Bye bye, SV1

After the horizon check, and a check of the VHF radio, the crew entered a code to tell P62 that they were about to shed the service module. The spacecraft was then yawed 45 degrees to the left. Since the CM’s thrusters could not impart translation motions, only rotations, it was the SM’s task to manoeuvre clear in order to minimise the risk of a collision.

Jettisoning the service module was an intricate task left almost entirely to automatic systems that not only severed the interface cables and pipes safely, they also ensured that the SM moved clear. However, the SM wns the primary source of power, oxygen and cooling, and inadvertently separating the two modules any earlier in the flight would have had disastrous consequences for the crew. Therefore, as with most other events that occurred only once yet demanded the highest reliability, the SHCS took care of disconnecting the umbilical lines between the two modules, cutting the ties that held them together and controlling the SM’s evasive manoeuvres, all of which occurred in just a few seconds.

The process began when the CMP applied power to the logic circuits of the SECS and armed the pyrotechnic system, connecting dedicated batteries to their control circuits. After the arming sequence was complete, the flip of a switch began the separation sequence. As w’ould be expected, this switch was guarded by a metal cover to avoid accidental operation. The SECS then assumed control. A command was sent to a controller box in the service module. This started a timer to trigger the RCS jets on the SM. ready to move the now unwanted hardware away from the CM after separation. Many systems on board the SM remained active to support the separation process. In particular. tw’O fuel cells continued to supply power for the jettison controller and the thrusters, and to fire the pyrotechnics that severed the steel lies holding the CM and SM together. First the eleetrieal connections across the umbilical w ere carefully disconnected by an arrangement of cams and levers in the CM, powered by a small explosive charge that literally unplugged the cables between the two modules and then another pyrotechnic charge drove a guillotine through all the cables and plumbing that ran between them. On command from the SM’s event controller, each of the three strong tension ties that ran through the heatshicld to hold the twro modules together w’as cut by two separate explosive charges in a manner that allowed springs beneath each of six support pads to push the modules apart.

As the service module came free, its controller fired its jets both to pull it away from the command module and to impart a spin that attempted to stabilise its

motion. On early missions those thrusters that were working to pull it away continued to fire until depletion or failure, but despite these efforts to take the SM well away from the CM the complex dynamics of the remaining propellant in the tanks caused it to turn around and approach the CM again.

During their debriefing, the Apollo 11 crew were asked if they saw their service module.

“Yes. It flew by us," said Mike Collins.

“It flew by to the right and a little above us, straight ahead," added Buzz Aldrin. "It was spinning up. It was first visible in window number four, then later in window number two, really spinning."

This problem w’as cured by shorter separation burns.

After the SM had gone, the crew quickly checked the pressures in their RCS tanks, safed the system that had fired the explosive devices, and checked that their batteries still had enough pow’er for the final leg of the mission. The CM w as yaw ed back 45 degrees in order to face backwards along their flight path and then pitched down to the correct attitude for atmospheric entry. At this point. Collins noted how the weak thrust from water vapour leaving the steam duct interfered with his attempts to yaw. The vapour came from the evaporator, now’ their only means of losing heat. “When I got a yaw’ rate started, the water boiler would fight me, the rate would reduce to near zero, and 1 would then have to make another input."

At this point, in a heads-down attitude with the lift vector up and with everything verified, the CMP pressed ‘Proceed’ on the DSKY to give the autopilot control of their attitude. Its display showed their impact coordinates and a check of their heads – up, down status at wliich point ‘Proceed’ was again pressed to pass control of the re­entry profile fully into the computer’s hands.