Gravity on a ringworld

[Sarevok]

Would it be possible to contain the atmosphere on a ringworld such as Halo through centrifugal gravity alone ? Or is some kind typical magical artificial gravity generators required ?

[Terralthra]

Would it be possible to contain the atmosphere on a ringworld such as Halo through centrifugal gravity alone ? Or is some kind typical magical artificial gravity generators required ?

You'd need magical forcefields to hold a ringworld orbiting a sun together at all. Smaller ringworlds likethe eponymous Halos and Culture Orbitals could hold an atmosphere to the surface through centrifugal gravity combined with retaining walls, assuming a heat source is somewhere nearby.

[Simon_Jester]

More or less as Terralthra says, The answer is "yes, sort of," but you'd have to calculate the amount of spin required based on a number of factors. A "ringworld" that is very very large (say, a radius of 1 AU, equal to that of the Earth's orbit) would need to be spun up to... I think the number Niven came up with was about 1000 km/s- which is about twenty or thirty times faster than the orbital velocity of a planet like Earth. At that point you do have one hell of a problem with centrifugal force.

Think about it. You want 1g of centrifugal force pointing away from the star, on the surface of the ringworld. That means, effectively, you need something that can hold the ringworld up against 1g. How much do a hundred million kilometers of ringworld weigh? How much strength would you need to hold up that weight against that 'gravity?" It's hard to imagine.

[Alerik the Fortunate]

Scrith. The smaller ones would do it as well, though you'd need a tangential rotation velocity of something on the order of a kilometer per second if you want to keep your entire atmosphere in place only with centrifugal pseudogravity and retaining walls (root mean square velocity of air molecules at room temperature and pressure is about 460 m/s; you'll need more than that to account for the higher velocity particles if you want to limit atmosphere escaping to less than a trickle). Assuming 1 km/s tangential velocity and 9.81 m/s^2 centripetal acceleration, you need a minimum of 102 kilometer radius, which is doable with a carbon nanotube structure, I think. However, atmospheric pressure at 1 g on Earth has about 100 kilometers of atmosphere above our heads, so you'd really need something about twice that size at least to maintain regular pressure unless you wanted to cap the atmosphere with some sort of pressurized membrane.

[Terralthra]

Scrith. The smaller ones would do it as well, though you'd need a tangential rotation velocity of something on the order of a kilometer per second if you want to keep your entire atmosphere in place only with centrifugal pseudogravity and retaining walls (root mean square velocity of air molecules at room temperature and pressure is about 460 m/s; you'll need more than that to account for the higher velocity particles if you want to limit atmosphere escaping to less than a trickle). Assuming 1 km/s tangential velocity and 9.81 m/s^2 centripetal acceleration, you need a minimum of 102 kilometer radius, which is doable with a carbon nanotube structure, I think. However, atmospheric pressure at 1 g on Earth has about 100 kilometers of atmosphere above our heads, so you'd really need something about twice that size at least to maintain regular pressure unless you wanted to cap the atmosphere with some sort of pressurized membrane.

The given tensile strength of scrith is comparable to the nuclear strong force. Like I said: magic.

[Alerik the Fortunate]

Yes, it requires magic technology to hold the ringworld together, but not to hold the atmosphere in :::)

[Ultonius]

The nearest you're probably going to get to a Niven-style ringworld would be a 'necklace' of Bishop Rings around a sun, perhaps linked by a maglev track running through them. Since they would be edge-on to the sun, they would either need artificial solar-powered lighting, or mirrors mounted between the rings that would reflect sunlight onto their inner surfaces. However, that still doesn't solve the problem of the sheer scale of such a project, in terms of size and the amount of material needed.

[Solauren]

Actually, if you are at the technology level to build a ring world, and the needed infrastructure capabilities, material should be a problem.

By that point, you probably have some sort of controlled fusion technology. Just mine the needed matter from the solar wind, local gas giants, local asteroids, and local comets/Oort Cloud material.

By the time you'd done, you're solar system will be free of all that pesky space debris.

[Simon_Jester]

If you're designing ring habitats that big, would it kill you to just put in a roof? There aren't a lot of reasons not to- spaceships cannot safely land 'through' the atmosphere in any case because of the velocity-matching requirement.

[Dave]

Simon, can you elaborate on the "velocity-matching requirement"?

I'm envisioning an Apollo-style capsule reentry, starting from a point beyond the ring, crossing the center and hitting the atmosphere on the inner side of the "far side" (from where you started.) From there, it's just angling your heat shield to deal with both your initial velocity towards the edge of the ring + the rotation of the ring. (Is that where the difficulty comes in?) Wait until you drop below supersonic speeds and pop your chutes.

In short: Kind of like that reentry scene from the first Halo game.

[Imperial528]

If you're designing ring habitats that big, would it kill you to just put in a roof? There aren't a lot of reasons not to- spaceships cannot safely land 'through' the atmosphere in any case because of the velocity-matching requirement.

Why land at all (or dock with a roof) when you could just dock with the existing retaining walls or the outer structure?

[Guardsman Bass]

Simon, can you elaborate on the "velocity-matching requirement"?

I'm envisioning an Apollo-style capsule reentry, starting from a point beyond the ring, crossing the center and hitting the atmosphere on the inner side of the "far side" (from where you started.) From there, it's just angling your heat shield to deal with both your initial velocity towards the edge of the ring + the rotation of the ring. (Is that where the difficulty comes in?) Wait until you drop below supersonic speeds and pop your chutes.

In short: Kind of like that reentry scene from the first Halo game.

I think that kind of re-entry is only necessary if you have to use the atmosphere to slow down for landing, like real-life manned spacecraft. If you have any ability to expend fuel to drastically slow down before entering the ring's atmosphere, you could just come in at a steeper angle. We don't do this because manned spacecraft today have to carry all their fuel into space with them, but the hypothetical ring-building civilization probably won't be limited by that.

Not that entering through the atmosphere is a good idea compared to simply docking with the structure.

Why land at all (or dock with a roof) when you could just dock with the existing retaining walls or the outer structure?

If you don't have to come in "through the roof" from space, then why even have it open to space at all? It would be much easier just to enclose the whole thing and have giant "windows" where light can enter the ring.

[someone_else]

Actually, if you are at the technology level to build a ring world, and the needed infrastructure capabilities, material should be a problem.

Being able to generate materials stronger than strong nuclear force, AND move outrageously bloody ridicolous amounts of mass around DOES NOT mean you can make outrageously bloody ridicolous amounts of materials stronger than strong nuclear force.

I mean we can move fucktons of stuff with trucks, boats, planes, but that has no impact on how fast we can porduce seriously strong materials like say depleted uranium.

IF they can make ringorlds, the material they are using is stronger than strong nuclear force AND cheaper-than-fucking-dirt.

Which makes their other objects pretty fucking durable as well.

I have the strong impression that if a civilization can build Ringworlds, then their youngsters build their own full-size planet for science classes.

Simon, can you elaborate on the "velocity-matching requirement"?

I'll do the leg work for him.
Spincalc will do number-crunching because I'm in the mood to set up a spreadsheet to do it.

Now, the Ringworld from the Larry Niven's book is the size of Earth orbit, so it's radius is 1 AU or 150 000 000 000 meters. They also say its gravity is around 99% of Earth's.
Spincalc tells me that its tangential velocity is 1206767 m/s or around 1.2 million km/s.
As a matter of comparison Saturn V had around 20 km/s of delta-vee (it could have reached that speed if going straight in a hypotetical vacuum).

As a sidenote, this thing is pretty cool for in-system travel. Also can defend itself pretty fucking well. Anything kicked out of their airlocks goes at 1.2 MILLION m/s. It's 1 AU every 34 hours. You can reach Jupiter in a fucking week (then it's a pain to slow down though).

Halo ringworlds are around 8000 km if Wikipedia isn't telling bullshit and I assume their gravity is normal because I saw no indication of the contrary.
Spincalc tells me their tangential velocity is 8857 m/s or 8.8 Km/s. The average rocket sending stuff to LEO has around this delta-v budget.

But you say who cares of these speeds? Aerobraking for a longer amount of time and reentry!

I'm sorry it does not. The issue here is that this piece of shit was designed to avoid having a feature planets have. This piece of shit lacks a gravity field (or its field has the wrong shape if it's from Niven's book).

How do you convince the capsule to stay inside the atmosphere of the ringworld long enough for it to slow down?
For a planet it's the gravity pulling the thing down once it did the reentry burn and the same force keeping it in orbit before, but here? You need an engine to thrust it down in the atmo for the whole braking maneuver. At this point it's easier to simply use these engines to match speed with the ring itself.

As a sidenote, the planet's "gravity pull" that allows you to shed speed does have a speed limit as well. You cannot go faster than the planet's escape speed or you end up slingshotting around.
Inner-system probes did use tricks like these to shed their speed and reach a stable orbit but had to do multiple passes on different planets (aerobraked a bit and then were slingshot on each of them until they finally reached an acceptable orbital speed for the last one)

In short: Kind of like that reentry scene from the first Halo game.

It was either bullshit or these are just capsules launched by a craft that already matched speed with the ring with the usual engines moving at plot-speed.

Why land at all (or dock with a roof) when you could just dock with the existing retaining walls or the outer structure?

This does not solve any of the above issues.
The only half-smart way to land would be to set up a docking ring in the middle of the ringworld. There the tangential speed would be less. Then you take the fucking longest elevator of the system to bring stuff to the surface.

This is why I hate ringworlds. They manage to be worse than terraforming and colonizing a fucking shitty planet like say Mars.

[someone_else]

Getto edit: a couple typos, they change nothing of what I'm saying really.

-1206767 m/s is 1.2 million m/s, NOT km/s like I claim above.

-a Halo ringworld has a natural gravity field like anything, but its size and strenght are pretty close to nothing when compared to Earth's.

[Alerik the Fortunate]

Full scale ringworlds aren't comparable to terraforming planets; each provides the habitable surface of millions of worlds using an amount of material found in a typical solar system. If you don't have the resources to do that, you can stick with plenty of smaller Bishop rings or O'neill cylinder type structures and get the equivalent surface of a few thousand planets just by cleaning up the comets, asteroids, and perhaps a very small moon.

[Imperial528]

Why land at all (or dock with a roof) when you could just dock with the existing retaining walls or the outer structure?

If you don't have to come in "through the roof" from space, then why even have it open to space at all? It would be much easier just to enclose the whole thing and have giant "windows" where light can enter the ring.

Because it's either the roof or the railings, and roofs aren't mandatory by OSHA.*

In seriousness, though, I'd imagine building a roof is just another level of complexity, especially since it'd need to be transparent and strong enough so that pieces of it don't break off and destroy cities on the ring. That is, if you want natural sunlight.

And if pieces of it ever did break off, you'd better hope that the rest of the ring (at the least the surface, at the worst the structure itself) can survive those pieces crashing into it at very high speeds. Now, if your ringworld is small, a roof would probably be much easier and more practical. If your ringworld is large, I'd just go with open top and retaining walls.

Why land at all (or dock with a roof) when you could just dock with the existing retaining walls or the outer structure?

This does not solve any of the above issues.
The only half-smart way to land would be to set up a docking ring in the middle of the ringworld. There the tangential speed would be less. Then you take the fucking longest elevator of the system to bring stuff to the surface.

This is why I hate ringworlds. They manage to be worse than terraforming and colonizing a fucking shitty planet like say Mars.

Actually, it really does. If you have strong enough materials you can make your vector tangent to the ringworld, and adjust your velocity so you line up with a docking bracket that will grab you and hold you onto the edge of it once your ship itself is lined up with the center point of the ring. Just make sure that everything's strapped down and the floors are facing the right direction. This works with either a dock on the outside edge of the ring or on the retaining walls.

And if you really feel like being gutsy, and you have powerful enough engines, you could try to match the spin of the ringworld itself. This would make the actual docking smoother but this is made up by the greater requirements on the ship itself.**

It also occurs to me that instead of building a dock in the center of the ringworld, build a tether or arm that grabs incoming ships so that they can match the ringworld's spin (or even have the arm spin them up) by accelerating tangent to the arm, then line up with a docking bracket and be released from the arm.

*At least, from what I remember of ten-hour OSHA training.
**I'm a bit iffy on this one as I don't know enough about orbital mechanics to be sure it would work, essentially you're substituting your own thrust in place of gravity to make a false orbit on the same axis that the ringworld is on.

[someone_else]

Full scale ringworlds aren't comparable to terraforming planets; each provides the habitable surface of millions of worlds using an amount of material found in a typical solar system.

The AMOUNT is the same, but the KIND OF MATERIAL is not anywhere close.

For terraforming you just need to mine a ridicolous amount of stuff from gas giants and other planets and dump it on a rocky planet.

For Ringworlds you need to turn the same amount of mass into a rabidly strong material. <-- it ain't anywhere as easy as just trucking it around.

So it's not the same as terraforming. Terraforming is actually easier.

If you don't have the resources to do that, you can stick with plenty of smaller Bishop rings or O'neill cylinder type structures and get the equivalent surface of a few thousand planets just by cleaning up the comets, asteroids, and perhaps a very small moon.

This also allows far better survivability of the whole civilization. As if you declare a quarantine on a station IT WILL BE QUARANTINED. Forest fires, shitty foreign species screwing up other ecosystems... and so on.

The same for a bunch of terraformed planets.

It's also easier to do from an economcial sense of the things. How many times you say "oh shit I really need a million worlds worth of surface to relocate the entire fucking galaxy's worth of races on it wit still tons of room to spare"? Most of the times growth is slower and you just add new smaller stations when needed instead of paying the huge cost of making something so awfully big and having it sit 99% empty for the next few dozen millenia.

Even terraforming makes more economic sense since you terraform much less space at a time.

A Ringoworld can be a Coruscant-like Galactic Empire Capital to show off how cool they are, but it's not practical to house a real population.

Actually, it really does. If you have strong enough materials you can make your vector tangent to the ringworld, and adjust your velocity so you line up with a docking bracket that will grab you and hold you onto the edge of it once your ship itself is lined up with the center point of the ring.

This works fine for stations of realistic sizes and aircraft carriers, where the tangential speed (or aircraft speed) is manageable to slow down in little space. For a Ringworld the tangential speed is 1'200 km/s, for Halo ringworlds it's 8.8 km/s. Bullet speed is 1 km/s. So if you get too close and you get hit by an antenna, that's an antenna going 1000 times faster than bullet speed (or just 8 times for Halo ringworlds). Gonna hurt.

Unless your spacecraft has a similar speed in the right tangential vector and direction, it will rip to shreds the docking system or (if it is unbreakable) subject the ship, crew and cargo to decelerations nothing could hope to withstand without the usual "acceleration compensators" from ST.

**I'm a bit iffy on this one as I don't know enough about orbital mechanics to be sure it would work, essentially you're substituting your own thrust in place of gravity to make a false orbit on the same axis that the ringworld is on.

At this power levels, orbital mechanics are irrelevant. The orbital speed at Earth distance is 30-ish km/s and a ringworld wants you to reach 1200 km/s to dock, you're well above the speed that would keep you in orbit of the Sun naturally, so it's all in your engine's hands. Same issue than trying to reenter in a Ringworld atmosphere, your engines have to do what gravity isn't able to.

You just place yourself to the side of the ring and start accelerating in the right direction, correcting the course as needed (the thing is a ring, so you will have to correct your course to stay at the same relative distance from its hull as you run around it). It's the same kind of maneuver Luke did before blowing up Death Star, with a bit more safety systems.

[Sarevok]

Guys I was thinking of smaller ringworlds like Halo which span only about 1200 km in radius iirc. Halo is said to produce its gravity from spinning yet retains an atmosphere without any known force fields to contain it. Thats what I am curious about.

[Eternal_Freedom]

I would think smaller ringworlds and oneill cylinders are better purely on the grounds of versatility. For one, you coudl concievably move them to another star if absolutely necessary (it's already self-contained, just add engines for an instant generation ship).

The same can't be true for a 1 AU radius ringworld. I know a sun-type star's lifespan is billions of years, but it does change. For instance, in one billion years the Sun will be 10% brighter and Earth will be lifeless. That's a big bummer for your ringworld which will take an awful long time to build.

[Imperial528]

Guys I was thinking of smaller ringworlds like Halo which span only about 1200 km in radius iirc. Halo is said to produce its gravity from spinning yet retains an atmosphere without any known force fields to contain it. Thats what I am curious about.

If Halopedia is to be believed, Halos are in fact either 10,000km in diameter or 30,000km, depending on the halo in question. (The first ones constructed were of the larger type, I believe all of the ones in the game are of the smaller type, the larger ones having been destroyed by then)
And while that's no 1 AU ringworld, it's no small fry either.

[someone_else]

Guys I was thinking of smaller ringworlds like Halo which span only about 1200 km in radius iirc. Halo is said to produce its gravity from spinning yet retains an atmosphere without any known force fields to contain it. Thats what I am curious about.

Its size is irrelevant. If its gravity is 1 gee (Earth-like) and the atmosphere is Earth-like its atmosphere won't escape by going "up" any more than Earth's atmosphere escapes by going "up" (up= perpendicular to Earth's or the ring's habitable surface). Now, this requires some caution when you set it up, but after everything is spinning air won't escape.

The main difference is that this thing is a ring, so air can escape from the edges of the ring to the contrary of Earth (that is a sphere so has no edge ). So you need side walls up to Earth's LEO heights, that's 120-150 km high walls.

Done. Air does not escape if you have 100+km walls at the sides AND Earth-like spin gravity AND earth-like atmosphere and pressure. (technically some air escapes, but it's a painfully slow process that isn't an issue)

At even Halo sizes, this is no less bullshit than magic force fields due to structural considerations, as said above.

I know a sun-type star's lifespan is billions of years, but it does change. For instance, in one billion years the Sun will be 10% brighter and Earth will be lifeless. That's a big bummer for your ringworld which will take an awful long time to build.

I think that whoever can build a 1 AU ringworld can build or at least keep alive a star indefinetly (refueling it and syphoning off heavier stuff) without a lot of effort.

[someone_else]

Now, this requires some caution when you set it up, but after everything is spinning air won't escape.

This means that the air has to spin at the right speed before the spin-gravity effect kicks in and keeps it where it should. This means that when you are building it you must convince all the atmosphere to start spinning together with your ring or it rushes into space.

So you have to put a roof to keep all air in the few km closest to surface (not a big deal, since most of the atmopshere is already there), and keeping it there as you fill the compartment. Then you have the thing spin (if it wasn't already) and then raise the roof until the pressure is close to vacuum (a few km), then open it to space and disassemble the roof. Maybe you can modify the walls to make this temporary roof by bending or somesuch.

[Alerik the Fortunate]

As per my original post, you are going to need something with at least a couple of hundred kilometers radius if you want unroofed normal atmospheric pressure at 1 g, since if your tangential velocity is comparable to typical random motion speeds of air molecules, many are going to achieve trajectories that will not be sufficiently constrained by the pseudogravity to be held in by the retaining walls and you'll have a lot of spill over the top, unless you wall all the way to the center. Halo should be more than large enough, provided the retaining walls are high enough.

[Ellindsey]

Now, this requires some caution when you set it up, but after everything is spinning air won't escape.

This means that the air has to spin at the right speed before the spin-gravity effect kicks in and keeps it where it should. This means that when you are building it you must convince all the atmosphere to start spinning together with your ring or it rushes into space.

So you have to put a roof to keep all air in the few km closest to surface (not a big deal, since most of the atmopshere is already there), and keeping it there as you fill the compartment. Then you have the thing spin (if it wasn't already) and then raise the roof until the pressure is close to vacuum (a few km), then open it to space and disassemble the roof. Maybe you can modify the walls to make this temporary roof by bending or somesuch.

I expect that it would be easier to add the air (and water and other contents) after you've spun it up. Build the structural frame, floors and walls first in a vacuum. Spin it up, then finish adding everything else. Sure, you've got to figure out how to get your supplies to a spinning ring, but you were going to need to figure out how to dock with it while spinning anyway.