Spacecraft life-support engineers close the oxygen loop by feeding metabolic CO2 into a reduction reactor that regenerates water for electrolysis. This simulator renders the two real reactor chemistries side by side — the flight-proven Sabatier reaction, which vents methane and needs steady hydrogen resupply, and the fully-closing Bosch reaction, which instead deposits solid carbon that fouls the catalyst bed. Pick a reactor, set crew size, mission length and bed temperature, and watch CO2 and H2 molecules stream through an instanced 3D catalyst bed, reacting (or passing through unconverted, off the ideal temperature window) while live readouts track hydrogen loop closure, cumulative mission resupply mass, and bed/catalyst status — including real carbon-bed swap events for the Bosch reactor.