Upthread: Recommendation to retain the Two-Stage Lunar Orbit Insertion (LOI) Maneuver (Aug 05, 1968)
See list attachedFebruary 5, 196969-PA-T-14APA/Chief, Apollo Data Priority CoordinationTwo-stage LOI looks good after C’
Just like in other fields of endeavor, it always seems possible to use actual flight results to prove how smart you were before the flight. I am writing this note to crow about how C' proved we “done right” in planning a two-stage LOI.
As you recall we originally considered manually backing up the GNCS during LOI to avoid an overburn using both burn duration AND the EMS ΔV counter. However, when we got down to detailed planning on how to do this, we concluded that we had insufficient confidence in the ΔV counter to wait for it to clock out since the consequences of an overburn are catastrophic. Furthermore, although it sounds simple, monitoring three data sources simultaneously and taking proper action at this critical time turned out to be messy. As a result, the final C' procedure was to backup the GNCS by manually shutting down the SPS if it exceeded the LOI₁ estimated burn duration by more than six seconds. This value was consistent with the 60 x 170 n.m. initial lunar orbit. If we had been using a one-stage LOI our rule would have had to be for the crew to shut down manually just about at the nominal burn duration (no delay) in order to avoid an unsafe pericynthion in the event of a high thrust engine.
On C' LOI₁ we actually experienced a burn duration 4.9 seconds in excess of that expected. Therefore, given a one-stage LOI on C' the crew would have shut down the SPS manually even though the G&N was operating properly and then they would have had to make a second burn of about five seconds duration to finish it off. (In addition to that, we would have been unable to utilize the flexibility of the two-burn LOI targeting to com- pensate for the trajectory dispersion following the last translunar mid- course correction and we would have ended up with a 64 mile altitude on the back of the moon rather than a 60 circular orbit.)
Incidentally, our other pre-flight conclusion, that is, lack of con- fidence in the ΔV counter was also proven correct on this flight by several in-flight anomalies including an erratic accelerometer!
Weren't we smart?