I am trying to make a space plane with several reserve tanks that will be boosted into orbit by some kind of launch vehicle. The plane is a wing mounted engine design with 4 sponsons of 2 reserve fuel tanks that are detachable. The sponsons themselves attach in 2 stages, first dropping the rear tank and finally the front tank. The correct fuel flow in the plane is 4 rear reserve tanks drain equally (detach), 4 front reserve tanks drain equally (detach), wingtip tanks drain equally, body drains. This works. I have a fuel line going from the rear reserves to the front reserves (1 to 1), from the front reserves going to the engines (2 to 1). These make sure the reserve tanks drain first in the right order before the main tanks are touched. Now. What I would like to be able to do is attach fuel lines from my launch vehicle to my plane such that I can use my plane engines during take off but without touching the fuel in my plane or reserve tanks, going purely on the launch vehicle fuel tanks while it is attached. To do this, I had assumed I merely needed to run a fuel line from the launch vehicle main tank to each of the rear most reserve tanks (so 1 to 4). I expected this to work and leave all of my reserve tanks untouched while the launch stage is attached. Alternatively, if it didnt work, I was expecting it to use my rear tanks fuel anyway. What I did not expect however, was that it was going to use 2 of the 4 rear tanks and leave the others untouched. (Attached for your engineering convenience is the plane with a proof of concept miniature launch stage and the fuel lining as described above)
