Unlike chemical and rocket systems, which must expell a propellant to create thrust, an electrodynamic tether generates thrust through Lorentz-force interactions with a planetary magnetic field. By using the space environment to create thrust, electrodynamic tether systems can dramatically reduce the cost of many space missions by eliminating the need to launch large quantities of propellant into orbit.
An electrodynamic tether is essentially a long conducting wire extended from a spacecraft. The gravity gradient field (also known as the "tidal force") pulls the tether taut and tends to orient the tether along the vertical direction. As the tether orbits around the Earth, it crosses the Earth's magnetic field lines at 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, which is called the "motional EMF", can be up to several hundred volts per kilometer.
In an "electrodynamic tether drag" system, such as our Terminator Tape and Terminator Tether products, 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 conducting ionospheric plasma at one end of the tether and expelling them back into the plasma at the other end of the tether, the motional EMF voltage will drive a current along the tether. This current will, 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 "electrodynamic drag force" will decrease the orbit of the tether and its host spacecraft. Essentially, the tether converts the orbital energy of the host spacecraft into electrical power, which is dissipated as ohmic heating in the tether.
Figure 1. Principle of electrodynamic tether propulsion.
In a "electrodynamic propulsion" system, the tether can be used to boost the orbit of the spacecraft. A power supply is added to the tether system and used to drive current in the direction opposite opposing the motion-induced EMF, the tether can "push" against the Earth's magnetic field to raise the spacecraft's orbit. The major advantage of this technique compared to other space propulsion systems is that it doesn't require any propellant. It uses the Earth itself, coupled through its magnetic field, as its "reaction mass." By eliminating the need to launch large amounts of propellant into orbit, electrodynamic tethers can greatly reduce the cost of in-space propulsion.
Applications of Electrodynamic Tethers:
Propulsion for LEO Spacecraft
ED tether systems can provide propellantless 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 reducing costs for missions that involve delta-V hungry maneuvers such as formation flying, low-altitude stationkeeping, orbit raising, and end-of-mission deorbit. TUI is developing several ED tether products, including the µPET Propulsion System and the Terminator Tether Satellite Deorbit Device.
Reboost of the International Space Station
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 within several months. NASA currently plans to launch several large rockets every year to carry fuel up to the station so that it can reboost its orbit. These launches, however, will be very costly.
Tethers 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 reboost system could counteract the drag forces or even raise the station's orbit. NASA and TUI's studies revealed that such a tether reboost system could reduce or eliminate the need for dedicated launches for reboost propellant, potentially saving up to $2 billion over the first ten years of the station's operation.
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