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One of the ISS' main uses is to provide a long duration microgravity environment for a wide variety of experiments. For some experiments residual acceleration needs to be much smaller than average.

This answer to How much electrical energy is required to dissipate heat? states:

What costs energy in a cooling system is moving fluid around. In the case of the ISS, it's around 7.5tons per hour.

That's one figure for one particular fluid system.

Fluid systems are loops, and while they could be routed like power DC power aboard a spacecraft with "supply" and "return" lines always immediately adjacent, they could also potentially be routed in very large space-station-sized loops.

If so, the mass flow would have an angular momentum that could potentially be significant in some microgravity acceleration budget scenarios.

I'm thinking of the angular momentum in the tape recorder in Voyager for example.

Question: Did or do space station engineers need to worry about angular momentum stored in circulating fluids? Was anything done during the design phase to address it, or does it show up in some issues during operation?

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    $\begingroup$ They normally don't turn on and off. $\endgroup$ – Organic Marble Apr 14 at 1:35
  • $\begingroup$ @OrganicMarble except when they do (e.g. maintenance) but your point is already the start of a good answer if one of the reasons they don't turn off is torque! So I gather for example when external ammonia lines are stopped for repair, the internal water lines keep flowing. It seems intuitive that turning them off would be bad as it depressurizes them (providing an opportunity for bad stuff) but it would be cool if a resulting torque disturbing microgravity were on some list somewhere as well. $\endgroup$ – uhoh Apr 14 at 1:50
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    $\begingroup$ I just remembered one of the shuttle water loops cycled on and off too, so it does happen. But the shuttle was a lot bigger than a probe. $\endgroup$ – Organic Marble Apr 14 at 1:59
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    $\begingroup$ also relevant is routing; if "'supply' and 'return' lines (are) always immediately adjacent" as opposed to one giant circum-spacecraft loop, then there's essentially no torque to worry about. $\endgroup$ – uhoh Apr 14 at 2:01
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    $\begingroup$ @qqjkztd this is angular momentum, the number describes a mass flow rate. We don't have enough information yet to calculate the torque on the ISS if the circulating fluid suddenly stops, but comparison to an impulse is probably not the right way to address the problem. $\endgroup$ – uhoh Apr 15 at 23:29

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