OMS Thrusting Sequence

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.