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This answer to the question Apollo-11 delta-v “due to residual pressure in the docking tunnel pushing the spacecraft apart” detected as it happened? Was it corrected? quotes the following passage from the Apollo 11 Mission Report; MSC-00171

The third problem area was the large number of trajectory perturba­tion in revolutions 11 through 13 because of uncoupled attitude maneuvers, such as hot firing tests of the lunar module thrusters, undocking impulse, station-keeping activity, sublimator operation and possibly tunnel and cabin venting. The net effect of these perturbations was a sizeable down­ range miss.

Question: What are "uncoupled attitude maneuvers" and why do them?

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The RCS thrusters on Apollo come in balanced pairs, one on each side of the vessel's center of gravity. Normally, if you wanted to change your orientation, you'd fire both thrusters in a pair, producing a rotation but not a translation.

However, these thrusters have a minimum power level. If you want to create a very small attitude change, you'd fire just one of them to start the rotation, fire the counteracting thruster in the other pair to stop, and accept that you'll pick up a bit of translational velocity.

Additionally, when testing the thrusters to ensure they work, you fire them one at a time. It's a lot easier to tell the difference between "one thruster firing" and "zero thrusters firing" than it is to tell the difference between "one thruster firing" and "two thrusters firing", and if one of the thrusters is under-performing, it's far easier to tell if that's the only one active.

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  • $\begingroup$ concise, clear and complete, excellent answer, thanks! $\endgroup$ – uhoh Jul 21 at 22:11

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