>>10948916One realistic approach would be to carve inside a trojan asteroid at Jupiter's L4 or L5 to use it as a structure and have a thousand drones working 24/7 3D print a layer of yet-to-be-invented void resistant material. Then, also use a ridiculous number of disposable ion thrusters to slowly build momentum.
To make this happens, you would need to
1/Have a space industry sustainable enough for sending men and material into space. That means completly mastering the cis-lunar region with a rich Lunar industry. Aluminium mines to build the biggest part of the rocket (tanks), Ice mines to create the heaviest component of a rocket (fuel) and industry advanced enough to build all the other electric component from redirected asteroid with REE. Maybe have a Moon based space elevator or slingshot to reduce the costs of fret.
2/Have a colonial outpost close enough of L4 and L5 to help the workers. You can't just send worker on a long term mission in deep space without their family and it would be too expansive to have recurrent Earth-L4-L5 trips. So, I suggest having the same settlement as the Moon but on Mars which is closer to L4 and L5 and maybe use Phobos as an operation center.
3/Have a fleet of drones using asteroid dust to 3D-print the habitacle layer inside the asteroid. And this will takes ages. You can justify the Moon Village as a miner outpost to secure redirected asteroids with REE. You can even push for a small Martian resort. And this will be what you have to solve to see your O'neill Cylinder.
Why build an O'neill Cylinder?
I have some fantasy justifications.
-You may want to secure the trojans asteroids. Because redirecting them is hazardous and a 100 quadrillions dollars asteroid worth of gold can be hijacked by space pirates on its way to the Moon.
-You are preparing for a mission to Jupiter and the Jovian system. There are plenty of reasons to go their, ressources of course. Chemicals and gas you can't find on Mars, water in abundance too.
