What mechanism was used to control the temperature and humidity inside the Apollo mission capsules? For example, did they employ an electric heater? Did they utilize solar radiation? Was humidity artificially controlled?

An electric heater might be the most convenient, yet seems like a scary option in a cabin using 100% O2 atmosphere.

This is a follow up to my earlier question, "What was the approximate cabin temperature and humidity of the Apollo mission capsules?" which found that the intended "temperature was 70° to 80°F" with "relative humidity of 40 to 70 percent" according to NASA documents.

I am trying to understand how artificial atmospheres and the technology that maintains them has developed over time.

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    $\begingroup$ The operation of a lot of electrical and electronic equipment within the 100 % oxygen atmosphere of the cabin could not be avoided anyway. The additional fire risk of a carefully designed heater is very small when compared with the fire risk of all other electrics. $\endgroup$
    – Uwe
    Feb 28, 2018 at 10:34
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    $\begingroup$ So heat could have been a byproduct of other systems? That makes sense as there are three people and lots of necessary equipment in a very small space. It was likely that the differential between ambient and desired temperature was smaller than I was expecting when I wrote the question. $\endgroup$ Feb 28, 2018 at 14:16
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    $\begingroup$ I'm having trouble selecting an answer because both existing answers have great, complementary information. I recommend reading both. $\endgroup$ Mar 1, 2018 at 17:52

2 Answers 2


The problem with manned spacecraft is not normally keeping the cabin warm; rather it is rejecting the heat. The tiny pressurized bubble of the spacecraft is surrounded by the perfect insulator, vacuum. All the power generated by the spacecraft and by crew metabolism ends up as waste heat and must be rejected into space to maintain a comfortable temperature.

(You may recall the crew in Apollo 13 getting cold, but recall that in that case, the spacecraft was powered down to below emergency levels.)

The major source of humidity in the cabin is water exhaled by the crew; a certain amount of it must be removed to maintain a comfortable humidity level.

NASA refers to the system which removes carbon dioxide and cools and conditions the air as the Atmosphere Revitalization System (ARS), and the heat rejection system as the Thermal Control System (TCS). I will concentrate on the ARS in this answer since that seems to be the focus of your question.

The ARS systems for the Apollo Command Module (CM) and Lunar Module (LEM) share the same functionality. The Shuttle ARS also looks very similar. The main difference with Shuttle was that the suit interfaces were not such a primary feature of the system, since the crew was not expected to remain suited for a high percentage of the mission.

A schematics of the Apollo ARS system is shown below for the LEM. The CM system performs the same functions, but I could not find a nice schematic that wasn't over-complicated.

Oxygen (Cabin Air) is drawn into the system from the cabin (6), passes through canisters which remove carbon dioxide and odor (4), and is circulated through the loop by fans (1). The oxygen passes through a heat exchanger (2) where heat is rejected to the spacecraft TCS. The cooled oxygen then passes through water separators (3) for humidity control - these are essentially specialized fans that centrifugally remove the water. The water is sent off to storage. There is then an interface with the suit loops (5) and the conditioned air is returned to the cabin (7).

Lunar Module

enter image description here

The LEM schematic is from the LM Orientation.

Command Module information is in the Apollo Operations Handbook.

  • $\begingroup$ You are correct that my understanding was influenced by the Apollo 13 account of them getting cold. It is fascinating that rejecting heat is the primary concern, but makes sense for all the reasons you outlined. $\endgroup$ Feb 28, 2018 at 16:33
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    $\begingroup$ The insulating properties of a vacuum was the "aha moment" in this answer for me. My assumption was that heat would be drained off into cold space. $\endgroup$ Mar 1, 2018 at 14:28
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    $\begingroup$ To be fair, some heat is lost by radiation, but getting rid of heat is the major problem. If the shuttle lost all its cooling loops, it had about a 10 minute life span after that. books.google.com/… $\endgroup$ Mar 1, 2018 at 14:32

Temperature and humidity within the Apollo Command Module was controlled with a system of heat exchangers, electrical heaters and water-glycol (62.5 % ethylene glycol and 37.5 % water) coolant loops. The coolant loop was used to cool the electronics mounted on cold plates and also the atmosphere of the capsule.

It was possible to rout the coolant at first to the electronics cold plates and then to the cabin heat exchanger if heating of the cabin was necessary. Waste heat of the electronics could be recycled for cabin heating this way. The coolant flow through the heat exchanger could be reduced proportional to the demand of heating or cooling of the cabin.

Different heat exchangers were used for cabin and suit oxygen circuits. The suit circuit heat exchangers removed also excessive humidity by condensation. The condensed water was removed by a pump from the exchanger and transported to the waste water tank.

Excessive heat could removed by radiation. There were two space radiation panels at the outside surface of the service module at a 130 ° arc. If one panel was exposed to the Sun, Earth or Moon, the other panel exposed to space was used instead. Each panel could remove up to 4,415 BTU per hour from the coolant loops, that is 1,294 W of heat energy.

A minimum flow of coolant through the radiators was necessary to prevent freezing of the coolant within the radiators. But if the coolant temperature after the radiators was too low there were a primary and a secondary electrical heater for the two coolant loops with 450 W each. If the temperature reaches 43°F the No. 1 heater comes on, and at 42°F the No. 2 heater comes on; at 44°F No. 2 goes off, and at 45°F No. 1 goes off. If coolant temperature was 45°F or higher, no electrical heating was required and done.

Waste water could be used in evaporators to remove heat from the coolant loops by evaporation of water into the vacuum of space. About 8,000 BTU per hour or 2,344 W of heat could be removed by the evaporators. Evaporators were used only if cooling by radiators was not sufficient. When the temperature of the coolant entering the evaporator rises to 48° to 50.5°F, the evaporator mode of cooling was initiated. The coolant outlet temperature was regulated to a temperature between 40° to 43°F by control of the water steam pressure valve at the vacuum outlet of the evaporator. Water flow into the evaporator was regulated to keep the evaporator wick between too wet and too dry.

The water-glycol mixture of the coolant loops was precooled before launch using ground equipment to be used for cooling during launch through the atmosphere when neither the radiation panels nor the evaporators could be used

All information from a NASA paper titled, "Environmental Control Subsystem".

  • $\begingroup$ The mixture of water and glycol does not freeze near 40 °F even -20 °F is possible. $\endgroup$
    – Uwe
    Feb 28, 2018 at 21:15
  • $\begingroup$ Your thoughts on the function of those heaters? $\endgroup$ Feb 28, 2018 at 21:34
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    $\begingroup$ Added somme sentences about heaters usage when coolant temperature at the output of the radiators was too low. $\endgroup$
    – Uwe
    Mar 1, 2018 at 14:20
  • $\begingroup$ During the trans lunar flight, passive thermal control - a rotating “barbecue”-like maneuver, was used to maintain spacecraft temperatures within desired limits. $\endgroup$
    – paj
    Sep 5, 2020 at 12:13

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