I have a space plane where all rockets fire at once, but fuel tanks are discarded from rear to front. However, the system seems to only accept draining fuel tanks from front to rear! I tried reordering the stages but it doesn't care. The only solution seems to be to manually disable flow, but that is very awkward, especially when trying to re-enable when entering orbit. My apologies if this has already been explained, I could not find it in the wiki or forum.