Engine Propellant Feed

The propellant tank isolation valves are located between the propellant tanks and the manifold isolation valves and are used to isolate the propellant tanks from the remainder of the propellant distribution system. The isolation valves are ac-motor-operated and consist of a lift-off ball flow control device and an actuator assembly that contains a motor, gear train and sector gear. One pair of valves (one fuel valve and one oxidizer valve) isolates the propellant tanks from the 1/2 manifold in the forward and aft left and right RCS. One pair of valves isolates the propellant tanks from the 3/4/5 manifold in the forward RCS; and two pairs of valves, in parallel, identified as A and B, isolate the propellant tanks from the 3/4/5 manifold in the aft left and right RCS.

The forward RCS tank isolation valves are controlled by the fwd RCS tank isolation 1/2 and 3/4/5 switches on panel O8. The aft RCS tank isolation valves are controlled by the aft left RCS tank isolation 1/2 and 3/4/5 A and B and aft right RCS tank isolation 1/2 and 3/4/5 A and B switches on panel O7. These are permanent-position switches that have three settings: open, GPC and close.

When the fwd RCS tank isolation 1/2 and 3/4/5 switches are positioned to GPC, that pair of valves is automatically opened or closed upon command from the orbiter computer. When the corresponding pair of valves is opened, fuel and oxidizer from the propellant tanks are allowed to flow to the corresponding manifold isolation valves. Electrical power is provided to an electrical motor controller assembly that supplies power to the ac-motor-operated valve actuators. Once the valve is in the commanded position, logic in the motor controller assembly removes power from the actuator.

A talkback indicator above each tank's isolation switch on panel O8 shows the status of that pair of valves. The talkback indicator is controlled by microswitches in each pair of valves. The talkback indicator shows op or cl when that pair of valves is open or closed and barberpole when the valves are in transit or one valve is open and the other is closed. The open and close positions of the fwd RCS tank isolation 1/2 and 3/4/5 switches on panel O8 permit manual control of the corresponding pair of valves.

The forward RCS manifold isolation valves are between the tank isolation valves and the forward RCS engines. The manifold isolation valves for manifolds 1, 2, 3 and 4 are the same type of ac-motor-operated valves as the propellant tank isolation valves and are controlled by the same type of motor-switching logic. The forward RCS manifold valve pairs are controlled by the fwd RCS manifold isolation 1, 2, 3, 4 and 5 switches on panel O8. When a switch is positioned to GPC , that pair of valves is automatically opened or closed upon command from the orbiter computer. A talkback indicator above the 1, 2, 3, 4 and 5 switch on panel O8 indicates the status of that pair of valves. The talkback indicator is controlled in the same manner as the tank isolation valve indication. The open and close positions of the manifold isolation 1, 2, 3, 4 or 5 switch on panel O8 permit manual control of the corresponding pair of valves. The fwd RCS manifold 1, 2, 3 and 4 switches control propellants for the forward primary RCS engine only.

The fwd RCS manifold 5 switch controls the manifold 5 fuel and oxidizer valves, which control propellants for the forward vernier RCS engines only. The switch is normally in the GPC position, but it can be placed in either open or close for manual override capability. Electrical power is momentarily applied through logic in an electrical load controller assembly to energize the solenoid valves open and magnetically latch the valves. To close the valves, electrical power is momentarily applied to energize the solenoids surrounding the magnetic latches of the valves, which allows spring and propellant pressure to force the valves closed. A position microswitch in each valve indicates valve position to an electrical controller assembly and controls a position talkback indicator above the switch on panel O8. When both valves are open, the indicator shows op ; and when both valves are closed, it indicates cl . If one valve is open and the other is closed, the talkback indicator shows barberpole.

The open, GPC and close positions of the aft left RCS tank isolation 1/2 and 3/4/5 A and B and aft right RCS tank isolation 1/2 and 3/4/5 A and B switches on panel O7 are the same type as those of the forward RCS tank isolation switches and are controlled electrically in the same manner. A talkback indicator above each switch indicates the position of the pair of valves as in the forward RCS. The 3/4/5 A and B switches control parallel fuel and oxidizer tank isolation valves to permit or isolate propellants to the respective aft left and aft right RCS manifold isolation valves 3, 4 and 5.

The aft left and aft right manifold isolation valves are controlled by the aft left RCS manifold isolation 1, 2, 3, 4, 5 and aft right RCS manifold isolation switches on panel O7. The open, GPC and close positions of each switch are the same type as the forward RCS manifold isolation switch positions and are controlled electrically in the same manner. The aft left and aft right RCS manifold 1, 2, 3 and 4 switches provide corre sponding tank propellants to the applicable primary RCS engines or isolate the propellants from the engines. The aft left and aft right RCS manifold 5 switch provides corresponding tank propellants to the applicable vernier RCS engines or isolates the propellants from the engines.

Each RCS engine is identified by the propellant manifold that supplies the engine and by the direction of the engine plume. The first identifier is a letter-F, L or R. These designate an engine as forward, left aft or right aft RCS. The second identifier is a number-1 through 5. These designate the propellant manifold. The third identifier is a letter- A (aft), F (forward), L (left), R (right), U (up), D (down). These designate the direction of the engine plume. For example, engines F2U, F3U and F1U are forward RCS engines receiving propellants from forward RCS manifolds 2, 3 and 1, respectively; the engine plume direction is up.

If either aft RCS pod's propellant system must be isolated from its RCS jets, the other aft RCS propellant system can be configured to crossfeed propellant. The aft RCS crossfeed valves that tie the crossfeed manifold into the propellant distribution lines below the tank isolation valves can be configured so that one aft RCS propellant system can feed both left and right RCS engines. The aft RCS crossfeed valves are ac-motor-operated valve actuators and identical in design and operation to the propellant tank isolation valves. The aft RCS crossfeed valves are controlled by the aft left and aft right RCS crossfeed 1/2 and 3/4/5 switches on panel O7. The positions of the four switches are open, GPC and close. The GPC position allows the orbiter computer to automatically control the crossfeed valves, and the open and close positions enable manual control. The open position of the aft left RCS crossfeed 1/2 and 3/4/5 switches permits the aft left RCS to supply propellants to the aft right RCS crossfeed valves, which must be opened by placing the aft right RCS crossfeed 1/2 and 3/4/5 switches to the open position for propellant flow to the aft right RCS engines. (Note that the aft right RCS tank isolation 1/2 and 3/4/5 A and B valves must be closed.) The close position of the aft left and aft right RCS crossfeed 1/2 and 3/4/5 switches isolates the crossfeed capability. The crossfeed of the aft right RCS to the left RCS would be accomplished by positioning the aft right and left RCS crossfeed switches to open and positioning the aft left RCS tank isolation 1/2 and 3/4/5 A, B switches to close . (Note that the aft left RCS tank isolation 1/2 and 3/4/5 A and B valves must be closed.)

There are 64 ac-motor-operated valves in the OMS/RCS nitrogen tetroxide and monomethyl hydrazine propellant systems. Each of these valves was modified to incorporate a 0.25-inch-diameter stainless steel sniff line from each valve actuator to the mold line of the orbiter. The sniff line from each valve actuator permits the monitoring of nitrogen tetroxide or monomethyl hydrazine in the electrical portion of each valve actuator during ground operations.

The sniff lines from each of the 12 forward RCS valve actuators are routed to the respective forward RCS nitrogen tetroxide or monomethyl hydrazine servicing panels (six to the nitrogen tetroxide servicing panel and six to the monomethyl hydrazine servicing panel). The remaining 52 sniff lines are in the left and right OMS/RCS pods. During ground operations, an interscan checks for the presence of nitrogen tetroxide or monomethyl hydrazine in the electrical portion of the valve actuators.

An electrical microswitch located in each of the ac-motor-operated valve actuators provides an electrical signal (open or closed) to the onboard flight crew displays and controls and to telemetry. An extensive program was implemented to reduce the probability of floating particulates in the electrical microswitch portion of each ac-motor-operated valve actuator, which could affect the operation of the microswitch in each valve.