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Old 02-04-2006, 07:56 AM   #41
Tom Kalbfus
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Default Re: GURPS Ringworld 1,000,000 AD

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I've improved my idea somewhat, its back to three rings, only this time the Atlas Ring is the same thickness of the Habitat Ring, but it has a radius of 100 AU and it is 100 time as long with 100 times the mass of the habitat ring, and it is made of carbon nanotubes. The outward force of the spinning habitat ring is tranfered to the Atlas ring, 100 AU out, by a stream of circulating pellets. A similar idea was proposed for holding up a space elevator on Earth, called a space fountain, by Robert Forward. The pellets are constantly thrown from habitat ring to Atlas ring and back to habitat ring over again. Using Newtons laws of motion the habitat ring is supported by throwing pellets, via magnetic fields, away from the Sun and toward the Atlas Ring. The Atlas Ring is held firm against its tensil strength by throwing the pellets via magnetic fields back at the habitat ring to be used again by that ring as reaction mass to hold it in place against its spin.

The picture of the entire would looks like that of a classic ringworld, the Atlas ring is not seen as its permanently in shadow, and neigther are the pellet streams. I believe this ringworld solution takes care of the gravity problem. The Atlas ring might have half the mass of the Sun, but it is spread out over a circumference of 628.3 AU, the Atlas ring no more has a tendency to collapse in out itself than the Ringworld does. The converging paths of the pellet stream concentrates the inward force of the entire 628.3 AU of the Atlas ring's tensile strength onto the smaller 6.28 AU circumference of the ringworld itself. I can adjust this to whatever is required to hold in the mass of the spinning habitat ring.

The great thing about this is that no superstrength materials are required, I'm using all know materials in this design. The outer ring can always be made larger and more massive as required and increasing the radius of the outer ring, the gravity is kept managable by this method, as the force of gravity decreases with the square of the distance as well as it increased proportional to its mass.

The inhabitants of the ringworld can then ignore whats going on beneath their feet, its all taken care of by the ringworld's automatic systems without resort to superscience or rubber physic. The engineering feat is still tremendous, but no new laws of physics need to be found and no new supermaterials of force fields need to be invented. It is at its essence the same core ideas as Transhuman Space, only it is applied differently.
If no one objects, maybe we can go onto discussing what maybe found on the ringworld rather that how it holds itself together, unless someone can come up with a flaw with this idea.
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Old 02-05-2006, 04:28 PM   #42
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Default Re: GURPS Ringworld 1,000,000 AD

I give up. You're not actually doing the math to test your "ideas" - you're just assuming that engineering ideas that work when dealing with small structures apply to large structures. For example your space fountain reference for an orbital elevator is a structure that is about 40,000 km long, and more or less one-dimensional, weighing only thousands of tons at most.

Your numbers for the ring itself are: a million miles across, 1 to 10 miles of dirt and rock on top. Even ignoring all of the other problems, you still haven't explained how you prevent all this dirt and rock from gravitationally collapsing itself.

If we treat the ring as 584 independant squares, each square with sides a million miles long, and with an average thickness of 2 miles, each square has the mass of over three and a half Earths.

Ignoring the centrifical force problem that you're trying to solve with a Dyson sphere, lets assume that these square chunks of dirt are in solar orbit - there's no orbital mechanics to deal with.

How are you stopping the the big flat square from collapsing into a small sphere under self-gravitation? Your ring-world turns into a few hundred planets in unstable orbits almost immediatly even if you don't try to spin it at 43 times orbital velocity.

Note that solving this problem - which cannot be solved with physical materials - is a pre-requisite for solving the centrifical force problem. And the centrifical force problem is orders of magnitude more difficult to solve.

Niven invoked Scrith prior to the existence of carbon nanotubes, but he was aware of diamond. Chemical bonds are not strong enough to create a ringworld. It doesn't matter how much fuzting with balance spheres that you do, there are insoluble problems with building non-spherical objects bigger than planets.

To give you a rough idea of the gap between scrith and nanotubes: Carbon-Carbon bonds energies are about 30 mega-Joules per kg, what you need is something more like what used to be called the "Stong Nuclear Force" and is now more commonly called the "nucleon-nucleon interaction" or "residual strong force". The bond energies for nucleon-nucleon interactions in H2 (molecular hydrogen) are around 90 Tera-Joules per kg, the carbon nucleus is close to a Peta-Joule per kg.

You're six to nine orders of magnitude short. IE: You need a material somewhere between a million and a BILLION times stronger than building the entire ring out of a single perfect crystal of diamond or nanotube. Please note that actual real materials are much less strong than imaginary perfect crystals the size of planets, but lets pretend that 1,000,000 AD humans can build the ring out of a single perfect crystal. They still need something a million times stronger.

If you want to do some stupidly impressive mega-engineering, take a leaf out of Peter F. Hamilton's Night's Dawn trilogy, where the Kiint built a ring of planets around their star. While you need some pretty impressive mass-shunting abilities, and you need to be able to make tweaks to planetary orbits without damaging the biosphere, at least the laws of physics don't say it's impossible.

One last time: you cannot build a ring world out of baryonic matter. It is many thousands of times too weak, perhaps millions of times too weak. It is more difficult than building the Golden Gate Bridge out of Jello.
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Last edited by Fnord-Fnairlane; 02-05-2006 at 04:35 PM.
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Old 02-05-2006, 04:39 PM   #43
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Default Re: GURPS Ringworld 1,000,000 AD

Actually - I'm exaggerating.

Steel is only about 5,000 times stronger than Jello.

Trying to build a ringworld using chemical bonds is more like trying to build a bridge out of water.
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Old 02-05-2006, 08:40 PM   #44
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Default Re: GURPS Ringworld 1,000,000 AD

Quote:
Originally Posted by Fnord-Fnairlane
Where's LWCamp when you need him....
Seattle. On business. And not near computers in my free time.

This is an interesting problem, because it replaces the traditional issue of tensile forces (as you get with a monolithic spinning ring) with compressive forces. This means that nanotubes will absolutely suck - nanotubes have great tensile strength but buckle under even small compressive loads. Diamond will be better. Heck, concrete will be better.

This essentially comes down to a force ballance issue. The inner ring is rotating fast enough that the centripital acceleration is 1 g = 9.8 m/s^2. The inner ring requires a centripital pressure of F = m/A x 9.8 m/s^2 to keep from flying apart, where m/A is the mass per unit area. Instead of using tensile strength to supply the centripital force, we are now considering using an outside weight to push down on the ring. If this outside weight has an areal mass density of M/A, and assuming it is thin enough that we can ignore the 1/r^2 variation of gravitational acceleration, we get that the compressive force is F = M/A x 9.8 m/s^2. To have mechanical equilibrium, the magnitudes of the forces must be equal, so that M/A = m/A. In other words, the mass of the outer ring must equal the mass of the inner ring. If everything is perfectly balanced, all forces are compressive. If we are content with a thin walled inner ring (say, 1 meter thick, or maybe 3 tons/m^2) we can have a fairly thin outer ring as well. This is well within the compressive strengths of known materials. More realistically, we want it thicker to support buildings, bridges, subway tunnels, small lakes, canals, sewers, roadways, and the like. The terrian would be boring - vertical relief such as mountains would require compressive strength beyond the ability of chemical bonds to handle, causing plastic flow and buckling.

Now the problems:

First, all methods of "levitating" the inner ring are lossy. Friction of some sort will always couple the motion of the inner and outer rings, slowing down the inner ring while speeding up the outer. This is even true if you have magnetic levitation, there are ALWAYS energy losses in any dynamical physical system. If you have even miniscule spinning of the outer ring, it will require enormous tensile forces to keep it intact, tensile forces that exceed the tensile strength of matter held together by chemical bonds, and it will fly apart. In order to counter this, you need a motor to continually spin up the ring with a source of energy. Perhaps you could use solar energy, but any active system that lasts for 1,000,000 years without suffering malfunctions strains my suspension of disbelief well beyond the breaking point.

Second, the system may be in force equilibrium, but it is in unstable force equilibrium. As others have noted, the system will tend to self-gravitate. Tiny variations from uniform density will cause the structure to break up into smaller chunks that coalesce into spheres under their own gravity. Whether a person walking around on the surface is enough to cause this runaway collapse I don't feel like calculating right now, but it would suck if in order to live on the ringworld you had to maintain an absolutely rigid posture for your entire life.

Finally, you have the unstable equilibrium of the ringworld with respect to the sun, which also requires active control mechanisms that last 1,000,000 years without malfunction to keep the ringworld's source of light, heat, and energy in its preferred location.

Luke
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Old 02-05-2006, 09:24 PM   #45
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Quote:
Originally Posted by Tom Kalbfus
Now the outward bottom of the middle ring and the inward top of the outer ring are magnetized and they both point their magnetic North Polarity toward each other, and as you know from high school physics, like magnetic poles repel each other.
However, high school physics does not cover Earnshaw's theorem, which says there is always a way for something supported by a static combination of magnets and/or electric charges to flip around, move around, or otherwise become unstable such that it is no longer is supported by the magnets/charges. Thus, putting north magnetic poles facing north magnetic poles will not stably levitate the ringworld sections.

There are ways around Earnshaw's theorem. Superconducting levitation, alternating fields, or even a clever arrangement of rotating magnetic poles along a track over which something magnetically slides - but you cannot do it by naiively pointing the north poles (or south poles) at each other.

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Old 02-05-2006, 09:35 PM   #46
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Quote:
Originally Posted by lwcamp
This essentially comes down to a force ballance issue. The inner ring is rotating fast enough that the centripital acceleration is 1 g = 9.8 m/s^2. The inner ring requires a centripital pressure of F = m/A x 9.8 m/s^2 to keep from flying apart, where m/A is the mass per unit area. Instead of using tensile strength to supply the centripital force, we are now considering using an outside weight to push down on the ring. If this outside weight has an areal mass density of M/A, and assuming it is thin enough that we can ignore the 1/r^2 variation of gravitational acceleration, we get that the compressive force is F = M/A x 9.8 m/s^2. To have mechanical equilibrium, the magnitudes of the forces must be equal, so that M/A = m/A. In other words, the mass of the outer ring must equal the mass of the inner ring.
Quibble: my back of the envelope calculation was that the outer ring had to be about 43 times the mass of the inner ring, because it's only under Solar gravity, not a full Earth G.

My numbers may be off because I derived this from the fact the inner ring is doing 43 times orbital velocity. Thinking about it, my estimate for solar gravity seems way too high - surely solar gravitation forces at Earth orbit can't amount to 2% of a G?

It could be my radial velocity to generate 9.8 m/s^2 is off: could you check the maths?

In any case, don't we require the same weight not mass? And the outer ring isn't under a Gee.
Quote:
Originally Posted by lwcamp
If everything is perfectly balanced, all forces are compressive. If we are content with a thin walled inner ring (say, 1 meter thick, or maybe 3 tons/m^2) we can have a fairly thin outer ring as well. This is well within the compressive strengths of known materials.
OP wants miles of terrain thickness....
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Old 02-05-2006, 09:53 PM   #47
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And the outer ring isn't under a Gee.
Good catch. A quick calculation of the acceleration due to gravity at 1 AU from the sun gives g = 6.1E-4 m/s^2. This means the outer ring must be 1600 times as massive as the inner ring.

Still, for thin inner rings, the compressional strength can be handled by known materials (since the maximum mressure is still m/A x 9.8 m/s^2). Again, if you want mountains and naturalistic terrain, you are out of luck, and you still need active control and maintenance, and the ring will still tend to buckle under its own gravity, breaking up and forming many self gravitating spheres.

Luke

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OP wants miles of terrain thickness....
Well, sucks to be him.
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Old 02-05-2006, 10:35 PM   #48
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Default Re: GURPS Ringworld 1,000,000 AD

What about his "plan B": what's the mass ratio where the balance ring is past Saturn? For arguments sake, we could ignore the amount of mass required for the pellets themselves.

The numbers required are such that I'm starting to wonder if even Scrith can be strong enough. It only has a few hundred tera-joules of bond energy per kilogram, assuming that it is a single chunk of neutronium, bound together via the residual strong force.

As I understand it, the ring (in tension) can be consider as a suspension bridge with a length = circumference of the ring, under 1G, with the initial drop angle being 90 degrees?

Niven's scrith was even stonger than this requirement: the terrain was "bump mapped" into the scrith, and then coated with a thin layer of dirt / rock. Of course, that's probably still less than the strength requirement implied by sold rather than hollow mountains.
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Old 02-05-2006, 10:51 PM   #49
Tom Kalbfus
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Default Re: GURPS Ringworld 1,000,000 AD

Quote:
Originally Posted by Fnord-Fnairlane
I give up. You're not actually doing the math to test your "ideas" - you're just assuming that engineering ideas that work when dealing with small structures apply to large structures. For example your space fountain reference for an orbital elevator is a structure that is about 40,000 km long, and more or less one-dimensional, weighing only thousands of tons at most.

Your numbers for the ring itself are: a million miles across, 1 to 10 miles of dirt and rock on top. Even ignoring all of the other problems, you still haven't explained how you prevent all this dirt and rock from gravitationally collapsing itself.

If we treat the ring as 584 independant squares, each square with sides a million miles long, and with an average thickness of 2 miles, each square has the mass of over three and a half Earths.
Spread out over 1 million miles! Remember over any particular point on the ringworld's surface, the gravity isn't that great. The gravity of any particular section is no greater than that at the surface of an asteroid 10 miles thick. The centrifugal force as you may recall is equivalent to the gravity at Earth's surface, most of this force is balanced by interactions with pellet streams underneath the ringworld's floor., the residule centripedal force keeps the ringworld taut, and completely overwhelms the self gravity of each section and its tendency to collapse into a ball. Its not just total mass that determines the gravitational pull, but also its distribution.

The mass of three and a half Earths if squashed into a ball would produce a sphere that is one and a half times Earth's diameter and have a surface gravity of 1.5 times that on Earth's surface, but as you just stated, this mass is spread out onto a square that is 1,000,000 miles, by 1,000,000 miles by 5 miles on average, the gravity on that surface is not going to be 1.5 g.

Quote:
Originally Posted by Fnord-Fnairlane
Ignoring the centrifical force problem that you're trying to solve with a Dyson sphere, lets assume that these square chunks of dirt are in solar orbit - there's no orbital mechanics to deal with.

How are you stopping the the big flat square from collapsing into a small sphere under self-gravitation? Your ring-world turns into a few hundred planets in unstable orbits almost immediatly even if you don't try to spin it at 43 times orbital velocity.
The self-gravity is very weak if you spread it over one million mile wide flat squares.

Quote:
Originally Posted by Fnord-Fnairlane
Note that solving this problem - which cannot be solved with physical materials - is a pre-requisite for solving the centrifical force problem. And the centrifical force problem is orders of magnitude more difficult to solve.

Niven invoked Scrith prior to the existence of carbon nanotubes, but he was aware of diamond. Chemical bonds are not strong enough to create a ringworld. It doesn't matter how much fuzting with balance spheres that you do, there are insoluble problems with building non-spherical objects bigger than planets.
As they say, there is more than one way to skin a cat. Someone with your kind or reasoning living at the time of Leonardo Da Vinci. Might have stated that man will never fly until we can control the force of gravity. and that all flying machines require the ability to divert the force of gravity around the machine so that it is light enough to fly or to negate gravity by generating an antigravity force, any other form of flight is for the birds.

You haven't proven that an as yet undiscovered form of matter is the only way to build a ringworld. All that detail is literally below the floor of the ringworld. I don't like a universe that is full of nearly indistructable materials such as scrith and General Products Hulls. I do not believe you have examined all possible ways to build a ringworld with existing known materials. The technology for building such structures is clearly beyond our ken, but not the physics of it, that's all that matters to me. I like hard science fiction, not a universe full of Force fields, Warp drives, and General Products hulls.

Quote:
Originally Posted by Fnord-Fnairlane
To give you a rough idea of the gap between scrith and nanotubes: Carbon-Carbon bonds energies are about 30 mega-Joules per kg, what you need is something more like what used to be called the "Stong Nuclear Force" and is now more commonly called the "nucleon-nucleon interaction" or "residual strong force". The bond energies for nucleon-nucleon interactions in H2 (molecular hydrogen) are around 90 Tera-Joules per kg, the carbon nucleus is close to a Peta-Joule per kg.

You're six to nine orders of magnitude short. IE: You need a material somewhere between a million and a BILLION times stronger than building the entire ring out of a single perfect crystal of diamond or nanotube. Please note that actual real materials are much less strong than imaginary perfect crystals the size of planets, but lets pretend that 1,000,000 AD humans can build the ring out of a single perfect crystal. They still need something a million times stronger.

If you want to do some stupidly impressive mega-engineering, take a leaf out of Peter F. Hamilton's Night's Dawn trilogy, where the Kiint built a ring of planets around their star. While you need some pretty impressive mass-shunting abilities, and you need to be able to make tweaks to planetary orbits without damaging the biosphere, at least the laws of physics don't say it's impossible.

One last time: you cannot build a ring world out of baryonic matter. It is many thousands of times too weak, perhaps millions of times too weak. It is more difficult than building the Golden Gate Bridge out of Jello.
The inhabitants of the ringworld don't see the underside very often. The Galaxy has a mass much greater than this, yet it doesn't collapse into a ball or black hole, and it is made out of ordinary material. If you can have a Galaxy then you can also have a ringworld. I don't think in numbers, but instead use some common sense reasoning. Figuring out the exact material strength and mass distribution of the ringworld does little for the actual game, as the PCs won't concern themselves with building a ringworld, but more about exploring it. The will find that if they try to drill a hole through the ringworld floor, that nanotechnology will dissolve their drill bit and any holes they've made will be closed up by that same nanotechnology. Underneath the ringworld floor is something holding it up, something the PCs can't see as they stand on top of it. If they attempt to go under it, they will be pelted with a hail of pellets going back and forth between the outer ring and the middle ring. The outer ring is somewhere out there, at some great distance. The PCs can't see it as it lies under the shadow of the middle ring, but perhaps it can be discerned by starlight alone.
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Old 02-05-2006, 11:05 PM   #50
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Default Re: GURPS Ringworld 1,000,000 AD

Quote:
Originally Posted by lwcamp
Seattle. On business. And not near computers in my free time.

This is an interesting problem, because it replaces the traditional issue of tensile forces (as you get with a monolithic spinning ring) with compressive forces. This means that nanotubes will absolutely suck - nanotubes have great tensile strength but buckle under even small compressive loads. Diamond will be better. Heck, concrete will be better.

This essentially comes down to a force ballance issue. The inner ring is rotating fast enough that the centripital acceleration is 1 g = 9.8 m/s^2. The inner ring requires a centripital pressure of F = m/A x 9.8 m/s^2 to keep from flying apart, where m/A is the mass per unit area. Instead of using tensile strength to supply the centripital force, we are now considering using an outside weight to push down on the ring. If this outside weight has an areal mass density of M/A, and assuming it is thin enough that we can ignore the 1/r^2 variation of gravitational acceleration, we get that the compressive force is F = M/A x 9.8 m/s^2. To have mechanical equilibrium, the magnitudes of the forces must be equal, so that M/A = m/A. In other words, the mass of the outer ring must equal the mass of the inner ring. If everything is perfectly balanced, all forces are compressive. If we are content with a thin walled inner ring (say, 1 meter thick, or maybe 3 tons/m^2) we can have a fairly thin outer ring as well. This is well within the compressive strengths of known materials. More realistically, we want it thicker to support buildings, bridges, subway tunnels, small lakes, canals, sewers, roadways, and the like. The terrian would be boring - vertical relief such as mountains would require compressive strength beyond the ability of chemical bonds to handle, causing plastic flow and buckling.

Now the problems:

First, all methods of "levitating" the inner ring are lossy. Friction of some sort will always couple the motion of the inner and outer rings, slowing down the inner ring while speeding up the outer. This is even true if you have magnetic levitation, there are ALWAYS energy losses in any dynamical physical system. If you have even miniscule spinning of the outer ring, it will require enormous tensile forces to keep it intact, tensile forces that exceed the tensile strength of matter held together by chemical bonds, and it will fly apart. In order to counter this, you need a motor to continually spin up the ring with a source of energy. Perhaps you could use solar energy, but any active system that lasts for 1,000,000 years without suffering malfunctions strains my suspension of disbelief well beyond the breaking point.
The inner ring does not need to be levitated, it is in orbit around the sun so gravity cancels out centrifugal force. If a solid rong cannot have a stable orbit around the sun, then its not a solid ring, but one composed of thin overlapping panels, its entire function is to block sunlight to the middle ring or to let most of it through. It does this to the entire middle ring all at once by electronically becoming transparent of opaque, their are materials like this today, but If I'm wrong, nanotechnology can certainly do this. The whole ringworld experiences night all at the same time and it illuminated all at the same time when its not night.

Quote:
Originally Posted by lwcamp
Second, the system may be in force equilibrium, but it is in unstable force equilibrium. As others have noted, the system will tend to self-gravitate. Tiny variations from uniform density will cause the structure to break up into smaller chunks that coalesce into spheres under their own gravity. Whether a person walking around on the surface is enough to cause this runaway collapse I don't feel like calculating right now, but it would suck if in order to live on the ringworld you had to maintain an absolutely rigid posture for your entire life.

Finally, you have the unstable equilibrium of the ringworld with respect to the sun, which also requires active control mechanisms that last 1,000,000 years without malfunction to keep the ringworld's source of light, heat, and energy in its preferred location.

Luke
Nanotechnology can do all of the above, it can be self-repairing, self correcting and can even error check itself, and its smallest component pieces are atoms themselves, it is an active structure, not an innert object, it can go on doing this for millions of years.
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