Complex sentence: The two superior sections, linked together contain 6 liters of nitrogen at 20 atm which it acts as a propellant, the lower half, which are also connected, containing 12 liters of fuel.
Split into simpler sentences: The two superior sections, linked together contain 6 liters of nitrogen at 20 atm which acts as a propellant. The lower halves, which are also connected, contain 12 liters of fuel.