Counterbalancing a Ringworld so it doesn't explode

Question

Ringworlds are a bit of a SciFi staple but as they're commonly portrayed, the large ones have a major problem... they'd, well... explode if they were built out of materials that actually existed. This, as you can probably imagine, makes habitation rather difficult...

The issue is that as the rings get bigger, the tensile strength needed to hold them together grows with the diameter (assuming the "artificial gravity" remains the same), at some point the ring would need to be spinning so fast for a decent surface gravity that it'd rip itself apart. As Gav and Dan demonstrate with this poor, innocent CD. Presumably this would result in greatly reduced house prices on the ring...

In fact, for an Earth-like surface gravity, an Earth-like day-night cycle and an average density about that of carbon, the tensile strength needed to hold the ring together is 350 times greater than the strongest material known to man, Graphene...

This got me thinking... what if, instead of having just the spinning ring, there was a slightly larger stationary ring weighing down on the rotating one? Given that a ring of this size would end up using at least one whole planet to build, there's got to be an awful lot of gravity trying to crush the whole thing down into a nice neat sphere (gravity is good like that).

Assume that there is a construction crew big enough for the job, that there's enough material to build both these rings (the outer ring can literally be made out of any old rubbish) and there's some way to build a friction-less bearing (super-maglev?) between the two that can handle the immense pressure between the inner and outer rings. We have plenty of energy (i.e. someone's sun) and a couple planets to play with but no new physics or material science.

So, assuming all that, is this way of building Ringworlds actually possible using current (albeit massively scaled up) science?

Update: A couple people have pointed out that there needs to be a way of keeping the stationary ring from touching the rotating ring while still balancing the huge forces involved. Superconducting Maglevs is the way to go, using the numbers from the chart above, there would be ~25 kilotonnes per square meter between the stationary ring's inner surface and the rotating ring's outer surface, don't get me wrong, this is a lot of pressure, but it's no more than the fuel injectors in a modern diesel engine and a whole order of magnitude less than a high end waterjet cutter. Using the lift formula from this wiki page on maglevs you'd need magnets that could generate 25 Teslas to balance the forces, a lot, but seeing as the current (non-exploding) magnet world record is over 40 Teslas, difficult but still doable.

Also, the inhabitants on the inside (along with the atmosphere) should not actually feel any effects from the mass of the rings as for some physics reason I don't fully understand, the gravity inside a hollow sphere (or ring, which is just a slice of sphere) is always zero when the mass is evenly distributed, so no worries about the air being sucked into the center, the only force it and everything else on the inside surface should feel is centrifugal (yes, I know) force trying to squash them flat against the inner surface. Apparently that only applies to a completely closed shell

I have no clue how much thicker and heavier the outer stationary ring needs to be to generate the same kind of forces as the inner ring, I have a hunch that you'd need more than one planet as the ring is more spread out so the gravity trying to crush the outer ring might not be as strong as if it was still in planet form.

P.S. the whole idea of building a counterbalanced Ringworld is a) it's cool, and b) the particular one in the chart above would have both Earth-like "gravity" (centrifugal-gravity?) for the inner-surface inhabitants along with a 24hr day-night cycle by default

Answer 1

I think you are neglecting a fundamental aspect here.

If the outer, stationary ring has to exert a force on the inner, rotating ring to balance it, it has to be in touch with it, or in other words it will receive a force from it.

This means that you are shifting the problem of the excessive load from the inner ring to the interface between the outer and the inner ring.

And if the material you have is too weak to sustain that load, you go KABOOM anyway.

Answer 2

You don't have to build a classic 'Ring-world' to get the same amount of living space.

What you can do is build a series of 'Bishop Ring' style habitats with diameters around the 400 mile range. Assuming you can produce enough of it carbon fiber nano-tubes will withstand the rotational stress. Just one of these gives you an surface area approaching that of say Argentina - and that's assuming there is only one 'floor' level per ring. Then you build a second ring (contra-rotating ring) and attach it to the first. Rinse and repeat as needed. You can then surround the rotating rings as per you diagram with a 'rubbish' cylinder for impact and radiation protection as needed. Then build hundreds or thousands of cylinders as needed.

The only down side is you have to take an elevator 'up' to the zero g core of the station and then 'across and down' to transfer from ring to ring and shuttles to transfer to other cylinders.

Answer 3

If you could keep the friction from the outer ring down with maglev(incredible feat on it's own. It could work fine because it will still produce inward pressure that you need to balance the constant threat of explosion.

You may want to consider what kind of maintenance must be done on that outer ring to keep it from falling apart or land sliding around to the inner ring, causing terrible destruction to the spinning section.

Another thing to think about is if you are using a full planet for this the outside of the ring is also habitable. Without having to spin it will have surface gravity near that of the original planet you pulled apart. Give it a slight counter spin and they can have a daylight cycle out there too.

This begs the question, "Who gets to live on the inner ring and who has to live on the outside?"

One last thing, air..how do you keep it on the surface and not bunched near the center of the ring in a pretty but unreachable bubble. If you are an android, no problem. But if you need to breathe you may be stuck indoors.

Answer 4

In fact, for an Earth-like surface gravity, an Earth-like day-night cycle and an average density about that of carbon, the tensile strength needed to hold the ring together is 350 times greater than the strongest material known to man, Graphene...

So, a Banks orbital, then, rather than a proper Ringworld.

This got me thinking... what if, instead of having just the spinning ring, there was a slightly larger stationary ring weighing down on the rotating one? Given that a ring of this size would end up using at least one whole planet to build, there's got to be an awful lot of gravity trying to crush the whole thing down into a nice neat sphere (gravity is good like that).

"Weighing down" is not really the best way to look at this. You can do much better.

The size of a rotating space colony is limited by the tensile strength of your building materials under the assumption that the materials need to hold up their own weight. That's only true if they are spinning rigidly along with all the living space that they are holding up. As soon as you decouple a stationary support section, you can add as much cross-sectional area of material as you need to support the hoop pressure of the inner spinning section without worrying about its own weight. And the amount of steel you would need for that, let alone the amount of perfect carbon nanotube cable, is far, far less than the amount of Dumb Mass you would need to produce enough gravity to balance the hoop pressure--and it's much more stable. Big self-gravitating ring don't much like to remain as big self-gravitating rings; they tend to be subject to bead instability.

If you are actually building a proper Ringworld, though (i.e., a ring encircling a star), then it does make sense to consider the gravitational weight of the counterbalance in the star's gravity. This gets more efficient the smaller the ring is--but then, so does everything about building a Ringworld, or a Bishop Ring / Banks Orbital.