# Apollo CSM RCS maximum number of simultaneously active engines?

Reading the RCS subsystem manual, it seems like each RCS quad of the Apollo SM consisted of four hypergolic engines. Am I correct in saying that all four engines of each of the four quads could be fired independently of one another, i.e. in any combination? Note that I am not asking whether the control software did it this way, but whether the physical RCS propulsion subsystem was capable of firing all four engines per quad, and all four quads (i.e. a total of 16 engines) independently and in any combination.

• Note that the CM needed to purge (burn) its hypergolics during abort or normal re-entry (after parachute deployment at 24000 ft) which it did by firing 10 of its 12 jets simultaneously (which might take a minute and a half to deplete the fuel and oxidizer after firing valve squibs to join the redundant systems). As for the SM, the automatic system's relays would never simultaneously fire opposing thrusters, but there was also full manual control of each of its 16 thrusters.
– amI
May 24 '19 at 6:24
• What documentation says there was full manual control of every individual thruster? May 24 '19 at 6:45
• @space_voyager- mis-comm: page 2.5-5 is in section 2.5.1, specifically figures 2.5-2 (auto) and 2.5-4 (direct).
– amI
May 24 '19 at 7:59
• @amI: You are correct that figure 2.5-4 shows the wiring for direct control. But look closely; there aren't switches for each individual thruster. The lower-right corner shows the limit switches for the two rotational hand controllers. The lower-left corner shows abort-mode logic. Lower-middle is for the direct ullage manuever. The "switches" in the middle are the transfer switch between SM and CM RCS. At top is the SM separation logic. (cont...) May 24 '19 at 22:03
• @amI: ... Also look at figure 2.5-5 on the next page, a table which shows which inputs fire which RCS engines. You will see that everything is exactly as I describe in my answer. You simply cannot fire both the +pitch and -pitch engines at the same time, because you can't move the control stick to the opposite extremes (up and down) simultaneously. (Similarly with yaw and roll.) So the best you can do is manually fire half the engines. May 24 '19 at 22:08

## Service Module RCS: 8 max

• There are 16 engines total, arranged in four groups of four ("quads").
• You can fire 8 of the engines by placing the Direct RCS switch to ON, and then turning either rotational hand controller to its extremes in all 3 axes (pitch, yaw, roll). Each axis of a rotational controller has limit switches that are activated when the controller is twisted 11° from the center position. These limit switches directly fire the corresponding RCS thrusters when Direct RCS is on, overriding the digital guidance computer and the analog stabilization and control subsystem.
• Various programs of the guidance computer could theoretically fire up to 8 of the RCS engines at a time, but rarely did so in practice.
• The stabilization and control subsystem also could theoretically fire up to 8 of the RCS engines at a time, but rarely did so in practice.
• The 4 aft-facing engines could be manually fired as a backup ullage maneuver, using the sequential events control subsystem.
• When the service module main engine was fired, the pilot could steer the craft using the rotational hand controller. Rolls would activate 4 of the RCS engines. Pitch and yaw was controlled by gimballing the main engine.
• During SM jettison, the SM jettison subsystem fired the 4 forward-facing engines.
• The minimum impulse control could disable (but not turn on) all of the RCS engines in a selected axis.
• I haven't found any way to activate all 16 engines at once.

## Command Module RCS: 10 max

• There are 12 engines total: 3 axes x 2 in each oppositional pair x 2 redundant systems. Two of these (the $$-$$pitch for both systems) are near the tip of the cone (fore), near the parachutes. The remaining 10 are along the sides of the cone, near the base (aft).
• The system was only used for launch abort and re-entry. The fuel and oxidizer tanks were kept unpressurized until these events occurred.
• A launch abort was initiated by turning the T-handle of the translational hand controller 17° counter-clockwise. Among many other things, this would dump the CM RCS fuel and oxidizer out two blowout holes.
• During re-entry, the rotational hand controller would control the CM RCS engines. At 24,000 feet, this manual control was disabled. After the parachutes were visually confirmed to be deployed, the RCS Dump button was pressed. This fired the 10 aft RCS engines simultaneously, depleting the propellants.
• What is your source? Your answer suggests that this is the engine count in "practical" application (by a human or an automatic control law). However, my question is not about whether the software ever fired 16 engines, but if the physical propulsion system is setup such that it could technically allow for it (wasteful or not). May 24 '19 at 6:44
• SM is from filename "CSM19_Stabilization_&_Control_pp189-204.pdf", entitled "Guidance and Control", pages 189-204 of some larger document. I downloaded it months ago and have lost the source URL. CM is from the link in your question. Both files appear to be sections of a larger document, probably the same document. May 24 '19 at 13:01
• As far as if it is physically possible, one can only speculate, because there's no way to generate the control signals to fire all 16 engines simultaneously. It would depend upon having enough pressure in the system to do that. Considering that the CM could easily fire 10 of 12 engines to depletion, it's likely yes that the SM could fire all 16 engines. May 24 '19 at 13:07