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01-02-2010, 06:45 AM
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electrodynamic tether full report

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ABSTRACT
Electrodynamic (ED) tether is a long conducting wire extended from spacecraft. It has a strong potential for providing propellant less propulsion to spacecraft in low earth orbit. An electrodynamic Tether uses the same principle as electric motor in toys, appliances and computer disk drives. It works as a thruster, because a magnetic field exerts a force on a current carrying wire. The magnetic field is supplied by the earth. By properly controlled the forces generated by this electrodynamic tether can be used to pull or push a spacecraft to act as brake or a booster. NASA plans to lasso energy from Earthâ„¢s atmosphere with a tether act as part of first demonstration of a propellant-free space propulsion system, potentially leading to a revolutionary space transportation system. Working with Earthâ„¢s magnetic field would benefit a number of spacecraft including the International Space Station. Tether propulsion requires no fuel. Is completely reusable and environmentally clean and provides all these features at low cost.
CONTENTS
1 REVIEW OF EXISTING ROCKET PROPULSION MECHANISM
2 INTRODUCTION
3 HISTORY OF SPACE TETHERS
4 PRICIPLE
5 WORKING
6 STABILIZATION OF ELECTRODYNAMIC TETHERS
7 ED TETHERS APPLICATION
8 ADVANTAGES
9 WHY TETHERS WIN
10 CONCLUSION AND FUTURE SCOPE
11 REFERENCE
1. REVIEW OF EXISTING PROPULSION MECHANISM
The existing rocket propulsion mechanism derives energy from rocket fuels. The rocket fuel is burnt inside a chamber and gas produced due to combustion is expelled out through a nozzle, which produces the upward thrust for rockets or spacecrafts.
The currently available rocket fuels are in solid liquid and as from Hydrogen peroxide is one of the commonly used rocket fuels. Cold gas is another gaseous propellant. The disadvantage of these rocket fuels is that it produces low thrust.
Kerosene is a liquid propellant. The liquid fuel requires cryogenic systems for their implementation. The combustion of these fuels produces toxic gases, which are expelled to the space to obtain the required thrust. Thus it creates pollution in the outer space. The system that use solid fuels are unregulated. They produce lower thrust also.
Nuclear energy can be used as a propellant. But it produces radiations, which are very harmful. These radiations can penetrate the atmosphere and affect the human kind and other living things. The effect of nuclear radiations lasts for years that can jeopardize life on earth. So the use of nuclear propulsion technique is very risky. An electrodynamic tether with its unique features put forward a better option for propulsion of rockets and spacecrafts.

2. 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.
What can, without rockets, deploy satellite to Earth-orbit or fling them in to deep space, can generate electrical power in space, can then catch and eliminate space junk String! Sounds impossible, but the development in space-tethers may be as significant to future space development as rockets were to its beginnings.
Called an electrodynamic tether provides a simple and reliable alternative to the conventional rocket thrusters. Electrodynamic tethers work by virtue of the force a magnetic field exerts on a current carrying wire. In essence, it is a clever way of getting an electric current to flow in a long conducting wire that is orbiting Earth, so that earthâ„¢s magnetic field will exerts a force on and accelerate the wire and hence any payload attached to it. By reversing the direction of current in it, the same tether can be used to deorbit old satellites.
3. HISTORY OF SPACE TETHERS
While space-based tethers have been studied theoretically since in the 20th century, it wasnâ„¢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.
Fig..History of tether

4.PRINCIPLE
The basic principle of an electrodynamic 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. If a current I flows through a conductor of cross-section A then
I = neAv where v is the drift speed of electronics n is number density in the conductor and e the electronic charge.
For an element dI of the conductor
Id = nAdIev
But Adi is the volume of the current element. Therefore, nAdI represents the number (N) of electrons in the element
Hence, nAdIe = Ne = q, the total charge in the element.
Therefore, IdI = qv
But, the force dF on a current carrying element dI in a magnetic field B is given by
dF = IdIB
i.e.,dF = qvB
This fundamental force on a charge q moving with a velocity v in a magnetic field B is called the Magnetic Lorentz Force.
4.1 Lorentz Force Low
The Lorentz Force Low can be used to describe the effect of a charged particle moving in a constant magnetic field. The simplest form of this low given by the scalar equation
F = QvB
F is the force acting on the particle (vector)
V is the velocity of the particle (vector)
Q is charge of particle (scalar)
B is magnetic field (vector)
NOTE: this case is for v and B perpendicular to each other otherwise use F = QvB (sin (X) ) where X is the angle between v and B, when v and B are perpendicular X =90 deg. So sin (x) =1.
Flemingâ„¢s left hand rule comes in to play here to figure out which way the force is acting
4.2 Flemingâ„¢s Left Hand Rules
For a charged particle moving (velocity v) in a magnetic field (field B) the direction of the resultant force (force F) can be found by
MIDDLE FINGER of left hand in direction of CURRENT
INDEX FINGER of left hand in direction of FIELD. B
THUMB now points in direction of the FORCE OR MOTION. F
The force will always be perpendicular to the plane of vector v and B no matter what the angle between v and B is. Just pretend the following picture is.

5. WORKING
An electrodynamic tether is essentially a long conducting wire extended from a space craft. The electrodynamic tether is made from aluminium alloy and typically between 5 and 20 kilometers long. It extends Ëœdownwardsâ„¢ from an orbiting platform. Aluminium 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 earthâ„¢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.
In the above figure the sphere represents the Earth and the unbroken lines represents Earthâ„¢s magnetic field. The broken line is LEO. As shown in the figure there is a drag force experienced in the wire in a direction perpendicular to the current and magnetic field vector.
In an electrodynamic tether drag system such as the terminator Tether, the tether can be used to reduce the orbit of the spacecraft to which it is attached. If the system has a means for collecting electrons from the ionospheric plasma at one end of the tether and expelling them back in to the plasma at the other end of the tether, the voltage can drive a current along the tether. This current bill, in turn, interact with the Earthâ„¢s magnetic field to cause a Lorentz JXB force, which will oppose the motion of the tether and whatever it is attached to. This electrodynamics drag force will decrease the orbit of the tether and its host spacecraft. Essentially, the tether converts the orbital energy of the host spacecraft in to electrical power, which is dissipated as ohmic heating in the tether.
Fig2. Principle of electrodynamic tether propulsion
In an electrodynamic propulsion system, the tether can be used to boost the orbit of the spacecraft. If a power supply is added to the tether system and used to drive current in the direction opposite to that which it normally wants to flow, the tether can push against the Earthâ„¢s magnetic field to raise the spacecraftâ„¢s orbit. The major advantage of this technique compared to the other space propulsion system is that it doesnâ„¢t require any propellant. It uses Earthâ„¢s magnetic field as its reaction mass. By eliminating the need to launch large amounts of propellant in to orbit, electrodynamic tethers can greatly reduce the cost of in-space propulsion
The tether is dragged through the atmosphereËœsâ„¢ ionosheric plasma. The rarefied medium of electrons through which the whole set up is traveling at a speed of 7-8km/s. In so doing, the 5-km. long aluminium wire extracts electrons from the plasma at the end farthest from the payload and carries them to the near end (plasma chamber tests have verified that thin bare wires can collect current from plasma). There a specially designed devise known as a hollow cathode emitter expels the electrons, to ensure their return to space currents in the circuit.
Ordinarily, a uniform magnetic field acting on a current-bearing loop of wire yields a net force of zero, since that cancels the force on one side of the loop on the other side, in which the current is flowing in the opposite direction However, since the tethered system is not mechanically attached to the plasma. The magnetic force on the plasma current in the space does not cancel the forces on the tether. And so the tether experiences a net force.
As the tether cuts across the magnetic field, its bias voltage is positive at the end farthest from Earth and negative at the near end. This polarization is due to the action of Lorentz force on the electrons in the tether. Thus the natural upward current flow due to the (negatively charged) electrons in the ionosphere being attracted to the tethers far and then returned to the plasma at the near end. Aided by the hollow cathode emitter. The hollow cathode is vital: without it, the wireâ„¢s charge distribution would quickly reach equilibrium and no current would flows.
Switching on the hollow cathode causes a small tungsten tube to heat up and fill with xenon gas from small tank. Electrons from the tether interacted with the heated gas to create ion plasma. At the far end of the tube. a so called keeper electrode, which is positively charged with respect to the tube. Draw the electrons and expels them to space. (the xenon ions, mean while are collected by the hollow cathode and used to provide additional heating). The rapid discharge of electrons invites new electrons to follow from the tether and out through the hollow cathode. Earthâ„¢s magnetic field exerts a drag force on a current carrying tether, decelerating it and the payload and rapidly lowering their orbit Eventually they re-enter Earthâ„¢s atmosphere.
6. STABILIZATION OF ELECTRODYNAMIC TETHERS
Electrodynamic tethers have strong potential for providing propellantless propulsion to spacecraft in low-earth orbit for application such as satellite deorbit, orbit boosting and station keeping. However electridynmic 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 electrodynamic 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 electrodynamic 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 iongitudinal elastic oscillations as well as coupling between in-plane and out-cf-plane motions an electrodynamic 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 electrodynamic force will drive transverse higher order oscillations in the tether which can lead to the unstable growth of Skip-rope modes.
Two new control schemes are developed to provide the ability to prevent the unstable growth of librations transverse oscillations and skip rope modes. These feedback control schemes requires as input penodic measurements of the locations of the tether end mass and/or several points along the tether. The feedback algorithm calculates a gain factor based upon the network that the electrodynamic forces will perform on the tether dynamics. The feedback is performed by varying the current in the tether system slightly according to the calculated gain factor.
A tether system deployed in orbit around the Earth will be pulled by gravity gradient forces towards an equilibrium configurations oriented along the local vertical. In an electrodynamic tether system, illustrated conceptually in figure currents in the tether flowing across the planetary magnetic field will generate JXB forces acting in a direction perpendicular to both the magnetic field and the tether. These forces will push the tether away from the local vertical orientation.

The first requires periodic measurements of the locations of several points along the tether. This algorithm is referred to as the Tether configuration feedback method. The second algorithm requires only periodic measurements of the acceleration of the tether end mass. This algorithm is referred to as the Endmass Acceleration feedback method. These stabilization algorithm forms the heart of the Electrodynamic Tether Stabilization System (EDTS) which will enable electrodynamic tethers to provide long-term propellantless propulsions while maintaining tether stability and efficiency.
7. ED TETHER APPLICATION
7.1 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.
a.The µPET Propulsion System:
Propellantless Electrodynamic Tether Propulsion for Microsatellites
TUI is currently developing a propulsion system called the "Microsatellite Propellantless Electrodynamic Tether (µPET„¢) Propulsion System" that will provide propulsive capabilities to microsatellites and other small spacecraft without consuming propellant.
Fig.. The microPET Propulsion System concept of operations.
Fig.. Deployment test of the microPET tether.
Electrodynamic tethers can provide long-term propellantless propulsion capability for orbital maneuvering and stationkeeping of small satellites in low-Earth-orbit. The µPET„¢ Propulsion System is a small, low-power electrodynamic tether system designed to provide long-duration boost, deboost, inclination change, and stationkeeping propulsion for small satellites. Because the system uses electrodynamic interactions with the Earth's magnetic field to propel the spacecraft, it does not require consumption of propellant, and thus can provide long-duration operation and large total delta-V capability with low mass requirements. Furthermore, because the µPET„¢ system does not require propellant, it can easily meet stringent safety requirements such as are imposed upon Shuttle payloads. In addition, the tether system can also serve as a gravity-gradient attitude control element, reducing the ACS requirements of the spacecraft.
Characteristics:
The mass, size, and power requirements of the µPET„¢ Propulsion System depends upon the size of the satellite and the propulsive mission. TUI has developed a prototpye of a µPET„¢ sized for a 125 kg microsatellite which could raise the orbit of this satellite from a 350 km drop-off orbit to a 700 km operational orbit within 50 days.
b.The Terminator Tether Satellite Deorbit System:
Low-Cost, Low-Mass End-of-Mission Disposal for Space Debris Mitigation
Fig.Concept of operations of the Terminator Tether„¢.
Fig.. The Terminator Tether„¢ Deployer.
Tethers Unlimited Inc. is currently developing a system called the Terminator Tether„¢ that will provide a low-cost, lightweight, and reliable method of removing objects from low-Earth-orbit (LEO) to mitigate the growth of orbital debris.
The Terminator Tether„¢ is a small device that uses electrodynamic tether drag to deorbit a spacecraft. Because it uses passive electromagnetic interactions with the Earth's magnetic field to lower the orbit of the spacecraft, it requires neither propellant nor power. Thus it can achieve autonomous deorbit of a spacecraft with very low mass requirements.
Concept of operations:
Before the spacecraft is launched, the Terminator Tether„¢ is bolted onto the satelite. While the satellite is operational, the tether is wound on a spool, and the device is dormant, waking up periodically to check the status of the spacecraft and listen for activation commands. When the Terminator Tether„¢ receives a command to deorbit the spacecraft, it deploys a 5 kilometer long tether below the spacecraft. This tether interacts with the ionospheric plasma and the geomagnetic field to produce currents running along the tether, and these currents in turn cause forces on the tether that lower the orbit of the tethered spacecraft. Over a period of several weeks or months, the Terminator Tether„¢ will reduce the orbital altitude of the spacecraft until it vaporizes in the upper atmosphere.
7.2 Electrodynamic Reboost of the International Space Station:
The International Space Station is the largest and most complex international scientific project in history. And when it is complete just after the turn of the century, the station will represent a move of unprecedented scale off the home planet Led by the United States the International Space Station draws upon the scientific and technological resources of 16 nations Canada, Japan, Russia. 11 nations of the European Space Agency and Brazil.
Its construction started at 1998 November 20 when Russia launched Zarya control module. More than four times as large as the Russian Mir space station the completed International Space Station will have a mass of about 1,040,000 pounds. It will measure 356 feet across and 290 feet long with almost an acre of solar panels to provide electrical power to 6 State-of-the-art laboratories. The station will be in an orbit with an altitude of 250 statute miles with an inclination of 51.6 degrees. This orbit allows the station to be reached by the launch vehicles of all the international partners to provide a robust capability for the delivery of crews and supplies. The orbit also provides excellent Earth observations with coverage of 85% of the globe and over flight of 95% of the population. By the end of this year about 500,000 pounds of station components will have been built at factories around the world.
Research in the station six laboratories will lead to discoveries in medicine, materials and fundamental science that will benefit people all over the world. Through its research and technology, the station will serve as an indispensable step in preparation for future human space exploration.
Examples of the types of U.S. research that will be performed abroad the station include:
Protein crystal studies
Tissue culture
Life in low gravity
Flames, fluids and metal in space
The nature of space
Watching the Earth
Commercialization
The international Space station (ISS) will experience a small but constant aerodynamic drag force as it moves through the thin upper reaches of the Earthâ„¢s atmosphere. This drag force will cause the stationâ„¢s orbit to decay. If nothing were done to counteract this, the station would fall out of orbit with in several months. NASA currently plans to launch several rockets every year to carry fuel up to the station so that it can reboots its orbit. These launches however, will be very costly. Tether unlimited, Inc. has helped NASA to explore the potential for using Electrodynamic tether propulsion to maintain the orbit of the ISS. By using excess power generated by the ISSâ„¢s solar panels to drive current through a conducting tether, a tether reboots system could counteract the drag forces or even raise the stationâ„¢s orbit. NASA and TUIâ„¢s studies revealed that such a tether reboots system could reduce or eliminate the need for dedicated launches for reboots propellant. Potentially saving up to $2 billion over the first ten years of the stationâ„¢s operation.
7.3 Power Generation in Low Earth Orbit:
Electrodynamic tethers may also provide an economical means of electrical power in orbit. Essentially, the tether can be used to convert some of the spacecraftâ„¢s Orbital energy in to electrical power. However, since converting the orbital energy in to electrical power will lower the orbit of the spacecraft (thereâ„¢s no such thin as a free launch), this technique is probably only useful for providing high power energy bursts to short-duration experiments.
7.4 Space junk cleanup:

Illustration of how an electrodynamic tether with attached "space sheepdog" would work.
Space junk is a big problem. There is nearly 2000 tonnes of space debris orbiting the earth. Pieces of derelict spacecraft, bits of launch vehicles and even tiny flecks of paint are orbiting the earth at tens of thousand of kilometres per hour causing huge damage whenever they impact on spacecraft or satellites. Scientists are trying to predict the orbits of all the rubbish so that companies launching satellites or spacecrafts know their vehicle will be out of danger but could the future involve clearing up the mess by using tethers attached to space sheepdogs .The most direct application of electrodynamic tether would be to get rid of space junk. Over the past half century of space exploration, the region around Earth has become cluttered with debris, which could take years, and in some cases centuries, to fall from orbit. The danger is that old satellite and rocket stages and trash thrown overboard by early space shuttles and orbiting space station.
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 satellite guidance system must be functional after sitting in orbit for 10 years or more. Often this is not the case, and the satellite ends up stuck in its operational orbit. Some organisations are currently planning on boosting their satellite to higher. graveyard orbits at the end of their mission. This also required that the satellite™s power, propulsion and guidance be working at the end of the satellite™s mission. Moreover, it doesn™t really solve the problem “it just delays it. Somewhat like a toxic waste dump. Recent studies have shown that satellites left in a higher graveyard orbit will slowly break apart down to lower altitudes. Thus satellites boosted to higher disposal orbits will eventually endanger operational satellites. Moreover, once the old satellites fragment in to smaller particles, it will be nearly impossible to clean up the debris. Consequently, it will be much more cost effective in the long run to deal with the problem properly from the start. And deorbit all old spacecraft.
Using a tether to deorbit would be inherently more reliable. ED tethers are much lighter are more compact than conventional thrusters: a tether system would account for as little as 2% of the satelliteâ„¢s total weight and could be easily bolted to the satellites. Once the end of the satelliteâ„¢s useful life is reached. The tether would unreel, and the tether-driven orbital decay.

8. ADVANTAGES
The operational advantages of electrodynamic tethers of moderate length are becoming evident from studies of collision avoidance. Although long tethers (of order of 10 kilometers) provide high efficiency and good adaptability to varying plasma conditions, boosting tethers of moderate length (~1 kilometer) and suitable design might still operate at acceptable efficiencies and adequate adaptability to a changing environment.
ED tethers used for propulsion in low-Earth orbit and beyond could significantly reduce the weight of upper stages used to boost spacecraft to higher orbit. Much of the weight of any launch vehicle is the propellant and It is expensive to lift heavy propellants off the ground.
Since ED tethers require no propellant, they could substantially reduce the weight of the spacecraft and provide a cost effective method of reboosting spacecraft, such as the International Space Station (ISS)
9. WHY TETHERS WIN
Normal Launch from ground
Circular velocity is about 8km/s at Low Earth Orbit (LEO). You loose around 2km/s from drag and climb. You get around 0.5km from the spin of the Earth. So 2 rocket has to provide a Delta-V about 9.5km/s. You need to circularize your orbit which means firing the engine again about 45 minutes after launch. This restart of the engine only needs to provide about 0.1 to 1.15 km/s depending upon the altitude of the orbit.
Air Launch from 20 km to tether at 100 km altitude
We need to be doing about 5 km/s when we get to the end of the tether. We loose about 0.5km/s from climbing from 20 km to 100 km and air drag. We get about 0.5km/s from spin of Earth. There is no need to circularize the orbit as the tether has a big ballast mass and is in orbit. Net is rocket needs to provide a delta-V of about 5 km/s.
What happened
The orbital velocity at 100 km high is 7.5 km/s but the centre of mass of the tether is at 600km high (so 500km from tip to centre of mass) the orbital velocity is 7.56km/s. We have saved 0.29km/s already.
Our final design uses a tether tip speed of 2.5km/s relative to the centre of mass. So relative to the centre of Earth it is moving about 5.06km/s(7.56-2.5). Between the two we are 2.79(2.5+0.29) km/s below orbital speed at 100 km
We get about 0.5 km/s from the rotational speed of the earth and so only need 4.s km/s after altitude and drag loss. Starting from 20 km high we donâ„¢t loose so much to drag. Our air launch will gives us a running start, perhaps 0.2 km/s. Reduced air pressure enables a more efficient rocket engine.
What is the result
We need around the half the Delta-V. We needed a two-stage before but we only need one stage rocket now. It is right to think of it as only being the second stage. The first stage could have 5-10 times as large as the second stage, so we have saved a lot.
Mass production
Another big savings is due to expected mass production or re-usability. Because we have a large number of small rockets, instead of usual few big rockets, we can use assembly line methods. Even better, because we only go halfway to orbit, making a re-usable single stage vehicle is comparatively easy.
10. CONCLUSION AND FUTURE SCOPE
Satellite Tugboat
Another idea is for the ED tether to be attached to an unmanned space tugboat that would ferry satellites to higher orbits. After being launched in to low Earth orbit, the so called Orbital Transfer Vehicle would grapple the satellite and maneuver it to a new altitude or inclination. The tug could then lower its own orbit to rendezvous with another payload and repeat and repeat the process.
Exploring the outer planets
Perhaps the most exotic use if ED tether technology would be to propel and power spacecraft exploring the outer planets. Existing vessels have relied on solar cells, but at distances far from the Sun, the power available is typically favourable to ED tethers: The planet has a strong magnetic field moving much faster than the spacecraft the tether would essentially be stealing energy from the planetâ„¢s magnetic field.
In theory tether could power the craft™s instruments and generates thrust at one and the same time. For a circular orbit close to the planet tether propulsive forces have been calculated to be as high as 50 N and power levels as high as 1MW. This level of power would sustain a whole new suite of science instruments such as high-power radar”but it also means having to deal with power conversion, energy dissipation, and tether overheating
Tethers are an exciting area of space research with many possible applications. Soon they may become common, replacing conventional deployment technologies, and improving access to space.
\

11. REFERENCE:
IEEE spectrum. July 2000
http://www.tethers.com
http://www.tuiengineering.com
http://www.google.com
http://www.ieee spectrum.org.

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21-02-2010, 12:49 PM
Post: #2
RE: electrodynamic tether full report

.doc  electrodynamic tether full report.doc (Size: 530 KB / Downloads: 173)


INTRODUCTION
The story of Space Tethers is relatively new in the illustrious field of space technology. This technology was thought about when the cost of manOeuvreing spacecrafts and other objects in space was proving to be very costly and the removal of space debris was a necessity.
A space tether is a long cable used to couple space craft to each other or to other masses, such as a spent booster rocket, space station, or an asteroid. Space tethers are usually made of thin strands of high-strength fibers or conducting wires. The tethers can provide a mechanical connection between two space objects that enables the transfer of energy and momentum from one object to the other, and as a result they can be used to provide space propulsion without consuming propellant. Additionally, conductive space tethers can interact with the Earth's magnetic field and ionospheric plasma to generate thrust or drag forces without expending propellant.
The effectiveness of this technology is that it does not use any propellants. The problem with propellants is that it is very bulky, dangerous and exhaustable. The use of space tethers is the answer to all these problems.
TYPES OF TETHERS
There are mainly two types of Space Tethers. They are: Momentum-Exchange Tethers
These allow momentum and energy to be transferred between objects in space, enabling a tether system to toss spacecraft from one orbit to another.
Electro Dynamic Tethers
These interact with the Earth's magnetosphere to generate power or pollution without consuming propellant.
PRINCIPLES OF OPERATION
The Lorentz Force
i
When some charge 'q" moves in a magnetic field of intensity B, with a velocity V at an angle '0' with the direction of field, a force 'Bqv Sin 0' acts on it in a direction perpendicular to the plane of 'v' and 'B'.
F = Bqv
when 9 ~ 90°. i.e, when v and B are perpendicular. Here F is the force acting on charge called Lorentz Force. The Right Hand Rule can be used to figure out the direction in which the force is acting.
Right Hand Rule
For a positively charged particle moving ( velocity v) in a magnetic field ( field B) the direction of the resultant force ( force F) can be found by:
1)THUMB of right hand indicating the direction of the velocity, direction of current 2)INDEX FINGER indicating the direction of the field.B 3)MIDDLE FINGER indicating the direction of the force, F
This is the Right Hand Rule used to find the direction of the force.
The force will AL WA YS be perpendicular to the PLANE of the vectors v and B no matter what the angle between v and B is. Just pretend the following picture is of your right hand:
Right Hand Rule
Electro Dynamic Tether
1
WORKING
An electro dynamic tether is essentially a long conducting wire extended from a spacecraft. The gravity gradiet field( also known as the "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 at 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 be up to several hundreds volts per kilometer.
In an "electro dynamic tether drag "system, such as the Terminator Tether, the
¦ i
tether can be used to reduce the orbit of the spacecraft to which it is attached. If the. system has a means for collecting electrons from the ionospheric plasma at one end of the tether and expelling them back into the plasma at the other end of the tether, the voltage can drive a current along the tether. This current will, in turn, interact with the Earth's magnetic field to cause a Lorentz force which will decrease the orbit of the tether and its host spacecraft. Essentially, the tether converts the orbital energy of the host spacecraft into electrical power, which is dissipated as ohmic heating in the tether.
In an "electro dynamic propulsion" system, the tether can be used to boost the orbit of the spacecraft. If a power supply is added to the tether system and used to drive current in j.he direction opposite to that which is normally wants to flow, the tether can "push" against the Earth's magnetic field to raise the spacecraft's orbit. The major advantage of this technique compared to other space propulsion system is that it doesn't require any propellants. It uses the Earth's magnetic field as its "Reaction Mass". By eliminating the need to launch large amounts of propellants into orbit, electro dynamic tethers can greatly reduce the cost of in-space travel.
Electro dynamic tethers work by virtue of the force a magnetic field exerts on a current-carrying wire. Andre Marie Ampere, a pioneer in the study of electromagnetic phenomenon, first observed the phenomenon in the 19th century. In 1895, Hendrik. Lorentz summarized the phenomenon in the equation that now bears his name. The force acts on any charged particle moving through a magnetic field (including electrons
moving inia wire), in a direction perpendicular to both the direction of current flow and the magnetic field vector.
The basic principle of electro dynamic tether thrusters was first described in 1965 by S.D.Drell, H.M. Foley and M.A.Ruderman. In essence, it is clever way of getting an electric current to flow in a long conducting wire that is orbiting Earth, so that Earth's magnetic field will exert a force on and accelerate the wire and hence any payload attached to it. The direction of current flow through the tether, either towards or away from Earth, determines whether the magnetic force will add to or subtract from the tether's orbital energy, and therefore raise or lower its orbit.
The tether forms a unique type of electric circuits, which NASA demonstrated in space with the Plasma Motor Generator in 1993 and the Tethered Satellite System in 1992 and 1996. All three of these missions deployed long conducting tethers from orbiting spacecraft and generated kilowatts of power. ProSEDS will take the technology one step further, to produce thrust and de-orbit a payload.
The tethers is dragged through the atmosphere's ionospheric plasma, the rarefied medium of electrons through which the whole setup is traveling at a speed of 7-8 km/s. in so doing, the 5 km long aluminium wire extracts electrons from then plasma at the end farthest from the play load and carries them to the near end. There a specially designed device known as a hollow cathode emitter expels the electrons, to ensure their return to space. Currents in the plasma complete the circuit.
Ordinarily, a uniform magnetic field acting on a current-bearing loop of wire yields a net force of zero, since the force on one side of the loop is cancelled by that on the other side, in which the current is flowing in the opposite direction. However, since the tethered system is not mechanically attached to the plasma, the magnetic forces on the plasma currents in space do not cancel the forces on the tether, and so the tether experiences a net force.
As the tether cuts across the magnetic field, its bias voltage is positive at the end farthest from the Earth and negative at the near end. This polarization is due to the action of Lorentz force on the electron in the tether. Thus, the natural upward current flow is due to the (negatively charged) electrons in the ionosphere being attracted to the tether's far end and then returned to the plasma at the near end, aided by the hollow cathode emitter. The hollow cathode is vital: without it, the wire's charge distribution would quickly reach equilibrium , and no current would flow.
Switching on the hollow cathode causes a small tungsten tube to heat up and fill with xenon gas from a small tank, which contains just enough gas for the experiment's two-week run. Electrons from the tether interact with the heated gas to create an ion plasma. At the far end of the tube, a so called keeper electrode, which is positively charged with respect to the tube, draws the electrons and expels them to space. (The xenon ions, meanwhile, are collected by the hollow cathode and used to provide additional heating.)The rapid discharge of electrons invite new electrons to flow from the tether and out through the hollow cathode.
Earth's magnetic field exerts a drag force on the current-carrying tether, decelerating it and the payload and rapidly lowering their orbit. Eventually, they revision-enter Earth's atmosphere.
A key advantage of the electro dynamic tether is that it does not tax a satellite's or spacecraft's on-board power sources. The hollow cathode and other ProSEDS instruments, for example, will run on batteries charged by some of the power generated by the tether. If the tether were being used to raise rather than lower the payload's orbit, the current would have to be forced to flow in the other direction; the addition of a small solar-cell riower supply would do the trick. Electron Capturing
All the same, scooping up electrons from Earth's ionosphere is more easily suggested than done. Previous electro dynamic tether experiments used as the end mass either a large metallic satellite or a second hollow cathode. Unfortunately, a law of diminishing returns soon sets in; the density of electrons in the plasma limits the amount of current that can be collected in the given volume, and that limit is quickly approached as the satellite gets larger. The hollow cathode collector hits the similar limit. To attract
electrons, it emits xenon ions, but as the xenon gas flow is increased to attract more electrons, a collection threshold is soon reached.
PrdSEDS will use a radically different collection scheme, one that promises to be much more efficient and easily scalable to practical applications. Instead of a satellite or hollow cathode, the naked metallic tether itself will collect the electrons. Previous experiments used insulated wire to prevent arcing between it and the space shuttle deploying it. But because the ProSEDS tether will be in contact with the space plasma for most of its length, and because the entire rocket stage will be electrically connected to the tether, arcing should not be a concern.
Plasma chamber tests have verified that thin bare wires can collect current from plasma. Calculations indicate that the ProSEDS tether will be at least five times more efficient in electron collection than were previous space experiments, which used insulated wire and a satellite or hollow cathode as the electron collection device. The 1996 Tethered Satellite System, for example, collected a peak current of 1.1 A. and the 1993 Plasma Motor Generator only 0.3 A, whereas ProSEDS should see an average current of over 1 A, peak currents of 5 A, and average power of 1.46 kW. It is also predicted that ProSEDS will generate an verage thrust of about 1 N, which should dramatically hasten the payload's orbital decay, compared to a tether less reentry.
PRINCIPLE OF ORBIT SHIFTING
Using Electro Dynamic Tether
i
Consider a satellite with mass Msat orbiting a central body with mass of mass Mcentrai. The central body could be planet, the sun or some other large mass capable of causing sufficient acceleration on a less massive nearby object. If the satellite moves in circular motion, then the net centripetal force acting upon this orbiting satellite is given by the relationship
Fnet = (Msat X Vsat2)/R
This net centripetal force is the result of the gravitational force which attracts the satellite towards the central body, and can be represented as
, Fgrav = (GX MsatX Mcentrai)/R2
Since Fgrav = Fnet, the above expressions for centripetal force and gravitational force are equal. Thus,
(Msat X v2) / R = (G X Msat X MCentral) / R2
Observe that the mass of the satellite is present on both sides of the equation; thus it can be cancelled by dividing through by Msat. Then both sides of the equation can be multiplied by R, leaving the following equation. Velocity of a satellite moving around a central body is
I V2 = (GX Mcentrai)/R
Where G = 6.67 X 10-11 Nm2/Kg2, Mcentrai = the mass of the central body about which the satellite orbits, and R = the radius of the orbit for the satellite. In our case MCentrai is the mass of the Earth.
The direction of the velocity vector at every instant is in the direction tangent to the circle.
Therefore V2a 1/R
When v increases R decreases, i.e, the radius of the orbit decreases, i
A conductive tether material acts as a long wire moving through a magnetic field. This induces an electromotive force and corresponding current to move through the wire, with the surrounding plasma completing the circuit. The electromotive force or voltage potential depends directly on the field strength, the orbital velocity, and the tether length.
EMF= V = $v~ XBttl Where,
V is induced e.m.f across the tether length v is the velocity of the tether B is the magnetic field induced dl is the differential length
Now the direction of current flow due to the induced emf is given by Fleming's Right hand rule which states that when the forefinger, middle finger and thumb are mutually perpendicular to each other then the forefinger represents the direction of the field, the
thumb represents the direction of motion; the middle finger represents the direction of induced current.
In our case as the satellite is moving the current will be flowing g outwards, i.e., away from the Earth.
Now taking Lorentz right hand rule stated earlier F = Bqv
When G 90°., i.e., when v and B are perpendicular.
Here F is the force acting on the charge called Lorentz Force. The Right Hand Rule can be used to figure out the direction in which the force is acting.
Clearly, forces are required to overcome the Earth's gravitational attraction and to propel a satellite into orbit, with higher orbits requiring greater forces. Consider a satellite moving in a circular orbit. If the satellite is subjected to some force in the same direction as its motion, it will be propelled into a higher orbit and will travel at a slower speed according to the equation derived for orbital velocity. Conversely, if the satellite is subjected to some force in a direction opposite to its motion, it will be driven in to a lower orbit and will move at a greater speed.
Now in our case as the current is flowing outwards, the force will be opposite in direction to motion. So the satellites descend in orbit.
Now for propelling the satellite, the direction of current will have to be reversed so that the force is in the same direction as the motion of the satellite. This can be achieved by adding a battery to the tether which will work on solar energy and drive the current in the opposite direction.
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The motion of the satellite can be controlled by the hollow cathode which completes the circuit.
Using Momentum Exchange Tethers
The Tether Launch Assist concept combines the techniques Tarzen used to swing through the jungle with the principles of a simple electric motor to create a system capable of repeatedly picking payloads up from a sub orbital trajectory and boosting them to higher orbits. In this concept, illustrated in the figure to the right, a long high strength is deployed from an orbiting facility, and the tethered system is set into rotation so that, at the bottom of its swing, the tip of the tether is moving much slower than the center of mass of the system. A grapple system attached to the tip of the tether can thus reach down below the facility and rendezvous with a payload moving in a slower, sub orbital trajectory. The grapple would then capture the payload and pull it into orbit along with
the tether system. Later, it could release the payload at the top of the swing, tossing it into a higher orbit. When the tether system captures and releases the payload. it transfers some of its momentum and energy to the payload.
Tether Transport Facility
¢ 420 km altitude
¢ 5 tons for deployer and winch
¢ +145 tons ballast (ballast mass can be
supplied by shuttle external tanks) i
Payload Capture
¢ 5 tons
¢ Captured from a suborbital tra jectory
Tether Payload Released
¢ 290 km long one-half revolution later
¢ 5 tons with+1.2 km/sec
C '
0¬
i I
-¦
» 4k
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 marks, dramatically reducing costs for missions that involve delta-V hungry maneuvers such as formation flying, low-altitude station keeping , orbit raising, and end-of-mission de-orbit. TUI is developing several ED tether products including the uPET propulsion system and the Terminator Tether Satellite De-orbit Device.
Power Generation in Low Earth Orbit
Electro dynamic tethers may also provide an economical means of electrical power in orbit. Essentially, the tether can be used to convert some of the spacecraft's orbital energy into electrical power. However, since converting the orbital energy into electrical power will lower the orbit of the spacecraft (there is no such thing as a free lunch), this technique is probably only useful for providing high power energy bursts to short duration experiments.
Electro Dynamic Revision-Boost of the International Space Station
The International Space Station (ISS) will experience a small but constant aerodynamic drag force as it moves through the thin upper reaches of the Earth's atmosphere. This drag force will cause the station's orbit to decay. If nothing were done to counter act this, the station would fall out of orbit within several months. NASA currently plans to launch several large rockets every year to carry fuel up to the station so that it can re-boost its orbit. These launches, however, will be very costly.
Tethers Unlimited Inc. has helped NASA to explore the potential for using electro dynamic tether propulsion to maintain the orbit of the ISS. By using excess power generated by the ISS's solar panels to drive current through the conducting tether, a tether re-boost system could counter act the drag forces or even raise the station's orbit. NASA and TUI's studies revealed that such a tether re-boost system could reduce or eliminate the need for dedicated launches for re-boost propellant. potentially saving up to $2 billion over the first ten years of the station's operation.
Space Junk Cleanup
The most direct application of ProSEDS would be to get rid of space junk. Over the past half century of space exploration, the region around Earth has become cluttered with debris, which could take years, and in some cases centuries, to fall from orbit. The
danger is that all satellites and rocket stages and trash thrown over board by early space travelers could collide with working satellites, space shuttle, and orbiting space station. The International Space Station, for example, maneuvers several times a year to avoid hitting debris, burning up precious rocket fuel each time. NASA and the European Space Agency have recommended that governments require future spacecraft to be able to take themselves otit of orbit at the end of their life spans.
To do that, a satellite could be loaded up with extra propellant, to thrust to revision-entry. But that would add as much as 25% to the satellite's weight. The propulsion system, too, would need to remain functional after sitting in orbit for ten years or more.
Using a tether to de-orbit would be inherently more reliable. For one thing, it is an electromechanical system, with no complex valves, plumbing, or circuitry that must stay operational and leak free for years. Also, ED tethers are much lighter and more compact than conventional thrusters: a tether system would account for as little as 2 % of the satellite total weight and could be easily bolted to the satellite. Once the end of the satellite's useful life is reached, the tether would unreel, and the tether driven orbital decay would begin. The satellite would then burn up during revision-entry. One of the commercial partners on the ProSEDS effort, Tethers Unlimited Inc., based in Clinton, Wash., is developing a commercial version of the de-orbit system, known as the Terminator Tether.
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Satellite Tugboat
Another idea is for the ED tether to be attached to an unmanned space tugboat that would ferry satellites to higher orbits. After being launched in to low Earth orbit, the so called Orbital Transfer Vehicle would grapple the satellite and maneuver it to a new altitude or inclination. The tug could then lower its own orbit to rendezvous with another payload and repeat the process. Conceivably, several such orbital revision-assignments could be performed without the need for rocket propellant, making the tug relatively inexpensive to operate. But because plasma density diminishes rapidly with distance from Earth, the tug could only operate below about 2,300km. That would still make it practical
for about half the payload now scheduled for launch, which have destined for orbits of around 2200km. The lowest usable altitude would be about 250km, below which the atmosphere would begin to exert too much aerodynamic drag.
Exploring the Outer Planet
Perhaps the most exotic use of ED tether technology would be to propel and power spacecraft exploring the outer planets. Existing vessels have relied on solar cells, but at distances far from the Sun, the power available is typically less than 100 W. Jupiter and its moons have an environment particularly favorable to ED tethers; the planet has a strong magnetic field and a rapid rotation rate, and its mass dictates high orbital velocities. With the magnetic field moving much faster than the spacecraft, the tether
would essentially be stealing energy from the planet's magnetic field.
i
In theory the tether could power the craft's instruments and generate thrust at one and the same time for a circular orbit close to the planet, tether propulsive forces have been calculated to be as high as 50 N and power levels as high as 1 MW. This level of power would sustain a whole new suit of science instruments such as high power radars, but it also means having to deal with power conversion, energy dissipation and tether, overheating.
To address the many remaining performances and operational issues, a follow-on experiment to ProSEDS will need to be flown. Such an experiment would demonstrate the tether's use in raising altitudes and changing orbital inclinations in a series of predictable, repeatable flight profiles. NASA researchers have proposed such an experiment, but it has yet to receive funding.
Momentum Exchange Space Tethers
Net Ten si en
stronger oentrifu gal force
Net \
Tension
A momentum exchange tether is a long thin cable used to couple two objects in space together so that one transfers momentum and energy to the other. A tether is deployed by pushing one object up or down from the other. Once the two objects are separated by enough distance, the difference in the gravitational force at the two locations will cause the objects to be "pulled" apart. This is called the "gravity gradient force". The tether can then be let out at a controlled rate, pulled by the tension caused by the gravity gradient force. Once the tether is deployed, if there are no other forces on the tether it will have an equilibrium orientation that is aligned vertically. There are a number of different concepts for momentum exchange using tethers. Some general categories are:
A stationary tether is one that connects two masses together and remains at constant length, except, of course, for deployment and retrieval. A stationary tether could drag a payload through the upper atmosphere of a planet and lower payloads to the surface of an asteroid. If the tether is conducting and is moving through electric or magnetic fields, then it can be used as a generator to provide electrical power, or as a motor to provide propulsion. If the tether and its masses are orbiting a massive body, then
typically the system will be gravity gradient stabilized, with the tether pointed along the radius vector to the massive body. Thus, although the tether is stationary in the orbital reference frame, it is really rotating once per orbit in inertial space, and so is a slowly rotating bolo.
A bolo is a long rotating cable anywhere in space that is used as a "momentum-energy bank". It could be used to "catch" a payload coming from any given direction (in its plane of rotation) at any given speed (less than its maximum tip speed), and then some time later, "launch" the payload off in some other direction at some other speed. A gravity gradient stabilized bolo orbiting some planet has the property that if the tether is cut, then one-half an orbit later, the separation distance between the two masses is seven times larger than the initial separation. This can be used to deorbit the lower mass, or throw the upper mass to a rendezvous or to escape.
A "rotovator" is a long bolo in low orbit around a planet (or moon) that is used as
a giant elevator to reach down from space to lift payloads from a planet or to deposit i
payloads onto a planet. To reach the surface of the planet, the orbital altitude should be equal to half the length of the rotating cable. By proper adjustment of the cable rotation period to the orbital period of the center of mass of the cable (plus or minus the planetary rotation period), the relative velocity of the planetary surface and the tip of the cable can be made zero at the time of touchdown, allowing for easy payload transfer. A half-rotation later, the payload is at the top of the trajectory with a cable tip velocity that is twice the orbital velocity. Although present day material strengths do not allow the construction of rotovators around Earth or the major planets, they can be built for Mars, Mercury, and most moons, especially including Earth's Moon.
Tip Velocity and Material Strength
The maximum tip speed of all these systems is a function of the "launcher to payload mass ratio" of the tether system and the "characteristic velocity" of the material used. The characteristic velocity of the material in a tether is given by the square root of the ratio of the design tensile strength T of the tether to the density D of the tether material, u = (T_d/D) Al/2. In practice, the design tensile strength is usually chosen to be 50% of the measured strength for metals and 25% of the measured short-term individual fiber strength for other materials. Thus, using imperfect materials with reasonable safety margins, the characteristic velocity of most metals and fibers is around 1 km/s, with" optimistic predictions for graphite and improved polymers reaching 3 km/s. With the development of a design for a high strength-to-weight tapered Hoy tether, the design tensile strength can be safely chosen to be 60% of the measured strength of the individual fibers, allowing commercially available fibers to have characteristic velocities up to 4 km/s.
EXPERIMENTS AND SUCCESSES
The first space tether flew with the Gemini II astronauts in 1996. that was a 30 meter long non conducting tether made of parachute webbing, and it linked the piloted spacecraft with a rockets upper stage. In doing so, the tether stabilized the spacecraft as it orbited the earth. Since then there have been at least 17 space missions with tethers. The Small Expendable Deployer System (SEDS), on which the upcoming Propulsive SEDS experiment is based, has flown successfully four times. The first two missions, in 1993 and 1994, were mainly intended to validate tether development. They used a 20 Km long non-conducting tether.
The deployer was most recently used in 1996, for the U.S.Naval Research Laboratory's Tether Physics and Survivability (TiPS) experiment. Two small end masses, nicknamed Ralph and Norton, were connected by a 4.2Km long non-conducting tether. TiPS was designed to demonstrate a tether's longevity. It remains in orbit today.
The idea of Ed tether has been around for 35 years, but it was the 1990s before one flew in space. The 1993 Plasma Motor Generator mission used an insulated ED tether equipped with a hollow cathode end mass to collect electrons.
In 1992 and 1996, NASA flew the Tethered Satellite System (TSS) on the space shuttle orbiter.
The p PET Propulsion System
Propellant less Electro Dynamic Tether Propulsion for Micro satellites
TUI is currently developing a propulsion system called the '"Micro satellite Propellant less Electro dynamic Tether (pPET„¢) Propulsion System" that will provide
propulsive capabilities to micro satellites and other small spacecraft without consuming
¦i
propellant.
How it works
ED tethers can provide long-term propellant less propulsion capability for orbital maneuvering and station keeping of small satellites in low-Earth-orbit. The uPET„¢ Propulsion System is a small, low-power ED tether system designed to provide long-duration boost, deboost, inclination change, and station keeping propulsion for small satellites. Because the system uses electrodynamic interactions with the Earth's magnetic field to propel the spacecraft, it does not require consumption of propellant, and thus can provide long-duration operation and large total delta-V capability with low mass requirements. Furthermore, because the uPET„¢ system does not require propellant, it can easily meet stringent safety requirements such as are imposed upon Shuttle payloads. In addition, the tether system can also serve as a gravity-gradient attitude control element, reducing the ACS requirements of the spacecraft.
Characteristics:
The mass, size, and power requirements of the uPETIM Propulsion System depends upon the size of the satellite and the propulsive mission. TUI has developed a prototype of a uPET„¢ sized for a 125 kg micro satellite, which could raise the orbit of this satellite from a 350 km drop-off orbit to a 700 km operational orbit within 50 days.
Electro Dynamic Tether
1
BARRIERS TO OVERCOME
One question is the long-term survival of the tethers. While the atmosphere at Low Earth Orbit (LEO) altitude is extremely thin -millions of times thinner than the air at sea level -it is largely composed of atomic oxygen, which is very corrosive.
High velocity micrometeorites pose an even more prominent problem. High velocity meteorites can easily rupture the tether and can tear the tether apart materials with greater strength are being developed to overcome this barrier.
FUTURE
To Explore Jovian System
Researchers at the Marshall Center also are investigating the use of ED tethers to
extend and enhance future scientific missions to Jupiter and its moons. In theory, Ed
tether propulsion could be used near any planet with a Previous visits to the largest planet
in the solar system - including the "Grand Tour" flyby missions of Voyager 1 and 2,
launched in 1977, and an orbital visit by the Galileo probe, which left Earth in 1989 and i
continues to tour and study the Jovian system today- were illuminating, but the fuel limitations and minimum maneuverability of those probes hampers long term, more detailed scientific study. Development of a propellant free, ED tether propulsion system would make it possible to put a long term probe in Jupiter's orbit - one that could leverage the planet's powerful magnetic field and magneto sphere to travel freely among the Jovian moons, providing new insight about them as well.
Tether Transport from Low Earth to the Lunar Surface
A concept developed by Tethers Unlimited wherein several rotating tethers in orbit around the earth and moon may provide a means of exchanging supplies between low Earth orbit facilities and Lunar bases without requiring the use of propellants.
Tethers for Rapid Autonomous Deorbit of Leo Satellites
Tethers Unlimited Inc. is currently developing a system called "Terminator Tether" that will provide a low cost, light weight and reliable method of removing objects from Low Earth Orbit (LEO).
CONCLUSION
Electrodynamic tether now becoming the most popular fuel carrier for space crafts. ED tethers can provide long-term propellant less propulsion capability for orbital maneuvering and station keeping of small satellites in low-Earth-orbit. Electro dynamic tethers may also provide an economical means of electrical power in orbit. TUI is currently developing a propulsion system called the "Micro satellite Propellant less Electro dynamic Tether (uPET„¢) Propulsion System". The universe is eagerly waiting for the application of these tethers in future.
REFERENCE
i
Websites:
> http://www.tethers.com
> http://www.spacedomain.com
> http://www.efymag.com
> http://www.ieee.com
Books an,d Magazines:
¢ THE TERMINATOR TETHER„¢ - HOYT, R.P., FORWARD, R.L
¢ DYNAMICS OF SPACE TETHER SYSTEMS - BELETSKII, V.V.. LEVIN,
¢ STABILIZATION OF ELECTRODYNAMIC - HOYT. R.P & HEINEN
SPACE TETHERS
¢ ELECTRON ICS FOR YOU - JANUARY 2005
¢ ELECTRONICS TODAY - JUNE 2004

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22-08-2010, 03:13 PM
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RE: electrodynamic tether full report
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