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
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
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.