Invention: Electrodynamic Tether for Satellites

Added to WordPress site Aug 31, 2011. Updated Jan 23, 2021, got distracted for a decade with renovations…

Simple Explanation of my proposed Electro-Dynamic Space Tether

An electrodynamic space tether creates a propulsion force by utilizing three physical processes:

The generation and supply of electricity from a solar panel (plus battery storage) to generate a current flow in the tether wire (electron pump).
The utilization of free electrons and ions in the Earth’s ionosphere (positively charged hydrogen, oxygen etc), which starts around 85 kilometres up, maximizes around 300 km, and tapers off pretty low around 2,000 km http://en.wikipedia.org/wiki/Ionosphere . This ionization is heaviest in the day, where the Sun’s radiation is constantly knocking off electrons from the thin layer of molecules that surround the Earth at this height.
The utilization of the Earth’s electromagnetic field, which is created by two factors: the “solar wind” and the Earth’s “internal field” caused by the slow churning of the Earth’s iron core. http://en.wikipedia.org/wiki/Magnetosphere

The act of passing an electrical current through the tether wire will collect electrons from space on one end (about one million per cubic centimetre in some regions), pump them down the wire, and emit them back into space where they are grabbed up by the positively charged ions flying around. Emitters and collectors are being refined to facilitate this process.

http://en.wikipedia.org/wiki/Electrodynamic_tether (this content has expanded considerably in 10 years!)

Spinning Electrodynamic Tether

This is a deployment method for an electrodynamic tether to be used to re-position a satellite or other space craft in orbit around the Earth WITH NO FUEL. I will make it point form for now:

As far as I have seen, all space tether experiments to date have used some deployment method to keep them stretched out in a straight line (see NASA STS-75). This required a complex deployment pod and scaffolding. Or they are limited to being vertical to the Earth due to the natural effects of gravity.
The method presented below is claiming to be a new idea where by rotating multiple tethers like a helicopter blade using centrifugal force, the scaffolding can be eliminated and the cost and weight of deployment can be reduced dramatically.
By using this process to generate force at the right times and directions, a space vehicle or satellite can alter its orbit, either by making it more elliptical, or higher/lower in space (boosting), or even from a complete polar orbit into an equatorial orbit (It would take some good old Newtonian physics to calculate how long this would take…weeks, years, centuries???)
The idea is to have two (or more) tethers spooled out in two segments at right-angles to a rotating shaft.
Un-deployed, the tether wire is contained on two spools on opposite sides of the main shaft. (much like a weed-eater lawn trimmer)
Further out on the shaft is a counter-rotating flywheel, with an electric motor that sits between them, its axis also along the shaft.
To deploy, the motor starts, which is powered by solar panels/batteries on the satellite, and the spools release the two tethers. The rotational inertia spins them out until they are fully deployed, at a distance to be determined by the optimal design for the specific application.
A flywheel counter-rotates, so the net rotational inertia is zero, and the satellite maintains its steady position.
The tethers would have weights at each end in the form of an electron dissipaters / collectors.
Electrical current would be induced to flow through the system while the tethers swing through approximately 30 degrees, or 1/12 of the time. Thus the traction force vector would go through a repeated curved segment.
When one is collecting, the other is dissipating. They are controlled independently so that in the event that one fails, you still have half of an operating system.
The tether is only powered for a small percentage or arc of a full rotation (say 15 to 30 degrees).
In order to reduce the speed of the flywheel, it may also use two or three weights which are spooled out in the same fashion, but far enough out on the extended shaft and not too far out in radius for the possibility of the counter-rotating wires to get entangled.
The plane of the spinning tether would typically be oriented to cut through the center of the earth and co-planar with the orbit’s plane. This way, the timing of the induction sequences could be such that the craft could be lifted, dropped, moved forwards, or backwards at any point of the orbit. However, the rotating tethers could be retracted and re-deployed in any orientation. It is my understanding that existing tethers can only operate in a stable fashion to lift or drop, and not restore inevitable orbital decay due to drag in the ionosphere. (further research required here….perhaps only up and down is required relative to the Earth’s center if timing is correct, but this would still require static tethers to be swung through 180 degrees, something that is automatic with the rotating system)
- Eric Meger of exactEarth (COMDEV) who created the first broadband satellite, informed me that due to the gravitational stability, the co-planar intersection is the only self-stabilizing solution currently envisioned. (I invited him as a speaker at a project management symposium in Waterloo ON)…more here later…
The deployment arms of the two spools could be canted back at say 120 degrees to the shaft (and micro-adjustable with motor driven threaded rods) such that their mass would be situated about the center of mass of the satellite (including the flywheel). This way, the traction force created by the tether would act on the center of mass of the overall system and not create skewing forces.
The speed of rotation would be just enough to keep the tether tight and counteract slack generated on the end which is currently pushing (call this the “trailing tether”). This could also be controlled by reducing the current flow in the trailing tether dynamically…just enough push to not slacken the tether. Tension could be measured with a strain gauge at the vehicle end of the tether providing the feedback.
The entire assembly could thus be bolted to the side of a satellite, or a completely new satellite could be designed with this as a core propulsion system for a specific set of missions, like hopping from one piece of space junk to the next to gather them up and fling them to Earth.
Re-deployment and re-positioning of the tethers would require reverse operations and retracting the devices, then using the on-board satellite momentum wheels to re-orient the position to potentially any orientation and re-direction of the propulsion.
Induction sequences could be done when the local ionization levels are within operating tolerances with respect to day and night conditions etc, and particular missions to re-position the craft to a new location/orbit would have to be recalculated on the fly. Rapid changes in solar flares would have to be monitored and protected against (was this what killed STS-75?)
Possible problems would include:
- Old fashioned wheel-balancing problems: run-out detection within the inertial frame of reference monitoring could control the spooling of each tether independently
- Tether wire breaks! Use a carbon fibre band (like in the proposed space elevator), to take the tension, and/or in combination with insulating some of the wire on the inboard sections to optimize. Another solution: tether wire is a multi-strand fishnet stocking like structure which all conducts…
- Frictional problems on the shaft causing non-net-neutral rotational independence from the craft. One solution would be another mini-tether on the flywheel at right-angles to the shaft, so as to not require the satellite on-board momentum wheels to increasingly speed up to compensate. (This could also be used to de-spin them). I have a potential design for this but left out for the moment.
- Economics: you need space missions where the cost of this additional component system is cheaper than fuel-based systems, and the overall payload weight is a small enough fraction. Could this keep a space-based telescope up longer?

NASA has conducted several experiments with Plasma Motor Generator (PMG) tethers in space. An early experiment used a 500 meter conducting tether. Get data

During the 1996 NASA mission # STS-75 on Feb 25, the connection to the Shuttle fried due to some electrical overload and it drifted off into space. There is also a big UFO following due to the visibility of some space debris and people were claiming that these were aliens trying to investigate the detached tether. http://www.youtube.com/watch?v=As-wYmFYb3I

Some links..

Space Debris: jer-chyi.liou-1@nasa.gov

Tethers Unlimited hoyt@tethers.com

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