Concept seems easy enough & the buildout seems doable, though lots of potential issues to account for. Though it does seem like having some sort of gravity would seem to be part of a healthy crew as we look towards longer missions. Are we anywhere close to trying this?
The main difficulty is having a large enough radius to avoid disorienting effects from high rotation rates and differences in pseudogravity force between foot level and head level. I’ve done a quick search and found many different conclusions about the necessary minimum radius, from about 20 meters to several hundred. I suspect that the latter is more realistic if we’re talking long-term occupancy. Granted, that wouldn’t necessarily require a large structure, just two small ones and a tether.
On a small scale (one that would not be viable for long term human habitation), a module for the ISS was in the proposal stage:
This would be primarily for scientific testing and possibly for sleep but too small for full time habitation.
For SF style habitats and long term effective use, the engineering challenges are steep.
It would be trivial to do.
You don’t have to build a ring, or cylinder. You just need a long cable and a counterweight. Put, say, the crew area in one ball, the engine in the other (because you won’t need them for many months) and tether them together and spin it. Without the limitations of ring size * you could set it to any gravity you want by changing the rotational speed and/or length of cable.
Yes, you have to worry about the cable breaking and flinging you out into the Void with no hope of rescue, but, then every crewed interplanetary trip has that risk, from many things.
I may be missing something technical, but I can’t see why it wouldn’t work, and I wouldn’t have a crewed Mars mission without artificial gravity.
*In 2001, the crew would be incapacitated with motion sickness in the too-small centrifuge. Made for a good visual, but wouldn’t work.
I would guess that the reason it hasn’t been done yet is because we spend millions of dollars to go into space to escape gravity. Not really a compelling reason to bring gravity up there with us at the present time.
It does create a single point of failure, that will be under constant tension. Cable breaks, and there is no way to recover, you are just all dead.
We do know that microgravity is bad for you, and we should not send astronauts to Mars feeling zero-g the whole way. What we don’t know is how much gravity we need to counter the problems. Even just 10% would make a lot of things much easier, and may slow the physiological damage one gets from being weightless.
Building a ship that can provide 10% Earth gravity is a whole lot easier than one that provides 100%.
We probably need higher for long duration, but for the trip out there, 10% may be enough to prevent serious health effects.
One advantage of setting up on the Moon for a few years is that it’ll give us hard data on the long-term effects of one-sixth gravity; if that’s enough to avoid the deleterious effects of long-term zero gravity, the problem is greatly simplified.
True, but there are a lot of single point failures in a two year Mars mission.
If it was 10% gravity, the rotation might be slow enough that, should the cable break, the drive section can be fired up and chase down the crew module. Worse case, if the damage is irreparable, they have to go zero-g the rest of the way.
But you just give them two cables! 
Zero-g the rest of the way to nowhere - with high likelihood probably this won’t be Mars (or whatever destination) anytime soon given they were spinning about the common center of mass. Firing up the drive section to chase them down also probably wouldn’t help unless it had a tremendous delta-v and vast quantities of backup fuel.
Think spinning a yo-yo around over your head while you are walking and suddenly the string snaps. The yo-yo probably isn’t going to end up flying in the direction you were going.
Basically, it’s not insurmountable, but the engineering challenges aren’t exactly simple for a tethered system.
There are some considerations that add to the complications, namely, any in-voyage maintenance or repair requires a more complicated EVA, making any orbital adjustments (not exactly uncommon on interplanetary or interstellar missions) are more complicated for a tethered system, any docking (at either end or in the middle) requires stopping the rotation entirely, if you need to do more than just visit the other module, you need a fairly complex system to transfer material between them while rotation is going on (or need a system to stop/start rotation frequently adding to the challenge), and the tethering itself still needs to be worked out.
On the last point, there have been several space missions using tethers, some very successful, some not. While not insurmountable, any manned mission would still need a lot of engineering work. Far from ‘simple’.
You know, there are a lot of problems with my idea…but I specified the drive section would go get them. The velocity at the end of the tether is not going to be a large percentage of the total speed on the way to Mars. It could easily be corrected.
My quick calculations say a four hundred foot cable spinning at 2 rpm gives a quarter g at the two modules, with a velocity of about 40 ft/sec. Hardly nothing compared to 24000mph.
For redundancy, we could have both modules crewed.
Yeah, and they should bring along centrifuges for mice that can create a 38% gravity like on Mars.
There are no studies on the effects of anything but 1G or 0G, nothing in between. I speculate that somewhere between 10 and 20% will allow people to stay mostly healthy for moderate durations, months but not years, long enough to get to Mars, maybe. If Mars gravity is enough for indefinite duration without health effects, that’s great, but…
Reproductive systems may be less tolerant to low gravity, and the first test subjects shouldn’t be human.
Closer to 0.5g from what I’m seeing and ~80 ft/s (meaning the other module has to do 160 ft/s plus additional corrections after catching up).
Back of the envelope calculation but for that amount thrust, that’s about 50% more than a de-orbit burn of the Space Shuttle, i.e. if these are roughly the size of the Shuttle and have something roughly equivalent to the maneuver system on that, it’s a bit heavier thrust than available there.
So, no, as I noted, not insurmountable, but the overall system is not as trivial as stated and there are very real engineering challenges that make the concept something that we could not simply mock up in short order.
I got the same numbers, then realized I was using 400 ft as radius, rather than diameter.
But 40ft/s of delta V isn’t anything to sneeze at either.
I’ve always thought that they best way would be for the module to have enough onboard propellant to self rescue, especially if you have craft with more than just one living module.
The movie STOWAWAY (2021) starring Anna Kendrick used this type of system for gravity. And, of course, became a major plot point.
Though the thing that I found interesting about that movie is that it was the first time I’ve seen cycler orbits used in a fiction setting.
That’s a permanent slingshot orbit between two planets, yes? They should do this between the Earth and the Moon.
More or less, though not all (or most) cycler orbits take advantage of a gravity assist. It’s just an orbit that passes near the Earth and then near Mars and then repeats.
The main benefit is that you can have a much larger station for the trip, something that would be able to use spin gravity, and you just use much smaller craft for delivering people and cargo.
That’s kinda the idea with the Lunar Gateway. It doesn’t really come down all that close to the Earth, but it is easier to get to at the far end of its orbit.
I don’t think we really have any idea how it would work out, since it’s never been tried. The brain can adapt to a lot. It may be that it’s just fine after a few days of adaptation, and that nausea, etc. can be avoided just by ramping up the rotation rate slowly.
Or it may be completely insurmountable. Impossible to say until we try it.
Let’s do it. Weekend project. I’ll provide snacks and beverages. Anybody have a Heavy Lift Rocket?