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Knudsen pumps are not used in space exploration because they do not function at the altitudes required by nearly all satellites, and do not function at all when completely outside an atmosphere.
As for why not, it's hard to parse the intended conclusion from the summary beyond "it may be possible at certain altitudes", but consider also some remarks about ...
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The propellants are gaseous methane and oxygen propellants, they provide their own pressure just as the nitrogen used in the cold gas thrusters does. No diaphragms, bellows, or bladders involved. A thruster of the same size will give around 5x the thrust for the same propellant flow rate as the nitrogen thrusters they'll start with, so there's really no need ...
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