Electrodynamic tethers (EDTs) are long conducting wires, such as one deployed from a tether satellite, which can operate on electromagnetic principles as generators, by converting their kinetic energy to electrical energy, or as motors, converting electrical energy to kinetic energy.NASA, Tethers In Space Handbook, edited by M.L. Cosmo and E.C. Lorenzini, Third Edition December 1997 (accessed 20 October 2010); see also version at NASA MSFC ; available on scribd Electric potential is generated across a conductive tether by its motion through a planet's magnetic field.
A number of missions have demonstrated electrodynamic tethers in space, most notably the TSS-1, TSS-1R, and Plasma Motor Generator (PMG) experiments.
Tether propulsion As part of a tether propulsion system, craft 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. When direct current is applied to the tether, it exerts a Lorentz force against the magnetic field, and the tether exerts a force on the vehicle. It can be used either to accelerate or brake an orbiting spacecraft. Electrodynamic tethers exchange momentum with a planetary magnetosphere or ionosphere via Lorentz forces on a long current-carrying conductor, enabling drag or thrust without propellant in suitable environments (e.g., low Earth orbit). As open systems, they conserve momentum by reaction with the ambient plasma and magnetic field. In operation, a conductive tether moving through a planetary magnetic field experiences a motional electromotive force; closing the circuit through the ambient ionosphere allows current to flow, and the resulting Lorentz force can provide either drag (for deorbit) or, with external power injection, thrust along specific orbital geometries. Beyond orbit changes, tethers can also be used for in-situ power generation at the expense of orbital energy, though practical systems must address current collection (e.g., plasma contactors), arcing, attitude control, and vulnerability to micrometeoroids or space debris.
In 2012 Star Technology and Research was awarded a $1.9 million contract to qualify a tether propulsion system for orbital debris removal.
Uses for ED tethers
Over the years, numerous applications for electrodynamic tethers have been identified for potential use in industry, government, and scientific exploration. The table below is a summary of some of the potential applications proposed thus far. Some of these applications are general concepts, while others are well-defined systems. Many of these concepts overlap into other areas; however, they are simply placed under the most appropriate heading for the purposes of this table. All of the applications mentioned in the table are elaborated upon in the Tethers Handbook. Three fundamental concepts that tethers possess, are gravity gradients, momentum exchange, and electrodynamics. Potential tether applications can be seen below:
ELECTRODYNAMICS Electrodynamic power generation Electrodynamic thrust generation ULF/ELF/VLF communication antenna Radiation belt remediation SPACE STATION Microgravity laboratory Shuttle de-orbit from Space Station Tethered Space Transfer Vehicle (STV) launch Variable/low gravity laboratory Attitude stabilization and control ISS reboost TRANSPORTATION Generalized momentum scavenging spent stages Internal forces for orbital modification Satellite boost from orbiter Tether Assisted Transportation System (TATS) Tether re-boosting of decaying satellites Upper stage boost from Orbiter
ISS reboost EDT has been proposed to maintain the ISS orbit and save the expense of chemical propellant reboosts. It could improve the quality and duration of microgravity conditions.
Electrodynamic tether fundamentals
The choice of the metal conductor to be used in an electrodynamic tether is determined by a variety of factors. Primary factors usually include high electrical conductivity, and low density. Secondary factors, depending on the application, include cost, strength, and melting point.
An 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 (deorbit 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.
Take, for example, the NASA Propulsive Small Expendable Deployer System (ProSEDS) mission as seen in above figure.Fuhrhop, K.R., Gilchrist, B.E., Bilen, S.G., "System Analysis of the Expected Electrodynamic Tether Performance for the ProSEDS Mission," 39th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, AIAA, 2003, pp. 1–10.Johnson, L., Estes, R.D., Lorenzini, E.C., "Propulsive Small Expendable Deployer System Experiment," Journal of Spacecraft and Rockets, Vol. 37, No. 2, 2000, pp. 173–176.Lorenzini, E.C., Welzyn, K., and Cosmo, M.L., "Expected Deployment Dynamics of ProSEDS," 39th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, AIAA, 2003, pp. 1–9.Sanmartin, J.R., Charro, M., Lorenzini, E.C., "Analysis of ProSEDS Test of Bare-tether Collection," 39th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, AIAA, 2003, pp. 1–7.Vaughn, J.A., Curtis, L., Gilchrist, B.E., "Review of the ProSEDS Electrodynamic Tether Mission Development," 40th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, AIAA, 2004, pp. 1–12. At 300 km altitude, the Earth's magnetic field, in the north-south direction, is approximately 0.18–0.32 gauss up to ~40° 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).
To further emphasize the de-boosting phenomenon, a schematic sketch of a bare tether system with no insulation (all bare) can be seen in below figure.
The top of the diagram, point A, represents the electron collection end. The bottom of the tether, point C, is the electron emission end. Similarly, and represent the potential difference from their respective tether ends to the plasma, and is the potential anywhere along the tether with respect to the plasma. Finally, point B is the point at which the potential of the tether is equal to the plasma. The location of point B will vary depending on the equilibrium state of the tether, which is determined by the solution of Kirchhoff's voltage law (KVL)
and Kirchhoff's current law (KCL)
along the tether. Here , , and describe the current gain from point A to B, the current lost from point B to C, and the current lost at point C, respectively.
Since the current is continuously changing along the bare length of the tether, the potential loss due to the resistive nature of the wire is represented as . Along an infinitesimal section of tether, the resistance multiplied by the current traveling across that section is the resistive potential loss.
After evaluating KVL & KCL for the system, the results will yield a current and potential profile along the tether, as seen in above figure. This diagram shows that, from point A of the tether down to point B, there is a positive potential bias, which increases the collected current. Below that point, the becomes negative and the collection of ion current begins. Since it takes a much greater potential difference to collect an equivalent amount of ion current (for a given area), the total current in the tether is reduced by a smaller amount. Then, at point C, the remaining current in the system is drawn through the resistive load (), and emitted from an electron emissive device (), and finally across the plasma sheath (). The KVL voltage loop is then closed in the ionosphere where the potential difference is effectively zero.
Due to the nature of the bare EDTs, it is often not optional to have the entire tether bare. In order to maximize the thrusting capability of the system a significant portion of the bare tether should be insulated. This insulation amount depends on a number of effects, some of which are plasma density, the tether length and width, the orbiting velocity, and the Earth's magnetic flux density.
Tethers as generators
An electrodynamic tether is attached to an object, the tether being oriented at an angle to the local vertical between the object and a planet with a magnetic field. The tether's far end can be left bare, making electrical contact with the ionosphere. When the tether intersects the planet's magnetic field, it generates a current, and thereby converts some of the orbiting body's kinetic energy to electrical energy. 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. As a result of this process, an electrodynamic force acts on the tether and attached object, slowing their orbital motion.
In a loose sense, the process can be likened to a conventional windmill- the drag force of a resistive medium (air or, in this case, the magnetosphere) is used to convert the kinetic energy of relative motion (wind, or the satellite's momentum) into electricity. In principle, compact high-current tether power generators are possible and, with basic hardware, tens, hundreds, and thousands of kilowatts appears to be attainable.
Voltage and current 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 potential 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.
When a tether is moved at a velocity (v) at right angles to the Earth's magnetic field (B), an electric field is observed in the tether's frame of reference. This can be stated as:
E = v * B = vB