A RESEARCH AND SEMINAR REPORT ON ‘ELECTRODYNAMIC TETHER’ -by ASHISH RACHMALE

Posted: October 8, 2012 in Uncategorized
ABSTRACT

Electrodynamic tethers (EDTs) are the conductors suspended from satellites which operate on electromagnetic principles for producing force and current.

Electric potential is generated across a conductive tether by its motion through the Earth’s magnetic field.

As part of a tether propulsion system, crafts can use long, strong conductors (though not all tethers are conductive) to change the orbits of  spacecraft. It has the potential to make space travel significantly cheaper. It is a simplified, very low-budget magnetic sail. It can be used either to accelerate or brake an orbiting spacecraft. When direct current is pumped through the tether, it exerts a force against the magnetic field, and the tether accelerates the 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.

TABLE OF CONTENTS

1. INTRODUCTION …………………………………………….8

2. WHY TETHERS WIN ! ………..…………………………….9

3. HISTORY……………………………………………………….11

4. FUNDAMENTALS OF EDTs………………………………………………….…………12

4.1.Principle

4.2.Lorentz Force Law

4.3.Fleming’s Left Hand Rule

5. WORKING…………………………………….….…………14

6. ENGINEERING OF ELECTRODYNAMIC

TETHER ………………………………………………………….16

6.1.Mathematics Of EDT

6.2 Voltage across conductor

6.3 Current in Conductor

6.4 Tether Current

6.5.Circuit Diagram

6.6.Physics in EDTs

7.APPLICATIONS……………………………………………22

7.1.Propellant less Propulsion for LEO Spacecraft

7.2.Power Generation in Low Earth Orbit

7.3.Electro Dynamic Revision-Boost of the International Space Station

7.4.Space Junk Cleanup

7.5.Satellite Tugboat

7.6.Exploring the Outer Planets

8. FUTURE SCOPE…………………………………………24

9.CONCLUSION……………………………………………25

10.REFERENCES…………………………………………..26

10.1.Websites

10.2.Books and Magazines

1.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 into 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.

Called an Electrodynamic tether provides a simple and reliable alternative to the conventional rocket thrusters.Functionally, electrons flow from the space plasma into the conductive tether, are passed through a resistive load in a control unit and are emitted into the space plasma by an electron emitter as free electrons. In principle, compact high-current tether power generators are possible and, with basic hardware, tens, hundreds, and thousands of kilowatts appears to be attainable.

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.

2.WHY TETHERS WIN….!
COMPARISION OF EDT WITH CONVENTIONAL METHODS

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.

COST-EFFICIENCY OF EDTs IN REMOVAL OF DEBRIS

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 Electrodynamic Space-tethers may be as significant to future space development as rockets were to its beginnings. Tether propulsion is completely reusable and environmentally clean,,provides all these features at low cost and requires no extra fuel.

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.

3.HISTORY

Tsiolkovsky once proposed a tower so tall that it reached into space, so that it would be held there by the rotation of the Earth. However, there was no realistic way to build it.

To try to solve the problems in Komsomolskaya Pravda (July 31, 1960), another Russian, Yuri Artsutanov, wrote in greater detail about the idea of a tensile cable to be deployed from a geosynchronous satellite; downwards towards the ground, and upwards away; keeping the cable balanced. This is the space elevatoridea, a type of synchronous tether that would rotate with the Earth. However, given the materials, this too was impractical on Earth.

In the 1970s Jerome Pearson explored synchronous tethers further, and in particular analysed the lunar elevator that can go through the L1 and L2 points, and this was found to be possible with materials then existing.

In 1977 Hans Moravec and later Robert L. Forward investigated the physics of synchronous and non synchronous skyhook tethers, and performed detailed simulations of tapered tethers that could pick objects off and place objects onto the Moon, Mars and other planets, with little, or even a net gain of energy.

In 1979 the USA NASA examined the feasibility of the idea and gave direction to the study of tethered systems, especially tethered satellites. In 2000, the NASA and Boeing considered a HASTOL concept where a tether would take payloads from a hypersonic aircraft (at half of orbital velocity) to orbit.

4.FUNDAMENTALS OF EDTs

4.1.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.

4.2.Lorentz Force Law

The Lorentz Force Law can be used to describe the effect of a charged particle moving in a constant magnetic field.

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,

Thus in above equation represents q is  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.

The simplest form of this law 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: In 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
4.3.Fleming’s Left Hand Rule
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.

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. The voltage thus created along its length can be up to several hundred volts per kilometer.
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.
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 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.ENGINEERING OF ELECTRODYNAMIC TETHER

6.1.MATHEMATICS OF EDT

A motional electromotive force (EMF) is generated across a tether element as it moves relative to a magnetic field. The force is given by Faraday’s Law of Induction:

Without loss of generality, it is assumed the tether system is in Earth orbit and it moves relative to Earth’s magnetic field. Similarly, if current flows in the tether element, a force can be generated in accordance with the Lorentz Force Equation

In self-powered mode (de-orbit mode), this EMF can be used by the tether system to drive the current through the tether and other electrical loads (e.g. resistors, batteries), emit electrons at the emitting end, or collect electrons at the opposite. In boost mode, on-board power supplies must overcome this motional EMF to drive current in the opposite direction, thus creating a force in the opposite direction, as seen in below figure, and boosting the system.

Illustration of the EDT concept

At 300-km altitude, the Earth’s magnetic field, in the north-south direction, is approximately 0.18 – 0.32 Gauss up to ~40º inclination, and the orbital velocity with respect to the local plasma is about 7500 m/s. This results in a Vemf range of 35 – 250 V/km along the 5-km length of tether. This EMF dictates the potential difference across the bare tether which controls where electrons are collected and / or repelled. Here, the ProSEDS de-boost tether system is configured to enable electron collection to the positively biased higher altitude section of the bare tether, and returned to the ionosphere at the lower altitude end. This flow of electrons through the length of the tether in the presence of the Earth’s magnetic field creates a force that produces a drag thrust that helps de-orbit the system, as given by the above equation. The boost mode is similar to the de-orbit mode, except for the fact that a High Voltage Power Supply (HVPS) is also inserted in series with the tether system between the tether and the higher positive potential end. The power supply voltage must be greater than the EMF and the polar opposite. This drives the current in the opposite direction, which in turn causes the higher altitude end to be negatively charged, while the lower altitude end is positively charged(Assuming a standard east to west orbit around Earth).

6.2.Voltage across conductor

With a long conducting wire of length L, an electric field E is generated in the wire. It produces a voltage V between the opposite ends of the wire. This can be expressed as:

where the angle τ is between the length vector (L) of the tether and the electric field vector (E), assumed to be in the vertical direction at right angles to the velocity vector (v) in plane and the magnetic field vector (B) is out of the plane.

6.3.Current in conductor

An electrodynamic tether can be described as a type of thermodynamically “open system”. Electrodynamic tether circuits cannot be completed by simply using another wire, since another tether will develop a similar voltage. Fortunately, the Earth’s magnetosphere is not “empty”, and, in near-Earth regions (especially near the Earth’s atmosphere) there exist highly electrically conductive plasmas which are kept partially ionized by solar radiation or other radiant energy. The electron and ion density varies according to various factors, such as the location, altitude, season, sunspot cycle, and contamination levels. It is known that a positively charged bare conductorcan readily remove free electrons out of the plasma. Thus, to complete the electrical circuit, a sufficiently large area of uninsulated conductor is needed at the upper, positively charged end of the tether, thereby permitting current to flow through the tether.

However, it is more difficult for the opposite (negative) end of the tether to eject free electrons or to collect positive ions from the plasma. It is plausible that, by using a very large collection area at one end of the tether, enough ions can be collected to permit significant current through the plasma. This was demonstrated during the Shuttle orbiter’s TSS-1R mission, when the shuttle itself was used as a large plasma contactor to provide over an ampere of current. Improved methods include creating an electron emitter, such as a thermionic cathode, plasma cathode, plasma contactor, or field electron emissiondevice. Since both ends of the tether are “open” to the surrounding plasma, electrons can flow out of one end of the tether while a corresponding flow of electrons enters the other end. In this fashion, the voltage that is electromagnetically induced within the tether can cause current to flow through the surrounding space environment, completing an electrical circuit through what appears to be, at first glance, an open circuit.

6.4.Tether current

The amount of current (I) flowing through a tether depends on various factors. One of these is the circuit’s total resistance (R). The circuit’s resistance consist of three components:

  1. the      effective resistance of the plasma,
  2. the      resistance of the tether, and
  3. a      control variable resistor.

In addition, a parasitic loadis needed. The load on the current may take the form of a charging device which, in turn, charges reserve power sources such as batteries. The batteries in return will be used to control power and communication circuits, as well as drive the electron emitting devices at the negative end of the tether. As such the tether can be completely self-powered, besides the initial charge in the batteries to provide electrical power for the deployment and startup procedure.

The charging battery load can be viewed as a resistor which absorbs power, but stores this for later use (instead of immediately dissipating heat). It is included as part of the “control resistor”. The charging battery load is not treated as a “base resistance” though, as the charging circuit can be turned off at anytime. When off, the operations can be continued without interruption using the power stored in the batteries.

6.5.CIRCUIT DIAGRAM

The below figure describes a typical EDT system in a series bias grounded gate configuration (further description of the various types of configurations analyzed have been presented) with a blow-up of an infinitesimal section of bare tether. This figure is symmetrically set up so either end can be used as the anode. This tether system is symmetrical because rotating tether systems will need to use both ends as anodes and cathodes at some point in its rotation. The V_hvps will only be used in the cathode end of the EDT system, and is turned off otherwise.

(a) A circuit diagram of a bare tether segment with (b) an equivalent EDT system circuit model showing the series bias grounded gate configuration.

6.6.PHYSICS IN EDTs

To calculate the in-plane and out-of-plane directions, the components of the velocity and magnetic field vectors must be obtained and the force values calculated. The component of the force in the direction of travel will serve to enhance the orbit raising capabilities, while the out-of-plane component of thrust will alter the inclination. In the below figure, the magnetic field vector is solely in the north (or y-axis) direction, and the resulting forces on an orbit, with some inclination, can be seen. An orbit with no inclination would have all the thrust in the in-plane direction.

Description of an in-plane and   out-of-plane force.

Drag effects on an Electrodynamic   Tether system

There has been work conducted to stabilize the librations of the tether system to prevent misalignment of the tether with the gravity gradient. The below figure displays the drag effects an EDT system will encounter for a typical orbit. The in-plane angle, α_ip, and out-of-plane angle, α_op, can be reduced by increasing the endmass of the system, or by employing feedback technology.Any deviations in the gravity alignment must be understood, and accounted for in the system design.

7.APPLICATIONS

7.1.Propellant less Propulsion for LEO Spacecraft
ED tether system can provide propellant less propulsion for spacecraft operating in low Earth orbit.

7.2.Power Generation in Low Earth Orbit
Electro dynamic tethers may also provide an economical means of electrical power in orbit.

7.3.Electro Dynamic Revision-Boost of the International Space Station NASA currently plans to launch several large rockets every year to carry fuel up to the station so that it can re-boost its orbit.

7.4.Space Junk Cleanup
The most direct application of ProSEDS would be to get rid of space junk.

7.5.Satellite Tugboat
Another idea is for the ED tether to be attached to an unmanned space tugboat that would ferry satellites to higher orbits. After being launched in to low Earth orbit, the so called Orbital Transfer Vehicle would grapple the satellite and maneuver it to a new altitude or inclination. The tug could then lower its own orbit to rendezvous with another payload and repeat the process

7.6.Exploring the Outer Planets
Perhaps the  most  exotic  use  of  ED  tether technology would be to propel and power spacecraft exploring the outer planets. Existing vessels have relied on solar  cells, but  at  distances far  from  the  Sun,  the  power  available  is  typically less than 100 W. Jupiter  and    its  moons  have  an  environment particularly favorable to ED tethers; the planet has a strong magnetic field  and  a  rapid  rotation  rate, and its mass dictates high orbital velocities. With the magnetic  field  moving much faster than  the spacecraft, the tether would essentially be stealing energy from the planet’s magnetic field.

8.FUTURE SCOPE

Researchers are investigating the use of ED tethers to
extend and enhance future scientific missions to Jupiter and its moons. In theory, Ed
tether propulsion could be used near any planet with a Previous visits to the largest planet
in the solar system – including the “Grand Tour” flyby missions of Voyager 1 and 2,
launched in 1977, and an orbital visit by the Galileo probe, which left Earth in 1989 and i
continues to tour and study the Jovian system today- were illuminating, but the fuel limitations and minimum maneuverability of those probes hampers long term, more detailed scientific study.

Development of a propellant free, ED tether propulsion system would make it possible to put a long term probe in Jupiter’s orbit – one that could leverage the planet’s powerful magnetic field and magneto sphere to travel freely among the Jovian moons,providing more information and new insight about them as well.
Tether Transport from Low Earth to the Lunar Surface
A concept developed by Tethers Unlimited wherein several rotating tethers in orbit around the earth and moon may provide a means of exchanging supplies between low Earth orbit facilities and Lunar bases without requiring the use of propellants.
Tethers for Rapid Autonomous Deorbit of Leo Satellites
Tethers Unlimited Inc. is currently developing a system called “Terminator Tether” that will provide a low cost, light weight and reliable method of removing objects from Low Earth Orbit (LEO).

9.CONCLUSION

Electrodynamic tether now becoming the most popular fuel carrier for space crafts. The use of space tethers is the answer to all the current problems as they don’t require propellents..ED tethers can provide long-term propellant less propulsion capability for orbital maneuvering and station keeping of small satellites in low-Earth-orbit..

Over the years, numerous applications for electrodynamic tethers have been identified for potential use in industry, government, and scientific exploration.

Electro dynamic tethers may also provide an economical means of electrical power in orbit. TUI is currently developing a propulsion system called the “Micro satellite Propellant less Electro dynamic Tether Propulsion System”

Beyond these,ETDs are opening new doors in space explorations and getting close to the answer of ARE WE ALONE in this universe..

The universe is eagerly waiting for the application of these tethers in future…..!

10.REFERENCES

10.1.WEBSITES:

http://www.tethers.com/
http://www.wikipedia.org/

http://www.ieee.com/

http://www.tuiengineering.com/
http://www.google.com/

10.2.BOOKS and MAGAZINES:

THE TERMINATOR TETHER – HOYT, R.P., FORWARD, R.L
DYNAMICS OF SPACE TETHER SYSTEMS – BELETSKII, V.V.. LEVIN,
STABILIZATION OF ELECTRODYNAMIC – HOYT. R.P & HEINEN
SPACE TETHERS
ELECTRON ICS FOR YOU – JANUARY 2005
ELECTRONICS TODAY – JUNE 2004

-PUBLISHED BY © ASHISH RACHMALE

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