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Bringing spacecraft to higher orbits than LEO is quite expensive and as rocket reusability gains traction, it will probably get even more expensive when compared to a standard LEO deployment since the first stage does not have enough surplus fuel to go back to earth (at least the current designs).

An electrodynamic tether is a device which can exchange electrical energy for kinetic energy and vice versa by exerting a Lorentz force against the Earth's magnetic field. Since a spacecraft using this technology would not need any fuel once it's in orbit, it would be significantly cheaper to deploy. Combined with a magnetorquer, it would prolong the lifespan of a satellite significantly.

Why is this technology not being used all the time? Are there any issues with practicality of such a system?

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  • $\begingroup$ There may be an error: "since the first stage does not have enough surplus fuel to go back to earth", did you think about the second stage? $\endgroup$ – Uwe Dec 7 '17 at 23:05
  • $\begingroup$ @Uwe If I understand correctly, the first stage needs to use all the fuel to get the rest of the rocket to its destination, so there is no fuel left for controlled reentry. $\endgroup$ – JohnEye Dec 7 '17 at 23:10
  • $\begingroup$ Space tethers have a...checkered... history. en.wikipedia.org/wiki/Space_tether_missions $\endgroup$ – Organic Marble Dec 7 '17 at 23:58
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    $\begingroup$ To be useful, it should be possible to propel a communication satellite from LEO to GEO within some month using electrical power of some kW. The tether's mass should be less than about 20 % of the satellites mass. It should be possible to build the tether with materials available today. $\endgroup$ – Uwe Dec 8 '17 at 18:33
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Parsing information from Wikipedia. Space tethers are extremely slow, and are somewhat unweildly. The main thought has been for use to deorbit them. In order to make them work, one has to have a very long tether.

Just to give a few numbers, as seen from this site, a tether to raise the ISS would be about 25 km long. It would produce a continual 0.5-0.8 N of thrust. But the chance of a collision is much higher for such a long tether, and it makes operations more difficult.

Also, the effect stops working at about 1500 km.

All in all, it might be able to save a bit of energy, but it hasn't been well tested. It has a lot of potential, but also a fair bit of risk, with such long thin structures dramatically increasing the likelihood of space collisions.

The break even point is actually probably fairly low. It could be used to save fuel for most of the low altitude satellites, like Spy Satellites and the ISS, that have a significant deorbit thrust that shows up over time. I don't have an exact number, but think large LEO satellites in general.

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  • $\begingroup$ Thanks for answering. Why did you remove the edit from the question though? $\endgroup$ – JohnEye Jan 15 '18 at 21:32
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    $\begingroup$ It wasn't relevant to the question, so... $\endgroup$ – PearsonArtPhoto Jan 15 '18 at 21:36
  • $\begingroup$ I guess, but it might look like a conflict of interest, you knew about the change and might have discouraged others from answering by removing it. I'm not gonna take it so seriously, just something that crossed my mind. I wish it was possible to edit the bounty message though, I seem to have broken formatting in it as well. $\endgroup$ – JohnEye Jan 15 '18 at 21:55
  • $\begingroup$ @JohnEye No, it doesn't look like a conflict of interest, just good moderation. Sometimes the structure and rigidity of the SE format seems sub-optimal on a case-by-case basis, but the system is proven to work amazingly well for the cacophony of us sub-optimal humans users. Once a bounty starts, the situation should probably remain stable and fixed. While relaxing it seems intuitively helpful, there can always be someone who might cry "foul!". Your posted question is great by the way, and it and your bounty have resulted in a great answer. $\endgroup$ – uhoh Jan 16 '18 at 0:53

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