SpaceX CRS-4 Launch

One of the more interesting parts of the ULA discussion of on orbit refueling capacity was how you might "settle" the tanks. Gravity does that wonderfully, and of course while you're accelerating you have it covered, but when you want to 'restart' you are in a microG environment and the fuel might not be at the end of the tank where you want it. Simple pressurization doesn't help, the fluid gets evenly distributed in the volume. Not much help. A fuel bladder might work but you'd something that was very light and compressed well. I also discussed with one of the ULA folks a movable "piston" on one end of the tank. Basically do it like a pneumatic cylinder without the rod, You just pushed it to the other end. He pointed out that there aren't any pneumatics that work with hydrogen. It is just too "small" a molecule for wall seal to hold. That leaves things like spinning to create a centripetal 'hold' on the fuel with a narrower 'top' than bottom so that an axially spinning tank will accumulate fuel to the bottom. Ok for payloads, not great for astronauts though.