A Cryogenic Boiloff Reliquefier for Orbital Propellant Depots: Heat-Leak, Carnot and Refrigeration-Power Budgets

This paper presents a computed design study, SIMULATED, NOT MEASURED, for reliquefied propellant returned to the depot tanks, with zero routine venting in orbit, using boiloff vapour from liquid hydrogen and liquid oxygen depot tanks. The operating principle is distinct from neighbouring production methods: a closed-cycle helium refrigerator intercepts boiloff vapour from depot tanks and condenses it back to liquid, replacing the usual practice of venting or burning off the excess. The physics is refrigeration thermodynamics: a measured-style heat leak sets the boiloff, the Carnot limit sets the least possible work, and a realistic fraction of Carnot sets the electrical bill. Hydrogen and oxygen are treated separately because their temperatures, and therefore their costs, differ by a factor of several. Sizing follows from budgets computed in the accompanying script rather than from analogy with terrestrial equipment. The headline figures are a specific energy of 44.7 megajoules per kilogram of product, an installed electrical power of 3.81 kilowatts, a production rate of 0.307 kilograms per hour, and an annual throughput of 2.43e+03 kilograms over 330 operating days. Rejecting 2.09 kilowatts of waste heat at 340 kelvin requires 3.14 square metres of radiator, a photovoltaic supply requires 9.33 square metres of array, and the installed mass is 676 kilograms. Moving feedstock from a 400 kilometre to an 800 kilometre orbit costs 217 metres per second of velocity change. The study is a computed study with no targeting or weapon application; it concerns civilian production equipment only, and none of its figures is a measurement.