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Allocating Functions to Inside or Outside the Spacecraft

When you're deciding whether a function should be performed inside or outside the habitat, you want to trade off the ease of a shirtsleeve maintenance environment with the cost of moving the function inside. In general, you'd like to move detailed maintenance functions inside, but that doesn't necessarily mean you need to move the operational functions inside.

On average, for a spaceborne habitat like the International Space Station, if you move a function inside, its weight goes up by a factor of six. That's because you now have to cool the equipment, provide racks and coverings, protect it from contamination from the interior environment, protect the habit from the contamination it creates, and provide access for maintenance. Having the functions inside also uses up pressurized volume, which always seems to be at a premium.

When you move something outside, you have to design it to be maintained by someone wearing welder's mitts on his hands and a fishbowl over his head. That generally means you want to work with big chunks of the habitat -- boxes and cabinets and structure rather than boards and chips and wires. Outside you usually also have to consider a wider variation in operational temperatures and the vacuum environment.

To cut atmospheric leakage as much as possible, you want to minimize the number of hull penetrations. In this respect, it is useful to have an entire system outside the hull and bring only its product inside. If the system produces data, you might be able to use optical transmission to bring the data inside, or send them outside, without increasing the number of hull penetrations at all.

If a manifest item needs frequent attention from a human being, then it needs to be inside. However, for almost everything else, you want to design it so that the parts requiring fine motor skills can be brought inside for maintenance, but leave them outside to do their work.

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