Eli5: How are submarines able to stay underwater indefinitely?
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Well, they’re *not* because they’re limited by the amount of food on board, and if it’s a regular diesel-electric submarine then they are limited by the amount of charge that the battery has.
Otherwise, they can produce their own oxygen through either having oxygen tanks or using electrolysis – separating oxygen from seawater. They can also desalinate (remove salt from) seawater to get fresh drinking water.
They have tanks that can be filled by either air or seawater, so they can maintain neutral buoyancy. By pumping air in or out, they can change that so they rise or fall. While moving, they can angle their fins, driving the boats up or down.
That’s not 100% accurate. A nuclear submarine can remain underwater non-stop during it’s mission, but it’s limited by how much food it can carry. So it can’t stay underwater *indefinitely*, but it’s functionally able to stay underwater for it’s entire mission if required.
During WW2 subs ran on the surface for the most part and only submerged to escape or attack targets. This was because they couldn’t produce breathable air and needed air for their engines to operate so they were very limited in how long they could stay underwater.
Nuclear submarines have effectively unlimited fuel, in that their reactor cores don’t need to be re-fueled in their lifetimes or 30-50 years depending.
Nuclear subs can also use electricity to split sea water into Oxygen and Hydrogen to make breathable air, which allows them to stay underwater longer.
This is compared to modern diesel subs that still need to surface periodically to run their engines (getting air from the outside) and recharge their batteries.
Former sub sailor here. The response from /u/DarkAlman was wonderful. I’ll focus on nuclear powered subs in my answer, and provide a little more info. Food is absolutely the limiting factor. As for atmosphere, there are sensors onboard which detect levels of gasses on the sub. There is a machine that eliminates excess carbon dioxide. If we need oxygen, we can take in oxygen and through electrolysis we can get the oxygen from the water. As for drinking water, we get seawater and desalinate. This is also how we get water for cleaning dishes and taking showers, and flushing toilets.
The main limiting factor is food. All other resources are made by the sub, even oxygen. And when the oxygen production isn’t enough then they have special oxygen producing “candles” they burn that produce oxygen.
The YouTube channel Smarter Every Day did a whole video series on nuclear submarines.
Or if you wantt to skip ahead to oxygen production here’s the link. https://youtu.be/g3Ud6mHdhlQ
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