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Space colonies in High Frontier are (mostly) limited to a radius of 1 km.  But I've been in a discussion recently with somebody who wants to think bigger — he proposed a cylinder with an 8 km radius.

He wants a big, expansive interior space.  But he wants his design to be realistic, so I spent a little time extolling the virtues of a 1 km cylinder.  I thought y'all might find this interesting (and please chime in with your thoughts!).

A 1 km radius still makes for a pretty huge internal space, especially if (as I recommend) we make use of multiple decks.

For example, if with radius=1 km and length=1 km, the area of the main (1G) deck is 6.28 km^2, which is really quite a lot of room.  And then the Mars deck (0.38G, radius 380 m) would have another 2.4 km^2, and the Moon deck (0.16G, radius 160 m) would provide another 1 km^2.  Plus a few more decks in between for agriculture/industry, leaving all the decks mentioned for living/recreational areas.

The envelope (pressure vessel) thickness scales with the size.  I have some notes about that here:


For steel, it happens to work out pretty close to some nice round numbers: 1 m thick to contain 1 atmosphere at 1 km radius.  It scales linearly with both air pressure and radius.  So an 8 km radius would require a shell 8 meters thick of solid steel -- the sheer mass of steel required stretches credulity.  And might not work at all; there is a point at which the stress on the structure from its own mass is greater than its tensile strength can bear.

There are advanced materials which can probably do the job better, while being considerably more expensive (at least per unit mass).  But how much better, and where do they fail?  I don't really know.  I'm not a structural engineer.  But 1 km is pretty widely acknowledged as "doable" since people much smarter than us studied this in the 70s, so we won't raise any eyebrows with that.

[He wrote that he wanted clusters of 20-story high-rise towers overrlooking parks and lakes, with gravity on the 20th floor still very close to 1G...]

Gravity scales linearly with radius too.  So, if your floors are 3 m apart, then your 20th floor is 60 m up, where the gravity is 940/1000 = 0.94 G.  That'd probably put a spring in your step, but I'm guessing it would be barely noticeable.  (And the sky -- the underside of the Mars deck -- would still be over half a kilometer above the top of the building, so you'd definitely have an outdoorsy feel!)

So, everyone, what do you think?  Is a 1 km radius space colony "big enough" even in the far future?  Or do you think we should keep pushing for 5 km, even 10 km radius habitats?
Surface to dispose of waste heat scales with R^2. Internal passages for access imply a similar limit on how much hardware you stuff inside.

Pressure vessel loading is as you describe.

The gotcha may be centrifugal stress, scaling with radius and gravity. Beyond a certain scale you are forced to reduce artificial gravity or get clever in your engineering, or the weight of the structural elements opposing gravity exceeds their own strength.
Yep, that's what I'm thinking too.  It's like building a tower, or better yet, a bridge (a tension structure) — for a given material, there is a limit beyond which you simply can't build any bigger, because it can't support its own weight.

So, thinking far-future for a moment, reducing gravity is probably a good idea.  Just because we evolved in 1G doesn't mean we'll necessarily always need 1G... the Mars advocates certainly don't think so!  And reducing the gravity level really helps with the centrifugal stress.

It doesn't help with the pressure vessel, though.  For that, we'd probably have to switch from a cylinder to a torus.  Then what matters is the minor radius, not the major radius, so you could go a lot bigger overall.

And of course you could also reduce the air pressure.  People live quite happily in Denver, which is at about 1/2 atmosphere, so that cuts your pressure vessel thickness in half right there.  (Not to mention making it twice as easy for space workers to come and go!
In my view you depend too much on industrial (mechanical) processes rather than energy processes. I would send a nickle-iron asteroid into a near solar orbit until it warmed to near melting point then kick it into Earth orbit. A small area can then be super heated and spun off in a thread of liquid metal. Spin that thread into a cylinder that will be several times stronger than blowing air into it inflating the metal like a balloon.

This may have the strength to allow for a larger cylinder or torus. Other problems need other methods to solve.