The Apollo Guidance Computer, was built on RTL which apparently can let off a lot of heat, and deployed in environments where we cannot rely on convection to pull the heat away (not only in low gravity conditions, but also it was hermetically sealed for some reason)!

It doesn't appear to have featured a fan or anything. Yet I can not figure out if anything was done to discard the heat generated by the AGC. Or was that even necessary?

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    same way you would cool the astronauts: blow air to it. – user3528438 Nov 25 '18 at 21:31
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    There's a conductive structure build into the printed circuit boards and chassis transferring heat to a cold plates on the spacecraft, where a water-glycol solution is circulated to carry heat away. (And sort of like the astronauts with their liquid cooled undergarments when in EVA suits.) There's a boat load of MIT documents on the AGC, including one that was classified confidential at one time describing reliablity and inflight repair (in the early days). – user9041 Nov 26 '18 at 8:40
  • Just out of interest, how would gravity affect convection? The air would be moving simply because the occupants are moving, no? – user6464 Nov 26 '18 at 12:08
  • @paxdiablo my understanding is that hot air is not as dense as cold air, and so is lighter than cold air if both are subjected to gravity. The hot air will thus move upward, and cold air will move into the space previously occupied by the hot air. This is the principle behind hot air balloons, which cannot operate in outer space. And this is also how convection cooling works – Wilson Nov 26 '18 at 12:12
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    @user9041 that seems more like an answer than a comment, doesn't it? – Wilson Nov 26 '18 at 12:14

Both guidance computers (command module and lunar module) were essentially the same. They were the first flight computers built with integrated circuits, all dual 3-input NOR gates. The top sides of the chips were attached to "cold plates" which were attached to extruded aluminum "cold rails" which had channels for a glycol coolant to flow through. The primary and backup coolant loops were part of the ship-wide environmental control system: GN&CS - ECS Interface.

The Environmental Control Subsystem (ECS) provides thermal stability for the temperature-sensitive electronic equipment of the GN&CS. The electronic equipment (except the IMU) is mounted on cold plates and rails through which ECS coolant (ethylene glycol-water solution) is routed to remove heat. To cool the IMU, the coolant flows through its case. The heat that is removed from the equipment is vented overboard by the ECS sublimators.

Apollo Operations Handbook: Lunar Module

Although the paragraph quoted above is for the lunar module, the command module has the same computer and glycol cooling loops.

Interestingly, the early plan for Apollo was to carry spare computers that the astronauts could swap out in case the computer failed. However, during testing, the above-noted liquid coolant would leak and was difficult to re-seal. This contributed to the decision to keep the computers permanently wired (along with weight considerations and the excellent reliability of the hardware). The swapable units that were already built were used on the unmanned Apollo test flights, with hardwired units for all of the manned flights.

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