All the current proposals for a space elevator seem to be for one to the GSO.However is there some proposal for a space elevator to a low earth orbit? I know that it would require continuous power to just maintain it in a synchronized position, but it still seems to be worth it.
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4$\begingroup$ Possible duplicate of What is a "space elevator"? $\endgroup$– kubanczykCommented Oct 2, 2016 at 8:12
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$\begingroup$ @kubanczyk I dont think the two questions are the same. $\endgroup$– user2277550Commented Oct 2, 2016 at 8:21
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$\begingroup$ @user2277550 I agree, its not a strict duplicate but much of the understanding from the suggested duplicate linked question can be read across to your question. See my answer for some thoughts about LEO. $\endgroup$– PuffinCommented Oct 2, 2016 at 9:50
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5 Answers
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The reason it's hard to get to orbit isn't that space is high up.
It's hard to get to orbit because you have to go so fast.
Space isn't like this:
Space is like this:
Any explanation I would produce cannot be possibly better, so I shamelessly copied it from the canonical source https://what-if.xkcd.com/58/
The answers to the existing question "What is a space elevator?" also answer your question; they explain that elevator needs to go all the way up to the GEO and beyond.
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While in a technical sense a space elevator to LEO can't exist because it would just fall down there is another option called a rotovator--and it's much easier to build, besides.
You take a long cable and put it in orbit, the whole cable is spinning on the short axis. You orient this so the end facing the planet is on the backwards part of it's spin. Note that the tip of this cable will be moving far below orbital speed, for the biggest rotovator it can even come to a complete stop. (But picking something up off the ground isn't practical for safety reasons.)
Unfortunately, the rotovator has two weaknesses:
1) You have to dock with it on the fly, you can't just board it like you can an elevator.
2) An elevator is in continual outward tension, so long as the weight in transit is less than the outward tension it's fine. It's like a printing press--continually turning out mass in orbit. A rotovator isn't anchored, it has zero outward tension. Everything you lift lowers the rotovator. Thus it functions like a bank--you can only withdraw what you put in. The only reason this is of any use is that what you put in and what you take out need not be the same thing. You can boost spacecraft and deorbit trash.
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5$\begingroup$ In addition to deorbiting trash or asteroids, you can also reboost the rotorvator via more efficient, lower-thrust means than a conventional rocket, e.g. ion propulsion or electrodynamic tether. $\endgroup$ Commented Oct 2, 2016 at 20:32
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A LEO based orbital ring is possible with today's material. Paul Birch authored a paper in the 1980s When SpaceX's Starship is operational a starter OR should be possible.
Also search on "low cost orbital" for a proposal for a starter OR.
Once the first one is in place then the system can be bootstrapped into bigger rings
https://www.orionsarm.com/fm_store/Paul%20Birch's%20Page.htm
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$\begingroup$ Can you please explain how this works instead of just stating that it can and providing a link? Readers shouldn't have to browse a bunch of files and read papers to see if you are correct or not. And telling the reader to search for their answer...no. That's not what this site is for. $\endgroup$ Commented Apr 23 at 16:50
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$\begingroup$ While this link may answer the question, it is better to include the essential parts of the answer here and provide the link for reference. Link-only answers can become invalid if the linked page changes. - From Review $\endgroup$– Ryan CCommented Apr 23 at 16:57
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$\begingroup$ You can also build a Tethered Ring which would need rockets to launch it. $\endgroup$– phil1008Commented Apr 24 at 3:17
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From the linked question in kubanczyk's answer you can understand the principle of operation of a space elevator. As to what to do about getting to another orbit such as LEO you have a choice.
As an example, if you had a "conventional" supersynch elevator and released something in a high MEO it would not enter a circular MEO orbit but an ellipse with its apogee in MEO and perigee at some lower point in LEO. You'd have to work out the target MEO release point in order to minimise the delta-V to attain your desired LEO orbit. This might be large, e.g. > 1km/s, depending very much upon your definition of "LEO", but you have at least sidestepped the ~9.5km/s to go straight to LEO from the surface.
Clarification
By "conventional supersynch elevator" I mean that, i.e. one that terminates well beyond GSO and is balanced appropriately, see geoffc's answer to the other question and the comments after it. Its entirely up to the user where on the structure to release an object.
Thought experiment: If it is released at GSO altitude then it will adopt a circular orbit. If it is released slightly sub-GSO then it will have a GSO velocity at an altitude for which a circular orbit would have to have a higher velocity. As a result it will adopt an elliptical path with a lower perigee. Keep on lowering the release point and explore what happens to the perigee.
Related questions
There are some helpful explanations and diagrams in CoAstroGeek's answer to this question Needed height from the release of a space elevator to get a stable elliptical orbit? and HopDavid's answer to this one If we build a space elevator from Earth surface to GEO, could I step off it at GEO and remain in GEO?
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$\begingroup$ Umm, but MEO-ended elevator cannot hang (from a satellite), it needs to stand and support its own weight. Plus you need to transport extra propellant to add about 2km/s of delta-v, so even more weight again. $\endgroup$ Commented Oct 2, 2016 at 10:59
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$\begingroup$ I didn't actually say "MEO-ended" but have added a clarification anyway. Once again, I did stress the increased delta-V and also that it would be less than the launch to orbit delta-V cost that one would be faced with if the elevator were not present. $\endgroup$– PuffinCommented Oct 2, 2016 at 15:48
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$\begingroup$ But the question seems explicitly about low height elevator (actually LEO-ended), not about an early release from a conventional elevator. $\endgroup$ Commented Oct 3, 2016 at 6:29
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$\begingroup$ Yes, if we read it strictly then you're correct. I took a guess that the real intention behind the question was to deliver something to LEO. $\endgroup$– PuffinCommented Oct 3, 2016 at 21:32
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No, a space elevator cannot be built in Low Earth Orbit, the idea is nonsense. Sufficient rocket propulsion could be provided to "hover" an object directly above a point on Earth's equator at an altitude of Low Earth Orbit but it would require a constant acceleration upwards of just under 9.8m/s/s. With conventional rocketry this might be maintained for several minutes before the propellants were depleted at which point the object would accelerate rapidly Earthwards and breakup in the atmosphere.
