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The RCS thrusters on the Apollo Service Module were positioned approximately around the center of mass of the ship, such that translational maneuvers wouldn't unnecessarily rotate the ship. While the position of the center of mass would vary as propellant and other consumables were used, a small amount of differential thrust from the RCS thrusters could maintain the ship's attitude against the resulting torque.

Those mass shifts were small, however, compared to the movement of the center of mass with the Apollo LM attached.

When the Apollo CSM and LM were docked en route to the moon, did the control systems automatically utilize the LM's RCS thrusters in combination with the CSM's to maintain attitude during translation maneuvers, or was the LM entirely dead weight?

After LM extraction, translational RCS maneuvers may not have been needed; any small midcourse corrections that were performed on the RCS could be done by rotating the ship into the correct orientation, then using pure forward thrust (i.e. thrusting through the shifted center of mass).

The Apollo 13 crew commented on the difficulty of maneuvering the joined ship using only the LM's thrusters; presumably the CSM RCS was entirely shut down at that point, but if the CSM were undamaged and powered-up, could the ship be flown with coordinated RCS using the LM's controls?

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    $\begingroup$ I don't have any prior knowledge specific to Apollo CM/LM but just thinking it through it would seem that a co-ordinated attitude control, if is to avoid one vehicle fighting the other, would have to involve connecting one set of thrusters to respond to commands from the other vehicle. Seems to provoke a detailed question on whether anything like that was in the design. Your point about "may not have been needed" seems like a more comfortable scenario. Also, whilst docked, what needs would there be for translation either a) at all or b) other than through the main axis? $\endgroup$
    – Puffin
    Feb 28, 2016 at 21:13
  • $\begingroup$ Thinking of the Apollo 13 attitude control example you gave, it would make sense that the LM attitude control loops would have several modes, one for each configuration: LM+CM, LM, LM ascent only, LM ascent + CM. The idea I'm thinking is that each mode should recognise the inertia matrix of the vehicle at that time. Presumably in your anecdote "the difficulty" suggests a slight departure between design and reality. $\endgroup$
    – Puffin
    Feb 28, 2016 at 21:19
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    $\begingroup$ "What needs" -- after various maneuvers on the SPS, the RCS would be used to null out the "residuals" -- the slight errors between the commanded and achieved ∆v for the burn. I think it could be slightly more fuel-efficient to do so with 3-axis translations than by rotation followed by main-axis translation. I don't know if the GNC would have particular control modes or simply a method to input the current center of mass position, etc. $\endgroup$
    – Russell Borogove
    Feb 28, 2016 at 22:03
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    $\begingroup$ I was envisioning a setup where the LM RCS could be actuated under the control of the CM computer with the LM only supplying power to the RCS valves, no other systems involved. I think I remember that the Gemini-Agena missions were set up so that the Gemini controls would operate the Agena RCS thrusters (though the Agena avionics may have been involved), so it's a thing they could have engineered in. $\endgroup$
    – Russell Borogove
    Feb 29, 2016 at 15:07
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    $\begingroup$ It is also possible that the CSM thrusters could simply be set to CSM-only or CSM+LM (or possibly even CSM + LM ascent/descent stage distinct from CSM + LM ascent stage only) mode. The computer should easily be able to account for the shift of the center of mass between those. The problem on Apollo 13 would then remain that the LM didn't have that capability, because nobody thought it would ever be needed -- the LM wasn't intended to be flown while still attached to the CM, let alone the CSM stack. Either way, good question and I'm looking forward to an answer! $\endgroup$
    – user
    Feb 29, 2016 at 16:11

2 Answers 2

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The CSM computer could not use the LM RCS and vice versa. The different control modes are for the RCS DAP filter gains, and the Thrust Vector Control. Additionally to the control modes (CSM, CSM/LM, CSM/LM ascent stage or off) the computer had to be given information about the CSM and LM mass. Then, depending on the vehicle configuration, the correct moments of inertia were calculated and used for the RCS or TVC Digital Autopilot.

So on Apollo 13 they indeed had to maneuver around the stack with full SPS propellant tanks, just with the LM RCS engines. If you are interested, here the document describing the three DAPs (CSM RCS, CSM TVC and CM RCS) of the Command Module computer: http://www.ibiblio.org/apollo/Documents/HSI-208472.pdf

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  • $\begingroup$ Hmm. I can't find much that confirms that for the RCS DAP, but for TVC, it says "The operation of the DAPs during an SPS burn is completely automatic, requiring no inputs from the astronaut. Before the burn, however, the astronaut may enter the CSM and LM weights or the estimated engine-trim angles in the pitch and yaw planes. (This entry is optional…" So at least one mode automatically compensated for this. $\endgroup$
    – Nathan Tuggy
    Mar 9, 2016 at 18:46
  • $\begingroup$ On page 3.2-40 in the document I linked it says: "For these calculations, the DAP takes into account the average thrust of the RCS jets, the moment arm, and S/C moment of inertia about each axis.". The CSM RCS DAP uses the same calculations as the TVC DAP. With the mass and vehicle configuration the moments of inertia are then calculated. The engine-trim angles are for the changing center of mass with a attached LM and changing propellant mass during a mission. $\endgroup$
    – indy91
    Mar 9, 2016 at 19:29
  • $\begingroup$ So how much of a hassle was it to fly the complete stack with RCS on only one end? Was the procedure just to always do translations on the X axis, after whatever necessary rotation? $\endgroup$
    – Russell Borogove
    Mar 9, 2016 at 20:32
  • $\begingroup$ Translations on the X axis was the primary mode of doing course corrections with the RCS. On Apollo 13 they only used this for the last MCC, after three previous maneuvers with the descent propulsion system of the LM. Mostly the problem with maneuvering around was that they had to conserve power and it was decided to power down the attitude indicator (FDAI) after the second MCC, I think. So they only knew their attitude through the computer display, which was very challenging. They talk about this in the debriefing beginning with page 8-17: hq.nasa.gov/alsj/a13/a13-techdebrief.pdf $\endgroup$
    – indy91
    Mar 9, 2016 at 21:08
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The book Digital Apollo: Human and Machine in Spaceflight, in the context of the Apollo 9 LM flight testing, says that the digital autopilot on the LM computer supported three modes: CSM+LM, complete LM, and LM ascent stage, which suggests that even at that point they were anticipating the option of flying the CSM+LM stack from within the LM, as done during Apollo 13.

It's not explicit that the CSM RCS thrusters would be driven from the LM computer, but it seems likely that they would be, since that sort of thing had already been done during the Gemini program.

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  • $\begingroup$ Intriguing. In the Gemini-Agena case you mention here and your earlier comments, how was the system physically and electrically connected up? Were there electrical connectors in the docking adaptor? Just thinking what I'd be content with if I was in the position of a Gemini astronaut, I think I'd want some means of checking that the Agena interface was electrically safe before bridging it to the Gemini system. Perhaps it just means I'd have little chance of astronaut selection! $\endgroup$
    – Puffin
    Mar 7, 2016 at 0:24
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    $\begingroup$ pages.erau.edu/~ericksol/projects/gemini/chap09.html The electrical connections were made automatically when the physical docking latches engaged: "The rendezvous umbilical pads mate with the rendezvous umbilical lever assemblies on the docking cone, at the same time the latch hooks engage the latch receptacles. The main umbilical receptacle mates with its umbilical plug during the rigidizing sequence of the docking maneuver. The umbilical connection provides direct hard-line control of the target vehicle. This control includes simple switching and encoded commands." $\endgroup$
    – Russell Borogove
    Mar 7, 2016 at 4:24
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    $\begingroup$ Prior to docking, commands could be sent to the Agena encoded over the rendezvous radar as well! $\endgroup$
    – Russell Borogove
    Mar 7, 2016 at 4:26
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    $\begingroup$ And there's even some information on the digital command protocol! geminiguide.com/Systems/command.html $\endgroup$
    – Russell Borogove
    Mar 7, 2016 at 4:33
  • $\begingroup$ They are both really interesting resources. This bit "Command link control is used as a means of positioning the target vehicle in the desired attitude and orbital path, ..." really brings it home. I can imagine that being hard enough with a joystick controller let alone having to send commands in octal (fond, if slightly anxious memories!). $\endgroup$
    – Puffin
    Mar 7, 2016 at 12:06

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