How is it that "blow-down" propellant storage systems work in micro/ zero-gravity?

Question

A "blowdown" pressure system is something that is extensively utilized on Earth -- with tons of applications ranging from kegs to high-tech hydraulics.

This architecture focuses on adding a pressurized, inert charge gas into a vessel that is filled with a liquid propellant. On Earth, the gas is less dense than the liquid and rises to the top of the tank, such that when the outlet is opened, the gas quite literally forces the liquid out, as it tries to expand to atmospheric pressure.

SO... in a zero-gravity environment, there is no preferential direction that a 'less dense' gas will want to move, relative to a 'more dense' liquid. Intuitively, I thought to myself that this architecture wouldn't work for spaceflight since there's no telling which of the materials (charge gas or liquid fuel) will be floating near the outlet.

In other words, you could get really unlucky and have all your liquid fuel floating on the opposite side of your tank and your charge gas floating right next to the outlet. In this case, you'd open the tank valve and could drain your tank of charge gas without expelling any significant amount of liquid fuel (which is what you're actually after).

That being said, there have been tons of high profile missions/ liquid fuel engines, etc that utilize this architecture. Cassini being a great example. What am I missing?

Answer 1

Various designs are used to ensure that propellant and not pressurant gas exits the tank.

One is bladders or diaphragms to separate the fluids.

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Other designs use capillary structures, galleries, or screens which leverage the propellant's surface tension to capture propellant near the tank exit.

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Reference - Spacecraft Propulsion page 191

Answer 2

Tanks that contain a liquid propellant need a pressurant (a gas) of some sort to expel the propellant from the tank. As you've noted, there's a potential problem in zero-g environments, which is that if allowed to, the gas and liquid tend to mix and form a foam. A number of approaches are employed to overcome this problem:

1. Use a flexible bladder inside the tank that separates the gas from the liquid. Cassini used this approach for its hydrazine-powered monopropellant attitude control system.
2. Use settling burns provided by a separate set of thrusters that don't need a settling burn. The job of the settling burn is to provide enough acceleration (not much is needed) to force the gas and liquid to separate. Cassini did not use settling burns. A number of other vehicles have, including the Saturn upper stage.
3. Use a propellant management device that provides enough propellant to the engines so as to cause the gas and liquid to separate. Propellant management devices typically rely on surface tension to pull the propellant toward a sponge or sump. Cassini used this approach for it's bipropellant main engine.