Problem If you were attempting to take over an O'Neill Cylinder without destroying it or the majority of its inhabitants/ecology how would you go about it? What benefits or detriments to an attacking or defending force would the gravity and the cylinder shape of the land have to a fighting force?

Assumptions The tech required to make an OC isn't too far off what we have now and so in terms of tech you can expect it to be similar somewhat to the those living onboard the Elysium (from the film of the same name).

The OC your attacking is of strategic or cultural importance and you intend to take it over and replace its government with your own whilst doing minimal damage to its contents.

Its current government has dug in with its own combat forces so you should expect some resistance, as in they expect you to make a move to take the cylinder.

Question What would combat look like inside an O'Neill Cylinder?

  • $\begingroup$ "Similar somewhat to the those living onboard the Elysium": that is, the president of France and his staff. $\endgroup$ – AlexP Aug 4 '20 at 12:05
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    $\begingroup$ @AlexP He's referring to the VIP-only space habitat from the titular movie. $\endgroup$ – The Daleks Aug 4 '20 at 12:16
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    $\begingroup$ One thing to consider is if you are really dealing with an isolated, spinning drum. I've always considered that idea somewhat rediculess, since you'd want at least 2 cylinders or a counter rotating hull to stabilise the station. Furthermore, there is a lot of value in not having industry, storage, space ports, agriculture and defenses in/on the habitat drum. I believe that most habitats would be encased in a jungle of infrastructure inside a hull or asteroid. For this reason I believe that habitats would want to use this armor and would be indescriminate in defending against approaching foes. $\endgroup$ – TheDyingOfLight Aug 5 '20 at 6:15

Fun thing about cylinders full of air: they make for really nice musical instruments.

An OC necessarily has a very tenacious hull. You could land some machines on its outside and have them bang at the hull without damaging it. Just keep banging rhythmically 24/7 until everybody goes crazy. They'll eventually give up.

If you want to be really annoying, you can instead vibrate the cylinder. Its walls will work like a speaker of biblical proportions. You can play whatever you want for the populace in the cylinder without having to hack anything. Play annoying music 24/7 and you'll have a victory in short time. If you wish to pick a specific genre for your playlist, I think Danger Music would be the most effective.

Edit: I just remembered that infrasound may supposedly cause hallucinations. You could make everyone go crazy by using the ends of the cylinder to produce some infrasonic standing waves. Let the populace go crazy until they beg for help, then you come in and take over.

This can be countered in small pockets by simple noise cancelling technologies. To cancel it as a whole, the cylinder inhabitants would have to vibrate the cylinder just as you are doing, but with opposing phases. This might require them to come outside, so you bring the battle off the cylinder.

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    $\begingroup$ If you knock on the hull "shave and a haircut" but never "2 bits" they will eventually surrender and then you can give them the 2 bits. $\endgroup$ – Willk Aug 4 '20 at 16:35
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    $\begingroup$ "Achy-Breaky Heart", 24-7. I think I'd walk out the airlock, even without my suit. $\endgroup$ – NomadMaker Aug 5 '20 at 1:04
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    $\begingroup$ Hmm, I'd never thought of large-scale rickrolling as a viable military tactic. $\endgroup$ – Gryphon Aug 5 '20 at 4:09
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    $\begingroup$ Slim Whitman's "Indian Love Call." akready proven to defend the earth against Martians, movies.stackexchange.com/q/44386/37379 $\endgroup$ – Criggie Aug 5 '20 at 4:36
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    $\begingroup$ @NuclearWang canceling it in a point in space is easy. Canceling throughout a cylinder, not so much. You'd have to send the counter signal from the same source the attackers are using. $\endgroup$ – The Square-Cube Law Aug 5 '20 at 18:13

To make Paris a city of ruins will not affect the issue.

  • Phillipe Petain.

The best way to not tear the place up with fighting is to not fight. In World War 2, the French surrendered Paris to the approaching Germans without a fight. Fighting seemed pointless, and would destroy their beautiful city.


As a last resort, Churchill proposed defending Paris, so that it would absorb German troops trying to subdue the city. According to Churchill's secretary, "The French perceptibly froze at this." Pétain replied, "To make Paris a city of ruins will not affect the issue." Continuing to resist the Germans, Pétain said, would mean "the destruction of the country." And so, to avoid destroying France, Pétain surrendered it.

It is easy to understand that viewpoint. Defeat seemed inevitable. And today Paris is still really nice.

Your approach is the same. You are the Germans in this scenario. Approach with overwhelming force. The occupants of the cylinder should understand that you are willing to destroy the cylinder from space, with all of its inhabitants, because you have done that before. They should understand that if they surrender they will not be massacred. Make clear that you, like them, value the historic cylinder but that you are going to take it. Make clear that by capitualating without a fight, the current rulers will be allowed to leave in peace and a Vichy-style government by locals will take control, with final jurisdiction of course with your conquering forces.

Perhaps less exciting than beloinclothed warriors wrestling each other street to street, but more practical. I am sure there is some way to incorporate the wrestling for the movie version.

  • $\begingroup$ Another similar historical situation existed, with the Mongols. They were known to levels city that did not surrender. $\endgroup$ – RomainL. Aug 5 '20 at 8:25
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    $\begingroup$ so your answer is simply "attack with overwhelming forces"? that's all you got? $\endgroup$ – carlo Aug 5 '20 at 10:46
  • $\begingroup$ "And today Paris is still really nice" - some other European cities were completely or almost completely destroyed during the war, but were then rebuilt and are really nice. Maybe even nicer than Paris which now has quite a number of ghettos. $\endgroup$ – vsz Aug 5 '20 at 10:50
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    $\begingroup$ @carlo This answer seems more complicated than that, though. Not just "attack with overwhelming forces", but simultaneously make it known a) you'll use those forces if you have to and b) you'd rather not. $\endgroup$ – ceejayoz Aug 5 '20 at 14:19
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    $\begingroup$ Great answer, +1. I expect this works great against a rational enemy. However some enemies are not rational. I could imagine an extremist enemy force might hypothetically exist who would prefer martyrdom over surrender, regardless of whether or not doing so has any chance of changing the outcome of the war. $\endgroup$ – Dast Aug 5 '20 at 17:43

Either Fast and dirty: Hack the I.T. systems, then hack again.

O'Neil Cylinders and indeed ALL archiologies are artificial, sealed, technology based environments. At the end of the day it doesn't matter how many soldiers the ruling authority has, it doesn't matter how strong its internal and external defenses are. If you can gain control the systems controlling light, temperature, oxygen content, artificial gravity etc then you can dictate your own terms - at least for as long as it takes the local administration to at least partially regain control.

Maybe they can isolate at least parts of the station, maybe not. Initially at least they will be on the back foot. That being the case whatever internal physical overrides there are you would be strongly advised to have organized at least some external reinforcements during the planning phase of your takeover.

Your takeover needs to be based on the premise that (along with all the other key systems like communications) you have control of external defenses and traffic control long enough for reinforcements to board and provide assistance.

Warning; A key issue will be whether or not you have any internal domestic support for your takeover. If the government in question is an unpopular, oppressive dictatorship no probs. If however it is an more or less enlightened democracy with a degree of popular support then your side is going to have to become the oppressive dictatorship - and then you get to see how long you last before someone overthrows you.

Which leaves one other option;

Or slow but gentle; If the society within the cylinder is 'open' and reasonably democratic stage a 'cultural' coup via social media, migration, influence buying and economic/financial manipulation.

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    $\begingroup$ Change the rotation speed of all station and make everything heavy and unable to move. $\endgroup$ – Rodolfo Penteado Aug 5 '20 at 3:24
  • $\begingroup$ This is a good one with the seige technique. Increasing the gravity or taxing the systems to make your inevitable footfall on the inside easier! $\endgroup$ – Phillip Roberts Aug 5 '20 at 14:07
  • Pinpoint strikes against any defensive emplacements on the outside.
    If you cannot even do that, a forcible takeover becomes problematic.
  • Land on the outer hull and secure it.
    Either you approach on the axis and then "climb down" or you send breaching pods with powerful engines to random points. Find some vacuum-proof superglue to stick in place. Your breaching pods contain airlocks and a working chamber that will seal tight against the hull. Pressurize the working chamber and start cutting your way in.
  • Plenty of decoy breaching pods.
    Only one in ten, perhaps only one in a hundred is genuine. The others are decoys. Simple robots start cutting into the hull, with no follow-up attack.
  • Release swarms of disposable attack microdrones.
    They go after internal sensors, communications relays, etc.

After that you might be able to attack the defense force "on the ground" with acceptable damage to the structure.

  • Alternate
    If the O'Neill relies on solar power for electricty or the heat balance, block that with a giant shade.
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    $\begingroup$ The sun shade is a clever idea and might work. Best bit is it forces the occupants of the cylinder to come to you, not the other way round. The rest, not so sure. Breaching pods look wonderful in SF but in reality they would be easy to detect on approach if only because they have to decelerate. An O'Neil Cylinder will by also default have a thick hull designed to prevent breaches. Also because its a complex machine there will be only so many points where you can breach and access living or work spaces e.g. breaching a sealed water tank or turbine pump gets you where? $\endgroup$ – Mon Aug 5 '20 at 1:33
  • $\begingroup$ And of course in order to take out the point defenses you have to get with range with systems capable of doing this - so the cylinder has the opportunity to shoot back. Your boarding parties are likely to take heavy losses going in and even heavier once inside because the defenders will know where its possible to breach and where not. Also initially at least they can potentially isolate/seal off boarded sections - and do all sorts of nasty things to people inside them. $\endgroup$ – Mon Aug 5 '20 at 1:35
  • $\begingroup$ This is more the kind of new way of fighting I was looking for I think. Being able to pop up anywhere from underneath on the battlefield makes large scale line warfare obsolete! I think however as soon as you pop up from underneath your at the mercy of whats on the inside, you'd have to hit multiple strong points at once or establish more of a stable beach head. $\endgroup$ – Phillip Roberts Aug 5 '20 at 14:03

All space craft (ships and habitats) share a common problem: heat dissipation. The OC will have large, vulnerable radiators. Damaging these would not destroy that habitat or its contents, but would force the inhabitants to come to terms so that they could make repairs before they all cooked in their own waste heat. It may not even be necessary to actually damage them at all.

Assuming their ships have sufficient reaction mass to keep adjusting their orbits, your attackers could stand off several thousand kilometres and use powerful long wavelength lasers to heat the panels sufficiently that they absorbed more energy than they radiated. Once the station surrenders, turn the lasers off.

The attacking ships can maneuver to prevent similar tactics being used back against them.

If that were not practical (it assumes the attackers have a lot of power generation) then short wave length lasers with more focused beams could be used to cut holes in the (presumably) fragile radiators. Given this is a habitat and not a ship with very limited or no maneuvering capability, even projectile weapons could work, despite the flight time of minutes or hours. (Effectively giant shotguns firing hundreds of thousands of tiny projectiles at very high velocities). Knowing what's coming is no good if you can't dodge.

After that just hope they don't have Casaba Howitzers!


While practically all of the life support systems would be in the cylinder and out of the immediate reach of the invaders, there would have to be some systems on the outside of the cylinder that are more easily accessible.

Since there is an invader it's a given that the occupants of the cylinder are not alone, so they would need communication systems. There would be sensors and surveillance systems. There would be airlocks and access hatches, robotic repair systems to maintain the hull. Most importantly, there would have to be some form of propulsion to maintain a constant rotation (in case it it changes due to outside influences).

If an attacker gains control over these outside systems, they can affect life on the inside significantly. They could stop rotation of the cylinder and thus end gravity on the inside. They could also accellerate the rotation to increase gravity. They just have to disable any existing propulsion systems and attach their own. They could make the entire cylinder deaf and blind to the outside world. In case light is not produced on the inside, but comes from the outside through windows, you can block the light, which would cause plant life to end or at least to suffer on the inside.

This is a situation similar to a siege of a medieval castle. As it happened in those times, the defenders would know the vulnerability of thir situation and would try to defend aginst the attacking force when they are already approaching. So a lot of the fighting would probably happen before the cylinder is even reached. If the defenders would have to retreat to the inside, they would be under siege from this moment. The attackers would either try to make the situation unbearable on the inside so that the defenders surrender or they would wear them down until they are so weak that they will not be able to resist when the attackers breach the hull and take the battle inside.

When the attacker makes sure to cause enough damage to the inside to weaken the defender, but not to cause damage beyond repair, they could assist in reconstruction of the cylinder after they have taken it over.

  • $\begingroup$ I also like the castle example from this answer, definitely sieges will be far more common than actual ground warfare, and combined with the tunnelling method i think this could be quite effective. $\endgroup$ – Phillip Roberts Aug 5 '20 at 14:04

Induction motors work by creating a rotating magnetic field, which induces a lagging field in a conducting armature. The armature doesn't have to be magnetic, or in a strict sense even ferrous, as long as it conducts. If you have the means to set up such a field from a distance (assuming you have plenty of energy available, so efficiency isn't a limitation), simply increase the spin of the ship itself...it's not likely they fitted powerful engines to maintain its rotational speed in vacuum, since only small corrections would normally be needed, and only occasionally. Get them up to about 3g and they should be ready to negotiate.


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