Apollo 9 SPS1 countdown Gimbal lock

[grummanonewire]

So Ive been waiting for SPS 1 after the RCS sep burn and seem to be flirting with gimbal lock and over the natural course of coasting I go into Gimbal lock Ive thought of a couple workarounds
1 do a V49 and position so I will not get close while waiting to manuever into burn attitude
2 let Gimbal lock happen and recover looking in the Gand C checklist from Apollo 15 I dont see a quick recovery procedure from GDC and I loose GDC attitude as well which makes me believe the GDC goes into gimbal lock as well
3 is there any documentation or source that indicated how they would have done a rapid recovery to proceed with CMC control of the Burn or would they have just proceeded with SCS and recovered the platform after ? or just delay the burn ? I know the the MCC burns were less time critical possibly but for a time critical burn such as LOI-1 or LOI-2 TEI or Retro

 

[indy91]

The flight plan has an item "maneuver to maintain visual contact with S-IVB" so that would definitely include "don't drift into gimbal lock" even if the sep maneuver attitude is fairly close to it. If you let the CSM drift going back to 0° yaw before that is definitely a good idea.

The GDC has no gimbals to lock, but from the schematics I concluded that keeping an accurate attitude will be degraded close to 90° yaw, because of an amplifier saturating. I have not been able to find good sources on this except for the fact that the GDC does in fact become less accurate beyond 60° yaw. The way I implemented it is that the GDC has near perfect accuracy up to 80° yaw and then it starts degrading.

There is a Rapid IMU Realign procedure assuming a good GDC alignment on page 7-1 of the Apollo 15 CSM G&C Checklist. But when the GDC also has lost its alignment then a P51 is probably the only thing you can do, maybe a Backup GDC Alignment but that isn't really quicker than a P51/P52. You would simply be no-go for LOI-1 or any other time critical maneuver until you recover, if that actually happened. Might be a reason why they maneuvered quite early to the LOI-1 burn attitude to prevent any chance of this happening close the ignition time.