Hard sci-fi question: rotational artificial gravity space station

Morrus said:

Sure it does. You have to provide air and power and stuff to your second station. You have to replicate the entire infrastructure, rather than just building a new floor. There's no way that building an entire new station is easier than adding a floor to an existing one.

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Modifying an existing, working structure can be very costly. But we're drifting off point here.

The discussion was never about adding to an existing design - that was just part of your example. It was about how to build the initial structure more efficiently. And for that, there's no particular advantage in building two concentric cylinders compared to using the same quantity of material to build a single larger cylinder.

I'd think that whether an existing structure could be extended would depend a lot on how the initial structure was build. If the initial structure wasn't built to be extended, adding to it might be very hard. I'm considering, for example, adding a whole new layer of roads to an existing bridge. I'm thinking that for most bridges, that would be very hard, because the bridge design was done as a whole, with the design fit very carefully to having just one roadbed. Similarly a space station might be hard to extend. But it would really depend on the initial engineering.

This seems to be a bit of a quibble. There seems to be no reason a-priori that you couldn't have multiple layers, or not have multiple layers. I would prefer a structure with several layers, if not a dozen or more, if the material physics allows it, simply because the structure seems more robust, and you get a lot more living space that way.

In any case, any modifications would need to preserve the mass distribution so the entire structure didn't wobble or have undue stress in any area. But that's no different in principle than adjusting weights in a plane or on a boat to adjust the center of gravity of the vehicle, and can mostly go unstated.

I don't know if three or four element structures would work out better or worse than two. I just know that one by itself has problems. I can imagine that the physics that applies to two cylinders could be applied to an ensemble of three or more, except I don't know how well balancing the rotations would work with an odd number of cylinders.

To keep this straightforward: Both a cylinder and a torus can be made to work, with the caveat that there is a minimum size which means the cylinder might have to be very big, and that a single spinning structure is hard to turn. (I wonder how that is fixed for a single torus, or if these would be designed to have two parallel parts spinning in opposite directions.) And there are a lot of options for the particulars of the structure, including how many layers and whether there is a corridor running down the middle, or if there are round or flat end caps, or if a torus is built with multiple parallel rings, some smaller and with lesser gravity and others larger and with more gravity.

A problem which we have not discussed is protection from radiation. That either limits where the structure is put (inside the magnetic belt of the earth) or requires quite a bit of shielding on the outside to provide protection (I've read that a structure could be built from material sent from the moon via linear accelerator, with unused/waste material put on the outside as a protective layer.)

What I've read as a way to bootstrap all of this is to build a base on the moon which would be a combination mine/solar power array/linear accelerator, to send materials to one of the Lagrange points, then build big O'Neal cylinders there as habitats and factories, with the eventual goal, in part, to manufacture power satellites which would transmit sunlight converted to microwaves to the surface.

Anyways,
TomB

Umbran said:

What may be false is that you can add that second layer at dramatically reduced cost (and weight), as outlined above - unless/until you have super-lightweight materials of arbitrarily high strength, making two separate modules may be more efficient than making one module be heftier.

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I see, so the cost question can be simplified to: (1) Does it cost less to launch the materials for an entirely new station into space? (2) Or does it cost less to strengthen the "suspension bridge cables" holding the station together under the centrifugal force?

My assumption was that #2 would be cheaper because in scenario #1 not only are you launching suspension bridge cables but you're also launching life support tanks, exterior structure, water reclamation systems, etc, etc.

But maybe I oversimplified what strengthening the "suspension bridge cables" would involve? Even though I'm the architectural field, this sort of engineering is something I have absolutely no familiarity with.

Anyone have anecdotes or insight into which would be cheaper, #1 or #2 and why?

Morrus said:

Sure it does. You have to provide air and power and stuff to your second station. You have to replicate the entire infrastructure, rather than just building a new floor. There's no way that building an entire new station is easier than adding a floor to an existing one.

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Yes, that was my thinking too.

I knew that strengthening "suspension bridge cables" wouldn't be quite as simple as I was imagining it, but I figured it couldn't be so much more involved that it would be more costly than making a new station from scratch.

MarkB said:

Actually, Babylon 5 had both. Its working and accommodation areas were mostly layered and enclosed, but a large portion of its interior was entirely open and landscaped, with a train running along the central spine.

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That's kind of how I was envisioning it would work. You wouldn't want to completely enclose a residential section for psychological / livability reasons. But having a separate low-grav section for industry, sports, and the tram makes sense.

MarkB said:

Modifying an existing, working structure can be very costly. But we're drifting off point here.

The discussion was never about adding to an existing design - that was just part of your example. It was about how to build the initial structure more efficiently. And for that, there's no particular advantage in building two concentric cylinders compared to using the same quantity of material to build a single larger cylinder.

Click to expand...

Sorry! That's my fault, since it's a precept of the scenario I'm working on that an old space station built around mining has grown overcrowded due to being relatively isolated and classism issues.

tomBitonti said:

I don't know if three or four element structures would work out better or worse than two. I just know that one by itself has problems. I can imagine that the physics that applies to two cylinders could be applied to an ensemble of three or more, except I don't know how well balancing the rotations would work with an odd number of cylinders.

To keep this straightforward: Both a cylinder and a torus can be made to work, with the caveat that there is a minimum size which means the cylinder might have to be very big, and that a single spinning structure is hard to turn. (I wonder how that is fixed for a single torus, or if these would be designed to have two parallel parts spinning in opposite directions.) And there are a lot of options for the particulars of the structure, including how many layers and whether there is a corridor running down the middle, or if there are round or flat end caps, or if a torus is built with multiple parallel rings, some smaller and with lesser gravity and others larger and with more gravity.

Click to expand...

So, if you have two rotating cylinders, how does that work? They spin in opposite directions to counteract one another's forces?

Wouldn't you still need a power source to keep them spinning just like you would with one cylinder?

And in the case of one cylinder within another, how the heck would one move between cylinders spinning in opposite directions?

A problem which we have not discussed is protection from radiation. That either limits where the structure is put (inside the magnetic belt of the earth) or requires quite a bit of shielding on the outside to provide protection (I've read that a structure could be built from material sent from the moon via linear accelerator, with unused/waste material put on the outside as a protective layer.)

What I've read as a way to bootstrap all of this is to build a base on the moon which would be a combination mine/solar power array/linear accelerator, to send materials to one of the Lagrange points, then build big O'Neal cylinders there as habitats and factories, with the eventual goal, in part, to manufacture power satellites which would transmit sunlight converted to microwaves to the surface.

Click to expand...

You're describing layering, which is passive protection. As I understand it, there are broadly two other categories of protection: active (using human-created magnetic or electrostatic fields) & planning (taking advantage of areas in space that require less shielding than others).

After reading over the 2015 Mars Mission radiation challenge it seemed like none of the theoretical designs were getting the radiation shielding results NASA was looking for, so maybe some kind of "silver buckshot" approach combining passive, active, and planning protection will be used in the future?

Quickleaf said:

So, if you have two rotating cylinders, how does that work? They spin in opposite directions to counteract one another's forces?

Wouldn't you still need a power source to keep them spinning just like you would with one cylinder?

And in the case of one cylinder within another, how the heck would one move between cylinders spinning in opposite directions?

You're describing layering, which is passive protection. As I understand it, there are broadly two other categories of protection: active (using human-created magnetic or electrostatic fields) & planning (taking advantage of areas in space that require less shielding than others).

After reading over the 2015 Mars Mission radiation challenge it seemed like none of the theoretical designs were getting the radiation shielding results NASA was looking for, so maybe some kind of "silver buckshot" approach combining passive, active, and planning protection will be used in the future?

Click to expand...

For multiple tori, I was imagining them side by side. I agree, putting them one inside the other is problematic.

If left alone, there would be no forces to cancel out. There is a force when you try to re-orient (turn) the spinning cylinder. The forces from two spinning cylinders can be made to cancel. (The force is perpendicular to how you are trying to turn the cylinder. Two of the ends will push in to each other; the other two wifi pull apart.)

Each cylinder will keep rotating forever, unless acted on by external forces. The two could act on each other, but presumable that would be prevented by keeping the cylinders apart. The connection struts would need a low friction coupling. I suppose there would be small losses, say, due to induced current from spinning in the earth's magnetic field. (Induced currents might be a huge problem, but I don't know hardly enough about that to say more.)

I don't think I'm qualified to say much about active shielding either, except that it only works for charged particles (I think). I don't think it works for high speed neutrons or high energy photons. In any case, I have no idea whether passive or active shielding is better for big space stations.

Thx!
TomB

Being able to turn the cylinders is important because they need to be turned so to keep them pointed at the sun. That is presuming big fixed mirrors are used, as seems to be typical for the big cylinder design.

Thx!
TomB

double post

tomBitonti said:

Being able to turn the cylinders is important because they need to be turned so to keep them pointed at the sun. That is presuming big fixed mirrors are used, as seems to be typical for the big cylinder design.

Thx!
TomB

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It seems like each option would have its challenges. If you have a single spinning cylinder, then once you get it up to speed it will essentially keep spinning indefinitely under its own momentum with only occasional corrective input required - whereas if you have two linked, counter-rotating cylinders there will inevitably be friction at the point of connection, which will require motors to maintain the spin and periodic maintenance for wear-and-tear.

I wonder if there would be an advantage to using a single rotating cylinder (or torus) with a compact, rotating counterweight to cancel out the angular momentum. It feels like that would be easier to maintain, but maybe I'm just not visualising it correctly.

I believe a problem with the idea of extending one station at a later point is that the assumption that you could just "reuse" the existing stations energy supply and air supply and what not might rely too much on the idea that the original station would have any "spare" supplies for that purpose. The primary problem is getting stuff into space in the first place. You will try to avoid bringing more than you need, since it's really costly.


What I wonder however about materials. Do we need a cylinder or torus? COuld we take a torus and take only, say 2 12th of it, put at opposing ends, so we have something like this: (-----) and have that rotate?
You would need to go through micro-gravity to get to the other half of the station, but you would be able to achieve a bigger radius while needing less material. But I have no idea if such a construction would really rotate as well as a cylinder or torus.

A sphere would be strange in many ways, for instance not only would things be "pulled downwards" but they would also be pulled towards the "equator". I am not wording that correctly but basically, the further something is from the "equator" the less aligned the "gravity" would be with the "floor". There are also some air circulation things that would happen.

Rendezvous with Rama is not a terrible read, if your into that sort of thing.