PNEUMATIC CONTROL SYSTEM
The pneumatic control system provides GHe (gaseous helium) pressure to operate all third stage pneumatically operated valves with the exception of those provided as components of the J-2 engine. GHe is supplied from an ambient helium sphere and pressurized from a ground source before propellant fill operations at 3,100 ± 100 psia at 70° Fahrenheit for valve operation. The sphere is located on the thrust structure and is pre-conditioned to above 70° Fahrenheit from the environmental control system before liftoff.
The pneumatic control system provides regulated pressure at 475 ± 25 psig for operation of the LH, and LOX vent-relief valves during propellant loading, LH2 directional control valve, LOX and LH, fill and drain valves during loading, and the GH2 engine start system vent-relief valve. It also provides operating pressures for the LH, and LOX turbopump turbine purge module, LOX chilldown pump purge module control, LOX and LH, prevalves, LOX and LH, chilldown shutoff valves, and the LH, continuous propulsive vent control module.
The pneumatic control subsystem is protected from overpressure by a normally open solenoid valve controlled by a downstream pressure-sensing switch. At pressures greater than 535 + 15, -10 psia, the pressure switch actuates and closes the valve. At pressures below 450 + 15, -10 psia, the pressure switch drops out and the solenoid opens, thus acting as a backup regulator.