Actually, that's a lot bigger than what I was expecting. After calculating it myself I got the following: t = 80 days (flight duration) n = 1.5 gallon/(person * day) (water requirement per person per day) ~= 5.7 litres / (person * day) Radius calculated from: That would place the diameter 4.4 meters or about 14'4" if I didn't just make a fool of myself by messing up the unit conversion. (EDIT: Turns out that I did, at least in a way. I wrote my radius approximation in feet and inches instead of diameter. It was originally 7'8") However, in a bit of a dick move, I have to say that a sphere of a diameter of 11 meters (just to be clear, I know that's a double of radius and used 5.5 meters for calculation) should contain roughly 700 m³ (or 700000 litres or about 185000 gallons)… so at the very worst both of us made some mistake. ;)That was a lot smaller than I was expecting.
P = 100 person (crew) V = P * t * n = 12000 gallons ~= 45600 litres = 45.6 m³ V = (4 * pi * r³) / 3 <=> r = cbrt((3 * V) / (4 * pi)) ; cbrt standing for cubic root r = cbrt((3 * 45.6) / (4 * 3.14)) = cbrt(136.8/12.56) = cbrt(10.89) ~= 2.2 meters