# Would it be possible to have an O'Neil cylinder used as a water “world” nature preserve?

Would it be possible to have an O'Neil cylinder large enough to house actual oceans and creatures like whales?

What might the energy requirements be to construct this?

• What do you mean with "what sort of energy"? – L.Dutch - Reinstate Monica Oct 17 '20 at 2:45
• How is that not off-topic as a real-world Question, thus barred by Worldbuilding SE? – Robbie Goodwin Oct 17 '20 at 21:42

Oceans? No. O'Neill cylinders max out around 1600 km3 (radius of 4 km, length of 32 km as per the original design). That is less than the volume of Lake Ontario, and almost certainly exceeds the structural limits.

A McKendree cylinder on the other hand has a theoretical maximum volume around 8,000,000,000 km3, almost six times the volume of all the oceans combined (1.35 billion km3). Assuming you only want an ocean contained, this should easily fall within the structural limits.

• O'Neill/McKendree cylinders assume a habitat that is mostly filled with air, not with a sea that's hundrets of meters deep (which would be required for whales to live in). The water would add an insane amount of mass to the cylinder, forcing a much smaller size. – cmaster - reinstate monica Oct 17 '20 at 23:44
• @cmaster-reinstatemonica Mass can be compensated for with a much slower spin (fish don't care about gravity), and sculpting the floor/hull to mimic changes in depth (most of the ocean floor is a desert). I'm sure this is within the capabilities of a McKendree. – rek Nov 17 '20 at 17:43
• My BOE calculation says an ocean 1/4 the volume of the Pacific would be a nearly 3x deep as the Pacific average, far more than needed to replicate an ocean if you work to minimize the 'deserts'. – rek Nov 17 '20 at 20:55

An existing O'Neill Station or a custom-built one?

• A spin gravity habitat puts stress on the hull. In a simple, non-spining station, the mass/inertia of objects is only noticeable if you want to move them relative to the station. In a spin habitat, centrifugal force pushes objects on the inner surface against the surface. Lots of water is heavy.
• Lifting material from the planet to orbit is expensive. Transporting material between different orbits is expensive. So a space station will be no stronger than required for the purpose, or you are wasting money. The O'Neill will be designed to hold a thin layer of soil, shallow lakes, and lots of air inside.
• But permanently inhabited space stations also need radiation shielding, which requires mass. Possibly the shielding material can be used as extra structural material.
• Whales are rather large animals. To house a genetically viable population (for a preserve and not a theme park) you have to produce awesome amounts of food. A single adult male sperm whale needs more than a ton of fish per day alone. Pods with females and young can be six to twenty animals.

Much easier to build an artifical whale preserve on Earth, and that is difficult enough.