Space Station Life Support Loop Simulator

Loop mode

Crew & hardware

Crew size 4

CO2 scrubber efficiency 85%

O2-recovery fraction 50%

Cabin O2

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Cabin CO2

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Resupply mass avoided (30-day mission)

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How it works

Each crew member metabolizes roughly 0.84 kg of O2 and exhales about 1.00 kg of CO2 per day. In a sealed cabin that CO2 has to go somewhere: a regenerable scrubber (like the ISS CDRA) pulls a fraction of it out of the recirculated air stream every cycle. The captured CO2 then feeds a Sabatier reactor, which combines it with hydrogen to make water and vents methane:

CO2 + 4H2 → CH4 + 2H2O   (Sabatier reaction)
2H2O → 2H2 + O2          (electrolysis)

Electrolysis then splits that water back into O2 (breathed again) and H2 (fed back into the Sabatier reactor). Stoichiometrically, 44 g of CO2 can yield at most 32 g of recoverable O2 — a ceiling of about 72.7% by mass. Real hardware never hits that ceiling: reactor conversion, leakage and venting losses cut it down, which is what the O2-recovery fraction slider represents (ISS's actual oxygen-from-CO2 recovery has historically run near 40–50%). Cabin gas balance is integrated every frame:

d(CO2)/dt = crew·CO2rate − scrubEff·flow·[CO2]
d(O2)/dt  = −crew·O2rate + recoveryFrac·0.727·scrubbed(CO2)

Loop mode — Closed-loop recycles scrubbed CO2 through Sabatier + electrolysis; Open-loop vents all captured CO2 straight to space and every gram of O2 must be resupplied instead.
Crew size — more people means faster O2 depletion and CO2 buildup, so the same hardware has to work harder to hold cabin air steady.
Scrubber efficiency — how much of the CO2 passing through the scrubber each cycle actually gets captured, rather than staying in the cabin air.
O2-recovery fraction — what share of the theoretical 72.7% mass ceiling the Sabatier/electrolysis chain actually delivers as breathable O2; the rest leaves as vented methane and is lost from the loop.

Real-world relevance: this is the actual chemistry behind the ISS's ECLSS (Environmental Control and Life Support System) — and closing this loop further, so a crew resupplies almost nothing, is one of the hard engineering problems standing between low-Earth orbit and a multi-year Mars mission.

CO2 O2 H2 H2O CH4 (vented)

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