Quote:
Originally Posted by Bob B.
Ullage motors are often solid propellant so you don't have this problem.
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Perhaps Mugaliens' point was that the ullage motors are a one-shot deal (because they are solid). If the liquid fuel hasn't settled by the time the ullage motor burns out, you don't get another chance to restart the liquid engine.
Of course, you could always include spare ullage motors, or use a restartable liquid fueled ullage motor that relies on a pressure bladder. (From Larry's link, it appears that the Apollo third stage did that, using hydrazine as the fuel.) With cryogenic fuels used for the main engines, a pressure bladder wouldn't work well because of the cold temperature and tendency for the fuel to boil into a gas.
BTW: To reduce splashing, the Saturn designers added baffles to the fuel tanks. At one point, they tried to reduce fuel sloshing by floating "beer cans" on the surface.