Every breath adds CO2 to a fixed volume of cabin air faster than a saturating chemical bed can pull it back out — a mass balance that governs any closed life-support loop:
dn/dt = R_prod·N_crew − R_scrub(t)
P_CO2 = n·R·T / V_cabin (ideal gas law, → mmHg)
R_scrub(t) = R_max · f_sat(t) (falls off as the LiOH bed saturates)
Each crew member exhales roughly 0.02 mol/min of CO2. The lifeboat module's own round canisters were sized for two people on a short hop, not three on a four-day return — so R_scrub falls behind R_prod and cabin CO2 climbs toward the ~15 mmHg level where it starts to impair judgment and breathing.
The spacecraft did carry spare LiOH canisters — cube-shaped ones built for the other module's square sockets. Historically (Apollo 13, April 1970) the ground crew designed a cardboard-and-plastic-bag adapter, radioed up step by step, to duct cabin air through a mismatched canister anyway. Clicking Build the Mailbox Adapter reproduces that fix: it swaps in the much larger square-canister capacity, and f_sat(t) resets so R_scrub can outrun production again.
- Crew size — sets CO2 production rate; three crew push the round canister to breakthrough far sooner than two.
- Time acceleration — the mission clock runs in simulated minutes per real second; CO2 is a slow process, so this plays hours in seconds.
- Build the Mailbox Adapter — a one-time, irreversible fix, exactly like the real one: once built, cross-format canisters stay online for the rest of the mission.
Lesson: redundant consumables are only real redundancy if their physical interface is compatible with every system that might need them — a spare that cannot be plugged in is not a spare.