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Is there some general trend for the operating pressure of reaction wheels? I can imagine low to no pressure has obvious advantages of low to no drag (more drag requiring more power, beyond that necessary to compensate for bearing friction), but sometimes lubricants, cooling, and electrical issues benefit from some atmosphere. It can help reduce migration of oil, help cool things, etc.

A poor analogy is a spinning hard disk drive. They usually require at least a half an atmosphere to maintaining proper head gap (the ALMA correlator (also All Systems Go for Highest Altitude Supercomputer) has no magnetic hard drives because of the altitude) so those would required sealed pressurization to work in low pressure or vacuum. Obviously reaction wheels have different constraints.

So I am wondering, do reaction wheels generally run in vacuum, or are they pressurized? Are they generally sealed or vented? I'm asking based on this comment.

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I've found several examples of reaction wheels that are designed to run in a vacuum.

As reaction wheels are generally enclosed in a vacuum enclosure, they are under stress at ambient pressure and this stress will affect the microvibration behaviour. It was therefore decided to make the new facility also compatible with testing equipments under light vacuum conditions.

Other reaction wheels run inside a pressurized enclosure. Gas pressure is usually in the region of 0.01 bar.

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  • $\begingroup$ OK I would say this is pretty darn clear, thank you! I love to look at nicely machined momentum wheels, they are really beautiful! $\endgroup$ – uhoh Oct 31 '16 at 12:20
  • $\begingroup$ This answer is incorrect; it implies that all reaction wheels are unenclosed. This is not the case. Only a small portion of them are unenclosed. Most reaction wheels are hermitically sealed and contain a low pressure inert gas. $\endgroup$ – David Hammen Oct 31 '16 at 15:13
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Most reaction wheel assemblies use ball bearings between the rotor and housing, with some kind of lubricant, typically liquid, coating the ball bearings. There's a problem with liquid lubricants: They evaporate in vacuum. For this reason, most reaction wheel assemblies are hermitically sealed with a low pressure inert gas inside the container.

Alternatives to the use of liquid lubricants are using solid lubricants or forgoing the use of ball bearings entirely, using magnetic suspension instead. Both can operate in vacuum, but both have disadvantages. Friction is greater with solid lubricants compared to liquid lubricants, and the lack of a housing creates contamination issues. System complexity is considerably greater for magnetically suspended reaction wheels than those that use ball bearings.

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  • $\begingroup$ Thanks for this - I had a feeling there was a trade-off here. I'm not sure it's exactly evaporation - I thought evaporation was governed mostly by the partial pressure of the liquid while boiling is governed by absolute pressure. Maybe it has to do with transport as well? $\endgroup$ – uhoh Oct 31 '16 at 15:14
  • $\begingroup$ I've asked this follow up question. $\endgroup$ – uhoh Nov 3 '16 at 3:25
  • $\begingroup$ This answer is wrong. A low pressure inert gas can not stop the evaporation of a liquid. This is governed by the partial pressure of the liquid within the gas, not the existence of the gas. It may have some small effect, or an indirect effect, but the mechanism needs to be explained. $\endgroup$ – uhoh Nov 4 '16 at 13:19

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