Closed-loop refrigeration cycles for liquid oxygen densification are disclosed. The disclosed refrigeration cycles may be turbine-based refrigeration cycles or a Joule-Thompson (JT) expansion valve based refrigeration cycles and include a refrigerant or working fluid comprising a mixture of neon or helium together with nitrogen and/or oxygen.

A Joule-Thomson cryocooler includes a pre-cooling cryocooler that includes a pre-cooling stage, a refrigerant circuit that includes a heat exchanger and a refrigerant pipe extending from the heat exchanger and cooled by the pre-cooling stage, and a heat conduction path that is provided separately from the refrigerant pipe and connects the pre-cooling stage to the heat exchanger to enable conductive cooling of the heat exchanger through the pre-cooling stage.