Vincent McConnell recently asked me about the descent orbit insertion maneuvers on Apollos 15-17. I explained that, yes, on these J missions the descent orbit insertion maneuvers were performed by the CSMs to conserve LM DPS propellant, permitting the LMs to carry more mass to the lunar surface. This extra mass included the lunar rovers, additional scientific equipment, an extra descent stage battery and other consumables for a 3-day surface stay.
But this always seemed inefficient to me as the CSM had to place itself as well as the LM into the descent orbit. Then, after LM separation for landing, the CSM had to burn still more propellant to return to its original circular orbit. The CSM's SPS engine was larger than the DPS and lacked a variable throttle so it had a slightly higher Isp of 314.5 sec vs 305 sec for the LM DPS, but this probably still didn't make up for the inefficiency of moving the CSM's extra mass to the descent orbit and back.
I assumed this was due to an early design decision that gave too much of the Saturn V's payload capacity to the CSM and not enough to the LM. But I still have some questions about this.
First, am I correct that performing the DOI with the CSM's SPS was less propellant-efficient than doing it with the LM's DPS?
Second, how was the Saturn V payload capacity originally divided between the CSM and LM? Given that the LM was overweight during most of its development, creating intense pressure to shed weight (and slipping the schedule and probably increasing risk) why wasn't the mass allocation revisited and adjusted earlier in the program? Did it quickly become too late to make any such changes?
