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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.
1 REVIEW OF EXISTING ROCKET PROPULSION MECHANISM
3 HISTORY OF SPACE TETHERS
6 STABILIZATION OF ELECTRODYNAMIC TETHERS
7 ED TETHERS APPLICATION
9 WHY TETHERS WIN
10 CONCLUSION AND FUTURE SCOPE
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.
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
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.
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-Vâ„¢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 Ã‚ÂµPET Propulsion System and Terminator Tether Satellite Deorbit Device.
a.The Ã‚ÂµPET Propulsion System:
Propellantless Electrodynamic Tether Propulsion for Microsatellites
TUI is currently developing a propulsion system called the "Microsatellite Propellantless Electrodynamic Tether (Ã‚ÂµPETâ€žÂ¢) 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 Ã‚ÂµPETâ€žÂ¢ 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 Ã‚ÂµPETâ€žÂ¢ 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.
The mass, size, and power requirements of the Ã‚ÂµPETâ€žÂ¢ Propulsion System depends upon the size of the satellite and the propulsive mission. TUI has developed a prototpye of a Ã‚ÂµPETâ€žÂ¢ 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 Tetherâ€žÂ¢.
Fig.. The Terminator Tetherâ€žÂ¢ Deployer.
Tethers Unlimited Inc. is currently developing a system called the Terminator Tetherâ€žÂ¢ 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 Tetherâ€žÂ¢ 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 Tetherâ€žÂ¢ 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 Tetherâ€žÂ¢ 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 Tetherâ€žÂ¢ 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
Life in low gravity
Flames, fluids and metal in space
The nature of space
Watching the Earth
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 satelliteâ„¢s power, propulsion and guidance be working at the end of the satelliteâ„¢s mission. Moreover, it doesnâ„¢t really solve the problem â€œit 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.
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.
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.
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
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 craftâ„¢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 radarâ€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.
IEEE spectrum. July 2000