[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.