# "Ringworlds" without super strong materials



## RX-79G (Feb 12, 2017)

In Niven's Ringworld or Banks' Culture books, super large rotating habitats have so much unsupported weight on them that they would have to be composed of exotic matter. The largest nanotube design is about 1000km in diameter, and one made from steel processed from asteroids would top out closer to 8km.

In all these designs, the tensile strength of the base material supports the load.

In my design, the ring(s) are supported entirely by the gravity of an object they are orbiting and magnetism.

In the case of the earth, the altitude of low earth orbit is good for .95G if you weren't orbiting. If you built two rings around the earth at that altitude in zero G, with one ring on the inside and the other on the outside, they wouldn't have to be strong at all. If you start to spin the inner ring faster, it is going to attempt to expand, tearing itself apart and attempting to push on the outer ring. And if you slow the rotation of the outer ring down, it will become "heavy" and push on the inner ring, trying to collapse toward the Earth.

While we can't make exotic matter, we can make solar powered magnetic levitation systems. This system could be distributed evenly between the inner and outer rings, acting as a bearing between them. Now when you speed up the inner ring, it will push magnetically on the outer ring. But if you simultaneously slow the outer ring by the same amount, it will push in with an equal force to the inner ring pushing out.

If the two rings are of equal mass, they will peak out at .95G, because that is what you get when the outer ring is motionless. All of the load of the outer ring, which is heavy pointed down at the earth is supported, through the magnetic repulsion, by the inner right which is heavy pointed out away from the earth by centrifugal force.

However, that is just the most obvious balance point. If your base gravitational object does not make the desired amount of gravity, you can produce more centrifugal force on the inner ring by balancing it with a heavier outer ring. In other words, if the inner ring's rotation gives it a perceived 1G of centrifugal force but the planet only has .5G to pull on the outer ring, then doubling the mass of the outer ring will counter the inner rings outward pressure. In the end, it is just a system of balanced weights pushing against each other, and different rotation speeds above and below weightless orbit speeds are like a scale with two beams - you can have a short (slow) beam and a long (fast) beam but balance the scale by using more weight on the short (slow) side. Or if the desired object has too much gravity at the desired orbit, you keep both rings spinning but have their differential above and below zero G consistent for a 1G effect.

Since it is just a balance of inner and outer weight, the materials don't have to be any stronger than the surface of the Earth, so they could be simply fused asteroid slag. What makes this all possible would be the amount of solar power that could be collected and turned into magnetic levitation. Which isn't to say that any of this is simple, but it does mean that all of it uses physics and engineering that we understand - just on a much larger scale.


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## J Riff (Feb 13, 2017)

And if you write it up as fiction, maybe someone will build it. Sounds good, I imagine the magnetic Ley lines matter - as in, if this is an alien planet, not the Earth... this effect could be much more powerful, more conducive to 'antigrav' devices?


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## RX-79G (Feb 13, 2017)

There is no such thing as "ley lines" and none of this is about "anti-gravity". This is just magnets and rotating mass.


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## Tannin (Feb 13, 2017)

How about a (relatively) low-tech version of your idea. Instead of magnetic bonds, use ordinary solid materials. Think of a number of pillars. You'd need some pretty impressive low-friction bearings, of course, but you'd be loading the pillars in compression rather than tension, which should be easier.


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## RX-79G (Feb 13, 2017)

Tannin said:


> How about a (relatively) low-tech version of your idea. Instead of magnetic bonds, use ordinary solid materials. Think of a number of pillars. You'd need some pretty impressive low-friction bearings, of course, but you'd be loading the pillars in compression rather than tension, which should be easier.


I don't follow. There are no magnetic bonds, it's a maglev. 

The velocity difference between the inner and outer ring is something like 50,000 km/h. A mechanical bearing is not possible.


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## tinkerdan (Feb 13, 2017)

Okay:
I can see the possibility for some complications in having two separate structures in some sort of synchronized orbit/rotation linked by mag-lev that may or may not help support the orbits of each object. But I'll assume all of that is worked out.

My question might be, what is the point or since I don't want to do the maths involved, would you be able so get enough centripetal velocity to simulate gravity with centrifugal force? Because other than that I'm not sure why a race of beings would go to such great lengths when they could just make individual stations in orbit.


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## chrispenycate (Feb 13, 2017)

The only reason there is so much tension in the system is to get one g on the surface. If we were willing to accept lunar gravity we'd be within present materials science (diamond, at any rate), though the mountain walls holding the atmosphere in would have to be a great deal higher - which would actually add to the tensile strength. 

Reductio ad absurdum, if you could put a transparent roof on it and accept microgravity on the surface, orbiting in one year (possibly not an Earth year, but the one you'd get with a planet at that distance from the star) any given lump would be in a stable orbit and there would be zero tension to accommodate. Which is probably how you'd build it, adding orbiting sections and then spinning it up when complete.


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## RX-79G (Feb 13, 2017)

tinkerdan said:


> Okay:
> I can see the possibility for some complications in having two separate structures in some sort of synchronized orbit/rotation linked by mag-lev that may or may not help support the orbits of each object. But I'll assume all of that is worked out.
> 
> My question might be, what is the point or since I don't want to do the maths involved, would you be able so get enough centripetal velocity to simulate gravity with centrifugal force? Because other than that I'm not sure why a race of beings would go to such great lengths when they could just make individual stations in orbit.


The point would be to provide a huge place to live with a precisely controlled environment and whatever gravity you want, and be able to do it without inventing new technologies. I used the example of spinning it around the earth, but that's pretty unnecessary. It would make more sense to build it as a Ringworld around a sun or around a planet that doesn't provide the right environment - like a gas giant or a small planet.

It is only "better" than a planet because you are creating it whole hog, so you don't have to fight a planet's weather with terraforming and you get to pick the gravity level. After that you are in the realm of Niven or Banks' rationale for such a structure, but without incredible materials your choices are either very small habitats or something like this - if you want full G.

From a writer's standpoint, this would be a megaobject that could be engineered without the super technology usually required in fiction.



chrispenycate said:


> The only reason there is so much tension in the system is to get one g on the surface. If we were willing to accept lunar gravity we'd be within present materials science (diamond, at any rate), though the mountain walls holding the atmosphere in would have to be a great deal higher - which would actually add to the tensile strength.
> 
> Reductio ad absurdum, if you could put a transparent roof on it and accept microgravity on the surface, orbiting in one year (possibly not an Earth year, but the one you'd get with a planet at that distance from the star) any given lump would be in a stable orbit and there would be zero tension to accommodate. Which is probably how you'd build it, adding orbiting sections and then spinning it up when complete.


If you use lunar G for a big ring with no object in the middle and had nanotube materials, you could build a ring at least as large as the earth in diameter. I was just presenting a way to build a very large structure with very crude technology. High mountains are only going to "help" if you have super strong materials. If you are using natural materials mountains are just more mass for the magnetic system to support.

And yes, you would build the rings in a stable orbit, then spin them up and down as needed.


Realistically, if you are going to have enormous banks of solar collectors, a third thin ring spinning at standard orbital speed between the inner and outer would provide an attachment point for the collectors and allow them to be hardwired to the inner and out maglevs directly.


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## Ursa major (Feb 13, 2017)

Just some musings....

 It seems to me, and at first glance, that there may be other forces to be considered, such as the effect of the mass of the star, and any moons, on the rings. These may play their part in deciding just how the two rings orbit the planet (in terms of how stable its position might be), as would considerations of the effect the rings have on the planet in terms, say, of solar radiation (i.e. the shadows it throw might throw onto the planet's surface.

But my biggest query would be why a (planetary?) society would construct such a thing. You mention extra space and the need for gravity akin to the planet's, but is gravity so important if the vast majority of the planet's population** stays on the ground, leaving the rings for making things (low gravity night help here***) and growing food (and/or fuel)? Humans don't really take up that much space on our planet (although it can seem otherwise in crowded parts of the world), but our way of producing food (and the types of food we prefer to eat) _does_ need a lot of land.


** - Those that want to live in different gravities might be better served on the type of structures seen in the Culture novels, rings that don't spin around only their own centre of gravity using their rotation to simulate gravity.

*** - And it would facilitate access to raw materials not found on the planet, Only what is made in orbit would be sent down to the planet surface.


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## RX-79G (Feb 13, 2017)

Ursa major said:


> Just some musings....
> 
> It seems to me, and at first glance, that there may be other forces to be considered, such as the effect of the mass of the star, and any moons, on the rings. These may play their part in deciding just how the two rings orbit the planet (in terms of how stable its position might be), as would considerations of the effect the rings have on the planet in terms, say, of solar radiation (i.e. the shadows it throw might throw onto the planet's surface.
> 
> ...


I used earth orbit as an example for the purpose of illustrating the forces involved. I would assume such a ring would be useful for places that don't provide suitable living for people. That includes planets too small for healthy gravity, and stars and planets too large. You could put a ring around Neptune that would make use of a planet that isn't useful for earth life because of the lack of surface and very high gravity.

This isn't the structure in Culture novels because those structures require entirely fiction materials or force fields to exist. This idea is made of boring materials and old technology.


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## Ursa major (Feb 13, 2017)

Thanks for the quick reply. 

I assumed that Earth wasn't your intended site. (Presumably, the solar energy density necessary to make the rings work puts some sort of (fuzzy) limit on how far from a star they can be.)

Oh, and I realise that the Culture novels' habitats are not currently (perhaps never) feasible. But then they are very large (though nowhere near as large as Ringworld), larger than is strictly needed for creating somewhere habitable with a standard gravity (whatever that standard might be).


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## RX-79G (Feb 13, 2017)

Ursa major said:


> Thanks for the quick reply.
> 
> I assumed that Earth wasn't your intended site. (Presumably, the solar energy density necessary to make the rings work puts some sort of (fuzzy) limit on how far from a star they can be.)
> 
> Oh, and I realise that the Culture novels' habitats are not currently (perhaps never) feasible. But then they are very large (though nowhere near as large as Ringworld), larger than is strictly needed for creating somewhere habitable with a standard gravity (whatever that standard might be).


I don't know what the furthest one could make use of solar energy - especially if you put collectors close to the sun and transmitted the energy in a focused or coherent form to the outer planets where it is being used.

I suppose the other reasons for building large structures would be immense populations in the trillions.


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## J Riff (Feb 15, 2017)

How are there not ley lines, if there are magnets? Or their equivelant? And, not to forget the effect of absolute zero temp, on any magnetic field. It is hard to write this kind of stuff up and keep laypeople interested in the story.


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## tinkerdan (Feb 15, 2017)

Ley lines are at best a theory that has never been established beyond fiction::


J Riff said:


> How are there not ley lines, if there are magnets? Or their equivelant? And, not to forget the effect of absolute zero temp, on any magnetic field. It is hard to write this kind of stuff up and keep laypeople interested in the story.


::they don't even rate up there with theoretical physics thought experiments.

But be wary of the Bermuda triangle because that is where the Schrödinger ley lines are stuffed under Pandora's box.


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## Mirannan (Feb 17, 2017)

J Riff said:


> How are there not ley lines, if there are magnets? Or their equivelant? And, not to forget the effect of absolute zero temp, on any magnetic field. It is hard to write this kind of stuff up and keep laypeople interested in the story.



I might be wrong, but I don't think there are temperatures low enough anywhere in the universe for some of the really weird effects (Bose condensates and the like) found at temperatures created in laboratories. Maybe superconductivity, but it's close.

Even if you build whatever it is in intergalactic space (a bit difficult!) the lowest you're going to get is the temperature of the cosmic microwave background. Which is slightly over 2.7K.


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## RX-79G (Feb 17, 2017)

Ley lines are not science and have nothing to do with magnets.

Absolute zero is not something associated with magnetism. Very low temperatures are associated with superconducting magnetic systems.


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## J Riff (Feb 18, 2017)

Errrrr..... okay. What do you call the magnetic lines of force then? And what are Ley lines, as defined by scientists for a long time? Mass attracts, magnetism is magnetism, the Earth has a magnetic field. Absolute zero allows for some great magnet tricks, maybe even hover shoes.


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## RX-79G (Feb 18, 2017)

Ley lines aren't defined by scientists.
Ley line - Wikipedia

Superconductivity occurs at temps close to, but not at absolute zero:
Absolute zero - Wikipedia

The earth's magnetic field doesn't have anything to do with the thread.

Magnetism - Wikipedia


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## J Riff (Feb 18, 2017)

Good, that's explained then. )


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