Why has no space station or near-future space station use centrifugal force? [duplicate]

Since centrifugal force can be used to replicate gravity using the equivalence principle, why has no space station actually utilised this? When space station staff have to endure long periods of free-fall that have detrimental health effects there should be significant reasons to consider such development. I have seen some concern about Coriolis force producing nausea or other unpleasant side-effects. Is this the primary concern, and if so, can it be countered?

• @RussellBorogove yes. Answer seems to be "size" Apr 30, 2021 at 17:27

This question brings up an area of active research and much discussion in professional circles. There is a lot of research yet to be done, mostly because as yet, nobody has been willing to pony up for the cost of actually doing experiments at true reduced-g levels. This Wikipedia article goes into a lot of the issues.

There is an unavoidable link among centrifugal acceleration, size, and rotation rate:

$$a=\omega^2r$$

where a is the magnitude of the centrifugal acceleration, $$\omega$$ is the rotation rate in radians per second, and r is the rotation radius.

With no other constraints, producing a given level of acceleration (often referred to as artificial gravity, "AG") could be easy and inexpensive: make it small and whirl it like a dervish! But as you mention, human physiology comes into play. At ~1.5 RPM (1 RPM = ~0.1047 radians/s) and slower very few people have problems with nausea. But as the rotation rate increases an increasing proportion of the population has such problems. Training can reduce peoples' sensitivity, but if you consider space tourists who might not want to undergo weeks of training and centrifuge desensitization, you're stuck with relatively slow rotation rates. In that case, the equation says that for a non-trivial level of acceleration, r must be relatively large. For a von Braun-style toroidal station, large r means large mass, and in the past large mass meant high launch costs and expensive in-space operations for assembly.

Some groups have proposed non-toroidal approaches but none have been successfully implemented at significant levels of AG. Rotating a two-element spacecraft with the elements connected by long cables would indeed provide AG, but also might have trouble with the added degrees of freedom of flexible dynamic systems.

The high launch costs are now being attacked by such entities as SpaceX and Blue Origin. May they be successful! A two-order-of-magnitude decrease in launch costs will completely change the landscape of AG research and implementation. Banking on those decreases, Orbital Assembly Corp. is proposing an initial experimental facility, the "Gravity Ring", to begin such research and tests of implementation methods in the next 2-3 years, followed by the "Voyager Space Station", a fully habitable rotating station producing lunar-g levels of AG, within this decade (given the capital infusion needed to support such a large project).

Disclaimer: I am closely associated with Orbital Assembly Corp.