Apollo 13 question regarding the Service Module separation

[Mushtang]

A few nights ago I watched the movie Apollo 13 and near the end when they separated the Service Module and Lovell was commenting that "one whole side of the spacecraft is missing" I wondered about the timing of the separation.

Why didn't they separate the SM sooner, before mid-course corrections had to be made, etc.? I'm assuming there was still *some* value to the SM such as some battery or other connections that weren't damaged so it was worth staying connected to that mass but I don't know for sure.

If I remember correctly they didn't shut all the power off until after the PC+2 burn which they assumed put them on the correct trajectory for a return to Earth and at that point it wouldn't matter if the unneeded mass was attached or not. But, when they realized they needed to do that mid course correction they were then using up more fuel and time than they would have if the SM was let go first.

Anyone know for sure why it was beneficial to bring the SM all the way to nearly reentry before cutting it lose?

 

[user 13]

Gene Kranz did not want to expose the CM heatshield to space for 4 days. It was not designed for that and keeping the SM attached would protect it from damage on the return trip.

My guess is Kranz weighed the threats and decided Keeping the SM was the safest option. Remember that they did not know the condition of the heatshield after the accident.

 

[Mushtang]

That's a great reason! Thanks for letting me know.

Also, the kid in your avatar was in the movie as Sy, so that's even better! Hahaha.

 

[SpaceManMat]

It was not certain how long the LM would work for, there’s a whole bunch of equipment on the SM that potentially could be useful if something unexpected cropped up. Basically no reason to get rid of it, so keep it as long as you can.

 

[Mushtang]

there’s a whole bunch of equipment on the SM that potentially could be useful

I wasn't sure how dead the CM was, it sounded like it was 100% useless as far as equipment, energy, propulsion, etc. so they powered it off for the long trip and discarded it at the end of the trip before reentry.

Basically no reason to get rid of it, so keep it as long as you can.

There actually was a great reason to get rid of it, but at least one great reason to keep it (insulating the heat shield) so they kept it.

 

[JStarStar]

I suppose if the course correction burns they had to do with the LM descent engine had used up all or most of their delta-V capacity, they might have ditched the SM to reduce the total weight of the vehicle, but as others have noted they didn't like the idea of the CM heat shield sitting exposed to space for three days.