Number of Decks on a Rotating Habitat

[Strato Incendus]

My current spaceship design with several ring habitats (6 in my case) works well for worldbuilding purposes, in the sense that the reader should easily be able to tell what types of facilities can be found where on the ship. That’s because the rings distinguish themselves from each other by function; in other stories (such as in “Braking Day”), the rings seem to be largely similar in structure, and there are simply several of those rings on the ship (meaning, every ring has habitats, work rooms etc.).

However, before building several ring habitats (6 to 8), of course the question is why one would realistically not simply add more decks to just 1 or 2 ring habitats. Since overall, that should require less mass than adding entire new rings.

In principle, this applies to both rings and cylinders — since the line between what counts as a ring and what counts as a cylinder is rather blurry. Basically, if a ring’s length exceeds its diameter, it starts becoming more like a cylinder — and/or if the ceiling height of the ring increases further and further. The main difference is that a ring still has some open space between the ship’s trunk and the ceiling (inner circle), whereas with cylinders, the ceiling often just is the ship’s trunk.

When it comes to the discussion of decks specifically, though, most depictions of cylinder habitats I have seen do not have several decks — merely an outer wall and an inner axis around which the cylinder spins, forming the ceiling — and then, there are regular buildings (all the way up to skyscrapers) being built on the inside of the outer cylinder hull. Hence, I guess my question primarily applies to ring habitats.

Due to the concern of keeping ship mass as low as possible, rings still sound like the superior alternative to me when it comes to spaceships, which are supposed to travel to another star. O’Neill cylinders, meanwhile, are permanent habitats of their own — so unless somebody wants to use them as vessels with which to explore other worlds, there is no reason to use cylinders as colony ships, because the cylinder itself already is the colony.

For spaceships, having at least 2 counter-rotating rings (or cylinders) seems necessary, so that they don’t flip around their own axis and thus change directions. A space station in orbit of a planet or moon, meanwhile, as far as I understand it, should also get by with a single ring habitat, even if that does result in the station spinning. (It might be a problem during reboosts, though, which the station has to do every now and then to stay in orbit?)The main reason why I did not add more than 5 decks to my ring habitats so far (with varying ceiling heights), is that centrifugal force (=meaning, in this case, “gravity”) increases with every further deck you add on the outside — or decreases with every further deck you add on the inside. However, assuming a ring diameter of 450 to 500 m (=a radius of 225 to 250 m) to create 1 g with 2 rotations per minute or less, this gradual increase or decrease in gravity per deck (if a ring has a standard ceiling height of 2.4 m to 2.6 m) seems to be pretty slow. Meaning, SpinCalc suggests you could add lots and lots of decks before the outermost one would reach 2 g, while the innermost decks still have 1 g.

Another reason for me to use fairly “flat” rings is the (albeit more unrealistic sounding) dismantling function of the rings. Meaning, for landing, they can detach from the central trunk, break down into sections, then the sections extend composite wings (the composite structure allows the wings to remain in a curved state while hidden inside the ring), and enter the atmosphere. This allows to transfer all the habitats and other rooms on the rings to the planet’s surface, because the rings become the first buildings on the planet. While of course these buildings could be tower-like or even skyscrapers, the part where they enter the atmosphere would become a lot more difficult with every further deck I add to the ring.Finally, there’s the relation between surface area of the ship and crew size. With five decks, a thickness of 32 m (=the width of the corridor + rooms on the ring), and an inner diameter of 500 m, my rings have a circumference of 1,461 m to 1,652 m (depending on which deck you’re standing on), making for a usable surface of about 5 x 50,000 sqm. That makes for about 0.25 square-kilometres per ring, times 6 equals 1.5 square-kilometres for the entire ship. This of course does not factor in the central trunk, as well as all the other areas with zero g. However, of course most crew members would spend most of their time in those areas where there is Earth-like gravity.

With a maximum of 1,500 people on the ship, if the ship reaches full carrying capacity, that makes for 1,000 people per square-kilometre. That’s about as densely populated as the Vatican — and only slightly less than Bangladesh, one of the most densely populated countries on Earth. Even if the crew only consists of 1,000 people — the first 250 couples, each of whom have two children — that’s still 666 people per square-kilometre, which would still make the ship more densely populated than Taiwan or Mauritius.

But of course, adding further decks to the rings would increase the surface area of the ship by about another 50,000 sqm with every further deck I add.

Hence my question: How many floors / decks do you think rotating ring habitats should have? Keeping in mind
a) the gradual increase / decrease in gravity as you add further decks outward / inward, and
b) the dismantling function that is special to the rings in my setting, so that as much of the ship as possible can be transferred to the target planet?

 

[sophiecentaur]

Concentric rings would not need to rotate at the same rate but at different angular velocities. That could allow a comfortable value of g for everyone. To go from one ring to another would involve a transfer pod in a void, along a helical track, 'orbiting' at a variable speed as the radius changed and keeping the G more or less constant during transfer. A book wouldn't need an actual diagram . . . .

 

[Melbourne Guy]

O’Neill cylinders, meanwhile, are permanent habitats of their own — so unless somebody wants to use them as vessels with which to explore other worlds, there is no reason to use cylinders as colony ships, because the cylinder itself already is the colony.

I used an O'Neill cylinder as an interstellar spaceship because it simplifies a lot of engineering. My ship was the asteroid Hektor hollowed out with the cylinder spun and held in place via superconducting magnets. The rocket engine needed a lot of hand waving (metallic hydrogen in this case), but the body of the asteroid provides protection from cosmic rays and a source of raw material, so it's not a bad plot device.

It was also as you note, a straightforward hollow interior with no additional floors apart from the structures on the inside rim.

Hence my question: How many floors / decks do you think rotating ring habitats should have?

In another novel the protagonist's ship had two 150m diameter counter-rotating rings spinning at 3 rpm for 0.75 gee on the rim. The habitat area was 12m in depth and had three levels, working on the simple premise that each level had a 3m ceiling height, which meant the apparent difference in gravity was mild between floors. There was also a multi-deck zero gravity area between the wheels with considerably more storage and living area.

But to answer the question, you need as many floors as the story needs. My crew was tiny, so had acres of space each in the ship, but you've a much larger population and they're packed in, so I'd go for as many floors as your structural engineering can provide, otherwise you're likely to have all sorts of societal mayhem ensuing.

 

[sophiecentaur]

O’Neill cylinders, meanwhile, are permanent habitats of their own — so unless somebody wants to use them as vessels with which to explore other worlds, there is no reason to use cylinders as colony ships, because the cylinder itself already is the colony.

A humanitarian point here: Would it be fair to launch a project involving generations of colonists and expect them to accept an horrific population density on the promise that their great grandkids might have a lot more living space 'at some point in the future'.

This is where I lose interest in the idea of colonisation on a shoestring. I would imagine that the colonists would spend at least as long in 'terraforming' the place they want to live in as on the journey there. Five generations to get there and fifty generations to get the place ship shape. That would be a minimum time scale. Could anyone suggest how a project lasting a thousand years could be assumed to remain politically stable or even to remember what the original plan was?

 

[Melbourne Guy]

Could anyone suggest how a project lasting a thousand years could be assumed to remain politically stable or even to remember what the original plan was?

This dynamic is the basis for many sci-fi stories as the ship social structure devolves over time. And @Strato Incendus, over many posts, has delved into this aspect because keeping the dream alive over such time scales in such a limited space is a narrative challenge.

 

[sophiecentaur]

keeping the dream alive over such time scales in such a limited space is a narrative challenge.

You mean it's hard to write a convincing story about the topic? As a non-romantic Engineer, I find that I tend to see the downside / practical issues with no rosy tinted spectacles.
The sort of Space Sci Fi that I get on with tends to talk in old fashioned terms (Philip K Dick and Asimov etc.) and just avoids the detailed Science. After all, it's all fantasy - which is totally fine for me.

 

[Strato Incendus]

A humanitarian point here: Would it be fair to launch a project involving generations of colonists and expect them to accept an horrific population density on the promise that their great grandkids might have a lot more living space 'at some point in the future'.

That's precisely the point ;) . Based on how some real-life physicists seem to completely ignore such concerns when ventilating the idea of generation ships, I would wager only about half of my crew (at best) consist of humanitarians. Others have more of a "the mission above all" mindset.

So I could use the fact that the ship starts out fairly densely populated as just yet another point to illustrate the core conflict of survival at all costs vs. well-being. If the ship were less densely populated (=if there were fewer people on board), average happiness might be higher, but the mission would be in greater jeopardy.

Also, starting out with a high population density - and having someone in the story explicitly bring that up - could serve as a defense against any sceptical reader, who might look at the size of the ship and initially think "there would be enough room for a lot more people on that ship!" Because the ship looks more massive than it is - just because it's 3 km long, that doesn't mean you actually have that much living space on it.

That way, any reader who would want to squeeze even more people into that ship would not only gain some sudden perspective when, somebody mentions it's already as tightly packed as Taiwan or even Bangladesh; but also, they might feel like a monster once they realize they wanted to shove even more people into that "tiny" can.

Thus, the reader can become an accomplice of the villains - in a similar vein as "Apocalypse Now" uses Ride of the Valkyries to get them hyped up for the attack - and then would be forced to reconsider just as much.

The main critical reader question I would then have to answer is: "Why did they not 'simply' put more decks on each of the rings? Why just five, instead of ten?"

Of course, in principle I could always handwave that away by using the overall mass of the ship as an excuse. But the obvious comeback at that would be, as outlined in the starting post: "Why not have fewer rings, but more decks, potentially for less mass and a much higher surface area overall?"

This in turn I could only explain with the need for the dismantling function - the rings still must be compact enough to split into sections, with dedicated engines, and enter the atmosphere.

I guess this leads us to the question: "What are the maximum dimensions something can have if it's supposed to survive entering the atmosphere unharmed?"

I have already established that the outside of the rings (which becomes the bottom once the rings break down and enter the atmosphere) are made of the same material as the ventral side of a space shuttle.

1. A space shuttle has a height of 17.86 m. My rings, at least those with 5 decks with standard ceiling height, only have a height of 13 m.
2. But of course, every section is 400 m long, whereas a space shuttle is only 37 m long.
3. Finally, a space shuttle has a wingspan of 23.79 m; my rings are at least 64 m in outer width (potential absolute minimum 32 m, which is the inner width). And the wingspan would still be added on top of that.

 

[sophiecentaur]

Others have more of a "the mission above all" mindset.

That would not be the way humans work. The major issue that's "above all" is for powerful individuals to gain power. If that conflicts with the mission then the mission loses.
Just take the World situation as it is now as proof of that.

But of course, every section is 400 m long, whereas a space shuttle is only 37 m long.

How could you contemplate such a tiny space for a stellar mission? The ship would need to be many (tens of) kms in extent to preserve the group sanity over generations.

 

[Strato Incendus]

That would not be the way humans work. The major issue that's "above all" is for powerful individuals to gain power. If that conflicts with the mission then the mission loses.
Just take the World situation as it is now as proof of that.

If you're talking about the over-representation of dark-triad personalities in positions of power, yes, of course these people still gravitate towards the top on board the generation ship, too. However, since personality tests are part of the vocational diagnostics (in order to find an optimally fitting job for each crew member, given that you can only recruit somebody for every required position from among the people you have), the dark triad is of course measured, too.

In fact, the reason the current commander was never promoted into the rank of captain was precisely because of her high psychopathy score. She still wound up as commanding officer anyway, because the previous captain died unexpectedly (she may or may not have had something to do with that... ).

It is then precisely because of her psychopathic nature that she can place the mission above all, without much regards for individual well-being. She cares about humanity (and its survival) as a collective; her opponents care about human beings (as individuals).

Most people without these dark-triad traits - those who aren't particularly competitive (low in agreeableness, high in conscientiousness) - don't actually want these positions of power, since they understand they come with a huge amount of responsibility. The problem about dark-triad types is that they're fine with having the power without minding the responsibility, i.e., striving for power merely to abuse it for their own selfish ends.

The commander, in contrast, despite her psychopathic traits, actually does feel responsible for the entire crew, as one would expect it of the captain. However, her narcissistic traits (dark-triad traits tend to correlate with each other) also make her assume she knew better what's best for her crew members than the crew members themselves do. This gives her the moral permission, in her own eyes, to override individual autonomy whenever it serves the mission. And at the end of the day, of course her personal history also plays into this - meaning, she actually also still has a selfish reason to commit so much to the mission.

How could you contemplate such a tiny space for a stellar mission? The ship would need to be many (tens of) kms in extent to preserve the group sanity over generations.

The ship has several kms worth of length, if you add together the ship trunk with the fore and aft spheres (3 km) with the circumference of all six rings (about 1.5 km x 6), plus the added consideration that every ring has 3 to 5 decks, so that's an additional length of 1.5 km for each floor.

You could take an extensive "stroll through nature" by going to the farm ring, walking one lap around deck 1, then going down to deck 2, where different plants and crops are grown, etc. And after that, there's still a whole second farm ring left.

That said, a lot of people simply resort to their personal virtual-reality chambers (one thing I always wondered about with Star Trek was how the holodeck wasn't constantly booked out, because everyone would want to have such a piece of tech). Those are inspired by closet-sized soundproof vocal booths. On the Exodus, every two-person quarter has such a VR chamber, 2 x 2 m in size.
They also have an optional middle separator screen panel, so that both people can use the chamber in parallel, running either the same or different programs. (Though usually, if two people are running the same program, they will remove the middle separator, so that they can be side by side for real, rather than merely seeing the other person's digital avatar on screen.) The VR chambers have trackballs in the ground that enable the user to walk or run on the spot, shifting their view on the screens surrounding them as they go.

 

[sophiecentaur]

in order to find an optimally fitting job for each crew member,

This is clearly a fictional scenario so you can do what you want in a story. The technology can be as good as you want but History is a good source of information about the way human societies work and there's no way to be sure that any group of people will behave any differently over generations. One of your "dark-triad personalities" will emerge, sooner or later.
There is similar to the fallacious belief in Terraforming some target planet and turn it into a replica of Earth. Evidence is that we are successfully messing up our present Earth so how could we trust a new society, somewhere, to do better. Human nature just can't be trusted so I, personally, wouldn't be keen on investing in any space colonisation.