The OMS thrusting sequence commands the OMS engines on or off
and commands the engine purge function. The flight crew can select,
via item entry on the maneuver display, a one- or two-engine thrusting
maneuver and can inhibit the OMS engine purge.
The sequence determines which engines are selected and then provides
the necessary computer commands to open the appropriate helium
vapor isolation valves and the engine gaseous nitrogen solenoid
control valves and sets an engine-on indicator. The sequence will
monitor the OMS engine fail flags and, if one or both engines
have failed, issue the appropriate OMS cutoff commands as soon
as the crew has confirmed the failure by placing the OMS eng switch
in the off position. This will then terminate the appropriate
engine's control valve commands.
In a normal OMS thrusting period, when the OMS cutoff flag is
true, the sequence terminates commands to the helium pressurization,
helium vapor isolation valves and two gaseous nitrogen engine
control valves. If the engine purge sequence is not inhibited,
the sequence will check for the left and right engine arm press
signals and after 0.36 second open the engine gaseous nitrogen
purge valves for two seconds for the engines that have the arm
press signals present.