April 1970: after the Apollo 13 oxygen-tank explosion, three astronauts crammed into the two-man Lunar Module "lifeboat" for the trip home. The LM's lithium-hydroxide (LiOH) CO2 canisters were sized for two men for two days — not three men for four. The Command Module carried plenty of spare canisters, but they were square and the LM sockets were round.
CO2 produced: dm/dt = n_crew · r_person (r ≈ 0.043 kg CO2 / person / hour, resting)
Partial pressure (ideal gas law):
P_CO2 = (m_CO2 / M_CO2) · R · T / V_cabin
M_CO2 = 44 g/mol, R = 8.314 J/(mol·K), T ≈ 294 K, V_LM ≈ 6.65 m³
mmHg = P_CO2[Pa] × 760 / 101325
Canister removal: dm/dt -= Σ (active, unsaturated canisters) × rate_canister
each canister absorbs LiOH + CO2 → Li2CO3 + H2O until its fixed mass capacity is spent
- Crew in the LM — 2 (design case) or 3 (the real April 1970 crowding); more crew means faster CO2 production.
- Mission clock speed — how many simulated minutes pass per real second, so you can watch a multi-day buildup play out.
- Build the mailbox adapter — unlocks once the LM canisters are exhausted and CO2 crosses the caution mark. It grafts a CM square canister onto the LM's round socket with a cardboard-and-tape duct (the real fix, built from a suit hose, a sock and duct tape), bringing two fresh scrubbers online.
The dashed mark on the gauge is NASA's ~7.6 mmHg (1%) caution threshold; concentrations above ~15 mmHg (2%) begin to cause headache and impaired judgement. In the real mission, CO2 was climbing toward dangerous levels before the improvised adapter — devised by ground controllers and built by the crew in about an hour — brought it back down.