Has anybody ever come across any pressure vessels (spacecraft or otherwise) that use more than one bladder; that is, to contain more than two fluids? Tanks with two fluids -- a pressurant and a propellant -- separated by a single bladder or diaphragm are common, but what about more?
The only thing I've ever heard of like that is from the petrochemical industry (you did say "otherwise"...)
It's a pressure vessel containing natural gas, oil, and water. Used at the outlet of wells to separate what comes out of the well.
Doesn't really have a bladder though.
It's called a "three-phase separator" although that is quite a misnomer IMHO.
Normally the propellants are stored separately, especially hypergolics, which spontaneously combust upon contact with one another. However SpaceX currently uses Composite Overwrapped Pressure Vessels (COPV's) filled with liquid helium inside of the liquid oxygen tank. The helium is heated and then released to pressurize the tank. So, technically the helium is stored inside the liquid oxygen tank. Also the S-IVB and I believe the S-II stage of the Saturn V rocket had common bulkheads. This means that the liquid hydrogen and liquid oxygen are just about stored in the same tank, except thy are separated by 2 thin sheets of aluminum and resin. This is all I could think of for 2 propellants/pressurants stored in almost the same tank.