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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?

uhoh
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cosmic_tintin
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    What edition/printing? – Organic Marble Apr 04 '19 at 03:55
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    @OrganicMarble It's in 7, 8, 9 but I'm not sure about earlier ones, I've made another edit. – uhoh Apr 04 '19 at 05:30
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    Am I missing something, or is this a complicated way of saying "Earth's gravity is weaker higher up"? – Ingolifs Apr 04 '19 at 05:33
  • @Ingolifs possibly not. "Attractions" is plural, and I'm wondering if the statement could refer to several additional effects including higher multipole moments of Earth as well as accelerations from the Sun and Moon. I think this will turn out to be a very interesting question. – uhoh Apr 04 '19 at 05:44
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    @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. – Everyday Astronaut Apr 04 '19 at 05:53
  • @EverydayAstronaut yes. Why is the "classical moon acceleration thing" related to distance to Earth rather than to the Moon? For that matter, what is the "classical moon acceleration thing"? – uhoh Apr 04 '19 at 06:15
  • @uhoh: For your 2nd question - very distant flyby moon gravity assists. An eccentric orbit around Earth, resonant with the Moon in such a way that Moon's gravitational influence consistently accelerates it. – SF. Apr 04 '19 at 10:47
  • @SF. As soon as you raise the orbit by a little bit, it will no longer be resonant, and so it will stop working, or perhaps even start lowering again. I'd like to see a reference to this before I believe it's an even slightly useful thing outside of scifi or space folklore. Let me know if you if you find something solid and I'll ask a new question for it. Related factoid: TESS is using a 2:1 lunar resonance to stabilize its orbit for many years. Of course the phasing is +/- 90 degrees, but I think this much more realistic than orbit raising. – uhoh Apr 04 '19 at 11:45
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    @uhoh: If it's resonant in, say, 86:1 and you raise it to be resonant 85:1? – SF. Apr 04 '19 at 11:47
  • @SF. waiting for a reference or a calculation now... please... because if you don't I'm going to be stuck thinking about this all weekend and trying to do it myself! Aaarrgghhh.... – uhoh Apr 04 '19 at 11:51
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    @uhoh: Sorry, I think the reasons you gave are why it's not being used in practice. – SF. Apr 04 '19 at 12:06
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    I can only think of a torque that wants to align the long axis with the planet. Or... if you drop a yo-yo on a stiff string, the center of mass of the satellite will shift in a linear way while the gravitational force on the yo-yo probe would increase by inverse square. Would that increase the orbital speed? Hard to imagine that as a practical application. ... I suddenly have a vague memory of reading about spinning things with arms in orbit, but I forget how that story goes. – Greg Apr 04 '19 at 18:45
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    I think we're talking about https://space.stackexchange.com/q/31620/21562 – Everyday Astronaut Apr 04 '19 at 19:59
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    @uhoh sorry, had it in my mind completely wrong. Was thinking about https://space.stackexchange.com/questions/31620/could-a-satellite-in-leo-pump-or-change-mass-distribution-to-gain-forward-mome#comment95513_31620 but it has nothing to do with it. – Everyday Astronaut Apr 04 '19 at 20:02
  • @EverydayAstronaut thanks for the updates, these are interesting and subtle questions, and the more discussion the merrier and the better! – uhoh Apr 05 '19 at 00:05
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    This one https://translate.google.com/translate?sl=auto&tl=en&u=https%3A%2F%2Fours-nature.ru%2Flib%2Fb%2Fbook%2F1990488644%2F22 - may be helpful. – Arris Apr 09 '19 at 00:32
  • @SohamPrajapati Are you still interested in this? I'm thinking of adding another bounty when this one expires. I think this one really needs an answer! – uhoh Apr 16 '19 at 11:13
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    @uhoh Always excited for such new ways of propulsions! – cosmic_tintin Apr 17 '19 at 12:05
  • @SohamPrajapati my guess is that it's a well-known orbital mechanical effect, not anything profound or new, but let's find out. I've added a new bounty. – uhoh Apr 17 '19 at 14:50

1 Answers1

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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.

Will Stevens
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