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This answer and this answer address the question Do reaction wheels generally run in vacuum, or are they pressurized? Sealed or vented? and they both mention that sometimes momentum wheels are sealed with a low pressure (inert) gas rather than vacuum.

edit: I found the following in Sinclair et al. SSC07-X-3, 21st Annual AIAA/USU Conference on Small Satellites:

Many small satellite wheels use a hermetic housing filled with a low-pressure gas. The internal atmosphere allows the use of bearing lubricants with modest vapour pressures as well as protecting parts from contamination or damage.

Gas would create additional friction for the system in addition to bearing friction (and possibly eddy currents), which means additional low power. It also means if power to the motor is not available for a period of time, angular moment would couple back to the spacecraft more quickly.

So there must be some benefit to the presence of gas in some applications.

This answer says:

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.

But my understanding is that evaporation is really more a function of the partial pressure of the material in question within the gas, not the total pressure of the ambient inert gas. I don't understand how the gas could inhibit evaporation.

So I don't understand why some reaction wheels would be sealed with a low pressure gas, and others with vacuum. What are the trade-offs and why is vacuum chosen in some cases and low pressure gas chosen in others?

The table below (open in new window for full size) is from Lubrication of Attitude Control Systems and was originally linked here. It's a helpful review of the subject, and the vapor pressures of various lubricants (some more vacuum-friendly than others) are listed near the bottom row. It's worth reading the PDF, it's a nice presentation of the issues.

The right hand side of the table is cut off in the original PDF.

enter image description here

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  • $\begingroup$ I suspect that evaporation may be more complex that what you have written here. In a high vacuum, escaping lubricant molecules likely fly off in straight lines until they impact the casing and then stick. Having the casing filled with gas at least means that "partial pressure" of the lubricant has a meaning. Another case maybe of continuum vs non continuum mechanics. $\endgroup$
    – Organic Marble
    Jul 2, 2018 at 23:34
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    $\begingroup$ @OrganicMarble sticking is temporary; if the molecule evaporated once, it will do it again, and again... so sticking doesn't mean permanently stuck. A low pressure buffer gas may change the time that the molecule spends before encountering a surface from milliseconds to seconds, but it will still encounter a surface fairly quickly and have the same opportunity to stick. So partial pressure is just as completely valid in either case, vacuum or not. Assuming it's all at the same temperature, the velocity at which it hits will also be similar. That said, see my message within the bounty banner. $\endgroup$
    – uhoh
    Jul 2, 2018 at 23:43
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    $\begingroup$ @uhoh This is indeed more complex than it may first appear. Lubricant is likely to be at a higher temperature "where it should be" than if it has been flung off onto the casing, so evaporation will be faster. Similarly, if the lubricant actually evaporates, then it will (mainly) stay within the gas, and if that gas reaches saturation (think of the dew-point with water) then it will impede further evaporation. $\endgroup$
    – MikeB
    Apr 13, 2021 at 15:44
  • $\begingroup$ @MikeBrockington well, saturation happens the same whether or not there is another gas present. Vapor pressure is vapor pressure, and to first order it doesn't matter much if it's a partial pressure (in the case of an atmosphere) or a total pressure (in the case of no additional atmosphere) Unless of course you're playing with Entonox :-) $\endgroup$
    – uhoh
    Apr 13, 2021 at 15:54

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I think you are right, the pressure of the gas does not inhibit the evaporation of lubricants. The same is true for air and water vapour. If we compress air from outdoors to 200 bar, a lot of water condenses, but the partial pressure of water is the same, it depends only on temperature, but not on ambient pressure. If the compressed air is expanded to 1 bar again, it is very dry. Not the rise of pressure lets the water condense, it is the loss of volume.

But if the lubricants are not sealed within the compartment of the reaction wheel, they would continue to evaporize into space until nothing is left in the bearings. But if the compartment is sealed with or without a gas inside, the evaporation of the lubricant will stop when the vapour pressure is reached within the compartment. The gas does not stop evaporation, but the closed compartment does.

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    $\begingroup$ Thanks - I am pretty sure the inert gas pressure does not prevent conventional evaporation. But my question is: "Why are some reaction wheels sealed with low pressure gas, others with vacuum?" There could be more complex transport mechanisms, or it could be for some thermal effect, I don't know. I would like to know why someone would put a low pressure gas around a wheel that's supposed to maintain a fixed spin speed for extended periods of time. Could drag be good for some reason? $\endgroup$
    – uhoh
    Nov 8, 2016 at 13:27
  • $\begingroup$ Cold welding may be a problem for some material combinations in the bearing. Some combinations may require a low pressure gas, others may be used in a vacuum. Heat transport from rotating parts within a vacuum by radiation only may cause bearings to overheat. Excess heat will evaporize the lubrication. $\endgroup$
    – Uwe
    Nov 8, 2016 at 14:17
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    $\begingroup$ OK thanks. It sounds like we're just speculating here. Let's wait for an authoritative engineering answer of precisely why gas is introduced into some sealed units, and others remain sealed under vacuum. Until then, @DavidHammen is still wrong :) $\endgroup$
    – uhoh
    Nov 8, 2016 at 14:50
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    $\begingroup$ @uhoh this question still feels relevant, has anyone had any further thoughts? $\endgroup$
    – Puffin
    Jan 20, 2019 at 16:37
  • $\begingroup$ @Puffin I'm still curious; I don't know what others have thought though. $\endgroup$
    – uhoh
    Jan 20, 2019 at 22:27

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