# Practicalities of docking with a large spinning space station

We've all seen countless examples of spinning habitats in fiction. However there are certain practicalities with these space stations that I can't get my head around, namely around how you'd dock with one in the first place.

I've done a cursory search and found this question, which partially answers mine, though I there are some further details that concern me.

First, we're assuming these structures are large, and are designed for habitating a large population of people for long periods of time, if not permanently. There are two scenarios I can think of that vary with the construction of the space station.

The entire station is monolithic and is rotating: Any craft trying to dock must somehow match velocities with a specific part of this station. The further you go from the centre, the harder this would be. Even at the centre, the docking craft would still have to "interstellar it" and match rotations. For stations with a larger radius, a lower rotation rate is needed to maintain ~1 G, and thus a lower rotation rate is needed for the docking spacecraft to dock.

Furthermore, the docking port has to be aligned with the centre of mass for both the station and the craft. This restricts the possible number of docking ports on the station to two, one on each side, and also restricts the design of the craft somewhat (If your craft is rotationally symmetric, like crafts tend to be, you'll have engines on one side, which means your docking port will necessarily have to be on the nose of your craft).

The station is composed of a rotating and a non-rotating section: With a sizeable non-rotating section, you have no limit to the quantity or design of the craft you can dock with. However the interface between the rotating and stationary parts presents a problem. First, no matter how well you grease it, there will always be friction that will gradually equalise the rotation rate between the two halves. I don't know if this can be rectified by the occasional spin-up of the RCS system or whether it's an insurmountable problem.

Also, there will need to be a pressurised interior path between the two halves of the station to let people and cargo transit between them. I suspect the engineering involved with having a rotating seal that is pressurised on one side, would be formidable to say the least.

Practically, how might these issues surrounding docking (matching rotations, design of monolithic rotating station,friction between parts of a 2-part station) be addressed?

• I can see the friction between a non rotating section and the rotating section being overcome with a small motor constantly adding the necessary torque to overcome any friction at the joint. In addition if we can create seals on high speed rotating shafts I'm certain we could make a rotating seal on a large diameter. Oct 9, 2018 at 7:27
• In the film 2001 the first scenario is shown beautifully. Oct 9, 2018 at 12:27
• The part in the center could be made not to spin.
– Muze
Apr 17, 2019 at 21:37