I have seen Isaac Arthurs video of O'Neill cylinders and read through many descriptions, but it still is not clear to me how exaclty the two counter-rotating cylinders are placed in regard to another and in regard to the sun.

I sort of imagined the cylinders to connect on their short site, when I first read about it. Than this reappearing picture shows them spread rather far apart; how do they even connect here?

Minute 8:50 in Isaac Arthurs video shows two cylinders connecting on the long-side, but unmoving. Is the long side ment to connect all along its length or merely in the middel? Do I have to imagine a stable shell, like a package of Twix, with the two twix counter-rotating inside?

Also, non of these graphics make it clear to me how I have to envision the position of the cylinders in regard to the sun; is the short- or the long-side facing the sun? I get the wireframe globes, the whole thing has to move, but how would a single unit be placed?

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    $\begingroup$ You might want to consider asking this question on the SFIA/Isaac Arthur Subreddit as well. The subreddit is one of the best places to discuss SFIA concepts. $\endgroup$ Commented Sep 25, 2019 at 21:04
  • $\begingroup$ @TheDyingOfLight Thank you very much for the tip! $\endgroup$ Commented Oct 2, 2019 at 8:12

2 Answers 2


You need two contra-rotating cylinders in order to neutralise the gyroscopic effects of a single spinning cylinder, and as a way to use the whole habitat as a momentum wheel. You need to keep the cylinders with one endcap pointing at the sun throughout their entire orbit, so as to keep the mirrors illuminated during station daytime. This means they need to be side-by-side, with some structural framework connecting their axes. In the picture you linked, there are a pair of white spars linking the near endcaps. That's pretty much all you need... not some kind of massive external shell.

From the wikipedia article:

First, the pair of habitats can be rolled by operating the cylinders as momentum wheels. If one habitat's rotation is slightly off, the two cylinders will rotate about each other. Once the plane formed by the two axes of rotation is perpendicular in the roll axis to the orbit, then the pair of cylinders can be yawed to aim at the Sun by exerting a force between the two sunward bearings. Pushing the cylinders away from each other will cause both cylinders to gyroscopically precess, and the system will yaw in one direction, while pushing them towards each other will cause yaw in the other direction. The counter-rotating habitats have no net gyroscopic effect, and so this slight precession can continue throughout the habitat's orbit, keeping it aimed at the Sun.

  • $\begingroup$ Do you happen to know if there is some sort of graphic or video, anything visual, going more into detail with this? $\endgroup$ Commented Sep 25, 2019 at 10:40
  • $\begingroup$ How do these two cylinders move in orbit; side by side or "on top of each other"? $\endgroup$ Commented Sep 25, 2019 at 11:05
  • $\begingroup$ @BackupPlan weirdly, I cannot find any examples of this. Such things clearly have existed in the past (check out this soviet-era diagram I found on project rho) but that seems to be about it. Maybe I'm searching in the wrong places. It seems bizarre that the videos you've already found don't show this basic designh principle of O'Neill's work. $\endgroup$ Commented Sep 25, 2019 at 11:05
  • $\begingroup$ @BackupPlan as your picture shows, and as my answer says, side-by-side. $\endgroup$ Commented Sep 25, 2019 at 11:06

Starfish Prime already explained the stabilisation and alignment aspects. This answer focuses purely on:

Do I have to imagine a stable shell, like a package of Twix, with the two twix counter-rotating inside?

An O'Neil cylinder doesn't have to look like this, but it is the most plausabile way to build them. You could of cause go for the classical pair just rotating in open space, but then you would not use the advantages of the environment properly. Not everything benefits from being under 1G all the time and the more mass your main drums have, the higher the stress and material strength required for them will be.

The smart thing is to optimise the setup. Make the main cylinders large and light and optimise them for habitation and recreation. Use smaller, low gravity cylinders in close proximity for industry which benefits from some gravity like farming and production. Use the zero G areas for everything you don't need gravity for. Some industrial processes will work better in zero G andstorage of almost everything is easyer and less demanding on the material here. Use the shell as a shield against radiation and impacts, for radiators, weapon systems, bulk storage (additional shielding), spaceports and energy generation (use solar or mirror arrays attached to on the shell). The fact that there is a Shell and periferal infrastructure doesn't change how the alighnment to the sun is maintained.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ Commented Sep 26, 2019 at 14:23

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