Jettison and separation

The integrity check was successful and the crew proceeded with the jettison. It was timed to occur w’hcn the stack, which was holding its attitude constant with respect to the stars, had the LM on the side of the CSM that was facing the Moon. This occurred only once per orbit. Л crewmember pressed a guarded push-button to send a signal to the pyrotechnic circuits. This detonated an explosive cord around the circumference of the tunnel to cut it cleanly.

“And, it’s aw’ay clean. Houston.” said Worden as the remaining air within the tunnel gave the LM a mild push away from the CSM. along with the tunnel and the discarded items in the cabin.

“Roger, copy,” empathised Parker. “Hope you let her go gently. She was a nice one.”

“Oh, she w’as at that,” agreed Worden.

But the trials of Apollo 15’s rendezvous day were not over. The procedures called on the crew to make an RCS burn to open the distance belw’een them and the LM, details of which had been entered into the computer earlier. Scott was then to use P41 to execute this burn.

“Houston, 15,” called Scott. “Question on the separation manoeuvre. Do you want us to burn residuals in P41, or just make 1-foot-per-second burn?”

“Roger. Dave,” replied Parker. "Burn them in P41. please.”

Scott looked at the DSK Y and was not happy with what it was telling them. The magnitude of the burn in the three orthogonal axes was displayed in front of him, and one component was telling him that the burn would take them forward, towards the very object they were trying to avoid. Additionally, the delayed jettison meant that it was difficult to keep a check on the LM because the discarded spacecraft was nearly in line of sight to the Sun.

“Houston, P41 says seven-tenths forward,” pointed out Scott. “Yes. seven-tenths forward, seven-tenths up.”

“Roger. Dave,” confirmed Parker.

“And forw’ard takes us right back to the LM,” reminded Scott.

”Stand by, Dave.’- said Parker, the MOCR’s only point of contact with the crew.

"We’re looking into that, of course."

‘’Okay. We got about a minute and 15 seconds or so.”

‘■Roger.”

Having pointed out the inconsistency. Scott continued with preparations for the burn, trusting that the flight controllers would know what was best. After a pause, he announced, “Average g is on.” The computer had begun to measure acceleration and was about to begin the burn.

‘’Ah, hold the burn, Dave.”

“Okay, we ll hold the burn,” said Scott.

The confusion bciween the MOCR and the spacecraft continued as each party began to use differing terminology to describe the position of the LM with respect to the CSM. Terms such as ‘in front of, ‘dead ahead’ and ‘trailing’ can have multiple meanings in the three-dimensional regime of space. Therefore when mission control suggested that Scott should simply point towards the LM and fire the thrusters to move away from it, Parker compounded the confusion by saying, “We need you behind him and then a firing of retrograde.” Unfortunately, the w’ord ‘retrograde’ has a precise meaning in orbital mechanics: opposite the velocity vector. Since they were leading the LM around the Moon at this point, the instruction’s strict meaning required that they manoeuvre the CSM to be trailing it. then slow’ down their orbital motion slightly in order to increase the separation, which was not what was intended. It wns an example of vehicle-based and orbital-based references being mixed. The MOCR eventually tightened up its language.

“Okay, Dave,” called Parker. “How about 2-foot-per-second posigrade, as long as you’re in front of him. Understand?"

”Okay; so that’ll be a minus-.v dclta-v for two feel per second at our present attitude, right?” checked Scott.

“Roger. That affirm. Dave."

‘’Okay. We’re all in the same frequency. We’ll do that.”