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The Transposition, Docking and Extraction (TD&E) maneuver was done during the Apollo Moon missions right after the translunar injection. The Command/Service Module (CSM) separated from the S-IVB, pitched 180 degrees, docked with the Lunar Module (LM) and extracted it from the upper stage.

TD&E, part 1 TD&E, part 2

I guess the reason for this delicate and potentially risky maneuver was that the 2 modules had to be stacked like that (CSM on top of the LM) on the Saturn V rocket due to aerodynamics, but during the mission the order had to be switched, so the service module engine could perform the Lunar Orbit Insertion. Is that correct?

Were there some other options considered? Was the engineering team trying to find a way how to avoid this maneuver somehow and simplify the mission profile? What led them to pursue this setup in the end?

I'm speculating that the chosen design meant the least possible load for the Launch Escape System (LES), because anything else I can imagine means the LES would have to carry more mass: CSM installed upside down (docked to the LM) would mean the LES must carry not only CM, but also SM. More mass for LES requires more fuel in the LES which is more mass to carry for the first stage with all the rocket equation problems. Unfortunately I cannot find a relevant resource to confirm this.

There is a plenty of online resources that describe the details of the procedure as such (which I already understand), but not the reasons why it must have been done in the first place. Even though I feel I probably understand at least part of the reason, a reliable reference would be nice.

Checked resources:
Apollo mission tutorial only describes the procedure.
Wikipedia only describes the procedure.
The Apollo 11 Flight Journal only describe the procedure.
How Apollo flew to the Moon book - I only have this preview, not the full book. The available sections again deal with the procedure, not the reasons behind.
Etc.

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Three factors come to mind from my reading over the years: Heat Shield integrity, Escape Tower limitations, and avoiding fuel over the CM on the pad.

Throughout this answer, keep in mind that the stacking order (from the top) is Escape Tower (ET), Command Module (CM), Service Module (SM), Lunar Module Shroud, Lunar Module (LM). See the full Saturn V diagram or this simplified diagram.

The ET, in order to keep its own mass minimal, only lifts the CM and itself. When it is triggered, the CM/SM joins are broken, and the ET's rockets fire.

The CM/SM connections all routed around the heat shield on the CM, none of them go through it. NASA has stated in several documents that this was to preserve the heat shield integrity.

The CM and SM do need to be connected — the capsule is only good for a few hours comfortable operation without the SM's support systems, and NASA's safety culture precluded any in-flight connections being made that were not mission essential. (A failure to dock to the LM would not be a mission failure, per se. It would abort the landing, but the LM was essentially just pushed along with the CM/SM.)

The shroud serves several purposes — one of which is transferring the load of the ET/CM/SM stack down to the S-IVB. The shroud also prevents damage to the LM before and during launch, and keeps the LM separated from the SM, as well as streamlining the stack, and preventing animals being accidentally carried. Note that the LM is not actually connected to the SM while inside the shroud — the CM/SM stack separates from the shroud, which then itself opens, revealing the LM.

The LM itself is a dual stage design — the Lunar Descent Module, and the Lunar Ascent Module.

Alternate Stacking

NASA did consider some alternate stacking schemes, but these were not for the Apollo CM/SM/LM as launched — these final designs were concepts approved well before production designs were completed. Two of the three other stackings considered (see link 1) are worth discussion.

One alternate stack was a CM/SM/LunarLaunch/LunarLanding stack — no on-orbit docking maneuvers. It would have, however, required a larger launcher than the Saturn V, and would have had the potential for losing all three astronauts at once at the moon, and would not have had the orbiting CM for comm relay and visual confirmation. The costs of designing an even larger launcher were considered prohibitive.

The second alternate stack given serious consideration was to launch the LM separately on a second Saturn V. This would have allowed a considerably more powerful LEM and a larger CM as well. It was not, however, considered cost effective, and so was rejected.

Discussion and Elaboration

The CM design could not have had the LM above it — it would have made the escape tower too massive, and the Apollo project was run almost to the safety limit already. Further, that would have put several tons of highly explosive fuel above the CM at load, which would have been rejected even by the (now relatively lax) safety standards of 1960's NASA.

The SM cannot be stacked above the CM for the same reasons, plus even more mass.

That the capsule need not be 2-axis balanced was apparently not considered until later in the program, in the 1970's.

The SM has to be able to use its engines; they're used for mid-course corrections, lunar orbit insertion and trans-Earth injection. Therefore, the LM cannot remain stacked underneath the SM if the SM is to have a single engine. The single engine is a matter of simplicity.

The LM cannot be used as the TLI/LOI engine, either — it would require more structural mass, and the LM engine is left on the surface of the moon.

The LM in the middle requires disconnecting and reconnecting the CM/SM, or having the CM be self-sufficient aside from the rocket motor, or having the connections run around the LM; for a variety of reasons, this was not practical.

So, for reasons of simplicity, the historic stacking mode was the best combination of habitability, safety, and price for the available technology set.

References

  1. https://smartech.gatech.edu/bitstream/1853/8042/3/SSEC_SE2_doc.pdf
  2. http://news.bbc.co.uk/dna/place-lancashire/plain/A3770174
  3. http://spaceflight.nasa.gov/history/apollo/apollo_mission.swf
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    $\begingroup$ This is an excellent answer, thank you. Just one clarification: you mention that SM engine was used for trans-lunar injection, but I think this was done by the Saturn-IVB. The SM engines were used for Lunar orbit insertion and trans-Earth injection. $\endgroup$ – mpv Mar 19 '14 at 9:39
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    $\begingroup$ @mpv -- That is correct. The SM engines were not used for TLI. The TD&E maneuver occurred after TLI. The SM engines were used for midcourse corrections along the way to the Moon (and of course lunar orbit insertion and trans-Earth injection). $\endgroup$ – David Hammen Mar 19 '14 at 17:24
  • $\begingroup$ Wasn't one of the reasons for rejecting the two-launcher option not just cost but complexity of managing and coordinating essentially two launch missions for one landing mission? $\endgroup$ – Anthony X Jun 17 '17 at 20:00
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Probably the biggest reason is they did not want to cut a hole through the heat shield, same reason the Soyuz has the ascent/descent element in the middle of the stack, instead of at the top.

But the Lunar module is not very robust, and probably needed the structural bracing for launch. So probably would not have worked well on top of the Command Module. It would have made the Launch Escape System much harder or more expensive.

Thus Soyuz is your counter example on a much smaller scale.

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    $\begingroup$ Would cutting a hole through the heatshield remove the requirement for transposition? I think it would be very difficult to connect the CSM and the LM in this way, because below the heatshield there is the service module and the huge nozzle. Hole in the heatshield would help only if the service module would be below the LM, but that would require even more complicated transposition maneuver (to free the LM and connect the CM with the SM). The structural strength of the LM seems more plausible as a reason, but I'm not sure if this was the main reason. $\endgroup$ – mpv Mar 16 '14 at 9:20
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    $\begingroup$ There were connections between CM and SM, for electric power, control signals, oxygen and water. These connections were made through the heatshield. These connections were cut just before reentry of the CM by a guillotine assembley in the SM. There was no hole cut in the heatshield, but the heatshield was build around the existing connections. It was not possible to separate CM and SM temporally. $\endgroup$ – Uwe Jun 25 '17 at 15:07
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I guess ... during the mission the order had to be switched, so the service module engine could perform the Lunar Orbit Insertion. Is that correct?

That's one of the reasons, but not the only one.

Before the TDE manuever, the modules are not connected at all. After the TDE maneuver, the CM and LM are connected to each other via an airlock, so the astronauts can move between the two without having to do an EVA. This removes a risky, time-consuming procedure from the mission and makes the journey much more comfortable.

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The Saturn V was equipped with a launch escape system. In case of a dangerous malfunction during launch, there was a smaller rocket on top of the Saturn V which had the job to drag the crew capsule away from the rest of the rocket.

This could only work when the crew was in the topmost part of the rocket. The LM could only house two crewmembers, so all crewmembers had to be in the command module during launch, so the CM had to be the topmost part of the payload.

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  • $\begingroup$ I'm not sure if you missed it, but Geoff already mentions LES. Perhaps you could find some way of expanding on your answer, touch points that Geoff hasn't already? I think there's more to say about it, even if some of it is fairly obvious to most of us. E.g., nobody yet described why the CSM and LM weren't docked (or in position to do so) from the start. Or adding some nice diagrams / photos explaining the maneuver, or something else entirely? $\endgroup$ – TildalWave Mar 15 '14 at 0:10
  • $\begingroup$ The Soyuz is also using Launch Escape System and they don't have the crew in the topmost module. There is the orbital module and below it the landing module with the crew. However if I imagine the LM below LES (and the CSM below LM), that seems kind of fragile. So another option would be to keep CSM on the top, but install it docked with the LM (the nozzle pointing upwards), connect that with the LES somehow reliably and cover under an aerodynamic fairing. Was that considered? $\endgroup$ – mpv Mar 15 '14 at 9:30
  • $\begingroup$ @mpv Maybe I'm reading your comment wrong (and of course it's been a while), but any configuration that does not attach the ET to the CM would incur a mass penalty for abort by ET, which means the ET itself would need more fuel, incurring a mass penalty for the launcher's first stage (since IIRC the ET was jettisoned before first stage cutoff; otherwise it would incur a mass penalty also for later stages). Also, two fully fueled rockets, one meant to be used while the other remains or is in close proximity, sounds like a risky setup to me. LM/SM/CM/ET stack order does not have that problem. $\endgroup$ – a CVn Aug 8 '15 at 22:24

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