Thrust Chamber Assembly Cluster

System B

Figure 2.4-1. Reaction Control Subsystem - Component Location propellants are blocked by normally closed fuel and oxidizer valve assemblies until a thruster-on command is issued. As the selected TCA receives a thruster -on command, its fuel and oxidizer valve assemblies open to route the propellants through the TCA injector into the combustion

I chambers, where they impinge and ignite. Switches on panel 2 (LGC THRUSTER PAIR CMOS) generate signals for the LGC, telemetry, and caution and warning talkbacks to indicate the status of the thrusters.)

2.4.2 SUBSYSTEM INTERFACES. (See figure 2.4-2.)

Transducer and valve position indicator switch outputs that originate in the RCS are processed by the Instrumentation Subsystem (IS). The operational measurements that are monitored throughout the mission are transmitted via the Communications Subsystem (CS) to MSFN. The IS also processes RCS caution and warning signals. The RCS propellant tanks are pressurized immediately before separation of the LM from the CSM, by firing of explosive valves by the Explosive Devices Subsystem (EDS). Interconnect plumbing between the TCA feed lines and the APS propellant tanks permits the RCS to use APS propellants during certain phases of the mission, thereby conserving the RCS supply. The RCS receives 28-volt d-c primary power through the Commander's and LM Pilot's buses of the Electrical Power Subsystem (EPS).

The Guidance, Navigation, and Control Subsystem (GN&CS) provides commands that select and fire TCA's for durations ranging from a pulse of less than 1 second to steady-state operation. The TCA's can be operated in an automatic mode, an attitude hold mode, or a manual override mode. RCS operation can be controlled by the primary guidance and navigation section (PGNS) or by the abort guidance section (AGS). A detailed description of RCS operation by the GN&CS is given in paragraph 2. 1. 3. 5. The control circuitry is shown in figure 2.1.17.

Normally, translation and attitude are controlled by the PGNS in the automatic mode, in which all navigation, guidance, stabilization, and steering functions are controlled by the LM guidance computer (LGC). Under AGS control, the abort electronics assembly (AEA) and the attitude and translation control assembly (ATCA) take the place of the LGC.

The attitude hold mode provides semiautomatic operation. In this mode, either astronaut can manually determine attitude changes by displacing his attitude controller assembly (ACA), and three-axis translation changes by displacing his thrust/translation controller assembly (TTCA). When the ACA is displaced, an impulse proportional to the amount of displacement is routed to the LGC. The LGC uses this impulse to perform steering calculations and to generate a thruster-on command. The thruster-on command is routed to appropriate jet drivers in the ATCA, firing selected TCA's. A display and keyboard (DSKY) input to the LGC determines whether the LGC commands an angular rate change proportional to ACA displacement, or a minimum impulse for each ACA displacement. When the ACA is returned to the detent position, the LGC sends a command to hold the attitude. For translation, displacement of the TTCA sends a discrete to the LGC, which sends a thruster-on command to selected TCA's. When the TTCA is returned to neutral, the TCA's are turned off.

In the attitude hold mode, under AGS control, the AEA generates attitude errors that are summed, in the ATCA, with proportional rate commands from the ACA and a rate-damping signal from the rate gyro assembly (RGA). The ATCA then performs the steering calculations and generates the thruster on and off commands. Two or four X-axis TCA's for translation maneuvers and a manual override for attitude control in each axis (2-jet direct) can be selected. The four upward-firing TCA's can be inhibited to conserve RCS propellants during the ascent engine thrust phase.

The manual override mode, under PGNS or AGS control, overrides the automatic mode. The four-jet hardover command from the ACA is a manual override command, which is applied directly to the TCA's. The hardover output fires four TCA's simultaneously.

For MPS propellant-settling maneuvers, two or four downward-firing TCA's can be selected. Under PGNS control, the selection is manually keyed into the DSKY for routing to the LGC. Under AGS control, the selection is made by setting the ATT/TRANSL switch (panel 1) to the 2 JETS or 4 JETS position. Under manual control, pressing the +X TRANSL pushbutton (panel 5) provides a command to operate the four downward-firing TCA's, which continue firing until the pushbutton is released. Firing two TCA's conserves RCS propellants; however, it requires a longer firing time to

Figure 2.4-2. Reaction Control Subsystem - Interface Diagram

settle the MPS propellants. Before staging, firing of four TCA's may be necessary because thermal impingement on the descent stage skin limits operating time for the downward-firing TCA's to 40 seconds.

2. 4. 3 FUNCTIONAL DESCRIPTION. (See figure 2. 4-3.)

Functionally, each RCS system (A or B) can be subdivided into a helium pressurization section, a propellant section, a propellant quantity measuring device, and eight TCA's. All RCS components are located in the ascent stage. Because RCS systems A and B are identical, only one system is described.

2.4.3.1 Helium Pressurization Section.

Approximately 1 pound of gaseous helium, at a nominal pressure of 3,050 psia at +70° F, is stored in the helium tank. Flow from the tank separates into two parallel paths. Each flow path contains a normally closed explosive valve that isolates the helium tank from the downstream components before RCS pressurization. A sensor (part of the propellant quantity measuring device) in the helium tank senses the pressure-temperature ratio of the helium. The sensor output is conditioned and routed to the A or B QUANTITY indicator (panel 2), which indicates the combined (fuel and oxidizer) percentage of propellants remaining. A pressure transducer at the outlet port of the helium tank monitors the helium pressure. It supplies a signal to the PRESS indicator (panel 2) when the TEMP/PRESS MON selector switch (panel 2) is set to He. (When the switch is set to He, the X10 light of the PRESS indicator goes on to indicate that the values displayed must be multiplied by 10.) The pressure transducer also supplies a signal that causes the RCS caution light (panel 2) to go on, when helium pressure at the tank outlet port drops below 1, 700 psia. When a caution or warning light goes on, a signal is routed from the caution and warning electronics assembly (CWEA) in the IS to light the MASTER ALARM pushbutton/ lights (panels 1 and 2) and to provide a 3-kc tone in the astronaut headsets. Pressing either MASTER ALARM pushbutton/light turns off both lights and terminates the tone, but has no effect on the caution or warning light.

When the MASTER ARM switch (panel 8) is set to ON and the RCS He PRESS switch is set to FIRE, the explosive valves open simultaneously to pressurize the RCS. Because of the redundant paths, failure of one explosive valve does not affect pressurization of the propellant tanks. Downstream of the explosive valves, the two flow paths merge and the helium flows through a filter that prevents contamination of downstream components by trapping debris generated by firing the cartridges in the explosive valves. A restrictor orifice, downstream of the filter, dampens the initial helium surge, thereby minimizing the possibility of rupturing the burst disk in the downstream pressure relief valve assemblies.

Downstream of the restrictor, the flow path contains two series-connected pressure regulators. The primary (upstream) regulator is set to reduce pressure to approximately 181 psia. The secondary (downstream) regulator is set for a slightly higher output (approximately 185 psia). In normal operation, the primary regulator provides proper propellant tank pressurization. A pressure transducer senses the pressure at the output of the regulators and provides an input to the PRESS indicator via the TEMP/PRESS MON selector switch (PRPLNT position). If one regulator fails closed, or if both regulators fail open, the downstream pressure decreases or increases beyond acceptable limits (minimum pressure of 165 psia; maximum pressure of 218.8 psia) and the RCS A REG or RCS B REG warning light (panel 1) goes on. (If the main propellant shutoff valves are closed, this warning light is inhibited and does not go on, regardless of the pressure at the helium regulator manifold.) Downstream of the pressure regulators, a helium manifold divides the flow into two paths: one leads to the oxidizer tank; the other, to the fuel tank. Each flow path has quadruple check valves in a series-parallel arrangement to prevent backflow of propellant vapors into the helium manifold if seepage occurs in the propellant tank bladders. The helium flows through the check valves into the propellant tanks. A relief valve assembly at the inlet port of each propellant tank protects the propellant tank against overpressurization. If pressure in the helium lines exceeds 220 psia, a burst disk in the relief valve assembly ruptures. When the pressure reaches 232 psia, helium is vented overboard through the relief valve vent port. When the pressure drops below 212 psia, the relief valve closes, permitting normal system operation.

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