Tank pressure control of the S-IVB (500 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 which are linked to the pressure sensors and a network of valves, provide a proportional control of the tank pressure.

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.

Four periods of pressurization can be distinguished. And it mainly requires four different modes of operation of the gas supply system:

  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 and second 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 burns 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.
  4. In Earth Parking orbit prior to the trans lunar burn.
    The propellant tanks have been depleted for about 20%. This amount of propellant was used during the first burn to bring the S-IVB stage, with the Apollo spacecraft on top, in an 185 km high parking orbit.
    This pressurization phase is called re-pressurization. During this phase of the flight the stage is orbiting the Earth and both propellant tanks have been partially depleted during the first burn. That implies that the pressure has to be maintaned in a larger volume. Maintaining the proper pressure, prior to the second burn to bring the Apollo spacecraft into a translunar trajectory, is done in two phases as explained below.
The operational mode in the Earth Parking Orbit (mode 4) has two sub modes:
  1. gas supply with a high flow rate during which heated Helium is used as a pressurization gas for both propellant tanks. The heat is provided by a small H2/O2 burner which is mounted against the thrust structure. This burner also generates a small thrust, enough to settle the propellants to the bottom of the propellant tanks. The Helium is supplied by 9 cold storage spheres which are located inside the cryogenic O2 tank.
  2. Gas supply with a low flow rate during which Helium is supplied from 7 ambient Helium storage spheres which are located around the conical thrust structure. Each cryogenic propellant tank has its own battery of ambient Helium supply: two storage spheres for the O2 tank and 5 storage spheres for the much larger H2 tank.
Mode 1
Mode 2
Mode 3
Mode 4a
Mode 4b

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Copyright 2005 by   Sander Panhuyzen
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