Pressurization System

Each RCS has two helium storage tanks, four helium isolation valves, four pressure regulators, two relief valves, two check valves, two manually operated valves and servicing connections for draining and filling.

The helium storage tanks are composite spheres and consist of a titanium liner with a Kevlar structural overwrap that increases safety and decreases the tank weight over conventional titanium tanks. Each helium tank is 18.71 inches in diameter with a volume of 3,043 cubic inches and a dry weight of 24 pounds. Each helium tank is serviced to 3,600 psia.

The two helium tanks in each RCS supply gaseous helium individually, one to the fuel tank and one to the oxidizer tank.

There are two parallel helium isolation valves between the helium tanks and the pressure regulators in each RCS. When open, the helium isolation valves permit the helium source pressure to flow to the propellant tank. The helium isolation valves are controlled by the fwd RCS He press A/B switches on panel O8 and the aft left RCS He press A/B and aft right RCS He press A/B switches on panel O7. Each switch controls two helium isolation valves, one in the oxidizer helium line and one in the fuel helium line. The switch positions are open, GPC and close. When positioned to GPC , the pair of valves is automatically opened or closed upon command from the orbiter computer. The open/close position permits manual control of that pair of valves.

Electrical power is momentarily applied through logic in an electrical load controller assembly to energize the two helium isolation solenoid valves open and to magnetically latch the valves open. To close the two helium isolation valves, electrical power is momentarily applied through the load controller to energize a solenoid surrounding the magnetic latch of the two helium isolation valves, which allows spring and helium pressure to force the valve closed.

A position microswitch in each valve indicates valve position to an electrical controller assembly and controls a position indicator (talkback) above each switch on panels O7 and O8. When both valves (helium fuel and helium oxidizer) are open, the talkback indicates op ; and when both valves are closed, the talkback indicates cl . If one valve is open and the other is closed, the talkback indicates barberpole.

The RCS helium supply pressure is monitored on panel O3. The rotary switch on panel O3 positioned to RCS He X10 allows the forward and aft RCS helium pressures to be displayed on the RCS/OMS press fuel and oxid meters on panel O3.

Helium pressure is regulated by two regulator assemblies, connected in parallel, downstream of the helium isolation valves. Each assembly contains two stages, a primary and a secondary, connected in series. If the primary stage fails open, the secondary stage regulates the pressure. The primary regulates the pressure at 242 to 248 psig, the secondary at 253 to 259 psig.

The check valve assembly, which consists of four poppets in a series-parallel arrangement, is located between the pressure regulator assemblies and the propellant tank. The series arrangement limits the backflow of propellant vapor and maintains propellant tank pressure integrity in the event of an upstream helium leak. The parallel arrangement ensures the flow of helium pressure to the propellant tank if a series check valve fails in the closed position.

A helium pressure relief valve assembly is located between the check valve assemblies and propellant tank and will vent excessive pressure overboard before it reaches the propellant tank. Each valve consists of a burst diaphragm, filter and relief valve. The non-fragmentation diaphragm provides a positive seal against helium leakage and will rupture between 324 to 340 psig. The filter prevents any particles of the burst diaphragm from reaching the relief valve seat. The relief valve relieves at 315 psig minimum. The relief valve is sized to handle, without damaging the propellant tank, helium pressure flow volume if a regulator malfunctions to a full-open position.