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Electrodynamic tether

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Electrodynamic tethers are long conducting wires (usually deployed from a spacecraft) which can operate on the well known electromagnetic priciples as a generator (via converting its velocity to electric energy) or engine (ultilizing electric energy for velocity). Electric potential is generated across a conductive tether by its motion through the Earth's magnetic field. The choice of the metal conductor to be used in a electrodynamic tether is determined by a variety of factors. Primary factors include low resistivity, high conductivity, and low density. Secondary factors include cost, strength, and melting point (pending the precise application).

Tether propulsion

Main article: Tether propulsion

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.

Tether generation

Electrodynamic tethers in the role of being a generator are attached to a object, said conductive tether being oriented at an angle to the local vertical between said object and another body having a magnetic field having magnetic field lines external to said conductive tether. When the tether cuts the planet's magnetic field, it generates a current, and thereby reduces the energy consumption. The conductive tether moving across said magnetic field lines of said external magnetic field at an angle to the local vertical to produce an electric current in said conducting tether and a resulting electrodynamic force acting on the tether and attached object. The tether's end can be left bare, and this makes electrical contact with the ionosphere via the Phantom loop. Functionally, electrons flow from the space plasma into conductive tether, are passed through a resistive load in control unit and are emitted into the space plasma by electron emitter as free electrons. In priciple, compact high current tether power generators are possible and, with basic hardware, 10-25 kilowatts appears to be attainable.

Voltage and current

When an a conductor is moved at a velocity (v) through a magnetic field (B), an electric field is generated in the tether's frame of reference. This can be stated as:

E = v * B = vB

where the magnetic field of the Earth (B), being tangent to the Earth's surface in the north-south direction, is at right angles to the velocity vector of the spacecraft (v), assumed moving in a west-east direction. The direction of the electric field (E) will be at right angles to both the tether's velocity (v) and magnetic field (B), or along the local vertical. It should be noted that this electric field exists in the moving frame of reference because athe assumed moving magnetic field creates an electric field. No object actually has to be there, but if it is, then the relative motion of the magnetic field of the Earth will not only apply magnetic forces to whatever material the object is made out of, but electric forces too. Note also that the velocity used in this equation is the relative velocity between the object and the magnetic field because of various considerations. As it is assumed that the Earth's magnetic field rotates with the planetary body, the motion of the magnetic field must be subtracted from the velocity of the object to obtain the relative velocity.

NASA has conducted several experiments with Plasma Motor Generator (PMG) tethers in space. An early experiment used a 500 meter conducting tether. In 1996, NASA conducted an experiment with a 20,000-meter conducting tether. When the tether was fully deployed during this test, the orbiting tether generated a current of 3,500 volts. This conducting single-line tether was severed after five hours of deployment. It is believed that the failure was caused by an electric arc generated by the conductive tether's movement through the Earth's magnetic field. It has been surmized that if deployed to Jupiter, that planet rotates so rapidly that a tether can produce power and raise orbit passively and simultaneously.

Conductor's voltage

With a long conducting wire of length (L), a 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:

V= E * L = EL cos tau = vBL cos tau

where the angle (tau) 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.

Conductor's current

Although a voltage can be build up between the ends of the electrodynamic tether, current will not flow unless an open circuit exists. Electrodynamic tether circuits cannot be completed with another wire, as another tether will have a similar voltage generated in it. Fortunately, free space is not "empty", and, in near-Earth regions (and especially in the vacinity of the Earth's atmosphere), there exists highly electrically conductive plasmas (which are kept partially ionized by solar sorces of radiant energy). The electron and ion density varies pending various factors, such as the ___location, altitude, season, sunspot cycle, and contamination levels. It is known that a bare conductor will readily pull electrons out of the plasma. Thus, to complete the citrcuit at the upper positively charged tether end a sufficiently large area of uninsulated conductor need to be made.

There are difficulties for tether in ejecting electrons from a wire or in collecting positive ions from plasma. It is plausiable that very large areas at one end of the tether could collect enough ions to complete the circuit. This was demonstrated during the Shuttle orbiter's TSS-1R mission, when the shuttle itself was used as a plasma contactor for over an amp of current. Improved methods include electron emitters (such as hot cathodes, plasma cathodes, plasma contactors, and field-emission devices). Upon having both ends of the tether open to allow the flow of electrons out of one tether end and into the other end, there will be enough conductivity in relation to the surrounding plasma to allow current to flow through the electrodynamic tether.

Tether current flow

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.

Inaddition, a parasitic load is needed. The load on the current may take the form of a charging device (wich inturn charge batteries). The batteries in will be used to control power and communication circuits activation, 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.

Challenges

Internal vibrations

Tethers have many modes of vibration. Electrodynamic tethers build up vibrations from variations in the magnetic and electric fields of the earth. Oscillations (also called vibrations or mechanical transients) can be sensed by radio beacons on the tether, or inertial and tension sensors on the end-points. Unless they are damped somehow, the vibrations grow large enough that the tether will fail in less than a month from mechanical stress. One plan to control these is to vary the tether current to oppose the vibrations. In simulations this keeps the tether together. The sensors to sense tether vibrations can either be an inertial navigation system on one end of the tether, or satellite navigation systems mounted on the tether, transmitting their positions to a receiver on the end. Over a few weeks, electrodynamic tethers in Earth orbit can build vibrations in many modes, as their orbit interacts with irregularities in magnetic and gravitational fields. Electrodynamic tethers can be stabilized by reducing their current when it would feed the oscillations, and increasing it when it opposes oscillations.

Surges

As mentioned earlier, conductive tethers have failed from unexpected current surges. Unexpected electrostatic discharges have cut tethers, damaged electronics, and welded tether handling machinery. It may be that the Earth's magnetic field is not as homogeneous as some engineers have believed.

External articles and references

Patents
Publications
  • R. I. Samanta Roy, D. E. Hastings and E. Ahedo, "Systems analysis of electrodynamic tethers". Journal of Spacecraft and Rockets, Vol. 29, 1992, pp. 415–424.
  • E. Ahedo and J. R. Sanmartin, "Analysis of bare-tethers systems for deorbiting Low-Earth-Orbit satellites". Journal of Spacecraft and Rockets, Vol. 39, No. 2, March-April,2002, pp. 198–205.
  • J. Peláez, G. Sánchez-Arriaga and M. Sanjurjo-Rivo, "Oribital debris mitigation with self-balanced electrodynamic tethers".
  • Cosmo, M. L., and E. C. Lorenzini, "Tethers in Space Handbook" (3rd ed). Prepared forNASA/MSFC by Smithsonian Astrophysical Observatory, Cambridge, MA, December 1997. (PDF)
  • R. D. Estes, E. C. Lorenzini, J. R. Sanmartín, M. Martinez-Sanchez, and N. A. Savich, "New High-Current Tethers: A Viable Power Source for the Space Station? A White Paper". December 1995. (PDF)
  • Savich, N.A. and Sanmartín, J.R., "Short, High Current Electrodynamic Tether". Proc. Int. Round Table on Tethers in Space, 417. 1994.
  • James E. McCoy, et. al. "Plasma Motor-Generator (PMG) Flight Experiment Results". Proceedings of the 4.sup.th International conference on Tethers in Space, pp.57-84. Washington, D.C., April 1995.
Other articles
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