Tank pressure control of the S-II stage

To obtain a proper flow of propellants towards the five J-2 thust engines, the tank pressure has to be kept within a certain range of pressure values. These conditions are achieved by using a pressure relief system and a gas supply system. Pressure sensors monitor the tank pressure and will trigger the relief system and the gas supply system respectively, when the pressure has become too high or too low.
During the second stage boost phase, the heart of both systems is formed by the pressure control modules. These modules are linked to the pressure sensors and a network of valves and make a proportional control of the tank pressure possible.
Prior to lift-off, during pre-pressurization, the pressurization gas is supplied from separate regulated ground sources for each propellant tank.

The relief system is basically a vent system. Each propellant tanks has two vent valves.

Three modes of operation can be distinguished for the tank pressurization:

  1. Prior to lift-off
    The pressurization process is called pre-pressurization. Cold Helium gas is used as the pressurization gas and is supplied by the ground source equipment (GSE).
  2. During the first stage boost phase
    Pre-pressurization. During this phase of the flight, no propellant is consumed from the S-II stage. The evaporation rate of the propellants is sufficient to maintain the pressure inside the cryogenic propellant tanks.
  3. During the the burn of the five J-2 engine of the S-II stage
    During these phases of the flight when liquid Oxygen and Hydrogen is consumed by the J-2 engines, the required high flow rate of pressurization gas is obtained by routing a small fraction of the propellants through heat exchangers which are mounted against the J-2 enigines. Gaseous Oxygen and Hydrogen are than routed back into their propellant tanks to provide the necessary tank pressure.
Mode 1
Mode 2
Mode 3

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Copyright 2005 by   Sander Panhuyzen
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