# Zero G area on rotating wheel space station

If I needed a large area for zero-G training, is there any place to put it on a rotating wheel space station if the hub was needed for docking of spacecraft?

I wanted a place on the station that people could easily go to, hang out and relax, that would be zero G. I wanted it to be a place where they could look out a window to either side of the station. The symbolism of this place is important to the story

Assume the station radius is 150 meters and the rotation rate is 2.4 rotations per minute.

• What's wrong with a separate, non-rotatig station? – dot_Sp0T May 29 '19 at 6:03
• Why don't you just do it outside? Or in a box floating outside? Or in a cylinder rolling around the outer edge sufficiently quickly (a 15 meter radius cylinder that is counter-rotating vs the station at 24 rpm) that it's stationary in space? – Caius Jard May 29 '19 at 9:15
• I wanted a place on the station that people could easily go to, hang out and relax, that would be zero G. I wanted it to be a place where they could look out a window to either side of the station. The symbolism of this place is important to the story. – Bob516 May 29 '19 at 11:53
• @Bob516 that's an important detail; please do add it to your question (and thanks for accepting the answer on the other linked question, too ;-) ) – Starfish Prime May 29 '19 at 12:08
• Incidentally, your relatively high rotation rate (to give a luxurious 1 whole G) may result in problems by giving you larger coriolis forces. These are slightly awkward things to work out and depend on what the subject is doing, but it might be worth looking in to. – Starfish Prime May 29 '19 at 12:46

You have multiple options, you'll be pleased to know.

1. Use a train

The artificial gravity is a feature of your angular velocity as the station rotates. If you move in the direction of rotation, your angular velocity increases and you'll feel heavier. Move in the opposite direction and you'll feel lighter. If you put your training facility in a big train carriage and have a dedicated track, you can dial the artificial gravity up or down as you see fit. The track could easily be on the outside of the ring and the train could be reached via an airlock, so as not to take up useful internal space.

It has the disadvantage that it will take some time to spin up and slow down and (like normal trains) is likely to have a schedule, so you won't just be able to visit it at arbitrary times.

As so often with rotating stations, you'll probably want to keep the centre of mass of the train in the same place as the centre of mass of your station. Do this with a pair of carriages at opposing sides of the track or in the limit, a complete circuit of carriages that goes all the way around the ring.

Note that this technique could also be used to provide artificial gravity on a large non-rotating station (such as an asteroid) or to provided an artificial-gravity-to-zero-gravity transfer inside a large rotating volume like a cylindrical or spherical station.

2. Build a cylindrical or spherical station, or just a bigger hub

The hub experiences zero artificial gravity in the very centre because there's zero angular velocity there. If you imagine being inside a giant spinning structure and gently pushing yourself away from the very centre into open air and towards the rim, what will happen to force of the artificial gravity you experience?

Nothing. Until you come into contact with a rotating part of the station you'll continue to feel weightless because there's nothing else to apply any force to you (except possibly some air currents, but that'll be very gentle for a station of the size and rotational speed you're describing). This means you can have a large zero-g volume centred on the axis of rotation... in the limit, this means almost the entire interior space of a spherical or cylindrical habitat.

Your zero-g "hub" can therefore be as big as you like... to get into it, you take a lift into a low-g area or the zero-g centre of your station and then either use jet propulsion to take you out into the training volume, or have a contra-rotating safety cage (not unlike the train) that you can just grab on to. You can still have a dock in the middle; just use the space around it for your needs. Some of the designs of big cylindrical habitats had rope safety cages (which rotated with the station) around the axis for zero-g activities; certainly you don't want anyone to drift too far from the axis and risk making contact with a fast-moving part of the station.

3. Have a non-rotating section of your station

This duplicates my answer to another question of yours, regarding where to fit solar panels on a rotating station. The rotating part of the station can be attached to non-rotating superstructure where you can do all your useful zero-g work and more importantly, dock spaceships without having to faff around with a ridiculous rotating docking bay which is just asking for trouble (as any newbie Elite pilot will tell you).

The engineering of this is harder, as you need bearings and those need care and maintenance because their failure is likely to be a Very Bad Thing, but the tradeoff with easier and safer docking facilities and whatever other bits of zero-G infrastructure you need (training volumes, manufacturing facilities, etc) is likely to be well worth it.

4. Have a separate unspun training station

You're in space already, so you've got plenty of room to expand. Just build a box next door and take a shuttle over there when you need to play. This may be slight inconvenient (you don't want to put the stations right next to each other after all; that sounds like a safety headache) depending on how easily, cheaply and quickly you can run the shuttles.

5. Put the gravity ring inside a zero-g station

Build yourself a huge, air-filled cylinder (or sphere) in to which you put all your zero-g facilities and interfaces with space. Inside this cylinder, place your AG facilities... maybe a train, maybe a ring, maybe a centrifuge... you may please yourself. You can no longer land or launch on the moving part of the ring, but you can now do shirtsleeves maintenance on the bearings that interface the AG facilities with the ZG bits. There are still safety issues, and you'll get some interesting winds in the rimward parts of the zero-g section.

• Would a contra-rotating train effectively "spin down" the station over time? If so I'm sure it could be mitigated but just curious if it would become an issue. – ben May 29 '19 at 16:01
• @ben I'm honestly not sure... I think possibly, depending on the bearings, but the act of spinning the train up in the first place should give the station a bit of a kick in the right direction. – Starfish Prime May 29 '19 at 16:06

The hub itself could be zero-G, by being connected to the rotating parts via frictionless bearings.

Truly frictionless bearings are impossible, but magnetic bearings in a vacuum get close enough. You would use these magnetic bearings to slow the rotation of the hub while maintaining the rotation of the habitat areas. Then you could do your zero-G training in the hub. Any time anyone wanted to enter or leave the hub, you would have to spin it back up, again using the magnetic bearings.

• Yes, but the OP asked for an area different from the hub... – LSerni May 29 '19 at 6:13
• If you add in extensions to the ‘hub’ (for example two arms extending out to either side) this can provide any amount of space. If you really wanted you could build a whole non-rotating station around your rotating station without much in the way of issues, especially if you have gyroscopes/flywheels/rotation correction thrusters/multiple rotating rings. Basically this lets the ‘hub’ be more than the central spoke of the wheel. – Joe Bloggs May 29 '19 at 12:05

Make the zero g hub of the space station a very long cylinder. Ships will probably dock at one or both ends. But there can probably be space enough to put in a zero g lounge somewhere along the length of a cylindrical hub that could be, for example, 20 meters wide and 100 or 200 meters long, and which might contain one or two spherical sections 50 meters in diameter.

There might be some sort of zero g workshop or even a small zero g manufacturing space in the hub, and there could be a place for the workers to relax adjacent to the workplace in the hub.

Your space station will be orbiting some other body.

Once you reach the hub an move a bit forward or backward, you will be in free fall with no rotation, thus you will be experiencing micro gravity.

Only caution is to depart/return always from/to the axis of rotation: you don't want to make contact with a moving surface, or have a tangential velocity component while leaving the station.

The axis of rotation of your space station extends indefinitely in both directions. No matter how far your hub extends along that axis, it will continue to be in microgravity. (At least until you start running into new and strange engineering problems along the lines of "my space station is trying to round itself under its own gravity".) If you need more space in the hub for zero-G operations, just build more.

• The problem with this is that the area within the hub that's truly zero-G is infinitesimally small. – Ryan_L May 29 '19 at 5:10
• @Ryan_L That's true. You'll have to arrive at a level that's "close enough" - is 1% gravity enough? 0.1%? - and then you can determine how far away from the axis you'll feel that amount of force. The bigger and slower the ring, the larger an area will feel microgravity and the more gradual the gradient of force will be across it. – Cadence May 29 '19 at 5:53
• @Ryan_L, not necessarily, if you had a station that has a 50 metre trunk, think an hourglass shape, that does not rotate, and then rotating areas are strapped to the central thinnest part then you have plenty of zero-g area available to you on each end, if you had mutilple different rotating rings, spinning in opposite directions that should keep the trunk from spinning by itself – Blade Wraith May 29 '19 at 6:41