Bipropellant Valve Assembly
Each OMS engine receives pressure-fed propellants at its bipropellant
valve assembly. The bipropellant ball valve assembly is controlled
by its gaseous nitrogen system. The nitrogen system consists of
a storage tank, engine pressure isolation valve, regulator, relief
valve, check valve, accumulator, engine purge valves, bipropellant
solenoid control valves and actuators that control the bipropellant
A gaseous nitrogen spherical storage tank is mounted next to
the combustion chamber to supply pressure to its engine pressure
isolation valve. The tank contains enough nitrogen to operate
the ball valves and purge the engine 10 times. Nominal tank capacity
is 60 cubic inches. The maximum tank operating pressure is 3,000
psi, with a proof pressure of 6,000 psig.
Each tank's pressure is monitored by two pressure sensors. One
sensor transmits the tank pressure to the N 2 , He, kit He switch
on panel F7. When the switch is positioned to N 2 , tank pressure
is displayed on the OMS press N 2 tank left, right meters on panel
F7. The other sensor transmits pressure to telemetry.
A dual-coil, solenoid-operated engine pressure isolation valve
is located in each gaseous nitrogen system. The valve is energized
open and spring-loaded closed. The engine pressure isolation valve
permits gaseous nitrogen flow from the tank to the regulator,
accumulator, the bipropellant ball valve control valves and purge
valves 1 and 2 when energized open and isolates the nitrogen tank
from the gaseous nitrogen supply system when closed. The engine
pressure isolation valves in each system are controlled by the
OMS eng left, right switches on panel C3. When the OMS eng left
switch is placed in the arm press position, the left OMS engine
pod's pressure isolation valve is energized open. When the OMS
eng right switch is placed in the arm press position, the right
OMS engine pod's pressure isolation valve is energized open. The
gaseous nitrogen engine pressure isolation valve, when energized
open, allows gaseous nitrogen supply pressure to be directed into
a regulator, through a check valve, an in-line accumulator and
to a pair of engine bipropellant control valves. The engine bipropellant
control valves are controlled by the OMS thrust on/off commands
from the GPCs.
A single-stage regulator is installed in each gaseous nitrogen
pneumatic control system between the gaseous nitrogen engine pressure
isolation valve and the engine bipropellant control valves. The
regulator reduces the gaseous nitrogen service pressure to a desired
working pressure of 315 to 360 psig.
A pressure relief valve downstream of the gaseous nitrogen regulator
limits the pressure to the engine bipropellant control valves
and actuators if a gaseous nitrogen regulator malfunctions. The
relief valve relieves between 450 and 500 psig and resets at 400
A pressure sensor downstream of the regulator monitors the regulated
pressure and transmits it to the CRT display and to telemetry.
The check valve located downstream of the gaseous nitrogen regulator
will close if gaseous nitrogen pressure is lost on the upstream
side of the check valve and will isolate the remaining gaseous
nitrogen pressure on the downstream side of the check valve.
The 19-cubic- inch gaseous nitrogen accumulator downstream of
the check valve and upstream of the bipropellant control valves
provides enough pressure to operate the engine bipropellant control
valves one time with the engine pressure isolation valve closed
or in the event of loss of pressure on the upstream side of the
Two solenoid-operated, three-way, two-position bipropellant control
valves on each OMS engine control the bipropellant control valve
actuators and bipropellant ball valves. Control valve 1 controls
the No. 1 actuator and the fuel and oxidizer ball valves. Control
valve 2 controls the No. 2 actuator and two ball valves, one fuel
and oxidizer ball valve in series to the No. 1 system. Each control
valve contains two solenoid coils, either of which, when energized,
opens the control valve.
The right OMS engine gaseous nitrogen solenoid control valves
1 and 2 are energized open by computer commands if the right OMS
eng switch on panel C3 is in the arm or arm/press position and
the right OMS eng vlv switch on panel O16 is on; the valves are
de-energized normally when thrust off is commanded or if the right
OMS eng switch is positioned to off . The left OMS engine gaseous
nitrogen solenoid control valves 1 and 2 are controlled in the
same manner, but through the left OMS eng switch on panel C3 and
the left OMS eng vlv switch on panel O14.
When the gaseous nitrogen solenoid control valves are energized
open, pressure is directed into the two actuators in each engine.
The nitrogen acts against the piston in each actuator, overcoming
the spring force on the opposite side of the actuators. Each actuator
has a rack-and-pinion gear; and the linear motion of the actuator
connecting arm is converted into rotary motion, which drives two
ball valves, one fuel and one oxidizer, to the open position.
Each pair of ball valves opens simultaneously. Fuel and oxidizer
are then directed to the combustion chamber of the engine, where
the propellants atomize and ignite upon contact. The hypergolic
propellants produce a hot gas, thus thrust.
The chamber pressure of each engine is monitored by a pressure
sensor and is transmitted to the OMS press left and right Pc (chamber
pressure) meter on panel F7.
When the computer commands thrust off or an engine's OMS eng
switch on panel C3 or eng vlv switch on panel O14/O16 is positioned
off, the solenoid control valves are de-energized, removing gaseous
nitrogen pressure from the actuators; and the gaseous nitrogen
pressure in the actuators is vented overboard through the solenoid
control valve. The spring in the actuator forces the actuator's
piston to move in the opposite direction, and the actuator drives
the fuel and oxidizer ball valves closed simultaneously. The series-redundant
arrangement of ball valves ensures engine thrusting is terminated.
Each actuator incorporates a linear position transducer, which
supplies ball valve position to a CRT.
Check valves are installed in the vent port outlet of each gaseous
nitrogen solenoid control valve on the spring pressure side of
each actuator to protect the seal of these components from atmospheric
Each engine has two gaseous nitrogen purge valves in series.
These valves are solenoid-operated open and spring-loaded closed.
They are normally energized open after each thrusting period by
the GPCs unless inhibited by a crew entry on the maneuver CRT
display. The two purge valves of an engine are energized open
0.36 second after OMS engine thrust off has been commanded and
permit gaseous nitrogen to flow through the valves and check valve
into the fuel line downstream of the ball valves and out through
the combustion chamber and engine injector to space for two seconds.
This purges the residual fuel from the combustion chamber and
injector of the engine, permitting safe engine restart. The purge
valves are then de-energized and spring-loaded closed. When the
purge is completed, the gaseous nitrogen tank pressure isolation
valve is closed by placing the respective OMS eng switch (panel
C3) to off. The check valve downstream of the purge valves prevents
fuel from flowing to the engine purge valves during engine thrusting.