With Liquid Oxygen being used as an Oxidizer in Starship and some other rocket engines, I was curious if LOX was also used for propelling satellites as well.
2 Answers
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In short: no.
LOX, or any cryogenic fuel will slowly warm up without active cooling, resulting in boil off. Since satellites normally want long duration station keeping, this is non-ideal. As such, hydrazine is common, and more recently electric propulsion has become more common, since the propellant for both are storable long-term. This document has information on the state-of-the-art on satellite in-space propulsion.
There are plans to do orbital propellant depots in the Starship program, and also plans Zero-Boiloff (ZBO) storage for Blue Moon but for satellites, it’s far more practical to use the known-reliable options.
There have been cryogenics used on satellites before, such as COBE (4 year mission) which used liquid helium, and JWST, though the latter is a closed system. COBE used multilayer insulating blankets to manage boiloff, but also started with a 650 liter tank.
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Although no satellites have been deployed using LOX for satellite propulsion, there is no fundamental (as opposed to practical) reason why it could not be done.
LOX will only boil off until it reaches its vapor pressure (at that temperature). For instance, at 90K it will equilibrate at 1 atm (red line in the graph). At 120K it will equilibrate at 10 atm (green line in the graph). Etc.
Passive cooling of the JWST mirror by the JWST sun shield brought the mirror down to 40K https://blogs.nasa.gov/webb/2022/02/10/webb-is-chilling-out/#:~:text=We%20expect%20that%20the%20primary,or%20%2D233%20degrees%20Celsius
The vapor pressure of oxygen (O2) at 90K is 1.0 bar.
So if a spacecraft were equipped with a JWST-style sun shield protecting the LOX tank from IR from Sun, Earth, etc., LOX could be stored indefinitely in a 1 atm tank with no boil-off.
Shield positioning would be a challenge in LEO since the separation angle between Earth and Sun would be constantly changing. If the craft were in a heliocentric orbit or an interplanetary transfer orbit, shield positioning would be simplified.
A Starship LOX tank is rated for 6 bar which correlates with an equilibrium temperature of about 115K. The combination of sun shield and boil-off for passenger O2 consumption should preserve LOX during Earth-Mars transfer.
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$\begingroup$ Passive cooling is probably not practical for typical satellites because as you said Earth-Sun angle becomes a problem, and anyway their propulsion needs are relatively low and can be handled with other methods. But eventually there will be space tugs in Earth orbit, used for relocating other satellites, raising orbits, debris removal, etc. These would probably benefit from cryo, and passive cooling might be more feasible as they could schedule their activities for workable beta angles. A methalox tug relying (primarily) on passive cooling seems like it might be feasible. $\endgroup$ Commented Feb 18 at 20:16
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$\begingroup$ @StevePemberton, a space tug's going to need to have electric propulsion. Nothing else is efficient enough to do a non-trivial plane change, and a tug stuck in a single orbital plane is of limited use. $\endgroup$– MarkCommented Feb 19 at 4:16
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1$\begingroup$ @Mark re "... nothing else ..." --> Until practical antimatter propulsion arrives. May be "some while". $\endgroup$ Commented Feb 20 at 3:09
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$\begingroup$ @RussellMcMahon, an electrodynamic tether beats antimatter propulsion for efficiency. $\endgroup$– MarkCommented Feb 20 at 22:00
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$\begingroup$ @mark. As the tether energy is "free" then competing with it on an energy basis is "a bit difficult". However antimatter annihilation, if you can manage to contain it and shield it is entirely able to do the job and flexible to boot. You can do Mars return with landing and take off and probably pull 1G continuous accelerating and decelerating bot ways. $\endgroup$ Commented Feb 21 at 9:20