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I'm working on a project involving an interstellar society, and I'm looking to incorporate as much actual science and economics into the project itself since I'm a stickler for numbers and logistics. However, because I'm a stickler for numbers and logistics, I've found myself in a bit of a quandary as I've inevitably encountered the subject of space elevators as an efficient means of moving goods and people into and out of orbit. The problem I've encountered specifically is what payload levels could be moved into orbit along a space elevator, and the amount of power that would be required to sustain such an activity.

My question is this:

  • How do I determine how much payload a space elevator can lift where fusion power is readily available?
  • How do I determine what the most reasonable ascent speed is for the space elevator?

As I've mentioned, as much as I love science and technology, it's not a passion that I've pursued outside of creative hobbies such as worldbuilding. If you could break down the details in a somewhat comprehensible manner, I'd be ever so grateful. Thank you!

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  • $\begingroup$ Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. $\endgroup$
    – Community Bot
    Mar 25 at 10:34
  • $\begingroup$ Please edit your question to be just one question and ask the second one separately, or ask bring the text in line with the title ("how much payload") which might involve other factors than power consumption/delivery. $\endgroup$
    – Zeiss Ikon
    Mar 25 at 13:56
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    $\begingroup$ How realistic is the idea of a space elevator in the first place? $\endgroup$
    – Daron
    Mar 25 at 15:09
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    $\begingroup$ I check a number of proposal papers a while back and the average payload in these papers was 15 tons per week. That convinced me that space elevators are severely overhyped and that skyhooks, laser launch, launchloops and ultimately orbital rings are vastly superior systems. All of those can actually be built with current materials, although high temperature superconductors would be helpful for the latter two. $\endgroup$ Mar 26 at 10:38
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    $\begingroup$ @TheDyingOfLight yeah, orbital rings would be basically better than all the rest, but they're also waaaaay more complex than even space elevators. I'm not entirely convinced of the practicality of skyhooks either, but I'd find it harder justifying my stance against those. FWIW, my gonzo megastructure favorite is the startram, I'd like to think it is easier than all the rest by a couple of orders of magnitude, and no less useful. $\endgroup$ Mar 26 at 14:41

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The maximum load of the elevator doesn't depend on your power technology, but your material technology.

  • the maximum load of a lifter on your space elevator is determined by the thickness of the lower end of the cable.
  • the thickness of the lower end of the cable will be kept as low as possible to reduce the weight of the entire elevator, and hence the difficulty in building the thing in the first place.
  • the thicker the bottom end of the cable, the fatter the middle of the cable, and the heavier and more complex to make the whole elevator is

Really, anything from 20 tonnes or a thousands tonnes or more is fine... handwave in whatever you want to suit your story needs. Lighter loads for lower tech levels, partially constructed elevators with only a thin cable that's still being expanded, higher gravity worlds, etc.

The speed of your elevator is is limited partially by how it is attached to the cable, and partially by how it gets power. Elevators are more efficient than rockets, but there's still a lot of energy required to lift a load up to the geosynchronous point.

Specific orbital energy is the number to care about here. The important parameters are $R$ the radius of the planet you are lifting from, $a$ the radius of a geostationary orbit, $M$ the mass of the planet and $G$ the gravitational constant. $\mu$ is the standard gravitational parameter, $GM$. The amount of energy you have to expend to raise the lifter is defined as $$\epsilon_a = -{\mu \over 2a} + {\mu \over R }$$

For Earth, you'll have an $\epsilon_a$ of ~57.77MJ/kg. A hundred tonne lifter would need ~5.777TJ. If your lifter managed an average speed of 600km/h, you'd make the trip from the surface to orbit in a little under 60 hours, and need a steady power supply of about 27 megawatts sustained for the whole journey.

If fusion power is effective, compact and safe, you could fit a mini reactor to the lifter. Given the weight requirements, this seems unlikely unless you have a really substantial cable and a very large lifter of a few thousand tonnes or more. If fusion reactors are big and heavy and awkward (which is likely), you might keep them on the ground (or in orbit) and use laser or microwave beamed power to drive the lifters motors.

If you wanted a faster trip, you might be able to handwave one in given suitable power supplies, though cable friction is going to be a problem. Some kind of fancy superconducting magnetically levitated linear motor might work, but all that extra weight on the cable has to be held up by something...

Please also remember that you get one lifter per cable. if you want multiple lifters, you'll have difficulty fitting them past each other, even if the cable was very large indeed. Here's a reasonable artist's impression of a lifter. See how it encompasses the cable, and is quite broad in order to accomodate beamed power collection arrays:

artist's impression of a beamed power lifter with two large circular collection arrays on either side of the cable

(image credit Liftport)


I'll spare you the gory details of elevator cable width, but if anyone really wants me to walk you through it and doesn't fancy reading eg. The physics of the space elevator to work it out themselves, do ask.

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  • $\begingroup$ Thank you for the detailed explanation. This is actually very comprehensive and touches on all of the questions I had regarding space elevators. I have no issue reading up on the gory bits if you have them to share. Thank you again. $\endgroup$
    – Vivaporius
    Mar 26 at 4:57

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