Engine 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 ball valves.

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 psig minimum.

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 check valve.

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 contamination.

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.