Ringworlds and Other Megastructures

[Johnny Pneumatic]

I read an interesting idea on how to build a Niven-scale ringworld recently. Imagine a ringworld of the same physical dimensions as in Larry Niven's books. The ringworld rotates at the speed needed to create one gee on the surface(or less if desired). What keeps the ring from flying apart from the outward force is there is another massive ring on the non-star facing side of the inner ringworld. This outer ring doesn't orbit the star below at all; so the outer ring is in compression from the star's gravity acting on it. This inward force balances out against the inner ringworld's outward force. Pretty neat hmmmm? No implausibly strong scrith needed; only maglev technology to keep the inner and outer ring from touching. The outer ring could be an inverted ringworld. It's light would come from artificial suns that orbit the star below. In that case the "suns" really would be moving across the sky!:)

I love Hard SF, can you tell?

[Bronze Dog]

I prefer medium-hard and gossamer-soft Sci-Fi. Between those two, meh. Depends on other aspects.

BTW, happen to know any useful resources for designing realistic, habitable planets?

[Johnny Pneumatic]

I've seen stuff but can't remember how I found it. Best to ask over on the Bad Astronomy Forums (www.badastronomy.com)

[c4ts]

Niven's science fiction is practically the same as fantasy, but if you like space operas, it's really good.

[Johnny Pneumatic]

Originally posted by c4ts
Niven's science fiction is practically the same as fantasy, but if you like space operas, it's really good.

Yes I know and yes to the latter as well. The reason I like Hard SF so much though compared to softer sci-fi is that Hard SF won't always be fiction. Computers aren't sentient yet, but they will be someday. Someday they'll be bigger than planets. "Telepathy" using neural implants and radio(ect.) links isn't possible yet; but it will be someday. "Telekinesis" control of robots as you would your own arm, possible using said neural implants.

[Bronze Dog]

Originally posted by SkepticJ
"Telepathy" using neural implants and radio(ect.) links isn't possible yet; but it will be someday.
I just hope the Laughing Man won't end up hacking my brain or anything like that.

[Stimpson J. Cat]

This outer ring doesn't orbit the star below at all; so the outer ring is in compression from the star's gravity acting on it. This inward force balances out against the inner ringworld's outward force. Pretty neat hmmmm?

Slight problem.

What is the gravitational acceleration due to the Sun at the Earth's orbit? It is clearly much less than 1g (my calculations put it at about 0.0006g). That means that to cancel out the centrifugal force trying to pull the inner ring apart, the mass of the outer ring needs to be roughly 1600 times as massive as the inner ring.

That's pretty big.

And then there's the question of its own gravitational pull, which I am suspecting would be quite significant.


Dr. Stupid

[MRC_Hans]

Well, the outer ring's gravitational pull would act together with the centrifugal force, thus lessening the need for rotational speed of the inner ring.

Still, I don't think the idea makes a ringworld easier to implement. We can imagine monomolecular structures with very high shear strenghts that just might be strong enough for a rotating ring, but a structure of a ductile strength that could support such a huge weight is rather difficult to imagine.

..... Perhaps the ring (a single ring again) should simply be made massive enough to supply a sufficient gravitational pull and be made to rotate just fast enough that it needed not support its own weight.

However, as I understand it, a more fundamental flaw with the ringworld concept is that the structure is inherently unstable; you cannot place a solid ring in a stable orbit around a central body.

Hans

[Matabiri]

Originally posted by MRC_Hans
..... Perhaps the ring (a single ring again) should simply be made massive enough to supply a sufficient gravitational pull and be made to rotate just fast enough that it needed not support its own weight.

Since I don't want to go through maths that someone else has already done... what does the gravity well for a massive ring look like? I don't think it cancels like a Dyson sphere, but wouldn't there be a similar effect reducing the effective gravity inside the ring?

[wollery]

Originally posted by Matabiri
Since I don't want to go through maths that someone else has already done... what does the gravity well for a massive ring look like? I don't think it cancels like a Dyson sphere, but wouldn't there be a similar effect reducing the effective gravity inside the ring? Within the plane of the ring it would cancel in the same way as a Dyson sphere. Off plane the gravitational centre would be the geometric centre, i.e. the axis point.

[Abdul Alhazred]

I don't think a Niven scale Ringworld will ever be built

But a more modest ring-moon in geostationary orbit might. An enclosed torus accessible via space elevators, of course. Zero G inside.

This avoids the centrifugal force problems. The only hang up material-wise is the space elevators themselves. The torus would be formed of material from the moon.

There'd be room for all the world's population many times over. The interior "floor space" would be much greater than the Earth's surface.

The Earth itself could be maintained for agriculture and nature preserves.

Might also have one around Mars of material from the asteroids.

[zakur]

Originally posted by BronzeDog
BTW, happen to know any useful resources for designing realistic, habitable planets? This one (http://www.amazon.com/exec/obidos/tg/detail/-/158297134X/102-2565539-4560151) is decent.

[Stimpson J. Cat]

Wollery,

Since I don't want to go through maths that someone else has already done... what does the gravity well for a massive ring look like? I don't think it cancels like a Dyson sphere, but wouldn't there be a similar effect reducing the effective gravity inside the ring?
Within the plane of the ring it would cancel in the same way as a Dyson sphere. Off plane the gravitational centre would be the geometric centre, i.e. the axis point.

It doesn't cancel in the plane of the ring. There will be a net pull towards the part of the ring you are closest to, which drops off as you move towards the center of the ring. That is why the ring is unstable. As soon as the sun is not exactly in the center, the net force will be pulling the part of the ring closest to the sun towards the sun (and likewise pulling the sun towards the closest part of the ring).

This is why no stable orbits are possible either. One could imagine looking for a stable orbit where the center of mass of the ring and the center of mass of the sun orbit each other, but as long as the sun is inside the ring, the net gravitational force between them is actually repulsive, rather than attractive. No stable orbits exist in a repulsive force field.


Dr. Stupid

[Bronze Dog]

Originally posted by zakur
This one (http://www.amazon.com/exec/obidos/tg/detail/-/158297134X/102-2565539-4560151) is decent.
Thanks. Looks pretty good at first glance.

[Johnny Pneumatic]

Originally posted by MRC_Hans Well, the outer ring's gravitational pull would act together with the centrifugal force, thus lessening the need for rotational speed of the inner ring.

Still, I don't think the idea makes a ringworld easier to implement. We can imagine monomolecular structures with very high shear strenghts that just might be strong enough for a rotating ring, but a structure of a ductile strength that could support such a huge weight is rather difficult to imagine.

..... Perhaps the ring (a single ring again) should simply be made massive enough to supply a sufficient gravitational pull and be made to rotate just fast enough that it needed not support its own weight.

However, as I understand it, a more fundamental flaw with the ringworld concept is that the structure is inherently unstable; you cannot place a solid ring in a stable orbit around a central body.

Hans [/B]

That's true.

Why not? No plausible material, even collapsed matter or monopolium isn't even close to strong enough to not fly apart from the outward spin at those speeds. What do you mean in the bold? No material in this double ringworld concept needs to be any stronger than stuff we have now, or will have long before a ringworld is made(diamond-like compounds ect.)

Could be, but that's a lot more mass to be kept centered around a star than two ribbons of matter.

That's why the ring would have solar sails at the edges that can turn from black to mirror to reflect photons to keep it centered. The ring couldn't take decent size impacts, but if you can build a ringworld you can vaporise asteroids and comets with lasers. Or just move them.

What's the show stopper?

[lylfyl]

Originally posted by Stimpson J. Cat

It doesn't cancel in the plane of the ring. There will be a net pull towards the part of the ring you are closest to, which drops off as you move towards the center of the ring. That is why the ring is unstable.

So why does it cancel out inside a Dyson sphere? or does it?
I wouldn't know where to begin with the math. Love SF, but never dared to check their numbers.

[MRC_Hans]

Originally posted by SkepticJ


Why not? No plausible material, even collapsed matter or monopolium isn't even close to strong enough to not fly apart from the outward spin at those speeds.

Monomolecular fibres, that is, molecules of essentially indefinite length, are theorized to be strong enough. When you tear apart something you are just separating molecules, but in a monomolecular structure, you would need to tear apart the very molecules, which requires forces of an entirely different magnitude.

What do you mean in the bold? No material in this double ringworld concept needs to be any stronger than stuff we have now, or will have long before a ringworld is made(diamond-like compounds ect.)

Perhaps ductile is not the right world. However, consider the weight of such a structure. If it is not rotating, the pull from the central star will be distributed longitudally, thus every part of it needs to be able to support the weight of the whole structure.

Could be, but that's a lot more mass to be kept centered around a star than two ribbons of matter.

That's why the ring would have solar sails at the edges that can turn from black to mirror to reflect photons to keep it centered. The ring couldn't take decent size impacts, but if you can build a ringworld you can vaporise asteroids and comets with lasers. Or just move them.

What's the show stopper?

I very much doubt that solar sails could provide the necessary force to keep it in place. Remember, it is not just a matter of maintaining a precarious equilibrium, the ring will be inhabited, with various masses being shifted around on it. Just imagine a train running along the ring; the solar sails would have to compensate for perhaps a hundred tons of shifting mass. Would have to be mighty big solar sails. This is no problem on Earth because we have a huge mass in a stable orbit, but in an unstable orbit, like a pin balanced on its point, even the slightest impetus will have to be countered to keep the system in place.

Hans

[Stimpson J. Cat]

lylfyl,

It doesn't cancel in the plane of the ring. There will be a net pull towards the part of the ring you are closest to, which drops off as you move towards the center of the ring. That is why the ring is unstable.
So why does it cancel out inside a Dyson sphere? or does it?
Because it is a spherical shell. A sperical shell is the only arrangement for which the gravitational pull cancels out at all points inside.

The math is pretty hairy. Basically you have to integrate the force exerted over the surface of the sphere.

Once you show that it cancels out for a sphere, though, it is pretty easy to show that it doesn't cancel out for other shapes, such as a ring.

Imagine dividing up the sphere into equal size peices. When you are off-center, the peices you are closer to are each pulling on you more strongly than the ones you are more distant from. But there are more pieces pulling you back towards the center than there are pulling you away from center.

For a sphere, these effects exactly cancel out, so the net force is zero. For a ring, the number of peices pulling you off-center is less than for the sphere, but the number pulling you back to center is much less than for the sphere. So the two effects no longer cancel out.


Dr. Stupid

[Johnny Pneumatic]

Could you give some hard numbers Han? I'm pretty familiar with carbon nanotubes, been reading on them for years, and their tensile strength is somewhere in between 60 gigapascals and 200 gigapascals. This is many orders of magnitude below what would be needed to make a Niven-scale ringworld that spins for "gravity".