GURPS O'Neill Cylinder Design
 

The O'Neill Cylinder is a classic of space station design. The elegant cylindrical outer structure combined with the vast open, air filled surface of the internal structure really captures the imagination. It also helps that these are completely within real physics.

So, how to make one in GURPS Spaceships?

First, size and mass. The design for this Island 3 is cylinder 4 miles (6.4 km) wide and 20 miles (32 km) long, with hemispherical end caps giving it a volume of 234.6 mi^3 (960.81 km^3). It has a “flat” area of 16 mi x 12.6 mi (25.6 km x 20.1 km) and end caps with an area of 30.5 mi^2 (78.2 km^2), giving a total internal surface of 231.6 mi^2 (592.9 km^2). Estimates for the mass put it around 4,500 million tons.

This gives it a mass based SM between +21 and +22 and a volume based SM of between +25 and +26. I will be going with an SM of +21/25, since it makes some things easier down the line.

This touches on one of the tricky aspects of the O'Neill cylinder, it's tremendously low density. While most craft in GURPS Spaceships are roughly analogous to modern watercraft and aircraft, the O'Neill cylinder is more like a metal balloon, mostly air. The O'Neill cylinder is roughly 4 times as dense as air, for comparison water is 816 times as dense as air.

I decided to tackle this by calculating the mass of the air and subtracting a proportional number of systems. This gave a mass of air of 1,177 million tons or about 8 (150 million ton) systems.

Next, dirt to plant all those trees and grass on. If we presume 1 meter of dirt everywhere, we have 592.9 * 10^6 m^3 of dirt. Wet dirt (good for growing) is about half the density of rock at 1,250 kg/m^3, giving us a mass of 741 * 10^9 kg or 741 million tons, or 5 systems worth of mass.

Finally, we have the hull. And SM+21 system will be spread over a SM+25 area. Using the smaller systems rules from GURPS Spaceships 7 (page 4) and following the 1-3-10 progression the dDR will be divided by 100. This handy, because a SM+21 armor system has 10 times the DR of a SM+15 system So just take any SM+15 armor system of your choosing and divide the dDR by 10. Thin skinned, but you can always put a cylinder inside something else, like an asteroid, for extra protection. Some designs include a counter-rotating shell, which could very well be mostly armor systems.

Now, presuming we have 3 armor systems, we have 4 systems left for customizing! This habitat is so huge that a single habitat system has 20 million cabins, 10 million with total life support. Devote half those to amenities and you still have comfortable living space for 5 million plus people! This may sound cramped, but the green area and living areas are separate. Spread evenly, those 5 million people would each have a square 12 yards on each side of park all to themselves on top of their personal cabin.

This leaves us with 3 SM+21 systems left to customize however we please. Using smaller systems, the station can have a huge variety of functions.

Re: GURPS O'Neill Cylinder Design
 

That's great!
How fast can you set it rotating and what G can it attain for the habitat surface?

Re: GURPS O'Neill Cylinder Design
 

You might find some matter of interest in this old thread: O'Neill Cylinders. Also perhaps some in this one: Nanocomposite and structural materials.

Re: GURPS O'Neill Cylinder Design
 

I'm not a material engineer, but I am told that this design is at the limits (with safety margin) of what could be done with TL8 steel at 1 g internal gravity.

A rough and ready way to estimate maximum internal gravity would be to take any substitute material for the hull, find out how many times stronger than steel it's tensile strength is, and you have maximum internal gravity.

A side note, to avoid motion sickness in the general human population the RPM needs to be kept under 2. This may not apply to aliens or gene modded humans, but here's a handy calculator:
https://www.artificial-gravity.com/sw/SpinCalc/

Re: GURPS O'Neill Cylinder Design
 

A classic O'Neill Cylinder is going to mass around 10 billion metric tons when you account for atmosphere, habitation, structure, and shielding, so it will be a SM+22 structure by mass (volume does not really matter for Spaceships). With a 4 km radius, the RPM is 0.47 for 1g.

Re: GURPS O'Neill Cylinder Design
 

That sounds like a Gasbag system (SS7, pp. 9-10). Of course, such a thing would be useless in microgravity (or at SM +21), but it might give you a way to account for the difference.

Disappointingly, SS7 doesn't address what should be the immensely increased SM of a vessel using these systems.

Aren't those Open systems, by definition? Then you could use the Ecological Life Support design switch (Pyr 3/49, p. 24) to calculate carrying capacity.

In fact, that whole article ("Generation Ships," by David Pulver) would probably be helpful.

Re: GURPS O'Neill Cylinder Design
 

It's one of those situations where if you go by mass SM, you wind up with an order of magnitude too little internal space, but if you go by size SM you get into the rough ballpark but everything else is orders of magnitude too large. The Open Space system makes the most sense as "A regular deck with lots of potted plants." rather than what you'd see in an O'Neill cylinder.

Re: GURPS O'Neill Cylinder Design
 

An O'Neil is probably going to have six steel armor components, six habitat components, six open space components, and two hanger bay components. It might have other systems, though they will likely be too small to matter much (SM+16 or smaller).

Re: GURPS O'Neill Cylinder Design
 

In general you'll be fine using mass SM -- most of a cylinder isn't 'open space' its "non-usable volume".

Re: GURPS O'Neill Cylinder Design
 

Yes, there is no such thing as volume-based SM in Spaceships. It confuses people enough that SM in Spaceships is different from SM in Basic. I would rather not see you inventing a new kind of SM.

Re: GURPS O'Neill Cylinder Design
 

You should probably do something like this in your base design for radiation protection, especially from GCRs over decades-long exposure.

Re: GURPS O'Neill Cylinder Design
 

The shielding is included in the 10 billion metric ton figure. The shielding cannot rotate at the same rate as the shell anyway, it is usually made from slag and lacks the structural strength, so it must usually rotate independently. Even at 10 billion metric tons though, the density is 0.0062 grams per cubic centimeter, so it is mostly empty space (47 cm of steel for the shell).

Re: GURPS O'Neill Cylinder Design
 

That works out to ~3.6 t/m2. The 1977 NASA/Stanford study (NASA SP-413) recommended (p. 43-46) a minimum of 4.5 t/m2. If anything, our increasing understanding of radiation hazards in space since then suggests that this is too low, not too high.

Re: GURPS O'Neill Cylinder Design
 

Well, the thick atmosphere and the dirt would provide additional shielding. I would personally not mind tripling the mass to provide additional stuff, so SM+23 would not be unreasonable (30 billion metric tons). Anything less than SM+22 would be asking for trouble though.

Re: GURPS O'Neill Cylinder Design
 

Put a layer of factory farms under the main land surface. More production, upper land freed for parks, radiation shielding. Yeah you get some mutated crops or animals, but what could go wrong?

Re: GURPS O'Neill Cylinder Design
 

How are you getting light to your farms?

In general I would not recommend trying to emulate a classic O'Neill cylinder and instead design an O'Neill-inspired cylindrical habitat, there are some known issues with the classic O'Neill cylinder.

Re: GURPS O'Neill Cylinder Design
 

I have started using the term "Oberth cylinder" for more general sense, because a lot of people reasonably take "O'Neill" as implying a lot of the specifics of Island Three, such as there being two cylinders, using precession effects to keep sunlight coming in through windows, etc. After all, Island Three is what they read about if they try to look up "O'Neill Cylinder" in Wikipedia for background.

One thing that I recommend not trying to emulate about Island Three is the windows. I calculate that you have to make nearly full used of the strength of steel to support the structure against the centrifugal effect; I don't think it would be practical to either fit windows that bore the radial strain or to support ones that didn't. If you want natural light I think you have to pass it in through the end-caps. Otherwise use artificial light.

Re: GURPS O'Neill Cylinder Design
 

Of course, if you aren't using the windows, there's no good reason for the cylinder design in the first place, you're probably best off with a torus or multi-torus configuration, as that vastly reduces the amount of material strength you need to resist internal pressure.

Re: GURPS O'Neill Cylinder Design
 

It makes it clearer that you're building apartment complexes in space rather than whole suburbs though. That seems so much less revolutionary. Probably also that your stucture's inhabitants are temporary too. Neither are what the O'Neill people want.

Re: GURPS O'Neill Cylinder Design
 

One problem with the whole concept for fictional/gaming purposes is 'why'. That is, why would whoever is putting up the money build this immense flying parkland, when high-density urban is more cost efficient? It's possible to think of reasons, but they're likely to involve a vastly wealthier and more advanced society than most of the O'Neil enthusiasts envision.

The classic O'Neil concept of the habitats building power satellites to pay for themselves is of course nonsensical. Solar power sats probably don't make sense on their own terms, and if you're going to build them, you don't need O'Neil habs, you need the space equivalent of a line shack or an oil rig. For comparison, you don't build a chain of mansions on site if you're starting a mine in a remote region, you build efficient living modules or something along those lines, if the workers are lucky, something less if they're not. It's cheaper.

Re: GURPS O'Neill Cylinder Design
 

Solar power sats make sense if you can get launch costs below a certain threshold (which we are nowhere close to).

Re: GURPS O'Neill Cylinder Design
 

Usually the mechanism given is the by-product from rare-metal asteroid mining or production on the moon, both of which are evading the launch cost of earth's gravity well by never going down it at all.



Or occasionally some mega-structure like an space elevator or orbital ring.

Re: GURPS O'Neill Cylinder Design
 

I think the argument was that if you don't get out into the open air in the big outdoors sometimes it cause Bad Things to happen to your mental health. And that at scale, this would be cheaper than having to fly your workers down to Earth for a month twice a year or whatever.

Re: GURPS O'Neill Cylinder Design
 

SPS is easily achievable at a cost of ~$300,000/ton launch cost and the development of lunar mines. As long as ~99% of the materials are taken from the Moon, the economics are actually fairly good, as it ends up resulting in a wholesale electricity cost of ~$10/MW-h. It is actually much better than contemporary renewable sources when you subtract the tax credits and other subsidies, especially since you do not have to bother with the environmental consequences of mining rare earths.

Re: GURPS O'Neill Cylinder Design
 

How do you get the power down to Earth without making a weapon?

Re: GURPS O'Neill Cylinder Design
 

Low density over a larger area. Typical plans I've read about suggest about half the energy density of sunlight. It's not a focused gigawatt blast from space.

Re: GURPS O'Neill Cylinder Design
 

By designing your microwave transmitter without the ability to focus to dangerous levels. A typical geostationary orbit transmitter is something like a 1 kilometer transmitter and a ten kilometer rectenna, which won't be useful as a weapon unless your power is in the tens to hundreds of gigawatts (and the physics simply doesn't permit a transmitter of that side to focus to smaller than that area).

Re: GURPS O'Neill Cylinder Design
 

I mean, you probably can still use it as an economic weapon to disrupt your opponent's weather patterns, but that's a slow process that takes time to bear fruit.

Re: GURPS O'Neill Cylinder Design
 

After digging around a little to figure out the trade offs between diffuse space microwaves and land-based Solar, I'm still pretty skeptical about this approach, but it's fine for gaming.

Orbiting gigawatt blasts are better for gaming mind you.

Re: GURPS O'Neill Cylinder Design
 

The primary economic advantage of SPS is you need a 1/5th as much solar gathering capacity, plus you do not need the storage capacity for 4x the hourly production as on the surface. In essence, 50% of the time is night and, for the day, you average 50% production, as solar intensities waxes and wanes throughout the day, plus you have cloud cover and storage inefficiencies, meaning that a nation like the USA would need 140,000 square kilometers of solar panels and 80 million metric tons of lithium ion batteries. Since the materials for lithium ion batteries cost $7,500 per metric ton, the material costs alone for the batteries would be over $600 billion, and that it before prices increase due to increased demand.

Re: GURPS O'Neill Cylinder Design
 

A lot of futuristic settings are over populated and/or have expansive nature preserves. Either of those would make reduced land area required for power quite desirable.

Re: GURPS O'Neill Cylinder Design
 

I understand that you can used the land under a large microwave rectenna for other purposes, as the rectennas are rather open structures and block little rain or sunlight, and are light enough to suspend well above the ground.

Re: GURPS O'Neill Cylinder Design
 

... if you have enough power sats you can redirect them to all aim at the same place, can't you?

Re: GURPS O'Neill Cylinder Design
 

If individually you had powersats that could deliver energy in concentrations equal to half of sunlight (though in longer wavelengths) aiming 10 powesats at the same area would enable you to boil water 5x as fast as leaving it out in normal sunshine.

Giant death rays are actually quite hard to build.

Re: GURPS O'Neill Cylinder Design
 

It would actually be decently dangerous, but so is taking control of the power supply of a small country...

Re: GURPS O'Neill Cylinder Design
 

I remember a scene in one novel, where the protagonists are moving through a rectenna's 'shade' and it's being used as grazing for cows.

Re: GURPS O'Neill Cylinder Design
 

The best use is vegetable and fruit crops actually. The wire mesh can be placed 8' above the ground and they absorb ~100% of the microwaves, though only 80% gets turned into electricity (20% into waste heat). With the waste heat, you can grow vegetables and fruits outside of their normal growing season (while you could also do it with grains, grains are less profitable than vegetables and fruits).

Re: GURPS O'Neill Cylinder Design
 

Depending on the design, you might not be able to fit harvesters between the rectenna supports, and people might be unwilling to work under them, for fear of being 'nuked', no matter the truth of the matter. Also, you can run cattle on land that's unsuitable for market gardens (and sheep on even worse land).

The point is, you can get dual-use out of the land by farming it with whatever works best on that land as well as having the rectenna converting microwaves to power.

Re: GURPS O'Neill Cylinder Design
 

I have an unplaceable memory of a story in which there was a coffee plantation under a power rectenna, which with the rectenna itself concealed some sort of villainous facility from observation.

Re: GURPS O'Neill Cylinder Design
 

I would avoid uses that run the risk of maintenance issues for the rectenna structure. Depending on the opacity of the rectenna, I might put a conventional solar farm under it -- there's some loss of efficiency because of light blocked by the rectenna, but you make up for it with simplifying your power grid.

Re: GURPS O'Neill Cylinder Design
 

It's true that cow-proofing your rectenna array's supports would probably raise the cost a bit. Proofing it against idiots driving farm machinery too fast would cost even more.

Re: GURPS O'Neill Cylinder Design
 

Have the farm machinery driven by computers, perhaps?

Re: GURPS O'Neill Cylinder Design
 

If you can make that cheaper than minimum-wage workers, perhaps. It might just not be worth the expense, and it's just easier to use Anthony's solar array option, or run something relatively unlikely to hurt the recenna and cheap/low labour, like sheep.

Re: GURPS O'Neill Cylinder Design
 

I'm not sure that the minimum wage workers on farms are the ones driving farm equipment anymore.

Re: GURPS O'Neill Cylinder Design
 

That is also a good RPG plot.

For OP's purposes, having folks in orbit for powersat maintenance is probably sufficient to justify the habitat, but adding powersat security to that is good adventure fodder.

Nuke the burrito from orbit, it's the only way to be sure.

Re: GURPS O'Neill Cylinder Design
 

If you have SPS arrays, you will likely need lunar mines for the construction materials. Since they would use mass drivers to deliver materials into orbit, you would want substantial security, meaning a decent population on the Moon, which will likely need to go back to experience normal gravity after 6 months on the surface of the Moon. It is cheaper, easier, and safer to send them to an orbital habitat than to the surface of the Earth, so your space habitat would also support your lunar mines.

Re: GURPS O'Neill Cylinder Design
 

Or you could have asteroidal mines sending regular packets of refined material in large, slow-moving transfer orbits, to be "caught" locally and then used.

I'm willing to believe that environmental / artistic / traditionalist concerns could scotch most attempts at mining the near side of the Moon, and that scientific concerns (radio-antenna placement and stability, etc.) might be enough to hinder far-side exploitation.

Then you have to have security way out in the Belt, and you've still got rotating stations out there, but it is a different dynamic.

Re: GURPS O'Neill Cylinder Design
 

In RPG settings, you generally want more potential mayhem, not less, so I say include both, and that they are in cutthroat competition with each other.

Re: GURPS O'Neill Cylinder Design
 

Lunar mines end up being a lot cheaper than asteroid mines due to the ability to use mass drivers for transporting materials into orbit. The close proximity also helps to catch mistakes before too much damage can be done. Of course, asteroids have more easily accessible precious metals, so they would likely exist and would produce industrial metals as a byproduct.

Re: GURPS O'Neill Cylinder Design
 

My point was not an economic one but an aesthetic one.

Attempts at lunar mining are going to run into a buzzsaw of opposition from traditionalists who can't imagine tampering with the "Man in the Moon," from romantics who can't imagine blemishing the light associated with countless love songs and poems, and from plain old curmudgeons who don't want an age-old component of human nights altered.

In a future with generous space development, asteroidal resources could be relatively cheap, if you don't mind a long delay between when they're ordered and when they arrive. A multi-kiloton "package" equipped with a small, automatic mass driver could gently nudge itself into the inner system over a period of several months.

Re: GURPS O'Neill Cylinder Design
 

Well, 8 Flora would be the ideal location for such mines, as it is the closest of the large asteroids at 2.2 AU, meaning that it could supply the entire Inner System with industrial minerals. 20 TW of energy production would require 60,000 square kilometers of solar arrays, which would be 60 million metric tons of materials. At $10 per kilogram of industrial materials, the industrial materials would generate $600 billion in revenue during the 25 years of production.

Re: GURPS O'Neill Cylinder Design
 

Would it be, though?

To make SPS work at all requires substantial reduction in launch costs, even assuming Lunar materials. If you can get the launch costs down that much, I'm not sure the cost of rotating your crews is all that prohibitive.

Further, the classic O'Neil plan concept was that the 'colonists', for want of a better word, built SPS systems to provide power to Earth which pays for the construction and maintenance of the habs. The trouble (well, one of several troubles) with the idea is that the resources and time spend building new habs are resources and time not going into building powersats. From an investor POV, it almost certainly makes more sense to focus the full energy of the effort on the powersats directly.

(Which is a separate question from whether powersats make sense on their own terms.)


Realistically, the powersats probably don't compete with ground-based solar. Even the analyses that show they do tend to make a lot of highly optimistic assumptions that wouldn't apply in reality.

The advantage of space-based solar is 24 hour sunshine without atmospheric interference. But if you want 24 hour sunshine you need high orbits, LEO gets lots of interruptions. In fact, convenience wise you want geostationary orbit, but that's pretty high and makes power transmission more expensive. It also adds to the cost of servicing and construction and protection. SPS systems would be very vulnerable to attack, basically big kilometer-wide masses of solar cells or the equivalent, and they'd need to be insured, and maintenance costs are not firmly locked down because we don't have enough information to do that. There are also a whole mess of ownership and national jurisdiction issues that would arise.

Ground-based solar gets only half a day of power at a time, and the atmosphere soaks up a good chunk of the energy before it reaches your collector. But offsetting that is ease of construction, ease of maintenance, relative ease of protection, ease of access to the power grids, relative ease of protection, minimum of unknown technology required, etc.

An SPS system requires a very massive infrastructure to build and maintain. That same infrastructure can be used to built big power collection arrays on the ground. When you consider the amount of land needed for a rectenna array, you could use that same land for solar collectors and get a lot of power.

Whenever you see any suggestion that something done in space can be done for cost 'x', you should usually mentally triple it. Space is expensive, and the proposed ways around that (Lunar mines, for ex) have a tremendous amount of expensive engineering and R&D work still to be done before anything definite can be said about their cost.

That calculation, though, assumes solar vs. solar. In practice, you'd have to make the economic evaluation based on a mix of energy sources. Coal, oil, natural gas, hydro, wind, uranium/thorium, all compete with solar in space or on the ground, and a workable cost projection has to be based on the presence of them all, and it has to take into account the legal and political factors, not just the technical ones.

Re: GURPS O'Neill Cylinder Design
 

Nothing beats SPS cost for cost when you get the launch cost down to $150/pound to LEO and set up the lunar/asteroid mines. You are talking about $10/MW-h without the government subsidizes of nuclear, surface solar, or wind, and, unlike surface solar or wind, it is always available and never needs battery storage. Unlike coal, natural gas, or oil, it does not release greenhouse gases, so it does not cause climate change. On every level, it is the best energy technology until we figure out fusion, which would only be better if it did not depend on tritium or imported helium-3.

Re: GURPS O'Neill Cylinder Design
 

That's not quite true. In David Pulver's Alternate Spaceships article (I forget which issue it was; August 2011), there's an “Armor and Volume” option for adjusting a spacecraft's surface area (in the form of a dDR multiplier for its armor) and its Target SM based on a lot of its systems being high-density. Normally, only armor systems would count, and only high-density ones at that. But I could see, say, a Singularity Drive counting as multiple high-density systems for this purpose.

And bringing it back to the topic at hand, I could see Open Systems having the opposite effect, potentially forcing armor systems to cover a higher-than-normal surface area, reducing the effective dDR they provide and increasing the ship's Target DR. You'd need to extend the chart into the “negative Armor systems” range, with each Open Space counting as -x armor systems for the purpose of the overall effect on the spacecraft's surface area. I would design an O'Neill habitat using almost exclusively Open Spaces, taking other systems only when absolutely necessary.

There's also rules for Spin Gravity, though they assume that the craft has the gear to quickly spin up or down at will. It tells you have much gravity you can have at a given SM (which would be the Target SM as described above). If the craft is set into rotation by tugboat-like spacecraft instead of on-board gear, there's no need to pay for the Spin Gravity design feature.

Re: GURPS O'Neill Cylinder Design
 

In larger objects, the initial spin requires delta-v rather than gearing, so the cost is still applicable in realistic settings.

Re: GURPS O'Neill Cylinder Design
 

Thanks for the reminder on the Spin Gravity option, I'd forgotten about it.

Radiation was mentioned before, so I figured I would take a stab at it.
Normal background radiation is 0.15 to 0.35 rads per year, so lets use that as our goal.

Cosmic rays are 1 rad/week, or 52 rads per year and divide PF by 100. a PF of 20,000 gets us to our goal without special anti-cosmic ray shielding.

Solar flares from Spaceships 5 (page 40) come in a variety of flavors, with small ones up to once every 2 months and big ones every few years. Flares are also fairly non-penetrating, multiplying PF by 20. For the small ones, a total of 900 rads (at Earth orbit) would be reduced to 0.1 rads by a PF of 450. For the big ones, a 6000 rad flare would be reduced to 0.1 rads by a PF of 3000.

Now, the radiation protection table on Spaceships 5 page 41 starts following the standard GURPS log progression but then abruptly changes pace and I don't recognize the new progression. So sticking with the standard progression, the O'Neill cylinder in the OP would have a PF of 7,000 per system, giving the inhabitants 3 systems worth of protection in each system between hull armor, dirt and remaining customizable systems, a total of 21,000 PF.

Our intrepid space colonists need not worry about glowing in the dark from cosmic rays or solar flares.

It was mentioned that the science of space radiation has advanced, so maybe they still need to worry about glowing in the dark. I'm far from up to date on that topic.

Side note, I'm assuming the sudden change to progression on the protection factor table is not just a typo, but I don't think I've ever seen it anywhere else so I can't follow it past the edge of the table. If you've seen it elsewhere, let me know.

Re: GURPS O'Neill Cylinder Design
 

Woops, I forgot to factor in that the PF would be smeared out over a much larger area, just like the armor would. This reduces the PF of the inhabitants to 210. They are going to need a nice thick shell around their O'Neill cylinder, good anti-rad drugs or maybe a good artificial magnetic field to protect them.

But as far as a nice rocky shell goes, a SM +26 rock shell with the O'Neill cylinder as the "upper stage" spread out as 2 systems from each section rather than all in one section gives it 4 systems of 'free' rock as radiation shielding. This gives you 50,000 PF and 1500 dDR per system. You even have 2 core systems left, good place to subdivide into smaller systems for docking areas and other exposed necessities.

Re: GURPS O'Neill Cylinder Design
 

At SM+26 (which is 1 trillion tons), two armor systems per section gives a PF 51,200 (800 for SM+14, doubled for every +2 SM, for a total of six doublings), which gives over 1 million PF against solar flares and 512 PF against cosmic rays. That leaves you with 12 hull systems and 2 core systems and, if you gave the spaceship 4 habitat components and 4 open space components, you would have 24 billion cabins, allowing you to easily support 4 billion people in a single colony. Of course, this makes sense because a SM+26 O'Neill has a spin radius of 20 km and a length of 200 km.

Re: GURPS O'Neill Cylinder Design
 

I'm not quite sure I follow you, AlexanderHowl. This is a SM+21 station, inflated to SM+25 due to it having a density more like a beach ball than a warship, with a SM+26 shell of rock (or more likely a mix of rock, ice, and other useful raw materials.) around it.

Unless you are just pointing out that once the cylinders get large enough, the need for a rocky shell goes away due to sheer bulk.

Re: GURPS O'Neill Cylinder Design
 

Or you could have 12 billion luxury cabins, which would closer approximate a home environment, I believe. That would support 24 billion people on an SM+26 station.

Of course, you'd have to have some services which would lower the number of people (sick bays, offices, that sort of thing).

It's still a boatload of people, no pun intended.

Re: GURPS O'Neill Cylinder Design
 

Spaceships SM is by mass, not volume.

Re: GURPS O'Neill Cylinder Design
 

Because it makes certain assumptions about ships generally staying within a certain density range, which O'Neill cylinders completely break by having a radically low density.

That article is about making a ships SM smaller than it's mass would indicate by making it mostly high density armor. I'm doing the opposite, making a ships SM much larger than its mass would indicate by turning it into mostly air.

Re: GURPS O'Neill Cylinder Design
 

Even making your ship/station 1 SM larger means you have to treat your armor as if it were a smaller system for the new SM. That would affect not only your dDR but your PF as well and I really wouldn't recommned it as a design decision. The Spaceships books havs multiple stations that are done at a standard SM.

However, if you feel like you just _have_ to do this that 1 SM change divides your average density by 3. That 21 to 25 chcnge youam ppear to eb contmeplating divides average density by _100_ and that is probably muhc more than is needed for the effect you want.

It would make more sense to me to build this as SM+25 and then just put in lots of Open Space.

Re: GURPS O'Neill Cylinder Design
 

Open Spaces can be used in many ways. One such use is as terrain upon which to build domiciles, apartment buildings, factories, and so on. That's why I'm saying go with Open Spaces instead of Habitats, for example: just let the inhabitants live and work in the Open Spaces.

Re: GURPS O'Neill Cylinder Design
 

I already did this in the previous posts. And yes, a factor of 100 is absolutely required, an O'Neill cylinder has a density 4 or 5 times that of air.

Re: GURPS O'Neill Cylinder Design
 

It really makes more sense to keep to the mass-based SM, as it avoids confusion. If you are using Open Spaces for habitation, you probably need one area per person, so it probably does not work out. A SM+26 spacecraft ends up with 1 million areas per Open Space, so you could only support 8 million people (assuming eight Open Spaces). The problem is that you need 800 million technicians to maintain the Open Spaces, so you need $4 quadrillion in Total Automation ($500 million/inhabitant).