Fuel Tanks are cylindrical containers used to store rocket fuel onboard rockets. Building fuel tanks into a rocket will increase the fuel storage capacity of the rocket.
There are three types of tanks to provide fuel to the engines. The monopropellant engine requires monopropellant fuel tanks. The bipropellant engine requires both a bipropellant fuel tank to store hydrogen and an oxidizer fuel tank to store the oxygen.
If a fuel tank is placed on top of or underneath another fuel tank, then the tanks will merge to become one larger tank.
Fuel tanks cannot merge sideways or diagonally.
If multiple fuel tanks are present on the rocket the fuel tanks will provide the same amount of storage as one large fuel tank.
There are no penalties for number of fuel tanks or distance between a fuel tank and an engine.