O'Neill Cylinders
 

I'm writing a bit of background material and I have come to the point where I want to briefly mention the construction of some really big habitats in space. I don't want to interrupt the flow of the text (which is historical overview material), but I would like to give the readers an idea of what the structures are: hollow cylindrical worlds spinning on their long axes, about the size of Arthur C. Clarke's Rama. That is, fifty kilometres long, twenty kilometres wide, and with an interior surface larger than the land area of Rhode Island or Luxembourg.

Now, the term "O'Neill cylinder" is sometimes used for this design in SF, and I propose using "O'Neill" as the in-setting term for these large cylindrical habitats (as opposed to the smaller "Stanfords", which are wheel-shaped rather than fully enclosed). But O'Neill's design was actually for a much more elaborate and specific design, with two cylinders counter-rotating, a separate agriculture ring, windows for natural lighting, etc.

Question: is "O'Neill cylinder" going to be misleading if used in the common sense without explanation?

Supplementary: anyone know off hand the limits for stability for a hollow cylinder rotating about its axis?

Re: O'Neill Cylinders
 

I don't think most people have read High Frontier. Plus calling all the big cylindrical habs "O'Neils" is just the kind of popular corruption that would reasonably propagate.

I think you'd be okay.

Re: O'Neill Cylinders
 

To be stable, the cylinder bust be rotating about the principle axis with the largest moment of inertia (this gives it the lowest rotational kinetic energy for a given angular momentum, meaning it cannot shed energy into other rotational modes).

A thin cylindrical shell - a hollow cylinder without endcaps - with mass M and radius R has a moment of inertia of MR^2 for rotation around its center axis.

A uniform circular disk - one of the endcaps of the cylinder - with mass M' and radius R has a moment of inertia of M'R^2/2 for rotation around its center axis. The cylinder will have two of these endcaps.

Thus, the moment of inertia for the entire hollow cylinder is MR^2 + 2 * M'R^2/2 = R^2 (M + M').

A hollow cylinder of length L, radius R and mass M without endcaps rotating around an axis perpendicular to its primary axis has a moment of inertia of M(L^2/12 + R^2/2).

A disk M' of radius R oriented perpendicular to its axis of rotation at a distance of L/2 - the endcap - has a moment of inertia of M'(L/2)^2 + M'R^2/4. Again, there are two endcaps.

Thus, for a hollow cylinder tumbling end over end, we have a total moment of inertia of ML^2/12 + MR^2/2 + 2 * (M'(L/2)^2 + M'R^2/4) = L^2 (M/12 + M'/2) + R^2 (M + M')/2

If the cylinder and endcap both have a uniform areal density D (probably equal to 1 ton/m^2, as this is sufficient to cut the dose from cosmic radiation down to levels without known long term health risks), then M = 2 * pi * R * L * D, M' = pi * R^2 *D.

For rotation about the cylindrical axis, this gives
I_z = pi * R^3 (2 * L + R) * D.
For end-over-end tumbling, on the other hand
I_x,y = pi * L^2 R (L / 6 + R / 2) * D + pi * R^3 (L + R/2) D
And we need
I_z > I_x,y
for stability. This gives us the condition
R^3 + 2 * L R^2 - L^2 R - L^3 / 3 > 0
Since it is late, I'm not going to solve this cubic inequality now (or check my work, for that matter - others may wish to look it over for accuracy), but will note that cubic equations do have closed form solutions, so you can find the allowed values of R in terms of L that give you a cylinder that rotates stably about its axis rather than tumbling end over end.
http://en.wikipedia.org/wiki/Cubic_equation

Luke

Re: O'Neill Cylinders
 

Nope. That _is_ the meaning of the term now, even if the meaning isn't proper given the origin of the word. Happens all the time.

Re: O'Neill Cylinders
 

I concur with other respondents: almost certainly not. The cylinder design (for purists, the double counterrotating cylinders, which allow you to maintain attitude control without using reaction mass) might be more accurately known as Island Three, but I don't see that getting into slang. "O'Neills" does seem a pretty likely candidate term for "free-floating space habitats large enough to be livable".

Re: O'Neill Cylinders
 

Agreed.

If you plug this equation into Wolfram Alpha, you can get solutions for R and L given the other number. Examples:

R = 20:
L < 33

L = 50:
R > 30

In other words, if lwcamp did his math right, your O'Neil cylinder isn't going to be stable if it's got a radius of 20 kilometers and a length of 50 kilometers.

Re: O'Neill Cylinders
 

Unfortunately, not so. Each cylinder individually acts as a gyroscope and if you try to keep both pointed at the sun, one will twist up, the other down (but with no net change in angular momentum). To get them to work as O'Neill described, you would need a massive and strong structure linking them, not the flimsy cables depicted by O'Neill. His design is badly understrength.

Re: O'Neill Cylinders
 

Yeah, I noticed. It's worse than that, actually, since I am going for a diameter, not radius, of 20 km. And the fact that my end caps are hemispheres rather than disks is not going to be anything like enough to save me. In fact, the inequality is rather discouraging. It looks to me as though cylinders have to be so short in relation to their length that roofing over the cylinder floor would be cheaper than capping the ends. That produces a Stanford rather than an O'Neill.

Which means I'm stuck with active stabilisation, I think.

Re: O'Neill Cylinders
 

Even with a ring rather than a cylinder, Larry Niven discovered he needed active stabilization. So scaling up doesn't seem to help.

The "natural" stability of L4 and L5 gets overrated too. You need to maintain your physical plant, you're going to need to maintain your stability too.

Re: O'Neill Cylinders
 

Sure, though that's a slightly different issue: translational stability of the centre of mass. I'm concerned about the thing tumbling, Niven's problem was with drifting off centre on the star.

Yeah, you're right. I guess it's not a big price to pay, except in engineering elegance.

Re: O'Neill Cylinders
 

I see two ways of giving your self more wiggle room:

1) Ditch the precession and use mirrors to keep the sunlight aimed in.

2) Increase the moment arm with some sizeable masses on spokes. These needn't be entirely deadweight, they can be high-gravity applications like wastewater separation or detention. Or if the yoke technology was realistic, you could mount the spokes on coaxial bands like barrel hoops and have them spinning at slower rates for the same apparent gravity, less gravity for invalids and sybarites, or adjustable if you like.

Re: O'Neill Cylinders
 

I never liked the look of the precession. And I think that natural lighting is overrated at the tech level I'm contemplating (GURPS TL 10, basically). I was going to sting a cable down the middle with some dampening to stop it from twanging or doing anything embarrassing like that and hang electric lights on that.

Good idea. I wonder whether plants (and maintenance workers) can stand high gravity in the agriculture/oxygeneration rings?

Re: O'Neill Cylinders
 

I'd think the axis would be naturally efficient as a general platform for large parts of all the systems needed to keep the interior livable, so that you'd probably want at least a semirigid lattice around an end-to-end transport system for solids as well as conduits for fluids, power, and bandwidth.

Re: O'Neill Cylinders
 

Anything fifty kilometres long is a cable.

Re: O'Neill Cylinders
 

Okay, so the Imperial Capitol in FLAT BLACK is going to be 20 km in diameter and 60 km long including hemispherical end-caps. Lit by a string of lights down the middle. It has a usable interior surface of nearly 3,000 square kilometres, making it larger than Luxembourg or Delaware. Most offices, residences, and such facilities can be under "ground", or rather, the interior can consist largely of a huge roof garden. Places with view windows will do best on the slopes of the end-caps.

With a population of, say 500,000, the Imperial Senate buildings and residences for a thousand senators and staff, can we spare room for a military academy? One campus of the military academy? Or should all military academies be put where cadets can bang off guns safely?

What's a good level of centripetal acceleration ("gravity") for such a place? 10 ms^-2 is a "standard gravity". 9.81 ms^-2 recalls the gravity of Old Earth. About 8.5 ms^-2 would be about average for people growing up in the colonies (weighted by population) and very convenient in GURPS rules. 10 ms^-2 would require that the place rotate at about 0.3 RPM.

Re: O'Neill Cylinders
 

Only a small portion of Imperial officers training involves personal weapons, and a lot of that can be handled in VR. I'd think live fires are infrequent enough to warrant field trips to the range.

Re: O'Neill Cylinders
 

By the way, does anyone know of a suitable ray-traced (not artist's impression) interior shot for something like this? I'd like one without Island Three-style longitudinal windows, and without the Rama-style spikes in the end-cap.

I already have these on Youtube:

• http://www.youtube.com/watch?v=swMAjaQ5AsQ
• http://www.youtube.com/watch?v=uhola...eature=related
• http://www.youtube.com/watch?v=rPj4y...eature=related

Re: O'Neill Cylinders
 

An experiment involving several generations of chickens and a centrifuge showed that they are healthier when raised under higher gravity. So farming should be OK.

Having your cadets train under varying g levels would also be useful.

Re: O'Neill Cylinders
 

Indeed. At minimum they need to train in free-fall, at lunar/asteroidal gravities, in gravity not above 0.65 gee, in gravity not below 1.07 gee, and in gravity about 0.86 gee. They also have to train extensively in a spaceship or ship-like habitat, on a moon or asteroid surface, and in several different planetary environments including urban, farmland, forest, and open land. And at sea, and underwater. Multiple campuses will be required.

Re: O'Neill Cylinders
 

My BLACK OPS players pointed out that other than the gravity variations, most of these could be had at a certain facility near Orlando, Florida.

Re: O'Neill Cylinders
 

Why can't you explain it? Do you abslutely have to write a 100% linear text? Put in a footnote, referring to the end of the article where you explain the term, or add a "see glossary article: O'Neill" in paranthesis. Or go fully hypertext, with clickable links and all that.


And using O'Neill as a general term for rotating cylindrical space habitats that aren't too large (several hundreds of kilometers) is fine.

Re: O'Neill Cylinders
 

I'm foreign. You can't expect me to recognise such oblique references to Disney World.

Re: O'Neill Cylinders
 

Because I was writing a two-page introduction.

Re: O'Neill Cylinders
 

Here is a render of a spin hab I did quite some time ago. You can see the fish-eye distortion due to the wide field of view - but that is inevitable if you are trying to take in a wide sweep of the hab at once.
http://panoptesv.com/Zoe/wheels/habitat.html

Luke

Re: O'Neill Cylinders
 

Not since 2385 they can't.

Re: O'Neill Cylinders
 

Thanks. I thought that having a point of view so very close to the inside surface would make the perspective very different from looking down a pipe, and am glad to see it's true.
Good point.

Re: O'Neill Cylinders
 

What about high-speed flywheels at the caps of the tube to provide stability?

Also, when I was doing similar thought experiments and web investigations, I found that you need a very large radius to approach proper "radial" gravitation, otherwise you can still feel rotating frame effects. These include differing ballistic trajectories depending on the direction you're firing, vertical drops being slanted, and weight varying with your direction of travel.

There was a nifty flash animation I found somewhere for simulating dropping or throwing a ball in a rotating frame.

Re: O'Neill Cylinders
 

Good thought.

I have 10 km radius here, and period 200 seconds. My reading suggests that that is ample to avoid dizziness, disorientation, and sickness. But there will be some noticeable Coriolis effects. A person running fast (10 ms^1) to the east will feel a 6% increase in "gravity", and a person running fast to the west a 6% decrease. The difference in long-jump distances will be the same. Rising in a high-speed lift (1 storey per second) you will feel a force of a 0.9% of a gee pushing you east.

I don't think it will affect anyone's Rugby skills.

Re: O'Neill Cylinders
 

If the pitch runs east-west, the east end team always is at a severe disadvantage.

If it instead runs north-south, play will tend to drift to the East side of the pitch, as it will be easier to pass east.

Re: O'Neill Cylinders
 

You always change ends at half time.

I wouldn't think that would matter much. Players' ability to pass a ball for distance already far exceeds the range where it is tactically wise to do so. A long pass in Rugby isn't called a "hospital pass" for nothing. In any case, it can't be worse than beach Rugby. Nothing is worse than beach Rugby. Nearly lost a PC in a game of beach Rugby once.

Re: O'Neill Cylinders
 

5-6%, or a 10-12% range in ability, would be quite significant in many situations. With guns, having to shoot higher or lower by that much (and it would be more at bullet velocities) would give a huge penalty to skill.

I think Rotating Frame G-Experience would be a valid enhancement if you want to go for realism.

Re: O'Neill Cylinders
 

Good point.

Re: O'Neill Cylinders
 

But it's right there by the spaceport!


Blown up or no, my point is that they don't necessarily have to go far afield to have those environments available for training, especially if there's a place purpose-built for high-budget, short-time-span tourism/dversion. Like the luxurious capital space habitat described.

Re: O'Neill Cylinders
 

Sure. I don't go in for single-biome planets, and even if I did you would be dead right about the possibility of building different sorts of environments as training ranges at a single campus. It is the gravity issue that binds. I think I need free-fall, asteroidal gravity, light, medium, and heavy. So I can get away with a light planet, a heavy planet, and multiple locations in this solar system.

Re: O'Neill Cylinders
 

The habitat is superior to Disneyworld in that lower gravity levels are available at locations closer to the axis, and if you have those pods I mentioned a while back extending further from the axis, then you've got heavier gravities, though they might not be roomy enough for operations there to count as field exercises.

Re: O'Neill Cylinders
 

What about also training in rotational gravities?

For slow RPM rotations, like in an O'Neill size habitat, the coriolis(sp?) effect should be neglible, but for much smaller habitats it will be an issue.

Re: O'Neill Cylinders
 

There are rotating habitats in the Solar System, others besides the Capitol.

Re: O'Neill Cylinders
 

But is it part of the training regimen? If not, should it be?

Re: O'Neill Cylinders
 

Certainly it should. The Fleet Protection deployment involves a lot of boarding and inspection on smaller-sized colonial habitats. (Besides ships and asteroids.)

Re: O'Neill Cylinders
 

Also, unlike Disneyworld, the O'Neill Habitat probably doesn't have that pernicious rodent problem.

:)

Re: O'Neill Cylinders
 

Different setting, same rodent problem. (Okay, it's a Cole Habitat, not an O'Neill Cylinder, but the point still stands.)

Re: O'Neill Cylinders
 

Yeah, that's what motivated me to do these models/renders in the first place. Here's a render from the surface of a ringworld showing what I was talking about with fish-eye lens distortion
http://panoptesv.com/Zoe/wheels/ringscenelg.html
what you would perceive when standing on the ringworld would be the ring going straight up, but in ray tracing you get a tilted looking projection unless the ring is right in the center of your field of view.

Luke

Re: O'Neill Cylinders
 

Having watched plenty of rugby and gridiron , I've noticed that when the wind is significant to one side of the pitch, it can drastically influence play strategies; also, switching ends doesn't even things out as much as might be implied; second half tends to be a bit slower for most of the teams I've seen play. A disadvantageous wind along the length of the pitch tends to help the team initially attacking from up wind a bit more than the second, as the first team with the advantage tends to make better use of it.

And a 15% easier in antispin vs 15% harder spin throw definitely means sufficient difference to alter play... and a lot more missed throws due to differences in ball performance.

Re: O'Neill Cylinders
 

I don't think that "15% easier" is an accurate characterisation. If you throw the ball the same it will go ~6% further. But since a good pass in Rugby is much shorter than the distance one can easily throw a Rugby ball that doesn't really help much. A cross-wind makes passes faster or slower, which does matter. The Coriolis effect only makes them come in a little low or high: that requires practice to adjust for, but equally to east or west.

Re: O'Neill Cylinders
 

That may be the most depressing thing I've seen in months.

Re: O'Neill Cylinders
 

Yeah, we don't do much with it in-setting. It's just there as background material and a Dire Warning To Future Generations to not let any other company pull the same sort of stunt.

Re: O'Neill Cylinders
 

I built a little calculator for the thickness of the structural hull, mass of the hull and contained atmosphere for a rotating cylindrical habitat, as functions of the length and radius, the "gravity" and air pressure, the mass-per-unit-area of the shielding, armour, landscaping, and fittings, and the strength and density of the structural material. It's an Excel workbook with no macros.

Just at the moment the list of possible building materials is a bit incomplete and some of the values for strength questionable.

I put it on Dropbox for review and personal use: https://www.dropbox.com/s/7iilf35hdx...ator.xlsx?dl=0