Did any of the Apollo-era spacecraft have any method of trimming the RCS jets? Due to variable mass placement (fuel tanks and sublimators emptying, astronauts moving, item stowage, location of moon rocks, etc) the center of gravity must have varied a little. Did the onboard computers compensate for any of this when firing the RCS thrusters?

If not, what was the magnitude of cross-contamination, like rotations introduced due to trying to only translate. Is this why an attitude hold mode was commonly in effect?

  • $\begingroup$ Too lazy to find citations, but if I remember rightly, the CG was periodically updated in the computer to account for fuel usage and moon rocks, though obviously it couldn't be constantly up to date with astronaut movement and the like. Besides error in the estimated CG, variable performance of the RCS thrusters themselves would have to be accounted for, and the guidance computer would be watching the attitude of the spacecraft during any maneuvers and correcting unwanted rotations in real time. $\endgroup$ – Russell Borogove Feb 20 at 20:44
  • $\begingroup$ @RussellBorogove My understanding so far is that the guidance would compensate by gimballing the engine during a burn (there is even a low-thrust period while this is carried out), however I dont find much about the RCS system only. This means that for transposition and docking, any translation probably introduced a little rotation, and vice versa. I am just wondering if this was in any way noticed, and if so, was it just handled by attitude hold (killing the rotations while the astronaut focused on the translations) $\endgroup$ – Innovine Feb 20 at 21:44
  • $\begingroup$ Yeah, I was talking about purely computer-controlled maneuvers, which would implicitly include attitude hold. In the close-in maneuvering of docking maneuvers (TD&E and LOR) I suspect the CMP would be aware of even very slow rotations and off-angles, and would manually correct them before getting out of the attitude hold deadband, but that's a guess on my part. $\endgroup$ – Russell Borogove Feb 20 at 22:34
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    $\begingroup$ There's a fabulous document on this ibiblio.org/apollo/hrst/archive/1151.pdf which is quite readable and would make a great answer if somebody wants to summarize it. It all sounds very familiar to a shuttle person - jet selection tables and all. Note the use of jet select tables means "Each group’s rotational effect on the vehicle is essentially independent" $\endgroup$ – Organic Marble Feb 20 at 23:06
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    $\begingroup$ @OrganicMarble I am actually writing some software to model an RCS system, so this document is DROOOL!! $\endgroup$ – Innovine Feb 20 at 23:13

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