If i understand what you are saying its that you what the external fuel tanks to be used up first so the can be dropped to lose weight... the game already has this..the FTX-2 External Fuel Duct. to use it 1. select the part and set in on the external fuel tank 2. the treat it like a strut and connect to the main tank 3. watch as the fuel is used up first if this doesn't make sense just look up the FTX-2 External Fuel Duct