Long-term space missions require minimized boil-off from liquid hydrogen and oxygen tanks to increase the payload mass fraction at launch. Vapor-cooled shielding technologies can use liquid hydrogen boil-off vapors to refrigerate and potentially eliminate boil-off from liquid oxygen tanks. Statistical thermodynamic calculations estimate that the amount of refrigeration provided by hydrogen for a liquid oxygen tank can be increased up to 50% through catalysis of parahydrogen-orthohydrogen conversion. Based on this prediction, a cryocatalysis hydrogen experiment facility was developed as a proof of concept. Hydrogen is liquefied in a noncatalytic condenser and vapor venting controlled via a proportional-integral-derivative controller. The effective heat capacity of the effluent is measured directly through a change in temperature of the known mass flow and applied heater power. The change in parahydrogen-orthohydrogen composition is validated independently with a thermal conductivity probe calibrated in situ. A maximum increase in the effective heat capacity of 35% was observed for hydrogen flow through activated ferric-oxide catalyst at a mass flow rate of 0.206 g∕ min. This result indicates that significant reductions to in-space venting of hydrogen and oxygen are possible through vapor-cooled shielding technologies that catalyze parahydrogen-orthohydrogen conversion. Nomenclaturenumber of molecules Nu = Nusselt number P = statistical weight Pr = Prandtl number Q = heat, W Q cond = heat via conduction, W Q rad = heat via radiation, W R = electrical resistance, Ω Re = Reynolds number T = temperature, K x = concentration (mole fraction) ε = energy of a state, J θ = characteristic temperature, K

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