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04-18-2010, 07:28 PM | #1 |
Join Date: May 2005
Location: Oz
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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? Last edited by Agemegos; 04-18-2010 at 07:42 PM. |
04-18-2010, 09:28 PM | #2 |
"Gimme 18 minutes . . ."
Join Date: Sep 2005
Location: Albuquerque, NM
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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. |
04-18-2010, 11:35 PM | #3 | |
Join Date: Nov 2004
Location: The plutonium rich regions of Washington State
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Re: O'Neill Cylinders
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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 Last edited by lwcamp; 04-18-2010 at 11:50 PM. |
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04-19-2010, 12:12 PM | #4 | |
Join Date: Sep 2007
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Re: O'Neill Cylinders
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04-19-2010, 12:50 PM | #5 |
Join Date: Sep 2008
Location: near London, UK
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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".
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04-20-2010, 11:15 AM | #6 | ||
Join Date: May 2008
Location: CA
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Re: O'Neill Cylinders
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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. |
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04-20-2010, 04:44 PM | #7 | |
Join Date: May 2005
Location: Oz
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Re: O'Neill Cylinders
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Which means I'm stuck with active stabilisation, I think. |
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04-20-2010, 06:01 PM | #8 |
Join Date: Aug 2007
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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.
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Fred Brackin |
04-20-2010, 08:00 PM | #9 | ||
Join Date: May 2005
Location: Oz
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Re: O'Neill Cylinders
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04-24-2010, 01:33 AM | #10 | |
Computer Scientist
Join Date: Aug 2004
Location: Dallas, Texas
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Re: O'Neill Cylinders
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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. |
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Tags |
flat black, o'neill cylinder, oberth cylinder, orbital habitat, space |
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