Why Am I Doing This To Myself?

Because I'm designing a surreal pulp science-fantasy tabletop roleplaying game, and I want to take the coolest elements of hard sci-fi and mix it with the punchiness of fast-paced slapstick fantasy action.

Limiting The Scope Of This Question

Magic Is Bullshit - To manage everyone's 'okay well then anything is possible' remarks, let me tell you this about the magic in this setting. It's all hardlight simulation, which means it only works so long as you're within the atmosphere of the Ring.

To keep things simple, the atmosphere, and thus the hardlight simulation, is kept in by the miles high walls on the inner and outer edges of the Ring. This means it's 'real' in the setting, because the players will never know any different, but it's not just some catch-all solution either.

Ancient Technology - Technology is actually quite limited. It's enormous and bulky, and typically only interfaces with the cadavers of ancient races or Artificial Intelligences, which are typically too weak to do more than activate teleporters, turn off security or open doors. AIs are used to fly ships around in the Ring's atmosphere.

Even weaker AIs are also possible, known as 'Fragment AIs'. Artificial Intelligence of this level is made from the fractured pieces of whole Intelligences. Being split off like this can be done as punishment, or as mercy. Of the 9 original AI programs built into the Ring, 5 have thought themselves to death, and three more have been fragmented. Nobody knows where the final AI is.

The Ring's Design - I know you can't have it all when you're designing these megastructures. I know about smaller versions of them, such as the ones used in Halo, and I know that I want my Ring's design to follow a few principles. Here are these limitations;

  • I want the players to be able to see the other side of the Ring, so the skybox looks awesome.
  • I want it bigger than the Rings in Halo, which are about the diameter of Earth. I don't want it anywhere near as big as the Ring from Ringworld, which is built to sit around the sun. I want this Ring to orbit the sun.
  • I think giving it a moon would be cool. Especially if the moon was a small-scale Shellworld megastructure.

What Do I Know So Far?

Research - I know you need big walls on the sides to keep the atmosphere in. I know that it needs to be off-kilter for some reason. I know that it needs to spin to keep itself in orbit and to simulate gravity. I know it needs to generate its own magnetic field, which is done using two giant cables running along the inner and outer radii, so everything doesn't die from radiation. I know the exterior probably needs a bunch of big thrusters to help it stay orbiting correctly. I know for this design, it would need to sit on a Lagrange point.

Unknowns - I don't know how big it should be. I was thinking something no bigger than 25,000 miles diameter, maybe even no bigger than 16,000. I was thinking a width of around 1,000 miles width (assuming 25,000mi diameter) or smaller, like 666 miles width (assuming 16,000.) This means I have no idea what the actual usable space of the Ring would be. It also means I don't know how high the walls would have to be, or how it would have to orbit, etc. I also don't know how the simulated day/night cycle would work, or if that would just be an inherent part of orbiting the sun.

And that's ultimately what the question is. Can you please help me address these unknowns using my parameters, and let me know the space I'd have to work with, how things would have to be etc? Thank you so much to anyone who's willing to tackle this problem with me!

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    $\begingroup$ Welcome bonglo, please take our tour and refer to the help center for advice on our ways. Enjoy the site. $\endgroup$ Commented Dec 11, 2021 at 22:37

1 Answer 1


Planet sized O'Neill cylinder.

What is a cylnder but a fattened ring? Your habitat is a hollow cylinder three times the diameter of Earth. One can see the lands sloping up and away at all sides but the far side is too far away to see. Atmosphere is confined because the cylinder ends are closed; exactly how is unknown because there be dragons.

It is also too bright to see across the cylinder. An artificial sun hangs in the center of the cylinder. It is half light and half dark, and provides a light / dark cycle by rotating. The nights are not that dark, as you might surmise.

Your cylinder is not in orbit of anything except perhaps the galactic center. It is in interstellar space. It might be a colony ship.

  • $\begingroup$ It's not what I was after, but this is still a really interesting solution. I bet this would make a great setting for a 'waking up from augmented reality' style sci-fi novel. I'd prefer it remain a huge ring however - but I bet some 'ark' style ship in the way you described would make an awesome adventure setting. $\endgroup$ Commented Dec 12, 2021 at 13:41
  • $\begingroup$ @bonglover666 - can you make it just a really short cylinder? $\endgroup$
    – Willk
    Commented Dec 12, 2021 at 15:11

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