Report 7th Sem

8/8/2019 Report 7th Sem

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,QWURGXFWLRQElectrodynamic (ED) tether is a long conducting wire extended fromspacecraft. It has a strong potential for providing propellant less propul-sion to spacecraft in low earth orbit. An electrodynamic Tether uses thesame principle as electric motor in toys, appliances and computer diskdrives. It works as a thruster, because a magnetic field exerts a force on acurrent carrying wire. The magnetic field is supplied by the earth. Byproperly controlled the forces generated by this electrodynamic tethercan be used to pull or push a spacecraft to act as brake or a booster.NASA plans to lasso energy from Earths atmosphere with a tether act aspart of first demonstration of a propellant-free space propulsion system,

potentially leading to a revolutionary space transportation system.Working with Earths magnetic field would benefit a number of space-craft including the International Space Station. Tether propulsion re-quires no fuel. Is completely reusable and environmentally clean andprovides all these features at low cost. Electrodynamic tethers are longconducting wires, such as one deployed from a tether satellite, which canoperate on electromagnetic principles as generators, by converting theirkinetic energy to electrical energy, or as motors, converting electrical

energy to kinetic energy. Electric potential is generated across a conduc-tive tether by its motion through the Earth's magnetic field. The choice of the metal conductor to be used in an electrodynamic tether is determinedby a variety of factors. Primary factors usually include high electricalconductivity, and low density. Secondary factors, depending on theapplication, include cost, strength, and melting point.


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+ LVWRU\ RI VSDFH WHWKHUVWhile space-based tethers have been studied theoretically since in the20th century, it wasnt until 1947 that Giuseppe Colombo came up withthe idea of using a long tether to support a satellite System (TSS) to

investigate plasma physics and the generation of electricity in the upperatmosphere. Up until the TSS the use of tethers in space has been li-mited. The best-known applications are the tethers that connect space-walking astronauts to their spacecraft. Astronauts can work and fly freeof the Space Shuttle using the Manned Maneuvering Unit (MMU), but formost work activities in the Shuttle payload bay (and during the assemblyof the International Space Station) astronauts still use a safety tether.

However, spacewalk tethers are very short and are not stabilized bygravitational forces. The TSS-IR mission and rocket-launched experi-ments, such as the SMALL expendable Deployer System (SEDS) and thePlasma Motor Generator (PMG), have increased our understanding of theway tethers behave in space. Each used different types of tether to deploysatellites and conduct research, demonstrating the usefulness of tethertechnology.


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3 ULQFLSOHThe basic principle of an electrodynamic tether is Lorentz force. It is theforce that a magnetic field exerts on a current carrying wire in a direction

perpendicular to both the direction of current flow and the magnetic fieldvector.

The Dutch physicist Hendrik Androon Lorentz showed that a movingelectric charge experiences a force in a magnetic field. (If the charge is atrest, there will not be any force on it due to magnetic field) Hence it isclear that the force experienced by a current conductor i n a magnetic fieldis due to the drifting of electrons in it. If a current I flows through aconductor of cross-section a then

I = neAv where v is the drift speed of electronics n is number density inthe 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 representsthe 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 isgiven by

dF = IdIB

i.e.,dF = qvB

This fundamental force on a charge q moving with a velocity v in amagnetic field B is called the Magnetic Lorentz Force.

4.1 Lorentz Force Low


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The Lorentz Force Low can be used to describe the effect of a chargedparticle moving in a constant magnetic field. The simplest form of thislow 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 useF = QvB (sin (X) ) where X is the angle between v and B, when v and Bare perpendicular X =90 deg. So sin (x) =1.

Flemings left hand rule comes in to play here to figure out which way theforce is acting

4.2 Flemings Left Hand Rules

For a charged particle moving (velocity v) in a magnetic field (field B) thedirection 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 nomatter what the angle between v and B is.


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: RUNLQJ An electrodynamic tether is essentially a long conducting wire ex-tended from a space craft. The electrodynamic tether is made from

aluminium alloy and typically between 5 and 20 kilometers long. Itextends downwards from an orbiting platform. Aluminium alloy isused since it is strong, lightweight, inexpensive and easily machined.

The gravity gradient field (also known as tidal force) will tend toorient the tether in a vertical position. If the tether is orbiting aroundthe Earth, it will be crossing the earths magnetic field lines orbitalvelocity (7-8 km/s). The motion of the conductor across the magneticfield induces a voltage along the length of the tether. This voltage canup to several hundred volts per kilometer.

In the above figure the sphere represents the Earth and the unbro-ken lines represents Earths magnetic field. The broken line is LEO.

As shown in the figure there is a drag force experienced in the wire ina direction perpendicular to the current and magnetic field vector.

In an electrodynamic tether drag system such as the terminatorTether, 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 elec-trons from the ionospheric plasma at one end of the tether and expel-ling them back in to the plasma at the other end of the tether, thevoltage can drive a current along the tether. This current bill, inturn, interacts with the Earths magnetic field to cause a LorentzJXB force, which will oppose the motion of the tether and whatever itis attached to. This electrodynamics drag force will decrease the or-bit of the tether and its host spacecraft. Essentially, the tether con-verts the orbital energy of the host spacecraft in to electrical power,which is dissipated as ohmic heating in the tether.


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7 HWKHUV DV *HQHUDWRUV An electrodynamic tether is attached to an object, the tether beingoriented at an angle to the local vertical between the object and a planet

with a magnetic field. When the tether cuts the planet's magnetic field, itgenerates a current, and thereby converts some of the orbiting body'skinetic energy to electrical energy. As a result of this process, an electro-dynamic force acts on the tether and attached object, slowing their orbitalmotion. The tether's far end can be left bare, making electrical contactwith the ionosphere. Functionally, electrons flow from the space plasmainto the conductive tether, are passed through a resistive load in a controlunit and are emitted into the space plasma by an electron emitter as free

electrons. In principle, compact high-current tether power generators arepossible and, with basic hardware, 10 to 25 kilowatts appears to beattainable.

NASA has conducted several experiments with Plasma Motor Generator(PMG) tethers in space. An early experiment used a 500 meter conduct-ing tether. In 1996, NASA conducted an experiment with a 20,000-meterconducting tether. When the tether was fully deployed during this test,the orbiting tether generated a potential of 3,500 volts. This conducting

single-line tether was severed after five hours of deployment. It is be-lieved that the failure was caused by an electric arc generated by theconductive tether's movement through the Earth's magnetic field.

When a tether is moved at a velocity (v) at right angles to the Earth'smagnetic field (B), an electric field is observed in the tether's frame of reference. This can be stated as:

E = v * B = vB

The direction of the electric field (E) is at right angles to both the tether'svelocity (v) and magnetic field (B). If the tether is a conductor, then theelectric field leads to the displacement of charges along the tether.


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6 WDELOL]DWLRQ RI (' 7 VElectrodynamic tethers have strong potential for providing propel-lant less propulsion to spacecraft in low-earth orbit for application

such as satellite deorbit, orbit boosting and station keeping. Howeverelectro dynamic tethers are inherently unstable. When a tether in anorbit carries a current along its length, the interaction of the tetherwith the geometric field creates a force on the tether that is directedperpendicular to the tether. The summation of these forces along thelength of the tether can produce a net propulsive force on the tethersystem, raising or lowering its orbit. The tether however is not a rigidrod held above or below the spacecraft it is a very long thin cable and

has little or no flexural rigidity. The transverse electrodynamic forcestherefore cause the tether to bow and to swing away from the localvertical. Gravity gradient forces produces a restoring force that pullsthe tether back towards the local vertical but this results in a pendu-lum-like motion. Because the direction of the geomagnetic field variesas the tether orbits the Earth the direction and magnitude of theelectrodynamic forces also varies and so this pendulum motion devel-ops in to complex librations in both the in-plane and out-of-plane di-rection. Due to coupling between the in-plane motion and longitudin-al elastic oscillations as well as coupling between in-plane and out-cf-plane motions an electrodynamic tether operated at a constant cur-rent will continually add energy to the libration motions, causing thelibration amplitudes to build until the tether begins rotating or oscil-lating wildly In addition orbital variations in the strength and mag-nitude of the electrodynamic force will drive transverse higher orderoscillations 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 pr e-vent the unstable growth of librations transverse oscillations andskip rope modes.

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ED tether system can provide propellant less propulsion for space-craft operating in low Earth orbit. Because the tether system doesnot consume propellant, it can provide very large delta-Vs with avery small total mass dramatically reduce the cost for missions thatinvolve 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 I PET Propul-sion System and Terminator Tether Satellite Deorbit Device.

a. The I PET Propulsion System:

Propellant less Electrodynamic Tether Propulsion for Microsatellites

TUI is currently developing a propulsion system called the "Microsa-tellite Propellantless Electrodynamic Tether ( I PET ) PropulsionSystem" that will provide propulsive capabilities to microsatellites

and other small spacecraft without consuming propellant.

The I PET Propulsion System is a small, low-power electrodynamictether system designed to provide long-duration boost, deboost, incli-nation change, and station keeping propulsion for small satellites.Because the system uses electrodynamic interactions with theEarth's magnetic field to propel the spacecraft, it does not requireconsumption of propellant, and thus can provide long-duration opera-tion and large total delta-V capability with low mass requirements.Furthermore, because the I PET system does not require propel-lant, it can easily meet stringent safety requirements such as are im-posed upon Shuttle payloads. In addition, the tether system can alsoserve as a gravity-gradient attitude control element, reducing the

ACS requirements of the spacecraft.

a. The Terminator Tether Satellite Deorbit System:

Low-Cost, Low-Mass End-of-Mission Disposal for Space Debris Miti-

gation

Tethers Unlimited Inc. is currently developing a system called theTerminator Tether that will provide a low-cost, lightweight, and re-liable method of removing objects from low-Earth-orbit (LEO) to mi-tigate the growth of orbital debris.


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The Terminator Tether is a small device that uses electrodynamictether drag to deorbit a spacecraft. Because it uses passive electro-magnetic interactions with the Earth's magnetic field to lower theorbit of the spacecraft, it requires neither propellant nor power. Thusit can achieve autonomous deorbit of a spacecraft with very low massrequirements.

Concept of operations:

Before the spacecraft is launched, the Terminator Tether is boltedonto the satellite. While the satellite is operational, the tether iswound on a spool, and the device is dormant, waking up periodicallyto check the status of the spacecraft and listen for activation com-mands. When the Terminator Tether receives a command to deor-bit the spacecraft, it deploys a 5 kilometer long tether below thespacecraft. This tether interacts with the ionospheric plasma and thegeomagnetic field to produce currents running along the tether, andthese currents in turn cause forces on the tether that lower the orbitof the tethered spacecraft. Over a period of several weeks or months,the Terminator Tether will reduce the orbital altitude of the space-craft until it vaporizes in the upper atmosphere.

Electrodynamic Reboost of the International Space Station:

The International Space Station is the largest and most complex in-ternational scientific project in history. And when it is complete justafter the turn of the century, the station will represent a move of un-precedented scale off the home planet Led by the United States theInternational Space Station draws upon the scientific and technologi-cal resources of 16 nations Canada, Japan, Russia. 11 nations of theEuropean Space Agency and Brazil.

Its construction started at 1998 November 20 when Russia launchedZarya control module. More than four times as large as the RussianMir space station the completed International Space Station willhave a mass of about 1,040,000 pounds. It will measure 356 feetacross and 290 feet long with almost an acre of solar panels to pro-vide electrical power to 6 State-of-the-art laboratories. The stationwill be in an orbit with an altitude of 250 statute miles with an incli-nation of 51.6 degrees. This orbit allows the station to be reached by


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the launch vehicles of all the international partners to provide a ro-bust capability for the delivery of crews and supplies.


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: K\ WHWKHUV ZLQNormal Launch from ground

Circular velocity is about 8km/s at Low Earth Orbit (LEO). You loosearound 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. Youneed to circularize your orbit which means firing the engine again about45 minutes after launch. This restart of the engine only needs to provideabout 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 tocircularize 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) theorbital 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 centreof mass. So relative to the centre of Earth it is moving about5.06km/s(7.56-2.5). Between the two we are 2.79(2.5+0.29) km/s beloworbital speed at 100 km

We get about 0.5 km/s from the rotational speed of the earth and so onlyneed 4.s km/s after altitude and drag loss. Starting from 20 km high wedont 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 butwe only need one stage rocket now. It is right to think of it as only beingthe second stage.


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6 XUJHVConductive tethers have failed from unexpected current surges. Un-expected electrostatic discharges have cut tethers (e.g. see TetheredSatellite System Reflight (TSS-1R) on STS-75), damaged electronics,

and welded tether handling machinery. It may be that the Earth'smagnetic field is not as homogeneous as some engineers have be-lieved.


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$ G DQWDJHV The operational advantages of electrodynamic tethers of moderate lengthare becoming evident from studies of collision avoidance. Although longtethers (of order of 10 kilometers) provide high efficiency and good adap-

tability to varying plasma conditions, boosting tethers of moderate length(~1 kilometer) and suitable design might still operate at acceptableefficiencies and adequate adaptability to a changing environment.

ED tethers used for propulsion in low-Earth orbit and beyond couldsignificantly reduce the weight of upper stages used to boost spacecraft tohigher orbit. Much of the weight of any launch vehicle is the propellantand It is expensive to lift heavy propellants off the ground.

Since ED tethers require no propellant, they could substantially reducethe weight of the spacecraft and provide a cost effective method of re-boosting spacecraft, such as the International Space Station (ISS).

1. Electro dynamic tether systems- In this type two heavy masses areseparated by means a long flexible extensive electrically conductive cable.This can perform various functions in space craft.

2. In low earth orbit tether can provide electrical power and positioningcapability for satellites and space crafts.

3. In the case of long term mission tethers could drastically reduce theamount of fuel needed.

Now tethers are made of electrically conducting materials like aluminiumor copper. This provides additional advantages to the tethers. An electrodynamic tether is a device use to convert orbital energy to electricalenergy and it works on the principle of electro magnetic induction. This

can be used for power generation. Tethers found applications in orbitraising, lowering and debris removal. Another application of tether isartificial gravity inside space crafts.


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Report 7th Sem

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