Spacecraft condensation

Prior to re-entry, the crews noticed how the area around the forward hatch up in the CM’s apex tended to cool and attract condensation from the cabin’s atmosphere.

‘"You know, I bet when we splash down out there.’’ said Tom Stafford, “this cold water runs all out in that…”

“Bet you’re right,” interrupted John Young. “That’s probably where all the water comes from.”

“I bet there’ll be water galore," said Stafford.

“Well, a lot of it’s condensing up the hatch, too," said Young. “That’s a good place for it; there ain’t no wires up here. I don’t give a shit if we get ice up here as long as there ain’t no wiring up there. As long as we don’t have to live up there.’’

“Good place to pul your feel up,’’ suggested Stafford.

“If I was designing the spacecraft,” continued Young, ever the hardened engineer, ”I’d make the bastard get the water out of it before it ever starts; but once it’s designed, that’s probably as good a place to have a water separator as anywhere.”

“Did the other spacecraft notice water under there?” asked Stafford.

“I don’t know if they ever noticed ice or not. We’ve got a lot of water up there now, a lot, a lot. Let me get my rag and go up in there and clean it out.”

Small amounts of water were not a problem in the cabin’s electrical system, partly as a result of the Apollo 1 fire. One of the changes made to the spacecraft was that all the electrics had to be hermetically sealed. When Odyssey, the Apollo 13 CM, re­entered, its wiring had been chilled for four days and had gathered condensation that covered every surface. Upon re-entry, large quantities of water rained down on the crew.

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 then shed to space by the two large radiators on the side of the service module or. if required, the primary and secondary water evaporators in the command module.

However, by design and a mere 15 minutes before re-entry, most of the elaborate systems for dissipating the spacecraft’s excess heat were about to be cast away 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 radiators and the primary and secondary water evaporators were used to cool the vvater/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 within the command module was only used to cool the spacecraft’s electronics. No attempt was made to actively cool the exterior during the 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 they were operated for the first time.