The first thing that comes to my mind is that you either have to commit to have the entire habitable portion of a station be rotating, or you have to have some kind of rotating joint that connects the rotating and non-rotating parts of the station. That joint has to not leak air and be very reliable (if it seized up, the station could tear itself apart from the sudden torque).
Having the whole habitable part of the station rotate may be fine most of the time, but it makes docking with ships more complicated. If the ship can't be spun, then you can either use some kind of rotating docking collar (which doesn't have to be perfectly airtight if it's only used once in awhile, but it still has to be pretty good) or you have put on a suit and do a spacewalk just to move things back and forth between the station and the ship which sounds kind of inconvenient.
(I guess there's actually another solution which is to stop the station spin whenever docking with a ship. That costs energy and/or reaction mass, though, and you'd have to deal with whatever disruption switching to zero-gravity brings.)
I can see why they might not have wanted to deal with this for ISS, but maybe for a bigger/more ambitious space habitat we'll want to do it.
Regarding your idea of stopping the station rotation: a simple way to do it would be to spin a flywheel contained inside the station. It wouldn't need to be very heavy if it spins very fast.
Another possibility is to have the docking station completely disconnected from the outer ring atmosphere, and to use small "elevators cabins" attached to robotic arms to go from the ships to the rings and vice-versa.
Speaking of "really huge" - Culture Orbitals are about the ideal: ~3,000,000 kilometres across, 1g at surface and rotation time of 24 hours so no need to stuff like the shadow squares of Ringworlds.
Sadly, they do rather require "magical" technology....
From a radius of ~300m onward you'll see green lights (= good for people) for all considered parameters. RPM drops from 1.7 to 0.5 for a radius of 3000m.
I've always been curious on this question because I don't have any physics background.
If you made something like a gravitron ride on the moon, would it take a slower rotation speed to reach perceived 1g than if you spun up a ring station in orbit? This calculator makes it seem like you could get pretty close to 1g with just a bullet train running on a 3.14 kilometer loop.
It seems like the main thing stopping earth trains from being faster is that most of our tracks were built a really long time ago and it's not worth the effort replacing them, but if metal is readily available and you're laying new track already, designing for ~300km/hr wouldn't be that much of a stretch no?
Yes, a little. In freefall, such as on orbit, you need 1 gravity of centripetal acceleration. a = v²/r; v = √(ar); if we assume a 3-meter radius (roughly Gravitron size) we need about 5.42 m/s tangential velocity to get one gee. The moon's gravitational acceleration is 1.62 m/s/s; to find the centripetal acceleration needed to get a Pythagorean sum of one gee, we take √((9.81 m/s/s)² - (1.62 m/s/s)²) = 9.67 m/s/s. That means that now our √(ar) tangential velocity is just 5.38 m/s, which is less than 5.42 m/s. Does that help?
The main thing stopping earth trains from being faster is politics, not engineering. Trains have been occasionally going over 300 km/h since 01955, decades before maglev. The Shanghai Maglev Train has been running at 430 km/hr since 02004. The Euroduplex regularly runs 320 km/hr on regular 1435mm standard-gauge rails and reached almost 575 km/hr in a test in 02007. 300 km/hr trains have been in regular service since 01989. There are several other train lines that run over 300 km/hr, in Taiwan, PRC, France, Belgium, Saudi Arabia, Japan, Germany, the UK, the Netherlands, Italy, Spain, Korea, and Switzerland. Soon India and the US will join them.
The big advantage of maglev is actually not smoothness or absolute speed but acceleration and deceleration.
4 rpm is about as fast as you can spin to avoid vertigo when turning your head. So for 1g that requires a diameter of 56 meters (about the size of the leaning tower of Pisa), which is big.
Other challenges include how to spin it up (and down) safely, how to dock with non-spinning things, how to deal with changes in mass distribution, and how to put thrusters on it for use when it's spinning. None of these is impossible, but together they create a serious engineering problem, and the size of the whole thing is ultimately the dealbreaker.
The ISS is 109m end to end so a 56 meter diameter isn't an impossible dream. I've toyed around with a design that uses basically a shell around Starship that would be bolted together in orbit to form the station. I was aiming for 2 RPM however.
Docking would be via a central hub. Ships would have to match rotation to dock, but it shouldn't be too hard. My conclusion is that if the money and/or political will were there we could start doing this today, but the project would be hugely expensive (even with SpaceX cutting launch costs to a fraction of what they were only a few short years ago) and once you have it built it will be looking for a purpose. It would be cool for people to basically commute up to the central part (via elevator) to do zero-g research stuff, then commute back to the ring to live and avoid the various health problems with long term zero-g living like bone density loss.
You can even build a simple starter station that has only two segments on opposite sides of the central hub. This is less cool since you don't get the jogging path around the station. If stability is an issue you could also include a computer controlled mobile counterweight on the ends. I also had the concept of building it as a double hull with a layer of water between the inner and outer to reduce radiation flux and absorb micrometeorite impacts.
But in the end you are still talking about a hugely expensive project that solves problems that aren't all that bad yet. About the only way I could see this being built is if Elon decides to go all in on space and liquidates his fortune to build it. The instant some annoying bean counters ask the question "is this the best way to spend this money" the project is dead.
