I don’t see the application of an O’Neil Cylinder within the Earth and Mars gravity wells given how expensive they would be to build next to better places to grow crops.
If one does get built, I would expect it in orbit around Jupiter or Saturn to support activity there.
One possible solution is a moon base. The moon is full of titanium and iron.
And then you could launch the stuff out of a weaker gravity well with no air resistance.
I don’t see the application of an O’Neil Cylinder within the Earth and Mars gravity wells given how expensive they would be to build next to better places to grow crops.
If one does get built, I would expect it in orbit around Jupiter or Saturn to support activity there.