We're all aware of the recently aborted SpaceX Demo-2 launch... While talking with friends we discussed the venting of LOX pre-launch.
I presume that the reason for this venting is largely to achieve (or maintain) a "full" tank of Liquid Oxygen. In part, I presume it's likely easier to permit some to boil off and replace it with additional cooled LOX, rather than try to cool and pressurise a closed tank "first time" - which blatantly isn't going to happen within the design's constraints.
Is this even a somewhat correct presumption?
Either way, the fact that some LOX is vented is undeniable.
Do we know how much is pumped beyond the ~287,430 kg capacity of the core and booster's tanks? or in other words: "How much is wasted?"
I appreciate that this figure will likely be significantly different to other models (for example, those that don't use sub-cooled LOX), and I'd be happy with any an answer, not necessarily specifically related to this launch.
During the conversation, I toyed with a ~130% figure (i.e: ~30% wasted), but honestly I pulled that out of thin air. If this isn't something that we know (perhaps it isn't public information), then I'd appreciate an informed estimate.