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Sutton and Biblarz's Rocket Propulsion elements says in Chapter 1, section 1.2 (editions 7, 8, 9, possibly earlier as well) that:

Totally different methods of producing thrust are used in nonthermal types of electric propulsion. As described below, these electric systems use magnetic and/or electric fields to accelerate electrically charged atoms or molecules at very low gas densities. It is also possible to obtain very small accelerations by taking advantage of the difference in gravitational attraction as a function of earth altitude, but this method is not treated in this book.

I find this intriguing, and so my question is; How?

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    $\begingroup$ What edition/printing? $\endgroup$ – Organic Marble Apr 4 at 3:55
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    $\begingroup$ @OrganicMarble It's in 7, 8, 9 but I'm not sure about earlier ones, I've made another edit. $\endgroup$ – uhoh Apr 4 at 5:30
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    $\begingroup$ Am I missing something, or is this a complicated way of saying "Earth's gravity is weaker higher up"? $\endgroup$ – Ingolifs Apr 4 at 5:33
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    $\begingroup$ @uhoh do you mean "accelerations"? "Attraction" is singular. I think Ingolifs might be right in that this is the classical moon acceleration thing. The quote refers explicitly to the distance to Earth. $\endgroup$ – Everyday Astronaut Apr 4 at 5:53
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    $\begingroup$ @uhoh: If it's resonant in, say, 86:1 and you raise it to be resonant 85:1? $\endgroup$ – SF. Apr 4 at 11:47
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I think that the highlighted sentence in the question is referring to the use of space tethers in gravity gradients for propulsion. (None of the section headings in the 9th edition of Sutton and Biblarz mention tethers, and neither ‘Tethers’ or ‘Space tethers’ appear in the index).

Geoffrey A Landis wrote a NASA report about this, available here:

Reactionless propulsion using tethers

An orbiting tethered satellite can propel itself by reaction against the gravitational gradient, with expenditure of energy but with no use of on-board reaction mass.

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  • $\begingroup$ Very nice answer! This makes a lot of sense. Can you double check that space tethers are not covered in Sutton? That would be necessary to be compatible with "...but this method is not treated in this book." Thanks! $\endgroup$ – uhoh Apr 25 at 20:51
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    $\begingroup$ Also, I think it's great that you've chosen to link to the original paper itself, rather than just a Wikipedia page. fyi you may also be interested in the question Could a satellite in LEO “pump” or change mass distribution to gain forward momentum? $\endgroup$ – uhoh Apr 25 at 20:53
  • $\begingroup$ Great! Also we can have the change in the plane or change in inclination of the satellite by spinning that tether about its longitudinal axis...(Gyroscopic precession) $\endgroup$ – Soham Prajapati Apr 26 at 3:52
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    $\begingroup$ I’ve checked that tethers aren’t mentioned in Sutton in Biblarz contents or index, and updated the answer. $\endgroup$ – Will Stevens Apr 26 at 20:02
  • $\begingroup$ The edit looks good, thank you. By the way I'd put two bounties on this question earlier, and each one expired after 7 days + 1 day grace period. As explained in answer(s) to Any way to award a second +100 bounty to a second answer to the same question? the Stack Exchange software requires each bounty to double. So this one ends up being quite large. You just missed the +200 bounty by less than a day I think. This is a bit unusual, I guess it's your lucky day! $\endgroup$ – uhoh Apr 27 at 12:56

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