Tank pressure control of the S-IVB (200 series) stage

To obtain a proper flow of propellants towards the J-2 thust engine, the tank pressure has to be kept within a certain range of pressure values. These conditions are obtained 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. The heart of both systems are 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.

The relief system is basically a vent system. Excess propellant gas can be vented through 7 different vent valves to allow for a propulsive or a non-propulsive vent.

Both the S-IVB and the S-II stage use liquid Oxygen and liquid Hydrogen as propellants. But the whole S-IVB pressure control system is far more complicated then the one which is used on the S-II stage. This difference is mainly caused by the fact that the J-2 engine of the S-IVB stage has to be restarted in space to bring the Apollo spacecraft into a translunar trajectory. An elaborate system is needed to preserve the proper conditions in space under weightless circumstances prior this translunar burn.

Three different ways of pressurization can be distinguished. And it mainly requires three different modes of operation of the gas supply system:

  1. Prior 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-IVB stage and pressure inside the liquid Oxygen tank can be maintained by a moderate cold Helium flow. To maintain the pressure inside the liquid Hydrogen tank the evaporation rate of the Hydrogen is sufficient.
  3. During the the burn of the J-2 engine of the S-IVB stage
    During these phases of the flight when liquid oxygen and Hydrogen is consumed by the J-2 engine, a much higher flow rate of pressurization gas is required. This high flow rate of pressurization gas for the LOx tank is obtained by routing large amount of cold Helium through heat exchangers which are mounted against the J-2 enigine. Heating up Helium not only causes a pressure increase of the pressurization gas but also provides a heat flow into the Oxygen tank which causes an increase in the Oxygen evaporation rate. In order to preserve the range of control of the pressure control module, cold Helium is added to the heated pressurization gas to partially compensate for this heat flow. The cryogenic Hydrogen tank is pressurized by gaseous Hydrogen which is obtained by tapping off some of the liquid Hydrogen flow to the J-2 engine. This Hydrogen is then routed through a J-2 engine heat exchanger to provide the necessary pressurization gas.
Mode 1
Mode 2
Mode 3

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Copyright 2005 by   Sander Panhuyzen
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