Why did they take the Service Module with them around the moon and back? They weren’t using it, its outgassing was making visual observations difficult, the outgassing was also adding a bit of unwanted yaw to the LM/CM/SM stack, and during maneuvers the mass of the SM was causing stress at the LM/CM juncture. I know after they finally separated from it they took photos of it through windows but couldn’t they have done that anytime earlier in the flight after it had failed?
Or is my question just another case of hindsight being 20/20?
They didn’t want to expose the command module heatshield to space until just before reentry. There were concerns about the heatshield getting too hot or cold and cracking from being exposed too long to space and sunlight.
There was some discussion of jettisoning it, for the reasons you mention, but they decided to keep it in place to protect the heat shield from meteoroid impacts and UV deterioration from sunlight. (Guess AndrewL types faster than I do!)
This site has a few reasons why keeping the CM and SM attached was a good idea. Mainly: No residual power from the SM fuel cells at the beginning of the crisis, thermal management difficulties for the CM, it was unknown how long term exposure to space would affect the heat shield, potential flight control issues.
This paper from the AIAA Space 2009 Conference lists these reasons for why NASA decided to not jettison the SM:
How much delta-v did they have left in the LEM’s tanks when it was all said and done? That’s the important thing. Those are the pros of keeping the SM attached, the con is that every burn you do with it attached costs more fuel.
They hadn’t fully emptied the descent tanks in the LM; IIRC, had they needed to, they could have jettisoned the decent stage and taken advantage of the ascent engine, no?
I had always thought that the SM was only partially disabled; some systems were dead, but others were working. Was it totally dead? (I guess if they were thinking of discarding it, it’d have to have been…)
Oxygen tanks were empty, and power cells were dead because of that, but they could still power the systems in the SM from the batteries in the CM and the LM. The RCS fuel tanks were still mostly full, and most of the RCS thrusters were still working (a few had valves jam shut in the explosion) so in theory they could have still used those. In theory, they could also have still used the main engine in the LM, it still had mostly full fuel tanks and they could power it from the CM. In practice, they weren’t confident that it hadn’t been damaged in the explosion. The high-gain antenna was damaged in the explosion, so not much point trying to use that. So other than the remaining RCS thrusters there wasn’t much they could confidently use.
Yes, but the ascent stage only has a few hours of battery life and oxygen in it - the consumables they needed to survive the trip back were all in the descent stage.