1. Topopolis tether

So a topopolis (think of a O'Neill Cylinder long enough to wrap around a star) would be unstable in the same way a Ringworld would be. Instead of giant stabilizing rockets to control its distance from the star, could a set of electromagnetic tethers do the job? Would they have enough power to move such a mass? (say, around 4e21 tonnes.)

2. Don't know if I should make this a separate thread, but... If the topopolis itself can extend and contract, could it stabilize itself without thrusters?

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Tethered how, to what?

4. Originally Posted by Noclevername
So a topopolis (think of a O'Neill Cylinder long enough to wrap around a star) would be unstable in the same way a Ringworld would be. Instead of giant stabilizing rockets to control its distance from the star, could a set of electromagnetic tethers do the job? Would they have enough power to move such a mass? (say, around 4e21 tonnes.)
4e21 tonnes is about 2/3 the mass of the earth, so you'd need a lot of power.

4e21 also happens to be the listed mass of Spaghetti, the first topopolis of Cableville (linked at your link), but it wraps 50 times around the primary, at 1.8 A.U.

ETA: quick question, if the topopolis rotate to produce artificial gravity, are they rotating about the tube axis then?
Last edited by grapes; 2018-Oct-08 at 01:24 PM. Reason: ETA

5. Originally Posted by grapes
.

ETA: quick question, if the topopolis rotate to produce artificial gravity, are they rotating about the tube axis then?
Yes, as I understand it. The entire line spins like twirling a string.

Originally Posted by thorkil2
Tethered how, to what?
Orbital tethers that react with the Sun's magnetic field. https://en.wikipedia.org/wiki/Space_tether

6. Originally Posted by grapes
4e21 tonnes is about 2/3 the mass of the earth, so you'd need a lot of power.
Power is available. "Spaghetti" = 42,150 times Earth's surface area facing the primary at any given time. Soak in the Sun.

7. I made the illustrations for that page, and I'm well aware of the genesis of the original concept (Bigger than Worlds, Larry Niven, 1974). But I've always been a little sceptical of the concept. The torus is supposed to be flexible enough to rotate in situ, even though the outer radius of the torus at any given instant is larger than the inner radius. The material of the torus is supposed to be 'stretchy' enough to accommodate the few extra kilometres each time the torus rotates. But even if this stretch is only a few thousandths of the length of the structure, this stretching would exert a braking force on the rotation - eventually the torus would stop rotating, and become a freefall habitat. So the torus must be continuously using energy in order to continue to rotate.

Note as well that the stretching would heat the torus slightly, and there would probably be continuous, minor tremors on the inner surface. As it happens the surface of the Earth flexes every day due to tides, but this is a tiny effect - I'm not sure how the two effects compare, but I think that the flexing on a Topopolis would be orders of magnitude larger.

8. Originally Posted by eburacum45
I made the illustrations for that page, and I'm well aware of the genesis of the original concept (Bigger than Worlds, Larry Niven, 1974). But I've always been a little sceptical of the concept. The torus is supposed to be flexible enough to rotate in situ, even though the outer radius of the torus at any given instant is larger than the inner radius. The material of the torus is supposed to be 'stretchy' enough to accommodate the few extra kilometres each time the torus rotates. But even if this stretch is only a few thousandths of the length of the structure, this stretching would exert a braking force on the rotation - eventually the torus would stop rotating, and become a freefall habitat. So the torus must be continuously using energy in order to continue to rotate.

Note as well that the stretching would heat the torus slightly, and there would probably be continuous, minor tremors on the inner surface. As it happens the surface of the Earth flexes every day due to tides, but this is a tiny effect - I'm not sure how the two effects compare, but I think that the flexing on a Topopolis would be orders of magnitude larger.
Could it be a mechanism? Outer non rotating skin for power collecting, inner (divided) compartments that rotate individually and independently. Like a line of O'Neill Cylinders laid end to end. It would of course require artificial lighting and a very effective and reliable self maintenance and repair system.

9. Also I wrote a short story set on this topopolis, if anyone's interested...

https://orionsarm.com/page/478

10. Using separate O'Neill cylinders seems to make a lot of sense. The necks between the cylinders could be joined together with a kind of powered universal joint; this joint could conceivably be used for stationkeeping across the entire hoop. An excellent idea.

Note that the instability of the Ringworld isn't really applicable to a topoplis- a Ringworld is unstable because it is assumed to be rigid, while an topopolis is both flexible and consuming energy constantly to maintain its configuration.
Last edited by eburacum45; 2018-Oct-15 at 11:23 PM.

11. Originally Posted by eburacum45
Also I wrote a short story set on this topopolis, if anyone's interested...

https://orionsarm.com/page/478
Neat!

Mine includes a "dark section" too, but due to damage, not design. Fixing it is part of the plot.

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Just wondering what would be the point of building a single giant structure this size instead of using the same material to build, say, 100,000 O'Neil Cylinders (or whatever the mass equivalent would be) and put each in it's own separate orbit around the same star?

13. A Topopolis rotates on the short axis for artificial gravity? Does it spin around its star, too? If yes, that sounds like a lot of dynamic motion to harvest energy from. If it was coordinated motion, could you shift the star's position like a gravitational tug? Slow as heck, I'm sure, but very cool.

14. Originally Posted by Dave241
Just wondering what would be the point of building a single giant structure this size instead of using the same material to build, say, 100,000 O'Neil Cylinders (or whatever the mass equivalent would be) and put each in it's own separate orbit around the same star?
Well, it dramatically cuts down on commute costs...

15. Originally Posted by Solfe
A Topopolis rotates on the short axis for artificial gravity? Does it spin around its star, too? If yes, that sounds like a lot of dynamic motion to harvest energy from.
Rotating along its axis wouldn't be free. You'd have to power it. I mean, you might be able to utilize incoming energy to induce the axial rotation, but its still an energy drain, not a source.

16. Originally Posted by Solfe
A Topopolis rotates on the short axis for artificial gravity? Does it spin around its star, too? If yes, that sounds like a lot of dynamic motion to harvest energy from. If it was coordinated motion, could you shift the star's position like a gravitational tug? Slow as heck, I'm sure, but very cool.
It orbits, I believe.

As for the point of it, I believe it's the Galactic Coolness Principle.

17. Originally Posted by Dave241
Just wondering what would be the point of building a single giant structure this size instead of using the same material to build, say, 100,000 O'Neil Cylinders (or whatever the mass equivalent would be) and put each in it's own separate orbit around the same star?
I just know that, like Ringworld, it's a great setting for fiction, it puts thousands of worlds within lifetime travelling distance for even the fallen primitives, and contains ancient wonders of hidden technology.

This setting also contains a swarm of O'Neill habs as well, but it's where the bad guys of the tale originate, so it's not nearly as awesome.

18. Originally Posted by DaveC426913
Rotating along its axis wouldn't be free. You'd have to power it. I mean, you might be able to utilize incoming energy to induce the axial rotation, but its still an energy drain, not a source.
But won't rotating around the star and it's magnetic field generate more energy than is required to spin the thing on it's short axis?

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Originally Posted by Noclevername
As for the point of it, I believe it's the Galactic Coolness Principle.
Oh, well THAT I can certainly understand!

20. Originally Posted by Noclevername
So a topopolis (think of a O'Neill Cylinder long enough to wrap around a star) would be unstable in the same way a Ringworld would be. Instead of giant stabilizing rockets to control its distance from the star, could a set of electromagnetic tethers do the job? Would they have enough power to move such a mass? (say, around 4e21 tonnes.)
My understanding is that conventional electrodynamic tethers would work effectively only within about 1000 km or so of Earth. You could probably do something similar for Jupiter. The sun is a much more complicated issue, and I don't believe it would work there. However, there are electric sail concepts that look a fair bit like tethers that would interact with solar wind. I don't think anything like that has been tested though. It would be dependent on how much solar wind there is (a star with a lot of solar wind might be a better candidate), but I think in all cases there would be a fair amount of variability over time. Solar sails might be a better option - more controllable, the amount of thrust is limited, but the source won't run out for a long time.

21. Originally Posted by Solfe
But won't rotating around the star and it's magnetic field generate more energy than is required to spin the thing on it's short axis?
That one might be a source of energy. But I strongly suspect that the harvesting of that energy will have the effect of braking the rotation.

My childhood bicycle harvested energy from the spinning wheels to drive a generator that powered a light. But it was not free. When generator was engaged, it was more difficult to peddle. And it absolutely killed the ability to coast. (i.e. it acted as a brake).

22. Originally Posted by Noclevername
Don't know if I should make this a separate thread, but... If the topopolis itself can extend and contract, could it stabilize itself without thrusters?
I think it could help.

It's an interesting idea, just some my fumbling thoughts about a topopolis: If you assume a hula hoop design in a circular orbit around a star, it could be somewhat stable gravitationally, and being able to stretch or contract part of it might help avoid the problem with a rigid structure like a ringworld.

If it's more complex than a hula hoop (not circular, multiple loops, etc.) I'd expect some self-gravity issues. Also, if there are remaining planets in the system or if it's a double star system, there would be perturbations on parts of the hoop that would add interesting complications.

As a second way to stabilize the hoop, perhaps consider controllable solar sails in a radiometer design: Have fins around the tube, with black solar panels on one side and a reflective surface on the other. That could help maintain the tube rotation, and they could be furled or angled to add or remove thrust for stabilization.

Then for cases where the sails aren't enough, ion thrusters might be used (powered by the solar panels), but that would mean consumables, so would have to limited.

Incidentally, one of the biggest issues I would suggest with an inhabited megastructure would be making up for atmosphere loss. If you're talking about something that's big enough to require much more oxygen and nitrogen than the biosphere of a good sized planet, it could become uninhabitable pretty quickly if there were any paths to atmosphere loss, and it might be difficult to find sufficient resupply. Keep in mind gas giants have little oxygen and nitrogen in their atmospheres (it's mostly hydrogen and helium) so even if there were continuing supply, it would be inefficient to get the gasses from them. For story purposes, you might assume transmutation, but fusing hydrogen or helium up to oxygen and nitrogen is radically more difficult than fusing deuterium and tritium. On the other hand, if in your story they can do that easily, they don't have to worry much about thrusters - they should be able to build excellent fusion rockets.

23. Per WP:

https://en.wikipedia.org/wiki/Electric_sail
The electric field of the wires extends a few dozen metres into the surrounding solar wind plasma. The penetration distance depends on the solar wind plasma density and it scales as the plasma Debye length. Because the solar wind electrons affect the electric field (similarly to the photons on a traditional solar sail), the effective electric radius of the tethers is based on the electric field that is generated around the tether rather than the actual tether itself. This fact also makes it possible to manoeuvre by regulating the tethers' electric charge.

A full-sized sail would have 50–100 straightened tethers with a length of about 20 km each.[citation needed] Compared to a reflective solar light sail, another propellantless deep space propulsion system, the electric solar wind sail could continue to accelerate at greater distances from the Sun, still developing thrust as it cruises toward the outer planets. By the time it reaches the ice giants, it may have accumulated as much as 20 km/s velocity, which is on par with the New Horizons probe, but without gravity assists.

24. As for solar sails, it would need to gather light for its inhabitants anyway, so it makes some sense.

25. Originally Posted by Van Rijn
Incidentally, one of the biggest issues I would suggest with an inhabited megastructure would be making up for atmosphere loss. If you're talking about something that's big enough to require much more oxygen and nitrogen than the biosphere of a good sized planet, it could become uninhabitable pretty quickly if there were any paths to atmosphere loss, and it might be difficult to find sufficient resupply. Keep in mind gas giants have little oxygen and nitrogen in their atmospheres (it's mostly hydrogen and helium) so even if there were continuing supply, it would be inefficient to get the gasses from them. For story purposes, you might assume transmutation, but fusing hydrogen or helium up to oxygen and nitrogen is radically more difficult than fusing deuterium and tritium. On the other hand, if in your story they can do that easily, they don't have to worry much about thrusters - they should be able to build excellent fusion rockets.
This worried me at first, until I remembered that there is more than enough oxygen and nitrogen in the Sun. Paul Birch had some fairly detailed plans to extract material from the Sun's atmosphere; by the time we can make topopoli, we can mine the Sun.

26. On the other hand, if you can do transmutation you don't need a sun. Fusing H-He into oxygen and nitrogen would be brilliantly exothermic; you could build a megastructure around your transmuter and live on the waste heat.

27. You can populate the outer system with Von Neumann robots, and mine comets for the necessary volatiles to deliver to the topo. Cheaper and easier.

28. Originally Posted by eburacum45
I made the illustrations for that page, and I'm well aware of the genesis of the original concept (Bigger than Worlds, Larry Niven, 1974). But I've always been a little sceptical of the concept. The torus is supposed to be flexible enough to rotate in situ, even though the outer radius of the torus at any given instant is larger than the inner radius. The material of the torus is supposed to be 'stretchy' enough to accommodate the few extra kilometres each time the torus rotates. But even if this stretch is only a few thousandths of the length of the structure, this stretching would exert a braking force on the rotation - eventually the torus would stop rotating, and become a freefall habitat. So the torus must be continuously using energy in order to continue to rotate.
For a ring around a star, the stresses would most likely be small in comparison to thermal stresses, however. Round numbers - for a 100-million-km radius loop around the star, with a 100km radius habitat, circumferential lengths are going to change by a millionth during one habitat rotation. That's an order of magnitude less than the thermal stress (for typical metals and concrete) associated with a 1K temperature change between the sunlit and dark sides.
Of course the thermal stresses make things worse - the inner curve is attempting to expand just as it needs to be at its shortest length. But the problem of frictional slowing would be there even if there weren't a geometrical consideration, for all but a thermal superconductor.

Grant Hutchison

29. A non-rotating outer skin can have permanent photoelectric or thermoelectric power, feeding it inwards to counter losses due to friction and thermal expansion.

30. Originally Posted by DaveC426913
That one might be a source of energy. But I strongly suspect that the harvesting of that energy will have the effect of braking the rotation.

My childhood bicycle harvested energy from the spinning wheels to drive a generator that powered a light. But it was not free. When generator was engaged, it was more difficult to peddle. And it absolutely killed the ability to coast. (i.e. it acted as a brake).
I can't really imagine both the rotation on the short axis and the long at the same time. Maybe like a giant, curved screw? So, you'd need some sort of thruster system all down the length to regulate the motion. That is a lot of energy, but you have a lot of space for reactors and such. Would the power have to be continuous?

Given the size of the structure, I don't think the issue would be energy per se, but fuel. Even ion thrusters which sip fuel is going to need a lot of mass to keep going and that mass will be impacting the structure all down the line, which is making drag as you say. I can't picture a system where there is only one bank of thrusters at one end or the other, it's have to be spread out. Would they have to switch off when pointed at parts of the structure? That is really complicated.

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