DOI with the LM

For Apollo 10’s rehearsal and for the first two landings, the CSM remained in its 110-kilometre orbit, leaving the LM to enter the descent orbit itself with a DOI burn. On Apollo 11, this was a 30-second burn, 15 seconds with 10 per cent throttle and the remainder with the throttle set to 40 per cent. Having a period of time at a low thrust setting allowed the gimbal mechanism on which the descent engine was mounted, to align the engine’s thrust with the spacecraft’s centre of mass. This DOI burn was carried out around the far side, 180 degrees from the desired perilune point, and was a retrograde burn – one that went against their direction of travel.