Electro Dynamic Tether

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WELCOME



CONTENTS

1 INTRODUCTION

2 EXISTING ROCKET PROPULSION MECHANISM

3 HISTORY OF SPACE TETHERS4 PRICIPLE

5 WORKING

6 ED TETHERS APPLICATION

7 ADVANTAGES

8 WHY TETHERS WIN

9 CONCLUSION AND FUTURE SCOPE



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 electrodynamicTether 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 acurrent carrying wire

Working with Earths magnetic field would benefit a number of spacecraft including the International Space Station

Tether propulsion requires no fuel. Is completely reusable andenvironmentally clean and provides all these features at low cost



EXISTING PROPULSION MECHANISM

The existing rocket propulsion mechanism derivesenergy from rocket fuels.

The rocket fuel is burnt inside a chamber and gasproduced due to combustion is expelled out through anozzle, which produces the upward thrust for rocketsor spacecrafts.

The currently available rocket fuels are in solid liquidand as from Hydrogen peroxide, Cold gas , keresenebut each produces many disadvantages.

Nuclear energy is also uses but it produces radiations.



An electrodynamic tether with its unique features putforward a better option for propulsion of rockets and

spacecrafts.



HISTORY OF SPACE TETHERSo since in the 20th century, it wasnt until 1947 that

Giuseppe Colombo came up with the idea of using a

long tether to support a satellite System (TSS) toinvestigate plasma physics and the generation of electricity in the upper atmosphere.

o The best-known applications are the tethers that

connect spacewalking astronauts to their spacecraft.



4. PRINCIPLE

y The basic principle of an electrodynamic tether is Lorentzforce. It is the force that a magnetic field exerts on a currentcarrying wire in a direction perpendicular to both thedirection of current f low and the magnetic field vector.

y The Dutch physicist Hendrik Androon Lorentz showedthat a moving electric charge experiences a force in amagnetic field. (if the charge is at rest, there will not be any

force on it due to magnetic field ) Hence it is clear that theforce experienced by a current conductor in a magneticfield is due to the drifting of electrons in it.



Lorentz Force Low

y The Lorentz Force Low can be used to describe theeffect of a charged particle moving in a constant

magnetic field. The simplest form of this low given by the scalar equation

y F = QvB

y F is the force acting on the particle (vector)

y V is the velocity of the particle (vector)y Q is charge of particle (scalar)

y B is magnetic field (vector)



Flemings Left Hand Rules

y For a charged particle moving (velocity v) in amagnetic field (field B) the direction of the resultant

force (force F) can be found by y MIDDLE FINGER of left hand in direction of

CURRENT

y INDEX FINGER of left hand in direction of FIELD. B

y THUMB now points in direction of the FORCE OR MOTION. F



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 CURRENTINDEX FINGER of left hand in direction of FIELD. BTHUMB 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. Just pretend the followingpicture is.



An electrodynamic tether is essentially a long conducting wire extended from a space craft. The electrodynamictether is made from aluminium alloy and typically between5 and 20 kilometers long. It extends downwards from anorbiting platform. Aluminium alloy is used since it is

strong, lightweight, inexpensive and easily machined.The gravity gradient field (also known as tidal force) willtend to orient the tether in a vertical position. If the tetheris orbiting around the Earth, it will be crossing the earthsmagnetic field lines orbital velocity (7-8 km/s). The motionof the conductor across the magnetic field induces a

voltage along the length of the tether. This voltage can upto several hundred volts per kilometer.



In the above figure the sphere represents the Earth andthe unbroken lines represents Earths magnetic field.The broken line is LEO. As shown in the figure there isa drag force experienced in the wire in a directionperpendicular to the current and magnetic field vector.



y In an electrodynamic tether drag system such as theterminator Tether, the tether can be used to reduce theorbit of the spacecraft to which it is attached. If the system

has a means for collecting electrons from the ionosphericplasma at one end of the tether and expelling them back into the plasma at the other end of the tether, the voltage candrive a current along the tether. This current bill, in turn,interact with the Earths magnetic field to cause a Lorentz JXB force, which will oppose the motion of the tether and

whatever it is attached to. This electrodynamics dragforce will decrease the orbit of the tether and its hostspacecraft. Essentially, the tether converts the orbitalenergy of the host spacecraft in to electrical power, which isdissipated as ohmic heating in the tether.



. This current bill, in turn, interact with the Earths 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.



propellant less propulsion for LEO spacecraft:

ED tether system can provide propellant lesspropulsion for spacecraft operating in low Earth orbit.Because the tether system does not consumepropellant



TUI is currently developing a propulsion system called the

"Microsatellite Propellantless Electrodynamic Tether (µPET)Propulsion System" that will provide propulsive capabilities tomicrosatellites and other small spacecraft without consuming

propellant.



y Characteristics:The mass, size, and power requirements of the µPET

Propulsion System depends upon the size of thesatellite and the propulsive mission. TUI hasdeveloped a prototpye of a µPET sized for a 125 kgmicrosatellite which could raise the orbit of thissatellite from a 350 km drop-off orbit to a 700 kmoperational orbit within 50 days.



Tethers Unlimited Inc. is currently developing a systemcalled the TerminatorTether 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.



ADVANTAGESy The operational advantages of electrodynamic tethers

of moderate length are becoming evident from studies

of collision avoidance. Although long tethers (of orderof 10 kilometers) provide high efficiency and goodadaptability to varying plasma conditions, boostingtethers of moderate length (~1 kilometer) and suitabledesign might still operate at acceptable efficienciesand adequate adaptability to a changing environment.



y ED tethers used for propulsion in low-Earth orbit andbeyond could significantly reduce the weight of upperstages used to boost spacecraft to higher orbit. Muchof the weight of any launch vehicle is the propellantand It is expensive to lift heavy propellants off theground.

y Since ED tethers require no propellant, they could

substantially reduce the weight of the spacecraft andprovide a cost effective method of reboostingspacecraft, such as the International Space Station(ISS)



. WHY TETHERS WIN?y Normal Launch from ground

y 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 2rocket has to provide a Delta-V about 9.5km/s. Youneed to circularize your orbit which means firing the

engine again about 45 minutes after launch. Thisrestart of the engine only needs to provide about 0.1 to1.15 km/s depending upon the altitude of the orbit.



y A ir Launch from 20 km to tether at 100 kmaltitude

We need to be doing about 5 km/s when we getto the end of the tether. We loose about 0.5km/s fromclimbing from 20 km to 100 km and air drag. We getabout 0.5km/s from spin of Earth. There is no need to

circularize the orbit as the tether has a big ballast massand is in orbit. Net is rocket needs to provide a delta-V of about 5 km/s.



y W hat happened?y 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 centreof mass) the orbital velocity is 7.56km/s. We have saved0.29km/s already.

y Our final design uses a tether tip speed of 2.5km/s relative to thecentre of mass. So relative to the centre of Earth it is movingabout 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

y

We get about 0.5 km/s from the rotational speed of the earth andso only need 4.s km/s after altitude and drag loss. Starting from20 km high we dont loose so much to drag. Our air launch willgives us a running start, perhaps 0.2 km/s. Reduced air pressureenables a more efficient rocket engine.



y W hat is the result?

y We need around the half the Delta-V. We needed a

two-stage before but we only need one stage rocketnow. It is right to think of it as only being the secondstage. The first stage could have 5-10 times as large asthe second stage, so we have saved a lot.



Satellite Tugboat Another idea is for the ED tether to be attached to an

unmanned space tugboat that would ferry satellites to higherorbits. After being launched in to low Earth orbit, the so called

Orbital Transfer Vehicle would grapple the satellite andmaneuver it to a new altitude or inclination. The tug could thenlower 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. Existingvessels 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 fromthe planets magnetic field.



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Electro Dynamic Tether