electrodynamic tethers

Seminar Report’06 Electrodynamic Tethers

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

Earth’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 Earth’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 Dept. of Electronics and Communication Vimal Jyothi Engineering College,chemperi


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 earth’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 20 th

century, it wasn’t until 1947 that Giuseppe Colombo came up with the idea of

using a long tether to support a satellite System (TSS) to investigate plasma

physics and the generation of electricity in the upper atmosphere.Up until the

TSS the use of tethers in space has been limited. The best-known applications

are the tethers that connect spacewalking astronauts to their spacecraft.

Astronauts can work and fly free of the Space Shuttle using the Manned

Maneuvering Unit (MMU), but for most work activities in the Shuttle payload

bay (and during the assembly of the International Space Station) astronauts still

use a safety tether.

However, spacewalk tethers are very short and are not stabilized by

gravitational forces. The TSS-IR mission and rocket-launched experiments,

such as the SMALL expendable Deployer System (SEDS) and the Plasma

Motor Generator (PMG), have increased our understanding of the way tethers

behave in space. Each used different types of tether to deploy satellites and

conduct research, demonstrating the

usefulness of tether technology.

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.

Fleming’s left hand rule comes in to play here to figure out which way the force

is acting



4.2 Fleming’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 ‘downwards’ 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 earth’s magnetic field lines orbital velocity (7-8 km/s). The motion

of the conductor across the magnetic field induces a voltage along the length of

the tether. This voltage can up to several hundred volts per kilometer.

In the above figure the sphere represents the Earth and the unbroken lines

represents Earth’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 Earth’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 Earth’s magnetic field to raise the

spacecraft’s orbit. The major advantage of this technique compared to the other

space propulsion system is that it doesn’t require any propellant. It uses Earth’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 atmosphere‘s’ 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 wire’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. Earth’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 Earth’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.


http://www.tethers.com/OrbitalDebris.html

http://www.tethers.com/OrbitalDebris.html


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.


http://www.nasaspaceflight.com/content/?cid=4370


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

Earth’s atmosphere. This drag force will cause the station’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 ISS’s solar panels to

drive current through a conducting tether, a tether reboots system could

counteract the drag forces or even raise the station’s orbit. NASA and TUI’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 station’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

spacecraft’s Orbital energy in to electrical power. However, since converting

the orbital energy in to electrical power will lower the orbit of the spacecraft

(there’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 satellite’s total weight and could be



easily bolted to the satellites. Once the end of the satellite’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 don’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 planet’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

www.tethers.com

www.tuiengineering.com

www.google.com

www.ieee spectrum.org.


http://www.tethers.com/

electrodynamic tethers

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