Service Propulsion System

Diagram of the Service Propulsion SystemCSMNEWSREF

The Service Propulsion System (SPS) is the Command/Service Module main engine and provides the impulse for all velocity changes (ΔVs) throughout a mission and the SPS abort capability after the launch escape tower is jettisoned. The engine uses Aerozine 50 (hydrazine/UDMH) fuel and nitrogen tetroxide oxidizer and has a maximum thrust of 20,500 lbf (91.2 kN). This hypergolic fuel combination is pressurized with helium during thrusting periods, so no propellant pump is needed. The propellant flow from the tanks to the thrust chamber is controlled by the engine injector valves, which are pneumatically controlled with nitrogen. The engine is gimbaled to allow thrust vector control during a burn.

Quickstart mode

In Quickstart mode the SPS engine works like a normal Orbiter main thruster. The engine can be throttled with the keyboard or Orbiter's joystick control as usual, although the real SPS engine isn't throttleable. The Interplanetary MFD (IMFD) autoburn program can be used, too. It's not necessary to use the panel displays and controls, the Orbiter engine displays can be used though the panel displays are enabled normally.

Engine thrust on/off logic

Location of SPS displays & controls on panel 1, lower part1

In Standard mode and Virtual AGC mode the SPS engine works like the real thing, that means as specified in the original documentationAOH. So the real SPS engine isn't throttleable, it only can be turned on or off by opening or closing one or both of the two pairs of engine injector valves (bipropellant valve systems A or B)3.D pneumatically. The gaseous nitrogen pressure of valve system A and B3.B can be monitored by switching the SPS-PRESS IND switch3.M to N₂ A or N₂ B.

In order to fire the SPS engine, the ΔV THRUST A and/or ΔV THRUST B switches 1.G are unguarded and switched to NORMAL ("single/double bank operation") to arm the control logic for engine injector valve system A or B.I Now there are 3 ways or thrust modes to fire the engine:

SPS thrust direct on mode

The SPS engine is designed to be very reliable, so the easiest way to fire the engine, just in case everything else failed, is to switch the SPS THRUST switch3.M to DIRECT ON. It can be stopped again by switching back to NORMAL.

CMC ΔV mode

The normal way to control the SPS engine is to use the Command Module Computer. The Virtual AGC uses program P40 to both provide the thrust on signal and to control the SPS thrust vector. The C AGC has a couple of fictional programs to support the different mission phases. For both CMC versions the SC CONT switch1.A is switched to CMC.

Location of SPS displays & controls on panel 3, upper part3

SCS ΔV mode

...is coming soon...

Thrust vector control

In order to control a ΔV manoeuver the SPS engine can be gimbaled and change it's pitch and yaw gimbal angles to provide thrust vector control. Currently only the Virtual AGC does thrust vector control.

The current SPS gimbal angles can be montitored by using the LV TANK PRESS/SPS GIMBAL meter1.C, the -5°/+5° scale is used, provided that the LV/SPS IND switch1.L is in GPI position.

...more is coming soon...

Propellant utilization

Location of SPS displays & controls on panel 3, lower part3

Thermal control

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#I The ΔV THRUST A/B switches receive power from the SPS PILOT VLV MNA/B circuit breakers on panel 8 to supply the engine injector valve system A/B. They also receive power from the SPS HE VLV MNA/MNB circuit breakers to power the injector prevalves A/B. Additionally they apply power for the SPS ready signal to the CMC.
#References
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External Links

• Apollo Operations Handbook: Service Propulsion System
• Apollo Experience Report - Service Propulsion Subsystem