Keeping cool

Over the final hour of a mission, as the crew prepared for re-entry, most of the systems in the command module were powered up. Throughout the mission the heat generated by these systems had been absorbed by a water-glycol solution (not unlike that found in the radiator of an automobile) and lost by two large radiators on the side of the service module or, if required, the two evaporators in the command module.

However, by design, most of the elaborate systems for dissipating the spacecraft's excess heat floated away 15 minutes before entry, along with the rest of the discarded service module, so a special provision had to be made to manage the heat generated within the command module during the half hour between separation and splashdown. Shortly before separation, a 'chill-down' process was begun, where both the radiators and the primary and secondary water evaporators were turned on, cooling the water-glycol to around 5°C. This didn't cool the cabin, which remained at about 24° C, but it prepared the coolant to absorb large amounts of heat from the electronics. This took advantage of the fact that water has by far the highest heat capacity of the common liquids. Although the total amount of heat that could be absorbed by the coolant was still quite limited, it was sufficient to last from entry to splashdown.

The water-glycol solution within the command module was only used to cool the spacecraft's electronics. No attempt was made to actively cool the spacecraft's exterior during its fiery plunge through the atmosphere, the heatshield being more than adequate to protect the structure.

One system that did not require to be cooled, but to be heated, was the command module reaction control system and its thrusters. These RCS thrusters had been exposed to the cold of space or the heat of the Sun for up to 12 days. Heaters ensured that they were all warm enough before being operated for the first time.

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