G'day folks

I'm hoping some people in the know might be able to help me with a story I'm planning.

The story is set on a giant space station which houses a few thousand people who are the last remaining people.

1. The space station will be rotating to produce artificial gravity. Would a more appropriate design be a cylinder or a torus? My impression is that a cylinder would be a more efficient design in terms of usable volume for materials used. Is that correct?

2. For story purposes, I'd like the space station to incorporate a fairly complete ecology, meaning the inhabitants grow food in soil, and live in houses. On that basis (which probably also makes the cylinder the shape of choice), roughly how much surface area would be needed to provide enough ground to grow crops and animals to feed a population of a few thousand? And what would be the appropriate ratio of width to circumference to provide a practical rate of rotation?

4. With a spinning space station like this, would its rotation need to be actively managed to maintain a steady spin? If not, how long would it take for problems to arise, what would be the likely nature of those problems (I'm guessing some form of coning) and what would be the consequences for the space station's inhabitants?

5. What other sorts of problems could you imagine arising over a period of a couple of centuries? (This last question because it might provide plot elements I hadn't thought of.)

Thank you for any assistance you might be able to provide.

'The High Frontier: Human Colonies in Space' Gerard K. O'Neill (1976).
There is way too much relevant material for me to attempt to summarize it.

-- Jeff, in Minneapolis

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Technically a sphere would be the most efficient shape, but spin would tend to dump everything into the equator, so a cylinder is probably best for human habitation.

To have a complex ecology would require several things-- a large surface area, a very durable outer shell (roots grow into the darndest places!) and a very reliable source of light-- meaning one with few or no moving parts. Moving that much soil also means that there has to be extensive space travel and space industry to esentially make the soil, shipping it up from Earth would be prohibitively expensive. And it also means space would likely have been colonized for some time previously, as early space habitats will undoubtedly use more efficient hydroponics to save both volume and mass. The people are going to have to be almost fanatically dedicated to keeping everything in balance and in perfect working order, and to population control.

Again, size matters. The bigger and more massive a station, the longer it will take for problems to arise. The more open interior space the station has, the more moving air and water will tend to slow the rotation.

Consequences of low gravity can include bone density loss and muscle atrophy, possibly increased height leading to an inability to use buildings and furniture designed for shorter 1-G people, and any gravity-assisted circulation systems for air or water will fail-- i.e., toilets will not flush correctly.

Centuries? Inbreeding, loss of knowledge, lack of maintainance, breakdown of vital technologies, loss of air and water from incomplete cycling or leakage, pollution buildup, any of tens of thousands of ecological imbalances, overpopulation, war, chronic agoraphobia leading to an inability to leave, impacts from space debris, solar flares, dirty windows from micrometeor abrasion cutting down the sunlight influx, clogged pipes.

For a station that can last that long, I'd recommend using what Larry Niven calls a "bubbleworld" or hollowed asteroid-- a nickel-iron asteroid melted by mirror-focussed sunlight and inflated into a capsule shape, then filled with air and its interior terraformed. A slight bulge in the center would give it a ring or band that can be filled with water to make a lake, which acts as both a resevoir and a heat-sink.

If the station's axis is perpendicular to its orbit, both ends can have a "flower" of focussing mirrors to reflect sunlight through windows into the interior-- there can be a line along the interior axis holding more mirrors to send that light "down" onto the farmland. The endcaps should be shaded to allow for cold areas where precipitation can take place. Streambeds carved into the shell will guide that water from the ends back into the central ring-lake.

And just a suggestion: building any station large enough to hold several thousand colonists will require a signifigant presence in space beyond just those few thousand-- industries, miners, construction workers, etc. If you want this station to have the only suvivors, you have to account for what happened to all the other people in space who made the station possible in the first palce.

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"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
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"The Mayan symbol for "book" looks a lot like a triple hamburger, but I've never seen them claiming it as proof the Mayans had Big Macs." - KaiYeves
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort

Three to five thousand people would need a large space cylinder.. I can not imagine soil as we know it as the best medium for plant nutrients to be transfered... a reservoir of people, a pool of genes... Medical, technical, Complexities as yet not imagined... a good story line.
'Jeff' is correct. This is a huge subject.
I would point you at the ISS. As that is paving the way. Building our knowledge base for future generations of space travelors... Your idea could be easily answered as humanities only chance... Last chance or no chance...

A cylinder is more efficient, but it's inherently unstable due to it's radius of gyration.

A torus is what you need for dynamic stability.

I used to think one could never have a cylinder, but many human systems these days are dynamically unstable and are kept stable by computers and feedback systems. Case in point: F-22.

On the other hand, if an F-22's stabiization systems go on the fritz, it's a half-second corrective action on the part of the pilot to leave the aircraft.

But if a cylinder's stab systems go on the fritz, a few thousand people may die, as the cylinder will slowly begin tumbling end over end, and all the farms, houses, people, and buildings that were firmly anchored to its inside wall will slide to the ends, probably to break through, flying off into space, and of course rupturing the seal.

So: Torus.

Just that the radius of gyration about the toroidal axis is about 30% larger than that about any other.

Yes, given that masses would be moving around inside, it would still have to be actively managed. It's quite easy, however, with jackscrew or cabled weights moved at various points throughout the structure to stabilize the mass distribution.

So long as they refrain from calling a Town Hall Meeting for all inhabitants at one point...

"Coning?"

The RPM would be managed with the most efficient thrusters. Probably hall-effect ions or VASIMIR.

Have you read Arthur C. Clark's Rendezvous with Rama? I think he fleshed many of them out back in 1973.

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If I set the budget, we'd have Ares and more. Unfortunately, I don't set the budget, and Ares is just too expensive and too far out for us to accomplish our goals within the budget we were given.

If we halt the ISS, all versions of Ares, and transport Orion and Altair aboard DIRECTv3's Jupiter family of Shuttle-Derived Launch Vehicles, we just might make it back to the Moon by 2020.

Arthur C. Clark got it right, I believe. The units can be balanced by pumping water ballast as required to different tanks to account for movement of personel . Give this gentleman credit. It could work quite well.

Best regards,
Dan

Note that a habitat is not a solid object. The radius of gyration depends on the mass distribution, and with the bulk of the mass at the "floor" of the habitat and very little along the axis, a cylinder could be nearly as stable as a torus. Slings/tethers extending from the station equator could make it even more stable than a simple torus, while reducing delta-v requirements for station rendezvous or departure or providing high-gravity environments for whatever uses might exist. There is also the possibility of an outer shell that is stationary or rotates in the opposite direction, allowing the whole to remain stable for much longer periods.

On the windows suffering from space weathering: put thin non-pressure-bearing panes on the outside. Replace them as needed, and in the meantime they function as something of a Whipple shield, breaking up any high velocity impactors that might otherwise get through inner panes. Also, if you focus sunlight through a light pipe arrangement, you could pretty much eliminate the possibility of space debris making a direct strike on the windows. The pressure-bearing windows could be quite small and well protected, leaving the easier-to-repair/replace mirrors to take the damage.


Electrical motors, once everything's spun up. The net angular momentum of the station is going to remain the same, aside from that lost or gained through tidal effects by planets and the sun or to uneven radiation pressure, effects that can easily be exploited to assist more than they interfere. An inner habitat shell, an outer, stationary or slowly counterrotating shielding/machinery shell, and perhaps some torque bars, tethers, or solar sails to tweak the station orientation with...

Except for the cylinder vs torus issue.

Who? AC Clarke? Or the OP?

Much of Clarke's pseudo-engineering was quite good. It was fleshed in in far greater detail by Gerard K. O'Neill.

Despite his amazing efforts towards cylinders, some concepts of which were twenty miles in length, cylinders are not stable.

However, they can be made stable by rigidly attaching distant weights to change the dominant radius of gyration...

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If I set the budget, we'd have Ares and more. Unfortunately, I don't set the budget, and Ares is just too expensive and too far out for us to accomplish our goals within the budget we were given.

If we halt the ISS, all versions of Ares, and transport Orion and Altair aboard DIRECTv3's Jupiter family of Shuttle-Derived Launch Vehicles, we just might make it back to the Moon by 2020.

Hi, I was refering to that which was described in 2001 Space odyssey.
In that we see two quite large wheels attatched by a small connecting cylinder. It is quite intuitive and may prove to be a practical design at some future date.

Best regards,

Dan

What technologies we do not have we soon will have... We could do this now. It as always comes to the COST of getting all that metal into space. Everything else we can do. As for the large tube or cylindrical station.. From just this brief discussion we can and could sort out issues as they present them selves. If the need was great enough... We can do anything if the want is to

The space shuttle is dynamically unstable and totally depends on computers. If the flight control computers all crash, the vehicle crashes. Likewise newer fly-by-wire airliners are highly dependent on computers. You can't eject from either the space shuttle or an Airbus A380.

Thousands of people trust their lives every day to computerized control systems -- even if they're unaware of it. I don't see the difference between a future space habitat being dependent on computerized stability control when the space shuttle and airliners already are.

The simple inertial guidance system which governs a rotating space station
is cheap enough to have several redundant systems . A dedicated trim system is not really very sophisticated at all, employing a few pumps which redistribute the water in different tanks within compartments.
It is a non- problem.

Best regards,
Dan

I think a sphere would be the best shape. It would just be like living in a wide valley, with sloping "mountainsides".

Arthur C. Clark was an instructor and later pilot in the RAF, and an talented and entertaining writer, but he wasn't perfect, danscope.

Any object, when rotated, will eventually, due to flexing and internal friction, will eventually wind up rotating about an axis that allows for the greatest moment of inertia to be rotating about that axis.

It's physics 101, these days.

This requires active controls, danscope, the failure of which would result in the catestrophic failure of the entire colony, which would be a very poor design.

A torus doesn't have that problem, as it's passively stable.

__________________
If I set the budget, we'd have Ares and more. Unfortunately, I don't set the budget, and Ares is just too expensive and too far out for us to accomplish our goals within the budget we were given.

If we halt the ISS, all versions of Ares, and transport Orion and Altair aboard DIRECTv3's Jupiter family of Shuttle-Derived Launch Vehicles, we just might make it back to the Moon by 2020.

O'Neill's solution to the inherent instability of cylinders was to attach two together side by side; they would be mechanically linked, and by adjusting that linkage he also proposed to keep the cylinders oriented towards the Sun.

In theory you could link a large number of cylinders together in this way, and this would make the whole array stable- but if you make the structure too big you have to start to consider the total mass. Self gravitation would start to cause a very massive array of habitats to collapse.

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For maximising living space the sphere is probably the least efficient design; it only has a narrow band of flat floor around the equator, and all the rest is at an angle.

J D Bernal recommended the use of a sphere because it is the strongest shape for a vessel under pressure - but since the pressure being contained is only one atmosphere, that is not really a major concern.

I'd recommend a thin section of a cylinder- a ring, with a square section, just like Arthur Clarke's space station in 2001. Maybe a bit fatter all round, so give a bit more living room.

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Folks

Thank you all for your information. It's been very useful, and given me ideas I hadn't had before.

Noclevername said:Something I'd completely missed.

Ah! That's part of the mystery the story explores.

Mugaliens asked:My apologies. A term I've picked up from Apollo. It was when the Passive Thermal Control rotation of the CSM got out of kilter, and the axis of rotation moved away from the spacecraft's centreline. I was imagining the first part of what you described earlier.

cjameshuff said:I was actually imagining the space station having no windows, so the inhabitants wouldn't be able to see out without using the station's sensory system. I was figuring on using a combination of solar and nuclear power to generate light for the space station in the form of a thick beam running along the axis of rotation. Only a part of the beam would be lit at any point, so that as the light moves along the beam, it mimics the motion of the Sun across the sky (another reason for using a cylinder instead of a torus).

eburacum45 said:Very interesting. Something which allows for a very interesting plot development.

No, the rest can be terraced, just like a mountainside. Even if you limit yourself to only the area where the slope is less than 30 degrees, it's still about twice as efficient as a cylinder.

If you want, you can simply use a single "flat" floor within a sphere, and leave the space underneath the floor empty. This would still be more efficient than a cylinder without the outer spherical shell.
For a single floor "natural" environment big enough to mostly provide its own radiation shielding, atmospheric pressure is the main concern. It amounts to 10 tons of pressure per square meter.

It's only when you build lots of floors into the design that artificial gravity structural strength begins to dominate. Assuming you use "natural" vegetation to supply food, this requires external greenhouses to feed all of the people. O'Neill's colony proposals included many floors and external greenhouse units; the picturesque park-like interior is only the "top" floor.

Perhaps you missed it because it's wrong. It doesn't matter how much or how little air/water is in the habitat. It will never slow down the rotation. Conservation of angular momentum ensures that the rotation rate won't slow down. Ever.

Unless, of course, something is involved which invalidates conservation of angular momentum. Tidal interaction with a nearby planet would be an important example. If your station is in orbit around a planet, tidal interaction will eventually cause the rotation period to equalize with the orbital period.

Eventually. The timescale for this to happen may be decades. And the cylinder may in fact be passively stable, with a mass distribution rather similar to that of a torus. The radius of gyration around its axis of a hollow cylinder of negligible cap mass is simply its radius, the radius of gyration around the perpendiculars is sqrt((6*r^2 + h^2)/12). If r is 1, and h is 2, the cylinder is as long as it is wide and the radius of gyration along the perpendiculars is 0.91...it's passively stable, though barely, and this is for a simple rotating cylinder without any additional stabilizing measures. A cylinder with a length half its diameter would be considerably more stable, a cylinder with a length 1.5 times its diameter would not be extremely unstable. Cap mass won't really be zero, but it will be considerably less than the "floor" mass.

And as I mentioned, it could be further stabilized with an outer stationary or counterrotating hull, or with equatorial tethers. They needn't be rigid, as you claimed in #8, a fully flexible equatorial tether will exert a restoring force as soon as the axis of rotation starts to change. A looped tether, connected at both ends and with a counterweight that can move or transfer mass to either side, could be another mechanism for active control, and might have benefits for tether traffic and damping of tether motions as well.

My windows weren't for an outside view. There would presumably be specific areas with such views, but it's not necessary for the interior of the main habitat. However, conversion of sunlight to electrical power is inefficient, and conversion of electrical power to high intensity light is difficult to do with high efficiency while maintaining good spectral characteristics and operating lifetime. Not saying electrical lighting wouldn't be used, but for the greenhouses and main areas, light pipes and diffusers seem like a far better arrangement in the long run.

In terms of volume contained. There's some point where the slope is too high to use as "ground", and if you replace the sphere section caps with flat cylinder caps, you at least have less cap area. If some support is from cables going the length of a stubby cylindrical habitat, it may well use less material. It may also be worthwhile to invert them, reducing internal volume and making them domes under compression from internal pressure.


Like I said, electric motors. Just have to maintain the proper momentum balance between the various components of the habitat.

Soil would not be the the best medium for plant growth in space, as you so rightly
point out. The nutrients in soil could be placed in an aquatic solution, reinforced by
human waste.......
All the misplaced who ha, about gravity. Gravity is not a force we cannot control.
We lack the understanding is all. Consider the gyroscope, and grasp the concept.
Artificial gravity is but a decade away if the right minds use their potential.
Nokton

What matters isn't cap area, but cap mass. With rounded caps, you minimize mass because much less thickness is required to contain atmospheric pressure. This is true whether the central zone of the habitat is cylindrical or spherical. You want rounded caps either way.

Anyway, if you want to avoid terraces, then you could simply have a single cylindrical "floor" within a spherical pressure hull. It's still requires less mass than a pure cylindrical habitat. This is assuming you want a "single floor" habitat. When you get into massively multi-floor habitats, artificial gravity loading may dominate over atmospheric pressure considerations.
Support from internal cables is about half as efficient than support from the outer shell. This is a reason why balloons don't include "extra" support from internal cables.

Balloons are trying to maximize volume for mass. Volume is useful for habitats, but is not the primary goal as it is for balloons...buoyancy isn't an issue. The overhead of those support cables would scale with their length, and so can not be a fixed factor as you claim: shorter cylinders would have shorter support cables and lower material requirements. Inward-bowing sides would have even shorter interior support cables, though the pressure hull area begins to rise again.

At some point, the material requirements for a given "land area" could easily be lower than those for a sphere, despite the lower enclosed volume.

The amount of volume "held in" by cables also scales with their length. You can decompose the functionality of a cylinder pressure hull into two basic components--a cylindrical tube which only supports "sideways" pressure and two endcap pistons held together by an internal cable. The internal cable only supports "endways" pressure. The mass of this cable scales with pressure x cap area x length = pressure x volume.

Anyway, the mass savings you're looking for simply aren't there. Theoretically, you could save on air mass by going with a torus instead of a sphere (the donut hole is sort of like a "thick" internal cable), but generally a torus ends up weighing more for a given usable living area than a sphere.

Hi Mugs, Arthur C Clark only invented the comunications satelite.

When was the idea conceived to use orbiting satellites to aid comunications...No. Artur C Clark did not 'Invent' that. It was very early in the cold war when Russian and American Military interests realized the value of a space program and the communication satellites was part of that top secret behind the curtain stuff... Do a search for your self. I did.

and hands up any one who thinks we will conquer gravity and have artificial gravity for space craft ?
A healthy understanding of gyroscopes and gravities relationship to mass leads me to conclude NO. Thats not going to happen. From that epic movie '2001 a Space Idiocy' came the concept of the large revolving space station... or did it ?
I have little doubt that A C Clark was an important contributor to our understanding of space and science fiction was enriched by him... BUT he had some unfortunate involvement with young people that has soiled his legend for some...

Did he write ;" I would like to know, but feel anxiety and fear of ridaquel... for asking.
To learn is a human instinct, To teaching is a human duty." ?
.

I'm a little concerned by the effect of coriolis forces on the atmosphere within; warm air would rise, and cool air fall, but they would each be deflected by the spin of the habitat. I'm not sure what this would mean for internal circulation of air, but potentially there could be strong winds at various altitudes at different times of 'day'.

Perhaps there should be internal baffles or even bulkheads to slow down or stop air from circulating too fast.

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Clarke is widely acknowledged in authoritative sources as the inventor of the idea of geostationary satellite communications.

His paper outlining this was published in 1945, before the Cold War. Of course he built on previous work, but he was the first to widely publish the idea in a complete form.

In honor of Clarke's contributions, geostationary orbits are sometimes called "Clarke orbits".