17 replies to this topic

[YOLF]

So I'm reading the Culture series and they use vast orbitals to live an entirely space faring lifestyle rather than colonizing planets. Orbital rings are nothing new, but I'm wondering how large a ring could be built using our resources in the asteroid belt and what the goldylocks zone for an orbital might be. Let's have a discussion!

[okok]

Just finished Matter. Very very good.

The wiki entry for Orbital (The Culture) gives a diameter of 3.71 million km for earth-like g and 24h. If you'd know the primary structure monocrystal density you could do the math. Asteroid belt is about 4% of moon mass.
Goldilocks zone? Why, same as earths.

Anyway, I'd like to win a trip to Masaq' Orbital one day. (oh well, or maybe a sequel).

[YOLF]

Hmmm... always thought there was more mass in the asteroid belt. I guess we'll just have to build them out of mars or Jupiter's moons Any estimates on how far 4% would go in building an orbital assuming we used materials that modern science believes will be available?

I just started reading Phlebus, great book, but I keep reading the internet instead. I'm surprised to see that they don't have more settings on orbitals in the culture series. I only see 4 mentioned from the whole series on wiki. I'd definitely like to see a sequel and getting a free trip to one would be great! So if you get two tickets

[taho]

Well, they are kind of big and way beyond anything that we could build anytime soon.

What could and probably will be built are simple iron rings or tubes, couple hundred meters wide. Everything you need to do that is already in space. Simply go to an asteroid, scoop up dirt, use magnet to separate iron, melt it with solar energy, make big plates and weld them together so that you get big airtight cylinder. Rotate it, put few meters of dirt on the "floor", put air in and bring the light in with mirrors and plant lots of plants and animals. Safe, self-suficient, independant island in space. And if you want to leave this solar system, then you can even put a fusion reactor on it with lots and lots of hydrogen and go. It will take couple of centuries to get anywhere, but we will cure ageing so we will be in no rush, right?

My guess is, that SpaceX will start building first one in 2030 on Phobos.

[YOLF]

I think that's all orbitals really are, it's jus a matter of scale. So it would just keep growing as segments were added. It wouldn't initially need to bea continuous taurus. It could just be a series of curved tubes separated by a thin tube with maglev train for going between big liveable tubes. As the need arose, space could be filled in in a balanced manner.

If we could build any part of it, it's already started!

[taho]

To build something like Orbital you would need incrediblly strong material.
You don't need to invent anything new to build smaller stations.

[YOLF]

Where/why all the extra strength needed? It's in zero G right? Would more material for strength not suffice?

[niner]

You wouldn't need a giant "orbital" to obtain 1 g acceleration. That's just a matter of getting the right rotational speed for a given radius. In the movie 2001 A Space Odyssey, which came out in the mid 1960's, they had a large rotating space station reminiscent of these fictional orbitals. It created an Earth-like simulated gravity, but it was not preposterously large like orbitals. At any rate, this is not a new idea. Worrying about attaining a 24 hour day from sunlight is a little ridiculous, given the ease with which we can simulate daylight using electric lighting. Considering the cost of building the structure, the cost of lighting is insignificant. It's also rather difficult to light the interior of a spacecraft with sunlight, which would require large windows or elaborate light collectors and pipes.

[taho]

Object in space is in orbit that has 0 g, meaning it has to resist 0 km/s/s force.
If you want to build a big ring that will rotate, you must have material strong enough to resist 10 km/s/s AND support itself. Yes, you can use more material, but the more you use the more you have to support, so it kind of cancels itself out.
That means, the higher your tensile strenght, the bigger the radius. For iron it is couple of km. Even carbon nanotubes wouldn't be enough for something the size of Orbital.
But, since there is plenty of space out there, you can build more of them with smaller radius and it would work just as same.
Mirrors are cheap and low tech. You can go solar -> electricity -> light, but why do that if you can use that 24/7/365 fusion reactor.

[YOLF]

You wouldn't need a giant "orbital" to obtain 1 g acceleration. That's just a matter of getting the right rotational speed for a given radius. In the movie 2001 A Space Odyssey, which came out in the mid 1960's, they had a large rotating space station reminiscent of these fictional orbitals. It created an Earth-like simulated gravity, but it was not preposterously large like orbitals. At any rate, this is not a new idea. Worrying about attaining a 24 hour day from sunlight is a little ridiculous, given the ease with which we can simulate daylight using electric lighting. Considering the cost of building the structure, the cost of lighting is insignificant. It's also rather difficult to light the interior of a spacecraft with sunlight, which would require large windows or elaborate light collectors and pipes.

The size still needs to be sufficient enough to create uniform gravity at ground level, not just gravity at the feet. We'd wind up elongated and weak on the upper half if that were the case.

Climinng structures to weightlessness might be sorta fun though.

Edited by cryonicsculture, 28 October 2013 - 11:11 PM.

[YOLF]

Object in space is in orbit that has 0 g, meaning it has to resist 0 km/s/s force.
If you want to build a big ring that will rotate, you must have material strong enough to resist 10 km/s/s AND support itself. Yes, you can use more material, but the more you use the more you have to support, so it kind of cancels itself out.
That means, the higher your tensile strenght, the bigger the radius. For iron it is couple of km. Even carbon nanotubes wouldn't be enough for something the size of Orbital.
But, since there is plenty of space out there, you can build more of them with smaller radius and it would work just as same.
Mirrors are cheap and low tech. You can go solar -> electricity -> light, but why do that if you can use that 24/7/365 fusion reactor.

So how big an orbital are we talking with steel if we use a curved pinwheel design to reinforce it and operate factories on them?

Solar is so cheap and efficient where there is a close enough star, I'd think solar would be a good way tto run things, and then have a backup fussion reactor for travelling.

[Lazarus Long]

To give some of you a practical idea of what you are talking about; have you ever taken a ride at a county fair or theme park called something like the "Round-Up"?

Usually it is some kind of a giant squirrel cage that you walk into and then stand up against the outer wall. The ride is about 15 feet in diameter (sometimes more) and spins at about 15 to 18 revolutions per minute. It is like a small ferris wheel on speed. The point is that the ride eventually has you pinned with over 1G against the wall at the top and that means it is generating more than 1G of centripetal force in order to keep you from falling. However not much more than 1G because that is how the ride delivers the feeling of weightlessness for an instant too.

The larger the diameter of the ring the slower it has to move to generate tangential momentum, which is where you are getting the 9.8 meters per sec² of acceleration that equals 1G In other words whether it is the equatorial zone of a sphere or the wall of a cylinder, the rotational velocity diminishes as the radius increases. I would have to go back to some basic physics texts, or probably Wiki someplace but I believe there is a formula for that relationship of rotational velocity (or RPM) versus the diameter (more probably radius) of a disk. This also basic engineering for all spinning disks like aircraft props, racing tires etc.

To give you an idea of how large things get and how fast, a theoretical ring construct of 1Km radius has a circumference of a little over 6.28 Kms in length. If you spun that ring at 1G and the floor span of the ring were 100 meters then the total surface area for 1G habitat would be .628 Km^2. This does not mean there aren't ring levels of less than 1G either and exposure there would be in shifts, or floor spans (think of that like a cylinder's height) greater than 100 meters. Actually if you simply make a larger floor of 160 meters, or a little larger than a good soccer field then the habitat area jumps up to over 1 square kilometer. In other words almost the size of many smaller communities.

This type of technology is well within our abilities now. Certainly in terms of structural mechanics but also in terms of material science. This does not have to rely on super exotic materials or anything more complex than the methods of advanced bridge building.

As to the ideal location for such a structure that will vary with its mission profile.

But if you are already catching asteroids then there are easier methods than building and spinning a wheel.


Here is an interesting paper on the physics of the ride that you can use to extrapolate to an orbital.
http://www.physics.s...m/3 Rides.1.pdf

Read the section 3.2 about rotation rides and here is a better paper on the physics of rotation.
http://theory.uwinni...circ/node6.html

The important part is that that 2Km diameter ring is turning more like 1 revolution per hour (maybe even slower) rather than per minute.

A really good classic Sci-Fi on this subject is Ringworld, by Larry Niven.
http://en.wikipedia.org/wiki/Ringworld

The novel opens in A.D. 2850 on Earth. Louis Gridley Wu is celebrating his 200th birthday. Despite his age, Louis is in perfect physical condition (because of a regimen of boosterspice) but is bored. He has experienced life thoroughly, and is thinking of taking a trip to and beyond the reaches of Known Space, all alone in a spaceship for a year or more. He is confronted by Nessus, a Pierson's Puppeteer, and offered one of three open positions on an exploration voyage beyond Known Space. Speaker-to-Animals (Speaker), who is a Kzin, and Teela Brown, a young human woman, also join the voyage.

They first travel to the Puppeteer home world, where they learn that the expedition's goal is to explore a ringworld: an artificial ring about one million miles wide and approximately the diameter of Earth's orbit (which makes it about 600 million miles in circumference), encircling a sunlike star. It rotates, providing artificial gravity that is 99.2% as strong as Earth's gravity through the action of centripetal force. The ringworld has a habitable, flat inner surface equivalent in area to approximately three million Earth-sized planets. Night is provided by an inner ring of shadow squares which are connected to each other by thin, ultra-strong wire (shadow-square wire).

[niner]

You wouldn't need a giant "orbital" to obtain 1 g acceleration. That's just a matter of getting the right rotational speed for a given radius. In the movie 2001 A Space Odyssey, which came out in the mid 1960's, they had a large rotating space station reminiscent of these fictional orbitals. It created an Earth-like simulated gravity, but it was not preposterously large like orbitals. At any rate, this is not a new idea. Worrying about attaining a 24 hour day from sunlight is a little ridiculous, given the ease with which we can simulate daylight using electric lighting. Considering the cost of building the structure, the cost of lighting is insignificant. It's also rather difficult to light the interior of a spacecraft with sunlight, which would require large windows or elaborate light collectors and pipes.

The size still needs to be sufficient enough to create uniform gravity at ground level, not just gravity at the feet. We'd wind up elongated and weak on the upper half if that were the case.

That wouldn't be a problem for any structure large enough for people to live in for longer than a few days. I don't think it would be possible to create a gravity gradient that would provide one g at our feet and zero at our upper body, at least not if the person had room to move. The idea of creating an inhabitable torus the size of Earth's orbit is ridiculous.

[taho]

You can't have it too small, because it would spin too fast. That would mess with your sense of balance. So it must be at least tens of meters wide, preferably hundreds of meters.
You can't have it too big, because you couldn't build it.
So if we look at movies, that would mean that Odyssey 2001, Babylon 5 and maybe up to the size of Elysium are quite possible. Large Cities in space.

[YOLF]

You wouldn't need a giant "orbital" to obtain 1 g acceleration. That's just a matter of getting the right rotational speed for a given radius. In the movie 2001 A Space Odyssey, which came out in the mid 1960's, they had a large rotating space station reminiscent of these fictional orbitals. It created an Earth-like simulated gravity, but it was not preposterously large like orbitals. At any rate, this is not a new idea. Worrying about attaining a 24 hour day from sunlight is a little ridiculous, given the ease with which we can simulate daylight using electric lighting. Considering the cost of building the structure, the cost of lighting is insignificant. It's also rather difficult to light the interior of a spacecraft with sunlight, which would require large windows or elaborate light collectors and pipes.

The size still needs to be sufficient enough to create uniform gravity at ground level, not just gravity at the feet. We'd wind up elongated and weak on the upper half if that were the case.

That wouldn't be a problem for any structure large enough for people to live in for longer than a few days. I don't think it would be possible to create a gravity gradient that would provide one g at our feet and zero at our upper body, at least not if the person had room to move. The idea of creating an inhabitable torus the size of Earth's orbit is ridiculous.

I think I may have miscommunicated. I read in one or another publication that with a particular design there would be 1G at the feet but only .8G at the head or something like that. So having an orbital of sufficiant size would be necessary to avoid varying bone/mineral densities.

[niner]

The size still needs to be sufficient enough to create uniform gravity at ground level, not just gravity at the feet. We'd wind up elongated and weak on the upper half if that were the case.

That wouldn't be a problem for any structure large enough for people to live in for longer than a few days. I don't think it would be possible to create a gravity gradient that would provide one g at our feet and zero at our upper body, at least not if the person had room to move. The idea of creating an inhabitable torus the size of Earth's orbit is ridiculous.

I think I may have miscommunicated. I read in one or another publication that with a particular design there would be 1G at the feet but only .8G at the head or something like that. So having an orbital of sufficiant size would be necessary to avoid varying bone/mineral densities.

I just did some half assed back of the envelope calculations, and I think this corresponds to a radius of 10 meters. Even at this rather tiny size, I don't think that kind of gravity gradient would be enough to distort our bodies, at least not greatly, but that's neither here nor there, since any sort of space colony would have a radius of at least a hundred meters, probably more. In this case, the gravity gradient would not be significant.

[johnf]

In the 1970s NASA Ames space colony summer studies, the largest habitat they worked out in any great detail were the cylinders 6km diameter.
These weren't the biggest we could build with then-available sorts of technology -more nearly 20km diameter for one G.
And this is just good steel for cables and some form of rock dust "concrete" whether it be sintered or a mix -something that doesn't outgas a lot, non-porous, that cures in the space environment, etc, etc... so it's hard to say without digging through mounds of papers on such subjects among the engineers today.

As to the "habitable zone", it goes from inward of Mercury's orbit if you want, to 'way out into the Kuiper belt if you want.
In O'Neills book on space colonies there's a version of his early Bernal Sphere model out at an arbitrarily defined "continental shelf" for our solar system. I beleive they settled on the mirror mass must be limited to no more than twice the rest of the colony, and it's got a 600 meter spherical habitat shielded with 4.5 tonnes per square meter of regolith concrete.
The outer distance to keep mass down to this "reasonable" extrapolation was about 4 light days. Near the heliopause. Out as far as Neptune, the Sun is only a particularly bright star. Go out much further and there are brighter ones to point your mirrors at, even if you're still orbiting this one.
In "Interstellar Migrations and the Human Experience" an article talks about nomadic interstellar travel: Space colonies eating cometary bodies, and using what delta-V they can apply so that by the time they're done eating this one, they're close enough to capture another. Large masses being pushed, tiny accelerations, very small relative velocity increments between such bodies so far out even if distances are vast.
For a colony of 20,000 to have as much energy per capita as the richest of today, such a colony would require a main mirror about the size of the continental US. WIth flying mountains of stuff freely available, what's a few microns of material?

[YOLF]

So we could be at the edge of the Sun's influence and get power from neighboring stars? I've never realized that. It would be interesting to see what the view out that way would look like.

So is the mirror attached to the orbital, or is it remote?