| MadSci Network: Engineering |
As with so many design questions, there are a multitude of answers to your question. Some of the questions that would need to be answered for a definitive, closed answer for the thickness would include but not be limited to:
In general, the cylinder wall thickness should be minimized while still having sufficient ballistic impact resistance to stop the particles with energy levels at or below the level of the accepted risk. Thicker walls produce a greater force by increasing the mass without providing additional engineering benefit. (Remember, you accepted the level of risk with the thinner wall and are now adding mass, cost, stresses, time, etc.) Therefore a 100 m thick wall is likely a waste of material and money and may prove to be incapable of supporting the stresses induced by the rotation of the cylinder.
An analysis done for spacewalks shows that 99.67% of the objects in Low Earth Orbit are 0.1 to 1 cm in diameter. Moving out to the Lagrange points should increase the fraction even more since deep space has fewer large particles than near a planet. Given the experiences of the International Space Station, Mir, other space stations and various spacecraft, a wall thickness of a few centimeters is likely to produce a sufficiently low probability of a rupture from a collision to be acceptable. It may be necessary to increase the thickness to accommodate stresses from the atmospheric pressure and radial accelerations or to provide sufficient rigidity, but it is very unlikely that an O'Neill cylinder will need a shell more than 1 meter thick.
From what should that shell be made? This is again an engineering question with an indeterminate answer until some more parameters such as the radius of the cylinder and rotational velocity are defined. However, general principles point in one of two directions.
If the material for the cylinder is being brought to the location of the cylinder, even if it is assembled elsewhere, the mass should be minimized. That points to several possible types of materials with high strengths and low densities.
If the cylinder is being built in place it makes a lot of sense to use an asteroid at the site if one is available. That eliminates the need to move large masses to the site. Such in situ resource utilization (ISRU) cuts down dramatically on cost and logistics. It is one of the reasons why ISRU is being considered for future Moon and Mars missions by NASA. Asteroid compositions vary, but those near the Earth tend to be made from silicate rock while those further from the Sun tend to contain large amounts of carbon. Both have sufficient structural integrity that they can be made into an O'Neilll cylinder though the specific design will need to be customized to each asteroid.
The question of replacing windows with something more solid was raised. Glass and other ceramics are not only transparent, they are quite hard and strong. Flaws such as scratches and bending are usually the downfall of glass. Try to pull apart a piece of glass without any visible flaws. You should not be able to do so. However, try bending a piece of glass after using a glass cutter to score the surface. The flaw causes local stress concentrations and also acts as a crack that propagates quickly through the glass. This si why glass does not meet its theoretical strength of 1-2 million pounds per square inch. Still, properly designed glass or other transparent ceramics such as alumina and sapphire are just as good as metal and other materials, so the relative amounts of transparent and opaque areas should be driven by the amount of sunlight needed and not impact.
The Internet Encyclopedia of Science states that O'Neilll proposed space colonies "...consisting of an immense rotating aluminum cylinder..." Given the time frame of O'Neilll's proposal (1974), this is likely correct. An alloy such as 6061 aluminum would provide high strength with low density. Aluminum alloys had also been used on aircraft and spacecraft, so there was a flight history for the materials to bolster their usage for this application.
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