Main Propulsion System Propellant Management Subsystem

Within the orbiter aft fuselage, liquid hydrogen and liquid oxygen pass through the manifolds, distribution lines and valves of the propellant management subsystem.

During prelaunch activities, this subsystem is used to control the loading of liquid oxygen and liquid hydrogen in the external tank. During SSME thrusting periods, propellants from the external tank flow into this subsystem and to the three SSMEs. The subsystem also provides a path that allows gases tapped from the three SSMEs to flow back to the external tank through two gas umbilicals to maintain pressure in the external tank's liquid oxygen and liquid hydrogen tanks. After MECO, this subsystem controls MPS dumps, vacuum inerting and MPS repressurization for entry.

All the valves in the MPS are either electrically or pneumatically operated. Pneumatic valves are used where large loads are encountered, such as in the control of liquid propellant flows. Electrical valves are used for lighter loads, such as in the control of gaseous propellant flows.

The pneumatically actuated valves are divided into two types: those that require pneumatic pressure to open and close the valve (type 1) and those that are spring loaded to one position and require pneumatic pressure to move to the other position (type 2).

Each type 1 valve actuator is equipped with two electrically actuated solenoid valves. Each solenoid valve controls helium pressure to an ''open'' or ''close'' port on the actuator. Energizing the solenoid valve on the open port allows helium pressure to open the pneumatic valve. Energizing the solenoid on the close port allows helium pressure to close the pneumatic valve. Removing power from a solenoid valve removes helium pressure from the corresponding port of the pneumatic actuator and allows the helium pressure trapped in that side of the actuator to vent overboard. Removing power from both solenoids allows the pneumatic valve to remain in the last commanded position. This type of valve is used for the liquid oxygen and liquid hydrogen feed line 17-inch umbilical disconnect valves (two), the liquid oxygen and liquid hydrogen prevalves (six), the three liquid hydrogen and liquid oxygen inboard and outboard fill and drain valves (four), and the liquid hydrogen 4-inch recirculation disconnect valves.

Each type 2 valve is a single electrically actuated solenoid valve that controls helium pressure to either an open or a close port on the actuator. Removing power from the solenoid valve removes helium pressure from the corresponding port of the pneumatic actuator and allows helium pressure trapped in that side of the actuator to vent overboard. Spring force takes over and drives the valve to the opposite position. If the spring force drives the valve to the open position, the valve is referred to as a normally open valve. If the spring force drives the valve to a closed position, the valve is referred to as a normally closed valve. This type of valve is used for the liquid hydrogen RTLS inboard dump valve (NC), the liquid hydrogen RTLS outboard dump valve (NC), the liquid hydrogen feed line relief shutoff valve (NO), the liquid oxygen feed line relief shutoff valve (NO), the three liquid hydrogen engine recirculation valves (NC), the two liquid oxygen pogo recirculation valves (NO), the liquid hydrogen topping valve (NC), the liquid hydrogen high-point bleed valve (NC), and the liquid oxygen overboard bleed valve (NO).

The electrically actuated solenoid valves are spring loaded to one position and move to the other position when electrical power is applied. These valves also are referred to as either normally open or normally closed, based on their position in the de-energized state. Electrically actuated solenoid valves are the gaseous hydrogen pressurization line vent valve (NC), the three gaseous hydrogen pressurization flow control valves (NO) and the three gaseous oxygen pressurization flow control valves (NO).

There are two 17-inch-diameter MPS propellant feed line manifolds in the orbiter aft fuselage, one for liquid oxygen and one for liquid hydrogen. Each manifold has an outboard and inboard fill and drain valve in series that interface with the respective port (left) and starboard (right) T-0 umbilical. The port T-0 umbilical is for liquid hydrogen; the starboard, for liquid oxygen. In addition, each manifold connects the orbiter to the external tank in the lower aft fuselage through a port 17-inch liquid hydrogen disconnect valve umbilical and a starboard 17-inch liquid oxygen disconnect valve umbilical.

There are three outlets in both the liquid oxygen and liquid hydrogen 17-inch manifolds between the orbiter-external tank 17-inch umbilical disconnect valves and the inboard fill and drain valve. The outlets in the manifolds provide liquid oxygen and liquid hydrogen to each SSME in 12-inch-diameter feed lines.

The prevalve in each of the three liquid oxygen and liquid hydrogen 12-inch feed lines to each engine isolates liquid oxygen and liquid hydrogen from each engine or permits liquid oxygen and liquid hydrogen to flow to each engine. Each prevalve is controlled by an LH 2 or LO 2 prevalve , left , ctr , right switch on panel R4. Each switch has open, GPC and close positions.

The 8-inch-diameter liquid hydrogen outboard and inboard fill and drain valves are also controlled by their own switches on panel R4. Each propellant fill/drain LH 2 , outbd , inbd switch has open, gnd and close positions, as does each LO2, outbd, inbd switch.

Each 17-inch liquid hydrogen and liquid oxygen manifold has a 1-inch-diameter line that is routed to a feed line relief isolation valve and feed line relief valve in the respective liquid hydrogen and liquid oxygen system. The LO 2 and LH 2 feed line rlf (relief) isol (isolation) switches on panel R4 have open , GPC and close positions. When a feed line relief isolation valve is opened, the corresponding manifold can relieve excessive pressure overboard through its relief valve.

The liquid hydrogen feed line manifold has another outlet directed to the two liquid hydrogen RTLS dump valves in series. Both valves are controlled by the MPS prplt dump LH 2 valve switch on panel R2, which has backup LH 2 vlv open , GPC , close positions. When opened, these valves enable the liquid hydrogen dump during RTLS aborts or provide a backup to the normal liquid hydrogen dump after a nominal main engine cutoff. In an RTLS abort dump, liquid hydrogen is dumped overboard through a port at the outer aft fuselage's left side between the orbital maneuvering system/reaction control system pod and the upper surface of the wing.

The MPS propellant management subsystem also contains two 2-inch-diameter manifolds, one for gaseous oxygen and one for gaseous hydrogen. Each manifold individually permits ground support equipment servicing with helium through the respective T-0 umbilical and provides initial pressurization of the external tank's liquid oxygen and liquid hydrogen orbiter/external tank disconnect umbilicals. Self-sealing quick disconnects are provided at the T-0 umbilical and the orbiter/external tank umbilical.

Six 0.63-inch-diameter pressurization lines, three for gaseous oxygen and three for gaseous hydrogen, are used after SSME start to pressurize the external tank's liquid oxygen and liquid hydrogen tanks.

In each SSME, a small portion of liquid oxygen is diverted into the engine's oxidizer heat exchanger, and the heat generated by the engine's high-pressure oxidizer turbopump converts the liquid oxygen into gaseous oxygen and directs it through a check valve to two orifices and a flow control valve for each engine. During SSME thrusting periods, liquid oxygen tank pressure is maintained between 20 and 22 psig by the orifices in the two lines and the action of the flow control valve from each SSME. The flow control valve is controlled by one of three liquid oxygen pressure transducers. When tank pressure decreases below 20 psig, the valve opens. If the tank pressure is greater than 24 psig, it is relieved through the liquid oxygen tank's vent and relief valve.

In each SSME, gaseous hydrogen from the low-pressure fuel turbopump is directed through two check valves to two orifices and a flow control valve for each engine. During the main engine thrusting period, the liquid hydrogen tank's pressure is maintained between 32 and 34 psia by the orifices and the action of the flow control valve from each SSME. The flow control valve is controlled by one of three liquid hydrogen pressure transducers. When tank pressure decreases below 32 psia, the valve opens; and when tank pressure increases to 33 psia, the valve closes. If the tank pressure is greater than 35 psia, the pressure is relieved through the liquid hydrogen tank's vent and relief valve. If the pressure falls below 32 psia, the LH 2 ullage press switch on panel R2 is positioned from auto to open , which will cause all three flow control valves to go to full open and remain in the full-open position.

The single gaseous hydrogen manifold repressurization line connects to the hydrogen line vent valve, which is controlled by the H 2 press line vent switch on panel R4. This valve is normally closed, and the switch is positioned to open when vacuum inerting the gaseous hydrogen pressurization lines after MECO and the liquid hydrogen dump. The gnd position allows the launch processing system to control the valve during ground operations.