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).