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4

Ullage for the Apollo command/service module and the lunar module was performed by those reaction control system engines which pointed in the same direction (+X) as the main engine. These engines are essentially digital in nature; they are either off or on. Some control modes allowed the RCS engines to be pulsed for finer control. However, ullage burns ...


1

Yes, it is only done on engine startup. Remember, ullage is done so that liquid propellant is fed to the engines, instead of gas bubbles. When a tank is allowed to sit in microgravity for a significant amount of time, there is a risk of some of the gas bubbles in a partially-empty tank migrating to the tank's outlet at the bottom. An ullage burn at the ...


8

If the upper stage is fired promptly after first-stage shutdown, it's possible that no ullage is needed at all. The thrust from the first stage engines settles the upper stage propellants. Any separation impulse between the stages tends to keep them settled, and there isn't enough time for any of the ullage gas to drift to the engine inlets before the upper ...


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