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In this universe, Earth had a couple orbital rings during the construction of the colony ships that were sent to Alpha Centauri. Humanity has achieved fusion power and efficient space flight (1g acceleration on ships).

For clarity, by orbital ring I mean a megastructure that has a greater circumference than the planet and orbits anywhere from 80km above to a geostationary orbit. They would have several tether systems which include elevators and trains.

Ideally, I would have maybe 3 around the main planet which is a binary planets system. 1 would rotate at such an angle that it can launch and recieve from the binary planet. The other ones might be on the stellar plane and maybe at a polar orbit. While this universe would be a fusion based society, I still think that atmospheric transportation would be costly. Perhaps the starship engines are banned from atmospheric use.

Additional information Humans left Sol around 2261 on a 182 year trip to the AC system. It was a colony fleet with 5 generation ships each with a population around 10,000 at the start of the trip. During the trip, some jobs were manufacturering of items needed for colonizing (mostly mining ships and shuttles). My initial thought was to have the ships orbit their respective worlds for between 50 to 100 years as they finish terraforming them and creating a planetary infrastructure. During this time orbital rings would have been constructed from harvested material nearby and in the outer reaches of each star (since at about 3 AU from each star is chaotic space I am putting an asteroid belt there). Both A and B stars have at least 1 habitable world and maybe 1 or 2 that are semihabitable. Around A is a subneptune(mini ice giant)although I am toying with it being a little bigger as I need some place to harvest vast amounts of hydrogen from. This planet will orbit in the outer reaches of the stable zone.

The basic question I am asking, is if a civilization is already fusion based space fairing, would they possess efficient space-to-ground capabilities in their ships that would render an orbital ring virtually useless?

I want to make sure I'm not ignoring any possibilities that come with fusion power and advances space flight. What do you think?

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    $\begingroup$ Since they built them around Earth, they have the knowhow to do it. What exactly worries you when it comes to "feasibility"? $\endgroup$ – Adrian Colomitchi Feb 29 at 3:17
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    $\begingroup$ You'd need to tell us about how long humanity had been there, how well established it is and what manufacturing infrastructure has been put in place. If it's immediatley on arrival of the ships, then they'd need to be awefully big ships carrying the pre-built pieces of the ring - maybe pre-built in the asteroid belt around Sol? What's the exact issue you're trying to solve here? $\endgroup$ – Tantalus' touch. Feb 29 at 3:18
  • $\begingroup$ Thanks for the response, I edited the question for more clarity and information. $\endgroup$ – Markitect Feb 29 at 12:53
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There'll be a couple of advantages they have at destination over how it all started on Earth - they are already in space with enough energy/power available (interstellar travel does require a higher redundancy and energy levels, so it is likely they have energy to spare once safely there).

The approach is to grab some asteroids at destination and start transforming them in space elevator tethers - very likely carbonaceous asteroids spun in carbon nanotubes. Once at least one (per planet) is done, the rest fall in place quickly.

One thing that can go awry with the Dyson rings: they are in an unstable equilibrium - i.e. if their rotation center drift from the center of the planet they orbit, the thingy gets outta whack with a positive feedback. It becomes important, because of that "binary planets system" which, based on your description, have a co-rotation plane that don't coincide with the stellar orbital plane.
How this matters? See, that rotational momentum of inertia is a pesky thing, 'cause precession period will vary based on the pull from the star and the other planet. I'm quite afraid an orbital ring at only 80 km altitude offers to little time for corrections when the things go ugly.

I'd rather go with a set of space elevators - the counterweight at the other end will keep the stability on track. Once out of the gravitational well, correcting for a destination not in the rotation plane is cheapish. I.e. trying to keep those orbital rings stable is going to cost more energy than climbing a space elevator tether and correcting the trajectory of the payload once launched.


On another line, orbital rings become a solution if you are severely constrained by the space available for processing. Otherwise they are awfully expensive in the amount of materials they require and don't offer advantages over space elevators when you have a low goods traffic. They may make sense on Earth, as a support for building those interstellar spaceships, massive amount of materials to process and put together... but you got to the destination with probably 100K people. Surely you won't want to leave soon, not before you get a bit more population on the planets you just started to colonize - probably the marching orders will be more on the line of "Make love, not orbital rings"

One on top of the other, I don't feel there will be a pressing need to get those orbital rings built starting soon after the arrival.

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  • $\begingroup$ Isaac Arthur talks about orbital rings and mentions that in order to stabilize them, several tethers to the planet should do the trick. I agree that around 80km might be too low. So maybe have them orbit closer to 150km (something where tethers and elevators can have a material with a plausible length. $\endgroup$ – Markitect Feb 29 at 13:15
  • $\begingroup$ About the planets, thinking more about it, I do believe they would be on the same stellar plane. So that would reduce a ring if used. The planets are about 700,000km apart from each other. $\endgroup$ – Markitect Feb 29 at 13:16
  • $\begingroup$ @user72655 your maximal constraint at the beginning will be low level of population, I don't think orbital rings are an efficient way of spending resources when there aren't enough people to take advantage of them. $\endgroup$ – Adrian Colomitchi Feb 29 at 13:43
  • $\begingroup$ That's fair. One of the larger factors that make it worth it to be is the ability to use the rings for space travel. Like a sling shot in the star systems. Also to transfer heavy materials to and from planets. I think, if I go with the rings and elevators, then I will make some sort of planetary law that doesn't allow the starship fusion engines to be used in atmosphere. $\endgroup$ – Markitect Feb 29 at 15:00
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I think you're in good shape using the orbital ring. Having a binary system is a complicating factor though.

I was thinking that you may need a bean shaped ring to account for the other planet's gravity, but the rotors on an orbital ring make them stiff so they don't need to follow an orbital path. However, the other planet will have an influence on the ring, so a lot of orbital maintenance will be required. Probably best to have a very low ring so that the parent planet is the dominant factor.

As far as getting to the other planet in the binary, any ring should work. You just have to release so that you pass through L1 (lowest energy trajectory) so the size and orientation of your ring will define heavy traffic lanes.

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