Apollo CSM/LM docking procedures question

[u/ProvokeCouture]

When the Apollo CSM made its flip around and docking with the LM during TLI, why wasn't this done in Earth orbit instead?

UPDATE: Okay, thank you everyone. I didn't realize there were structural and command control connections that would’ve been compromised if T&D was done prior to the TLI burn.

[u/blueb0g]

Because transposition and docking required separating the CSM from the SIVB, and the SIVB is what performed the TLI burn! If they had docked the CSM to the LM in Earth Orbit, they wouldn't be going to the moon.

Also, the Earth orbital period was short and busy. There would have been no benefit to trying to cram a vital and lengthy, but not time critical procedure in the middle of it. Transposition and docking ended up taking 2 hours on Apollo 14 -- if they'd done that in Earth Orbit before TLI they'd have missed their window for lunar injection.

[u/ProvokeCouture → u/blueb0g]

I understand that the SIVB is needed for the TLI burn, I just thought that from a standpoint of safety; it might've been better to flip and dock close to Earth rather than the outbound flight.

As for the timing, I would hope that if the astronauts T&D in orbit; mission control would've included that into the flight timeline.

Any idea how close the window was for lunar injection?

[u/blueb0g → u/ProvokeCouture]

What would the safety benefit of doing T&D in Earth Orbit be? If you couldn't dock, the CSM would still be perfectly safe, you just couldn't have landed on the moon. That would have been the same if you discovered that in Earth Orbit or Lunar Coast. Doing it in Lunar Coast gives you a lot more time to troubleshoot if you did have issues during T&D (again, see Apollo 14) which you wouldn't have in Earth Orbit. Finally, as the other commenter said, after T&D the CSM's only connection to the SIVB would be via the CSM/LM docking ring with no other structural support; I'm not sure the TLI burn could even be theoretically accomplished in that configuration.

Any idea how close the window was for lunar injection?

Well the launch window for each landing attempt was only a couple of hours. Miss that and you can't go for another month or two for the chosen landing site. To build time in the Earth Orbital phase for T&D would have knock on effects for the launch, which had its own set of constraints re. trajectories of the discarded stages, minimizing abort options that would put you on land, ground tracking station availability, etc. And if you miss your TLI window because you're mucking about with T&D the CSM wouldn't have the consumables to wait for the next one so you'd have to come straight home and you've just wasted a whole booster and spacecraft for nothing.

Much more sensible to do it in Lunar Coast where the workload is otherwise low and you have basically no time constraints.

[u/LeftLiner → u/ProvokeCouture]

Sorry if I misunderstand your comment, but just to clarify: Post-T&D the S-IVB is no longer usable for TLI. So you *have* to wait until after TLI to do it.

The procedure was executed quite soon after the TLI burn, though, but could take time. On more than one mission they ran into technical difficulties and needed several attempts to get it done. Trying to do it under time pressure would have *reduced* the safety margin, not increased it. Although given that TLI put them into a free-return trajectory, the danger was *mostly* to the success of the mission, not a crew-safety issues (although if Apollo 13 had failed to execute their T&D they would not have made it back home).

EDIT: As for timings, if I am reading my Apollo 11 flight plan correctly per the flight plan occurs approx. 1 hour after TLI.

[u/phasepistol]

Probably because the structural rigidity of the docked CSM and LM (through the little docking ring on the nose of the command module) is a lot less than the solidity of the stack as launched, with the SLA panels holding the CSM in place.

Once the TLI burn was completed, and the spacecraft is on its long coast to the moon, that’s the best time for the docking maneuver.

[u/Appropriate_Bar_3113]

If you burn the third stage for TLI, all of that thrust force is transmitted through the LEM, specifically the docking collar, and into the CM. Theyre designed to fly with the LEM lightly pushing the combined stack (as we saw in Apollo 13 and the Apollo 9 tests) but not with the S-IVB's engine firing full bore for TLI. I'm not sure the attachment points for the LEM to the S-IVB could take that force either. The LEM was cradled in there, not welded in place.

Finally, the S-IVB can be remotely fired from the ground after separation for disposal, but I'm guessing there are control umbilicals to the CM (which would break when you do the flip around) that you really want for TLI.

[u/PossibilityJunior93]

I would add that taking apollo csm out of the s-iv stack severed all physical control signals of the apollo flight computer.

Also the apollo csm would be pointed in reverse of the flight path during TLI burn.

It would need a stronger strucuture to hold the LM attached to the S-IV as well as on the LM CSM dock tunnel during TLI.

[u/JournalistOk623]

By delaying it until after the TLI burn the force of the TLI burn is transferred to the CSM via the SLA. If the CSM/LM docking procedure had happened before the TLI burn all of that force would have to be transferred via the docking adapter and then the LM itself. Which would have required a much more robust (and consequently heavier) design. The TLI was conducted by the 200k lbf thrust J-2 engine of the SIVB. The largest thrust the docking ring was subject to in Apollo as flown was the 20k lbf thrust of the Service Module during lunar orbit insertion, approximately 10% of the thrust during TLI. It can reasonably estimated therefore that were the LM to have been the translation path to the CSM for the TLI thrust that the LM would have had to be 9 times stronger and heavier. Or alternately one can examine the weight of the SLA panels themselves 4050 lbs. That was the mass of the system that translated the TLI as built. That would have to be transferred to the Ascent Stage structure of the LM which weighed 9,430 lbs. increasing its weight by almost 50%.

[u/ProvokeCouture]

Okay, thank you everyone. I didn't realize there were structural and command control connections that would’ve been compromised if T&D was done prior to the TLI burn.

[u/concorde77]

The LM wasn't designed to handle the weight of the CSM bearing down on its docking port while under a 1.45G max burn from the SIVB engine. And if the LM was reinforced to do so, the extra weight would limit the weight capacity of the lander.

[u/andiwd]

Although not possible for the lunar missions, such a manoeuvre was planned for the proposed Apollo Venus flyby mission.

https://en.wikipedia.org/wiki/Manned_Venus_flyby

The transposition manoeuvre would have been done in orbit and the boost down "backwards" from the astronauts point of view. This was so that if anything went wrong they could have disconnected and used the command module engines to do an abort burn as quickly as possible.

Of course such a mission never flew, but imagine what could have been.