V’s with a very small total mass dramatically reduce the cost for missions that involve delta-V hungry maneuvers such as formation flying low-altitude station keeping orbit raising and end-of-mission deorbit. TUI is developing several ED tether products including the µPET Propulsion System and Terminator Tether Satellite Deorbit Device[3].