The LM Environmental Control System: Keeping Astronauts Alive in a Tin Can

Two human beings produce roughly 2.2 pounds of carbon dioxide per day, exhale about a liter of water vapor per hour, and generate approximately 400 BTU per hour of metabolic heat—each. Put two of them in a pressurized aluminum box 92 cubic feet in volume, seal it shut, and within about 90 minutes the carbon dioxide concentration will reach lethal levels. Within hours, the humidity will condense on every cool surface, shorting electrical equipment. The heat will build until the cabin temperature exceeds what the body can shed, and hyperthermia sets in.

The Lunar Module’s Environmental Control System prevented all of this—continuously, automatically, for up to 75 hours—using a combination of lithium hydroxide canisters, water sublimation, heat exchangers, fans, and a plumbing network that circulated oxygen and cooling water through the cabin, the suits, and the avionics bays. The ECS weighed approximately 170 pounds and consumed about 300 watts of electrical power. It was the system that kept the atmosphere breathable, the temperature survivable, and the electronics from overheating in a vehicle where opening a window was not an option.

The Atmosphere: Pure Oxygen at 4.8 PSI

The LM cabin used a pure oxygen atmosphere at 4.8 psi—the same composition and pressure as the Command Module. This was roughly one-third of sea-level atmospheric pressure but with a higher oxygen partial pressure (4.8 psi vs. the 3.1 psi of oxygen in standard air), providing more than adequate respiration. The single-gas atmosphere simplified the ECS enormously—no nitrogen to manage, no mixed-gas regulators, no oxygen-nitrogen ratio to maintain.

Oxygen was stored in the ascent stage in two tanks as a supercritical gas—oxygen pressurized and cooled to a state between liquid and gas, giving it the density of a liquid but the uniform properties of a gas. The tanks held enough oxygen for the cabin atmosphere, suit supply, and EVA preparation operations for the planned mission duration plus reserves.

The cabin pressure was maintained by a pressure regulator that continuously metered oxygen from the supply tanks into the cabin loop. As the crew breathed, consuming oxygen and producing carbon dioxide, the total cabin pressure would drop slightly (CO2 was removed by the scrubbing system, and the oxygen consumed was replaced from the tanks). The regulator maintained a steady 4.8 psi by adjusting the flow rate.

A cabin relief valve prevented overpressurization. If the cabin pressure exceeded about 5.4 psi—from a regulator malfunction or a sudden oxygen release—the relief valve opened and vented gas overboard. A cabin dump valve allowed the crew to intentionally depressurize the cabin for EVA preparation—opening the forward hatch to exit onto the lunar surface required the cabin to be at vacuum first.

Carbon Dioxide Removal: Lithium Hydroxide

The crew’s exhaled carbon dioxide was removed by lithium hydroxide (LiOH) canisters—cylindrical cartridges packed with granular lithium hydroxide that chemically absorbed CO2 from the circulating atmosphere. The reaction was:

2 LiOH + CO2 → Li2CO3 + H2O

Lithium hydroxide reacted with carbon dioxide to produce lithium carbonate and water. The reaction was exothermic—it generated heat—and irreversible. Once the lithium hydroxide was consumed (converted to carbonate), the canister was spent and had to be replaced.

The LM carried a set of LiOH canisters sized for the planned mission duration plus margin. The ECS circulated cabin air through the active canister using a fan, and the crew replaced spent canisters on a schedule provided in the flight plan. Each canister lasted approximately 24 hours under nominal metabolic loads.

The ECS included a CO2 partial pressure sensor that displayed the cabin CO2 level on the instrument panel. Normal operations kept CO2 below 3.8 mmHg (about 0.5%). Above 7.6 mmHg, the crew would begin experiencing headaches and impaired cognitive function. Above 15 mmHg, the situation became dangerous. The canister replacement schedule was designed to keep CO2 well below any threshold, but the crew monitored the sensor as a cross-check.

On Apollo 13, the LiOH canister compatibility between the CM and LM became a life-threatening problem. The LM’s square canisters couldn’t fit the CM’s round canister receptacles, and with three crew members (the LM was designed for two) consuming CO2 removal capacity faster than planned, the LM’s canisters were being exhausted ahead of schedule. The famous “mailbox” adapter—built from cardboard, plastic bags, and duct tape according to instructions radioed from Mission Control—allowed the CM’s round canisters to be used in the LM’s air loop, solving the problem. The incident led to canister standardization for subsequent missions.

The Suit Circuit: Life Support Within Life Support

The ECS operated in two modes: cabin mode and suit mode. In cabin mode—used during pressurized cabin operations when the crew had their helmets and gloves off—the ECS circulated oxygen through the cabin volume, cooling and scrubbing it through the LiOH canister and heat exchanger.

In suit mode—used during EVA preparation, depressurized cabin operations, and as a backup if cabin pressure was lost—the ECS circulated oxygen directly through the crew’s suits via umbilical hoses. The suits were sealed systems with their own internal volume, and the ECS maintained oxygen flow, CO2 removal, and cooling within the suit loop independent of cabin pressure.

The suit circuit used a higher flow rate than the cabin circuit to ensure adequate CO2 washout from within the helmet. The human head is surrounded by a boundary layer of exhaled gas, and in the confined volume of a space suit helmet, CO2 can accumulate around the face unless there’s sufficient airflow to sweep it away. The suit circuit’s flow rate—approximately 8 cubic feet per minute per suit—was set to maintain CO2 concentrations below safe limits even during peak metabolic activity.

The suits connected to the ECS through umbilical hoses that plugged into ports on the crew station. Each umbilical carried oxygen supply, oxygen return (exhaled gas returning to the scrubber), and cooling water. The connections were quick-disconnect fittings that the crew could plug and unplug with gloved hands—an ergonomic requirement that drove the connector design to be operable without fine motor skills.

Thermal Control: The Water Sublimator

The LM generated heat from three sources: crew metabolism, avionics electronics, and solar radiation absorbed by the spacecraft’s surfaces. The total heat load varied by mission phase but typically ranged from 2,000 to 5,000 BTU per hour. This heat had to be rejected to space to prevent cabin and equipment temperatures from rising to dangerous levels.

In the vacuum of space and on the lunar surface, there is no air to carry heat away by convection. The only mechanism for heat rejection is radiation—emitting infrared energy from a warm surface into the cold of space. But radiation alone was insufficient to reject the LM’s full heat load within acceptable temperature limits. The ECS needed an active cooling system.

The primary heat rejection device was the water sublimator—one of the most elegant thermal control devices ever used in spaceflight. The sublimator worked by feeding water to a porous metal plate exposed to the vacuum. The water seeped through the plate’s pores and, upon reaching the vacuum side, flash-froze into ice. The ice then sublimated—transitioned directly from solid to gas—absorbing heat from the plate as it did. The water vapor was vented overboard, carrying the heat with it.

The sublimator was self-regulating. The rate of sublimation depended on the plate temperature: hotter plate, faster sublimation, more cooling. Cooler plate, slower sublimation, less cooling. No active control system was needed—the physics maintained the plate temperature at approximately 40°F, which was the effective sublimation temperature at the water’s feed pressure. The ECS circulated a water-glycol coolant loop through the avionics bays and the suit heat exchangers, picking up heat from electronics and crew metabolism, and then passed the warmed coolant through the sublimator to reject the heat.

The sublimator consumed water—approximately 0.5 to 1.0 pounds per hour depending on the heat load. The LM carried a water supply sized for the mission duration’s cooling requirements. The water was a consumable, like oxygen and propellant—when it was gone, active cooling stopped, and the vehicle’s temperature would begin to rise.

The Cooling Loop: Water-Glycol Through Everything

The water-glycol coolant loop was the thermal circulatory system of the LM. A mixture of water and ethylene glycol (essentially antifreeze) was pumped through a network of tubing and cold plates that contacted the major heat-generating systems:

Avionics cold plates: The AGC, signal conditioning electronics, communications equipment, and other avionic boxes were mounted on aluminum cold plates through which the coolant flowed. The cold plates conducted heat from the electronics into the coolant stream.

Suit heat exchangers: The crew’s metabolic heat was picked up by the suit ventilation loop and transferred to the coolant loop via a gas-to-liquid heat exchanger. This kept the suit atmosphere at a comfortable temperature.

Battery cold plates: The ascent and descent stage batteries generated heat during discharge. Cold plates under the battery cases kept them within their operating temperature range.

The pump that drove the coolant loop was a critical single-point component—if the pump failed, cooling stopped for the entire vehicle. The ECS included a redundant pump that could be activated by the crew if the primary pump failed. Pump failure during powered descent or on the lunar surface would have been a serious emergency, requiring mission replanning and potentially an early return.

Humidity Control: Managing Water in Zero Gravity

The crew exhaled water vapor with every breath—about 2 pounds per day per person. In a sealed cabin, this moisture would rapidly increase humidity to saturation, at which point water would condense on every cool surface: window panes, instrument panels, electrical connectors, and circuit boards. Condensation on electronics could cause short circuits. Condensation on windows would blind the crew during critical phases.

The ECS managed humidity through a water separator in the suit/cabin air loop. As the circulating air passed through the heat exchanger, it was cooled below its dew point, and moisture condensed onto the heat exchanger surfaces. A centrifugal water separator—a spinning drum that flung the condensed water outward by centrifugal force—collected the liquid water and fed it to the water storage system, where it could be used for drinking or fed to the sublimator for cooling.

In zero gravity, water doesn’t drip or flow downhill. It forms floating globules that cling to surfaces by surface tension. The centrifugal separator was specifically designed for zero-gravity operation—it used centrifugal force to substitute for gravity in collecting and directing the condensed water. The separator had to work continuously and reliably, because any interruption would allow humidity to build and condensation to form on cabin surfaces within minutes.

170 Pounds of Survival

The ECS was not glamorous hardware. It had no dramatic moments, no famous alarms, no decision points that were chronicled in the press. It ran in the background, circulating oxygen, scrubbing carbon dioxide, cooling electronics, and removing humidity, every second of every mission. When it worked—which was every mission except the improvised operations of Apollo 13—nobody talked about it. The crew breathed, the electronics stayed cool, the windows stayed clear, and the mission proceeded.

The system’s reliability was a product of its relative simplicity. The ECS had fewer failure modes than the guidance system, fewer components than the propulsion system, and less computational complexity than the software. But its criticality was absolute. A guidance failure might be worked around with ground-based solutions. A propulsion failure might have abort options. An ECS failure—total loss of CO2 removal, total loss of cooling, total loss of oxygen—had no workaround that didn’t involve immediately leaving the LM and returning to the Command Module.

The Environmental Control System kept 24 human beings alive while they worked on the surface of the Moon. It processed their breath, managed their heat, controlled their humidity, and maintained the thin shell of atmosphere between them and the vacuum outside. One hundred seventy pounds of pumps, fans, canisters, and plumbing, doing quietly and continuously the one thing that mattered most—keeping the crew alive long enough to do everything else.