Life support during spaceflight.
This is a complex task with three parts. The first is the atmosphere: air pressure and composition. If the crew remain in spacesuits all the time, there is no need to pressurize the ship. Oxygen must be added to the air mixture continuously, and carbon dioxide must be removed. There are several ways to do this. A ship can carry oxygen in cylinders and chemicals that absorb carbon dioxide. This is the simplest system, but it uses the most consumables. Consumables can be produced or purchased. One unit is enough for one person for one in-game day. For example, with a crew of three, 30 units will last ten days. During the last five days, the crew will ration supplies severely and make them last one more day. After that, the crew will die unless they replenish supplies, dock with another ship or station, or land on a planet with a suitable atmosphere. The second option is partial recycling. It uses water as a source of oxygen; carbon dioxide is separated and vented overboard. This is a more complex system that needs much more power but consumes only water. An even more complex, expensive, and power-hungry system is full recycling. Carbon dioxide is split and oxygen returned to the living quarters, minimizing the water needed to produce oxygen. A system like this can maintain a breathable atmosphere for years, but it needs maintenance.
The second part is temperature control. A thermal-regulation system maintains it and, when functioning properly, generally requires only power and working heat exchangers. The permitted temperature range is usually listed in the ship's general specifications.
The third part is food and water. There are allowances for food, drinking water, and water for everyday use. With a recycling system, only food and drinking water are needed. These are combined into a ration measured like life-support supplies: one unit per person per day. Different rations offer different levels of luxury. In game terms, they provide a luxury bonus for passengers and a morale bonus for the crew.