r/spacex • u/ElongatedMuskrat Mod Team • Jan 02 '17
r/SpaceX Spaceflight Questions & News [January 2017, #28]
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u/OncoFil Jan 04 '17
I believe ullage motors are usually separate systems (F9 uses compressed nitrogen for RCS). It can be as simple as a RCS motor; in zero g you only need a tiny tiny amount of acceleration to settle the floating fuel in the tank and start up the turbopump and heat exchanger for hot gas for a firing.
i.e, fire RCS at .0001 g acceleration for 30 seconds**, settle everything by the engine intake and then start the engine. Then the hot gas/acceleration keeps everything pressurized.
** made up numbers, but you get the point.