GURPS O'Neill Cylinder Design

[Tyneras]

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.

[Donny Brook]

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

[Agemegos]

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

[Tyneras]

Originally Posted by Donny Brook

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

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/

[AlexanderHowl]

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.

[Fred Brackin]

Originally Posted by AlexanderHowl

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).

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.

[dataweaver]

Originally Posted by Fred Brackin

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.

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.

[thrash]

Originally Posted by Tyneras

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.

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.

Next, dirt to plant all those trees and grass on.

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.

[Tyneras]

Originally Posted by thrash

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.

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.

[AlexanderHowl]

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).