LH2 Continuous Propulsive Vent System

The continuous vent system is used to provide a thrust force required to position propellants at the aft end of each tank during coast. The system con­sists of a vent line originating at the vent-relief valve, terminating at two low’ thrust nozzles located 180° apart, and facing aft on the forward skirt. Continuous venting is controlled and regulated by a pneumatically operated continuous propulsive vent module.

At the completion of the first burn engine cutoff, APS ullage engines are activated to settle the liquid propellants in the aft end of the tanks during the shutdowm phase. LHZ tank pressure is then vented through the continuous propulsive vent system, providing a continuous propulsive thrust to the stage. This maintains control of the propellants within the tanks. The APS engines are shut off after the transition is complete and the propulsive venting continues throughout the coast phase. The continuous propulsive vent module controls vent­ing from a maximum of 45 pounds to a minimum of approximately 7 pounds.

LH, Feed System —Prior to vehicle liftoff and prior

to engine restart, all LH2 feed system components of the J-2 turbopump assembly must be chilled to assure proper operation, Chilldown of the LH2 sys­tem is accomplished by a closed loop, forward-flow recirculation system. On command from the IU, the prevalve in the LH, feed duct closes and the chill – down shutoff valve opens. An auxiliary, electrically driven LH, chilldown pump, mounted in the LH2 tank, circulates the LH, within the system and is capable of a minimum flowrate of 135 gpm at 6.1 psi.

LH, is circulated from the LH2 tank through the low pressure feed duct, through the J-2 engine fuel pump, the fuel bleed valve, and back to the tank through a return line. Recirculation chilldown con­tinues through the boost phase and up to J-2 engine ignition. In the event of an emergency shutdown requirement, the chilldown system shutoff valve is closed upon command from the IU. LH, is sup­plied to the J-2 engine through a vacuum-jacketed, low-pressure duct at a flowrate of 81 pounds per second at -423° Fahrenheit, 28 psia. The duct is located in the fuel tank side wall above the common bulkhead joint and is equipped with bellows to compensate for thermal motion. Signals from the engine sequencer energize the LH2 feed valve, as required to obtain steady-state operation, A com­plete description of engine operation may be found in the J-2 Engine section.