No, Machine Elf is correct; at 14.7 psig with a cylinder of 8 km diameter, the hoop stress would be at 36 ksi if it were 64.3 in thick; at a factor of safety of three, that works out to be more than sixteen feet. (Hoop stress increases in proportion to radius which is obvious when you think about how the pressure load is resisted; the tube is reist8ng the normal force of internal pressure through the almost perpendicular circumferential strength, so a very large diameter will have high stresses even with modest pressures.) This doesn’t even account for centrifugal or other stresses on the structure. Of course, you wouldn’t build this out of basic construction steel; you’d want to use a high strength-to-weight material to take the internal loads and line it with an impermeable material inside, similar to how a composite overwrapped pressure vessel is designed, with the overwrap taking the majority of the load.
Extruding massive rings of steel as suggested upthread is not feasible for a number of reasons not the least of which are the vast energy requirements and the internal residual stresses that would result from differential cooling of such a thick steel ‘shell’. It isn’t any more practical to build a cylinder that way than it would be to hog a car chassis out of a block of aluminum.
This is why stories in which people just hollow out asteroids and spin them up to 1-gee don’t work - rock is a lot less strong than steel (especially in tension!)
I did the ASME pressure vessel calculations (which has the factor of 3 built in the allowable stress) and it works out to be around 15 ft. So Machine Elf and Stranger are correct. I used A-285 and not A 36 because A 36 is structural steel not meant to hold pressure. I also used a design temperature of 100 F.
Here is the funny part : None of these steels can withstand the cold temperatures of space without getting brittle. You will have to use other materials like Nickel Steel or Stainless steel or Aluminum. And all these metals too will need Heat tracing for the near zero kelvin temperature of space.
You can do a rough ASME pressure vessel calc here :
I know this because we have designed many Air Separation and Liquefied Natural Gas Plants and once you hit the -100 deg C temperature, pretty much all carbon steels are useless. Aluminum though can go far down but even that has a limit.
Space is near zero Kelvin only farish from a star or in shadow for a long time. The temperature of space near Earth’s orbital radius is around 283 Kelvin.
Add to that that asteroids have been playing a game of high velocity bumpercars for 4.5 billion years. So even if the asteroid is nominally solid, it’s probably shot through with fractures which will fail when stressed by being converted to a hollow pressurized cylinder.
“But this relatively mild average masks unbelievably extreme temperature swings. ”
“This solar radiation heats the space near Earth to 393.15 kelvins (120 degrees Celsius or 248 degrees Fahrenheit) or higher, while shaded objects plummet to temperatures lower than 173.5 kelvins (minus 100 degrees Celsius or minus 148 degrees Fahrenheit).”
That’s why you need to look at my second link, as I already mentioned. And a critical point is that an object that size is likely to be in its own solar orbit where it will never be shaded, so that part is irrelevant. And again, nothing is going to get anything close to zero Kelvin unless it is billions of miles from the sun or on the far side of a large object that is tidally-locked to the sun. The time in the shadow of an orbit around a planet just isn’t long enough.
And that’s just for a “dead” object in space. Anything filled with humans and their industry is going to have a problem with having too much heat, not too litle.
Ignoring the much smaller tidal forces this would not work, Objects in orbit are following the geodesic through space time and are not being subject to acceleration. For all intensive purposes an orbiting object is traveling in the equivalent of a straight line. This would not be the same for a singular object around the star, while it would work for a ring of particles.
When not ignoring the smaller tidal forces and the impacts of frame dragging you would have to deal with a tendency of it to break apart due to factors like rotational splitting or just the speed of causality.
Consider the amount of time it would take for a rotational force on one portion of the ring to reach the other side, which cannot be FTL as an example.
Space is elastic, and particles in it will exchange energy with it.
My own idea on this is altering the orbit of a mostly nickel-iron asteroid so it passes very close to the sun. It would take a while, but I’d think you could get a small metallic asteroid completely melted into a liquid drop with silicates as dross on top. Then bump its orbit back out and using nukes or vapor injection methods to create bubbles of open space inside as it cools. Just need to find an asteroid of whatever size is appropriate for your planned ship/habitat.
I’d assume if you were in space it would be better to make the factory up there, use solar power and drag asteroids in for processing.
Even if it was not the most efficient way to build this cylinder once you had the factory there you could use it to build other things making it worthwhile.
Two side questions: is 1 full G an absolute requirement? We should find out what the minimum maintenance requirement for humans is. We might fare as well at half a G, which could reduce the hoop stress load.
And what happens to a spinning gas in a cylinder? Will it naturally rarify along the axis and concentrate at the wall? In other words, we assume a gas volume of around 1.6 trillion cubic meters, but how thin will it be in the center (and would it make sense to have a 2km radius displacement cylinder in the center to reduce the gas requirement)?
No one knows. It absolutely is interesting to know, and to know whether we would need some level of modification to survive indefinitely, or if our bodies would adapt. One obvious downside, is that even if we do just fine in 1/3 gravity, coming back to full gravity would be problematic.
OTOH, plants probably don’t care. I can see how you would have farming cylinders at much lower spins. If wheat grows in .1 g just fine, then why have more? Easier o the structure, as well as on the farm machinery.
Yes, it will rarify. How much though depends on many specific factors of how fast the spin and the pressure at the walls. Something 8km in diameter would probably not have enough to make it unbreathable at the axis(assuming STP at the walls), though it would be noticeably thinner.
I don’t know that a displacement cylinder would be useful for that purpose. It would probably have more structural mass in it than the air it would siplace. OTOH, there are some uses for such a thing. Put a giant LCD screen on it, and now you have a sky. Use the interior for farming at lower g. If we want to keep marine environments, it also makes more sense to put them near the axis, where the water causes less forces on the structure.
The gas will rarefy in the center, and be denser at the walls. The rate at which the atmosphere gets thinner will be related to the “scale height” Scale Height - Definition of Scale Height - NAAP - but there are a couple of subtleties I’d have to think about a bit (particularly the fact that the magnitude of gravity would decrease closer to the center at a fast enough rate that this would have to be considered (for Earth-like planets, the atmosphere’s thickness is small enough that gravity can be considered constant).
We don’t know at what fraction of Earth surface gravity is sufficient for long term health because no astronauts have been exposed to a fraction of normal gravity for more than a few days but space phsyiologists suspect that 0.5 g is likely thr lower bound to minimize deleritous effects. Reduced centrifugal forces will reduce the centrifugal stress accordingly biut will do nothing to reduce hoop stress due to internal pressure. Even if we assume an internal pressure less than half of seal level with a 40% oxygen content, that would still require a pressure shell to be more than three feet thick with no safety factor, which is far thicker than any method can produce steel pipe either by rolling and seam welding or drown-over-mandrel (DOM) manufacture, and again, you cannot extrude thick wall pipe like Play-Doh.
A couple astronauts have spent more than a year in space. Spending a few months at a time in orbit is fairly common these days.
Astronauts spend a few hours a day doing exercises meant to ward off the deleterious effects of long times in zero gravity. It helps but I think some loss of bone mass and some other issues still occur.