Haven't got a sodding clue what I'm on about ? Then you must be unaware of my small involvement in a project to simulate the voyage of an interstellar, multi-generational spaceship. This began as an estimate of how many people you'd need to avoid extinction due to inbreeding and whether any breeding controls would be necessary (hence the Nazis), and expanded to consider how much food the population would need. But you can read all about that here.
Having established how many crew you need (about 100 to start, sustainable for millenia at a few times this) and how much food they require, this latest paper looks at air and water. Air consumption depends mainly on mass, whereas water is also strongly temperature dependent, so the code now allows you to set the spaceship temperature.
For this paper, the authors decided to move away from the minimum possible number and use a 1,000-strong crew, about the same as the starship Enterprise (D). Why this number, I'm not sure, but we're now well into the range of guaranteed survivability. The crew begins as a gender-balanced bunch of thirty-somethings (with a few older and younger) and sets them to have different activity levels (which affect food and water consumption) for different age groups. Temperature varies randomly between 18 and 21 C.
The result of this is that the crew need about 300 tonnes of oxygen and 1,000 tonnes of water per year - just for the humans, never mind the plants and animals. Add in nitrogen to the air and the total air mass is not that far off the water.
It's at this point I have to say I respectfully disagree with the authors on the rest of their approach. Once you establish a minimal level needed per some time period, the next step should be to estimate how this is affected by your recycling capabilities. It's already pretty obvious that our spaceship must be in the range of many millions of tonnes to sustain the crew, but obviously, you want to minimise the mass of water and oxygen as much as possible (especially since we may expect the mass to be totally dominated by the requirements of agriculture). So you could then consider the different cycles in play : how much is lost over different timeframes, e.g. some oxygen is combined with carbon in each breath, plants absorb nitrogen, water is lost rapidly through sweating and breathing but much less frequently through urination. Different recycling procedures will be necessary in all of these, so there will be different recovery timescales and efficiencies at work. Accounting for these would get you a handle on the important number : how much of each you need aboard the ship at any given moment.
Somehow I just wasn't able to persuade the authors of this. So instead they look at other techniques of water and oxygen production via chemical reactions. But these are, I have to say, both unnecessary and counterproductive. Water and oxygen can be stored indefinitely with literally zero risk of contamination, because you're in deep space... and, as we all know :
It is very cold in space.
Star Trek II: The Wrath of Khan (1982) - Yarn is the best way to find video clips by quote. Find the exact moment in a TV show, movie, or music video you want to share. Easily move forward or backward to get to the perfect spot.
So the only effect of producing water and oxygen chemically is to bring along a significant amount of extra mass. You're better off by far simply storing the entire supply needed for the journey and assuming no recycling at all, which, we've established, is already a bad idea. Now I do like the approach suggested of integrating the different processes, which sometimes share different resources and produce outputs the other requires, but this is not much developed, and again, it would be better by far to just recycle. Nor do I understand why they object to bringing in water and other supplies from Solar System bodies prior to the mission - the need to do this is absolutely unavoidable, and the mass of bringing in anything other than pure water must be larger than the optimum case of bringing in pure H2O. It doesn't make any sense.
So how much water will the Nazis need ? Dunno. I can tell you they'll need to use a thousand tonnes per year, but how much they'll actually have to bring along could be completely different. No idea at all. Perhaps future papers will look at that.
Water and air consumption aboard interstellar arks
The architecture of a large interstellar spaceship, which is capable of serving as a living environment for a population over many generations, is mainly dictated by the needs of said population in terms of food, water and breathable gases.
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