Sabatier Loop Closure Optimizer

Loop Controls

Crew size 4

Electrolysis current (A) 800 A

Sabatier reactor temp 350 °C

Catalyst degradation 5%

Loop State

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H₂ : CO₂ molar ratio

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Water loop closure

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Vented gas rate

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Water balance

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How it works

Two coupled reactions run in a spacecraft's oxygen-recovery loop. Water electrolysis makes breathing O₂ (and byproduct H₂) from Faraday's law; the Sabatier reactor then reacts that H₂ with the crew's exhaled CO₂ to recover water, closing part of the loop:

Electrolysis: 2H₂O → 2H₂ + O₂         ṅ(H₂) = I / (2F)
Sabatier:     CO₂ + 4H₂ → CH₄ + 2H₂O   (ideal ratio H₂:CO₂ = 4:1)
Conversion η(T) ≈ exp(-(T-350°C)² / (2·70²)) × (1 - degradation)

Whichever reagent is scarcer limits conversion. Too little H₂ (current too low) leaves CO₂ unscrubbed; too much H₂ leaves it unreacted — both vent overboard as wasted mass. Reactor temperature away from ≈350 °C, or an aged catalyst, drops the conversion efficiency η even at the perfect 4:1 ratio.

Crew size — sets the fixed CO₂ production load (≈1.04 kg/person/day).
Electrolysis current — sets H₂ (and O₂) supply via Faraday's law.
Reactor temperature / catalyst degradation — set the Sabatier conversion efficiency η.
Goal — maximize water loop closure and minimize vented mass by matching current to 4× the crew's CO₂ molar rate at the reactor's best operating temperature.

This is the real chemistry behind ISS's OGA + Sabatier assembly and every closed-loop deep-space ECLSS design study.

CO₂ H₂ O₂ H₂O CH₄ vented waste

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