Plane change manoeuvre

When the entire Apollo stack of LM and CSM arrived at the Moon, it was placed in an orbit that would pass over the landing site at the time of landing. After the LM had set off for the surface, the CSM returned to a 110-kilometre orbit if it wasn’t already there. While the surface crew carried out their exploration, the Moon continued to rotate on its axis and. in most cases. Look the landing site away from the orbital plane of the CSM. The exception was Apollo 11. for which the landing site and the CSM’s orbit were more or less aligned with the equator, with the result that the landing site did not significantly stray from the CSM’s ground track. On all the other landings there was sufficient tilt in the CSM’s orbit to require a plane change manoeuvre, and given the LM’s minimal fuel reserves, it was most efficient for the CSM to make it. Therefore, at some point while his colleagues were on the surface, the CMP in orbit executed a plane change manoeuvre.

Changing the plane of the orbit required a burn of the SPS engine of between 10 and 20 seconds. Unlike height adjusting burns that added or subtracted energy from the orbit by firing along the spacecraft’s direction of motion, a plane change burn was usually made at righi-angles to the orbital plane, often near the point where it crossed the Moon’s equator. Preparations for this burn w ere just the same as for any other SPS burn, except that it was usually made using only one of the two engine control systems in view of its short duration and, similar to the circularisation burn, everything had to be done by the CMP alone. In mission control, FIDO calculated ihc details of a burn that would achieve the objectives with minimum use of propellant. This information was written on a PAD and read up to the lone crewman, ready to be entered into the computer under Program 30.

Richard Cordon was the first CMP to fire the spacecraft’s big engine alone in lunar orbit. Rather than make the burn just before the LM returned, he carried it out the previous evening, at the end of the day they landed. "I realised at this time that it had been a real long day and I was tired and more prone to make mistakes. I certainly didn’t w-ant to be making mistakes during an SPS burn.’’ Normally for any SPS burn, two of the crewmen worked together through a checklist using the ‘challenge and response’ technique designed to ensure that no step w:as missed a luxury the solo CMP did not have. Gordon’s solution was to have mission control listen to him as he went through each step. Fortunately, unlike most SPS burns, the plane change was made while in communication with Harlh and he had 14 minutes between acquisition of signal (AOS) and the actual burn.

“When I came around this lime and had AOS. I chose to go to VOX operation and read the checklist as I performed it. to the ground so they could monitor exactly where I was, exactly what I was doing, and would be abreast of the status of the spacecraft at all times.” VOX meant that his transmissions were controlled by a voice-operated switch. Each time the CMP spoke, his words were transmitted to Earth, and there was no need to operate a push-lo-lalk button. "It gave me the assurance that I was reading the checklist correctly, not leaving anything out. Now, 1 would think that the ground probably appreciated this. They knew exactly where I was in the checklist, what I was doing, and if I was behind and if I was ahead, so if any particular problem came up. they knew that I was with it or behind it.” Without the weight of his two crewmales and their lunar module, the SPS burn felt much more sporty, as Gordon noted post-flight: ”’l’he acceleration, of course, is much more noticeable than with the LM docked.”

CSM to the rescue

Given a normal mission, the role of the CMP might seem to be minimal in the upcoming orbital ballet of rendezvous, but NASA’s defence-in-depth policy ensured that he would have plenty to keep him occupied. It is true that the LM was always the active participant, as it was its responsibility to get off the Moon, into lunar orbit, then find, track and pull alongside the CSM. But the CMP had the role of rescuer in case the LM failed to execute the rendezvous. For this possibility, he had practised a wide range of scenarios where the CSM would become the active spacecraft and w’ould hunt down an ailing LM.