The final stage of the TD&E process was the ejection of the lunar module from the top of the S-IVB, which was not a simple process of just throwing a switch and watching it happen. Throwing the switch would come at the end, but first, they had to get the signal from the switch in the CM, down past the LM to the pyrotechnically fired spring thrusters that pushed the LM free. That meant that the CMP had to connect two umbilical cables that ran power and signals between the two spacecraft.
However, to do that, he had to get into the tunnel, which is a short void between the command module's forward hatch and the lunar module's overhead hatch. Immediately after the docking, the tunnel was still a vacuum and the CMP had therefore to bleed air from the CM into the void and, in doing so, he also fed air into the LM cabin. Prior to launch, the dump valve in the LM's overhead hatch was left open so that, as the Saturn V lofted both spacecraft to orbit, the atmosphere within the LM was gradually exhausted, leaving the interior essentially a vacuum. By
docking with the LM, the tunnel had been placed over the hatch and, therefore, also over the dump valve. Consequently, when the CM cabin's oxygen was fed into the tunnel, it also entered the LM. Once the air pressure on both sides of the forward hatch was equalised, the hatch itself could be removed, the umbilicals could be connected to feed power to the pyrotechnic devices for freeing the LM, and a check could be made to ensure that all twelve latches had properly engaged. Finally, the switch could be thrown to eject the LM from the S-IVB and allow the Apollo stack to continue its journey to the Moon.
None of these steps was simple, as each had its own checklist of items that had to be set or verified to ensure that the crew did not configure the spacecraft in a way that might endanger their lives. Each step was carried out in a slow, methodical fashion of checks, verification and cross-checks, just to allow them to throw one switch.
In some romantic sense, the S-IVB stage had the most bittersweet, almost tragic fate of all the Saturn components. These large, perhaps elegant stages had been faithful servants to their Apollo masters, who they dutifully sent onwards to the Moon. They were spared the ignominious crash into the sea that befell their larger brethren, the SIC and the S-II. Instead, they were sent away from Earth to meet a celestial end. Of the ten manned Saturn V third stages - nine of which were Moon bound - half were sent to impact the Moon's surface at high speed in the name of science and lunar seismometry, while the others flew away from the Earth-Moon system to follow lonely orbits around the Sun.
Once the Apollo spacecraft left them, they became spacecraft in their own right, controlled from Earth and by their own internal systems in the Saturn's instrument unit until either their batteries ran out or the ground stations ceased to track the receding hulks. The people who controlled the S-IVB from Earth had to use what little residual propulsion the stage had left to achieve these final ends.
After translunar injection, both the S-IVB and the spacecraft were on very similar trajectories which were basically long, elliptical orbits. However, the intervening gravitational influence of the Moon determined the final fate of both craft. While the Apollo spacecraft continued on a path to lunar orbit, the S-IVB was given one of two fates.
For the early lunar Apollo missions, a decision was made to ensure that the S-IVB would be taken well clear of the spacecraft and, in effect, dumped in solar orbit. To achieve this, its remaining propulsion was used to slow it down, and while the spacecraft passed the Moon's leading hemisphere, the stage was targeted to pass the Moon's trailing hemisphere. By doing so, it received a gravitational slingshot that threw it out of the Earth-Moon system. This was the fate of four of the Apollo S-IVB stages and they are out there, drifting still.
Although the Apollo 9 mission never went to the Moon, its S-IVB was nevertheless sent out of Earth orbit as a rehearsal and it, too, orbits the Sun. Like the others, it is slightly inside Earth's orbit and periodically catches up with Earth.
Apollo 17's S-IVB after extraction of the LM.
As Apollo 17 headed out from the Moon, the crew saw something in the distance flashing at them regularly. Jack Schmitt had seen it earlier and Cernan had caught a glimpse of it. ''Hey, Bob, I'm looking at what Jack was talking about,'' said Cernan to Robert Parker, their Capcom, ''It is a bright object, and it's obviously rotating because it's flashing. It's way out in the distance. It's apparently rotating in a very rhythmic fashion because the flashes come around almost on time.''
They discussed the idea of turning the spacecraft around to enable them to look at the object with their optics, which were mounted on the opposite side. What could be seen out of the windows could not be viewed through the optics any more than windows at the front of a house could be used to look around the back. Anyway, Schmitt was in the habit of using a 10-power monocular to view Earth's weather patterns and when he trained it on the object, he reckoned it to be their S-IVB, some way off.
''One unique thing about it, Bob, is that it's got two flashes,'' said Cernan. ''As it comes around in rhythmic fashion, you get a very bright flash; and then you get a dull flash. And then it'll come around with a bright flash, and then a dull flash.'' ''That's the side of the S-IVB,'' said Schmitt, ''and then the engine bell, Gene.'' Cernan didn't believe him. ''The commander doesn't think that I can see the engine bell on that thing,'' commented Schmitt.
''Roger, Jack. Is that with the monocular you're looking at it?'' asked Parker. ''He couldn't see the engine bell if he had ten monoculars,'' said Cernan wryly. ''Bob, couple of revolutions ago when I was looking at it, I had a much brighter view and I believe I was looking at it broadside,'' said Schmitt. ''It looks to me like it may be flashing more or less end-on now. But it's not as bright now as it was a while ago. I just hadn't put it together as maybe being the S-IVB. I thought it was just some other particle out there.''
''Hey, Bob,'' said Cernan later. ''We got two of those flashers out there. They could be SLA panels. I don't know. They're alike in intensity and pretty regular in the bright and dim flashes they come out with, and they're widely separated.''
We'll never know whether Cernan and Schmitt were seeing the S-IVB stage or a couple of SLA panels. But it was not the last meeting the human race would have with an Apollo cast-off.
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