Tether Propulsion

The MXER tethers use electrodynamic tether propulsion to move back to the right orbital altitude after slinging payloads into higher orbits. However, this novel means of propellantless propulsion can in principle be used on any spacecraft that spends at least part of its orbit below an altitude of 1000 km (600 miles) (Earth's magnetic field strength and the ionospheric density are too low higher up).

Tethers Unlimited is developing what they call the Microsatellite Propellantless Electrodynamic Tether (PET) Propulsion System (since the Greek letter stands for ''micro'' but is pronounced ''mu,'' ''PET'' thus leads to the funny pronunciation ''mu-pet''). It is designed to provide small satellites with a long-term propulsion capability for orbital maneuvering and stationkeeping in low Earth orbit. In addition, the tether system can also serve as a gravity-gradient attitude control element, stabilizing the satellite's attitude with respect to Earth. The mass, size, and power requirements of the PET Propulsion System depend on the size of the satellite and its mission, but TUI has developed a prototype sized for a 125-kg (276-pound) microsatellite. It is able to raise the altitude of the spacecraft's orbit from 350 to 700 km (220 to 440 miles) within 50 days.

An electrodynamic tether propulsion system could also be designed to be a stand-alone spacecraft, able to dock to other satellites and move those into other orbits. TUI's previously mentioned Debris Shepherd, a reusable tether propulsion spacecraft for de-orbiting space junk, is an example of such a system. In principle its design could also be used to intercept healthy satellites, change their orbits (altitude, eccentricity, inclination), and then undock and fly to another target.

A similar design is the Electrodynamic Delivery Express (EDDE), studied by the companies STAR and Tether Applications in collaboration with the U.S. Air Force Research Lab. It consists of two spacecraft connected by a conducting tether, but does not rely on gravity-gradient stabilization. Instead, it slowly spins around its center of mass (located in the middle of the tether) for increased attitude stability; as with a spinning top, the resulting gyroscopic effect keeps the rotational axis of the spinning system in the same direction.

However, the spin does introduce the problem that a current initially flowing upward through the tether with respect to Earth's magnetic field will be going downward half a rotation later. This results in a Lorentz force aimed forward for half a circle, then backward for the next half rotation, adding up to a zero net force. To ensure that the net electrodynamic Lorentz force keeps on pushing in the same direction, the direction of the current flowing through the tether has to be changed twice per rotation. A benefit of the spin is that with the proper varying of the current versus the tether orientation, the net thrust direction of the EDDE can be selected over a wide range. This is not possible with the always vertically orientated gravity-gradient stabilized tethers, where the thrust is always in either an eastern or a western direction (depending on whether the current flows up or down the tether).

Because of the periodically changing current direction, the EDDE must be able to collect and emit electrons at each end of the tether. Instead of one collector at each end, the EDDE has multiple collectors distributed over the entire tether, effectively resulting in a chain of individual but connected electrodynamic tethers. The benefit is that it increases the EDDE's capability to collect electrons from the ionosphere, raising the maximum operating altitude by allowing adequate current levels at lower plasma densities. It also makes it possible to vary the current levels and therefore the strength of the Lorentz force over the different parts of the tether. If this force is made stronger on one side of the EDDE tether (with respect to the rotational center) than on the other side, the rotational velocity of the tether will change. The rotation will speed up if the stronger force on one side acts in the same direction as the rotation, but slow down if the force acts in the other direction. The rotation rate of the EDDE tether can thus be accurately controlled.

NASA and TUI have also studied the possibility of using an electrodynamic tether for reboosting the ISS. As this huge station orbits Earth, it experiences a small but constant aerodynamic drag force from the thin upper reaches of Earth's atmosphere. This drag force needs to be counteracted to prevent the station from falling out of orbit like the Skylab station did in 1979. Regular flights of Russian Progress and European ATV spacecraft bring propellant to the station to fire its orbital maneuvering rocket engines and restore the orbit, and while they are docked these cargo vehicles also help out with their own engines. Without these orbit restoring boosts, the ISS would fall out of the sky after only several months. However, the reboosts cost lots of propellant, which needs to be brought from Earth and is expensive to launch. Instead, an electrodynamic tether propulsion system using excess power generated by the ISS solar panels could be used to constantly counteract the aerodynamic drag force without propellant. It could even raise the station's orbit. TUI estimates that their design could save up to $2 billion in propellant launch costs over 10 years of the station's operation.

In 2000 there was a plan to launch a similar electrodynamic propulsion tether to the Russian Mir space station. The station was falling out of orbit and desperately needed a reboost to keep it operational for commercial activities (including space tourism) then being planned by a company called MirCorp. The tether, named Firefly, was designed by Joe Carroll of Tether Applications, Inc., and Russian space station engineer Vladimir Syromyat-nikov. It was built in the United States, and it was ready in 2000 to be delivered to Russia for launch aboard a Progress cargo spacecraft. Firefly, formally known as the Mir Electrodynamic Tether System (METS), consisted of a 150-kg (330-pound) assembly with a 5-km (3-mile) aluminum wire that was to be installed by cosmonauts on the end of the station's Kvant 2 module. A surplus spacewalk rocket backpack was to be attached to the end of the tether, to act as a countermass to help deployment and stability using the gravity gradient principle. A few kilowatts of electricity from Mir's solar arrays would be channeled through the tether to provide electrodynamic propulsion and slowly raise the station's orbit. However, there were problems winning approval from the U.S. State Department for an export license for the U.S.-designed tether under the International Traffic in Arms Regulations (ITAR). After more than a year, the State Department finally cleared export of the tether, but by then MirCorp had already run out of money and Russia had just announced that the Mir station would be de-orbited (it burned up in Earth's atmosphere in March 2001). Firefly was never launched.

Tethers Unlimited's most ambitious plan for the ISS is to use an electrodynamic tether to move the station's orbital inclination (the angle with which the orbit intersects Earth's equator). The current orbit of the ISS has an inclination of 51.6 degrees, which was necessary to enable Russian Soyuz rockets launched from Baikonur Cosmodrome to reach the station. The downside of this relatively high inclination is that it is not optimal for launches from NASA's Kennedy Space Center in Florida or Europe's Kourou launch site in French Guiana, which lie much closer to the equator than Baikonur. For flights from Kennedy Space Center, an orbit inclination of 28.5 degrees would be ideal, allowing the Space Shuttle or the future Ares I rocket to carry a maximum payload. The new Soyuz launch site built in French Guiana now makes it possible for Russian cargo vehicles to be launched from there, and with a launch pad upgrade also crewed Soyuz capsules could be launched from the European base. Thus, it would no longer be necessary to facilitate ISS launches from Baikonur, and the ISS's orbital inclination could be lowered to maximize the payload mass capabilities for Kennedy Space Center and Kourou launches.

However, changing the orbit inclination of the ISS from 51.6 to 28.5 degrees with conventional rocket engines would require a mass of propellant nearly as large as the total mass of the whole station itself. All this propellant would need to be launched from Earth, making the whole idea economically unviable; the launch mass benefits gained from the inclination change would not outweigh the costs for first launching all the necessary inclination change propellant. Electrical rocket propulsion systems (ion engines) could do the job for much less propellant, but would require enormous power supplies to provide a reasonable level of thrust. An electrodynamic tether with a modest power source could change the ISS's inclination without propellant. TUI proposes using a 50-km (30-mile) conducting tether and spinning flywheels to store energy generated by the ISS's large solar arrays. TUI projects that this system could move the station's orbit to the ideal inclination in 4 years.

Yet there are currently no plans to implement any type of tether propulsion system on the ISS, due to the required investments and the previously described safety issues related to long tethers in the vicinity of crewed space stations. NASA is moving its attention and the bulk of its crewed spaceflight budgets to its new lunar exploration plans, and the future of the ISS after 2015 is uncertain. It is therefore unlikely that there will be money and time available for developing tether applications for the space station. Moreover, there are currently no firm plans to launch crewed Soyuz rockets from Kourou (which would require expensive additions to the current launch pad), and as long as Russia needs access to the ISS from its Baikonur launch site, moving the station's inclination is out of the question.

For interplanetary travel, electrodynamic propulsion tethers could be embedded into solar sails like the spokes in a bicycle wheel, thus combining two very innovative space technologies. The solar sail would be used to propel the spacecraft through space, and the electrodynamic tethers to generate propulsion for maneuvering the spacecraft in orbit around a planet with a strong magnetic field (like the Earth and Jupiter). The tether system could further be used to generate electrical power for the spacecraft while slowing down in the planet's magnetic field using the electrodynamic drag principle.

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