It's popularly understood that the Apollo program only selected lunar-orbital rendezvous as a mission profile after first thoroughly rejecting the direct ascent profile. It seems as though the decision to go with LOR was highly controversial at the time, because it was seen as very risky relative to the other options (earth-orbital rendezvous and direct).

My question is -- why? In retrospect, of course, lunar-orbital rendezvous not only turned out to be the "right" solution from a design perspective, but it was largely a non-issue (notwithstanding the challenges of Gemini 4). To me, rendezvous seems to be primarily a problem of orbital mechanics, which is purely basic Newtonian physics. Of course, as Gemini 4 demonstrated, it isn't a trivial problem... but why was it not seen as a solvable engineering problem? Isn't it much less complex than the risky and dramatic engineering needed to build bigger and more reliable rockets?

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  • $\begingroup$ It seems to me that bigger rockets have a tendency to go boom :) $\endgroup$
    – kgutwin
    Aug 3 '18 at 15:13
  • $\begingroup$ See these two questions: 1, 2. $\endgroup$
    – Uwe
    Aug 3 '18 at 15:33
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    $\begingroup$ When LOR was proposed in 1961, there had been 0 rendezvous carried out in space. Proving the technique took most of the Gemini program. $\endgroup$
    – Hobbes
    Aug 3 '18 at 17:03
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    $\begingroup$ The cornerstone for this was Aldrins thesis about rendezvous (dspace.mit.edu/handle/1721.1/12652), which is from 1963. In 1961, they had not figured out how to do rendezvous, had not done it successfully before, and simply were not comfortable trying to do it in lunar orbit. $\endgroup$
    – Polygnome
    Aug 4 '18 at 7:30
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    $\begingroup$ Space is big. Really big. $\endgroup$
    – Mark Adler
    Aug 4 '18 at 15:35

It wasn't a non-issue at all, it was a complex technological problem to solve. When the Apollo program was conceived space flight was still in its infancy, humans had been in orbit, just. Orbital rendezvous was theoretically possible, but required technologies, techniques and procedures that nobody was sure could be developed in the time-frame:

  • Precision guidance
  • Miniaturized computers
  • Space radar system
  • Approach and docking mechanisms

At the time the proposed rocket designs had the power for direct ascent, and there was confidence they could be built (at least in some circles), so direct ascent seemed less risky. Once the realities of engineering and paying for the Nova size rockets set in, and there was more confidence in being able to develop the underlying technologies, LOR won out.

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    $\begingroup$ To add on a bit: In the early 60's it wasn't at all clear that tracking could be done accurately enough fast enough to do this. Error's of several 100's of feet/sec (those were the units of the time) only came down after multiple tracking passes, but there wasn't time for that after multiple maneuver burns. Trying to bring two Gemini capsules together with a 200fps excess close rate was shown to exceed the capsules maneuver capacity, and (amazingly, given their small size), still was a risk of collision. It took lots of work on procedures & tech to get past that. $\endgroup$ Aug 3 '18 at 16:04
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    $\begingroup$ useful reference: nasa.gov/centers/langley/news/factsheets/Rendezvous.html $\endgroup$
    – Hobbes
    Aug 4 '18 at 7:33
  • $\begingroup$ The development of space radar could use about 20 years of experience with radar in aircrafts. Radar for measurement of LM to CM distance used a transponder at CM. The tyranny of the radar equation was reduced from 1/r^4 to 1/r^2. Much less radar transmitter power was necessary and bigger distances were possible by the use of a transponder. $\endgroup$
    – Uwe
    Aug 5 '18 at 11:09

In addition to the complexities of actually performing the docking there is the additional detail that the abort options are significantly reduced. If one always has the fuel for the return trip, one can abort at any time. By only having the means to return in orbit, one would have to first rendezvous in Lunar orbit, which means the abort options were somewhat limited.

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    $\begingroup$ The service propulsion system of the Apollo spacecraft carried plenty of fuel for a direct abort at any point on the trip to the moon as did the lunar module's descent engine, leaving 2 direct abort options. The LM's descent engine was even considered for a direct abort on Apollo 13, however it would have required ditching the damaged SM before the burn. There was a concern that exposing the CM's heat shield to space for so long could damage it, so free return was used instead. With direct ascent, if an engine fails and you've only got one, how much fuel you have isn't the issue. $\endgroup$ Aug 3 '18 at 18:41
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    $\begingroup$ Am I missing something @JustinBraun? Getting the LM into a return isn't really a viable abort option without some way of docking to a CM for re-entry. $\endgroup$
    – user20636
    Aug 3 '18 at 20:56
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    $\begingroup$ I'm more talking about abort situations once on the Moon itself. If you have to dock back up with the orbiting spacecraft, you can't just launch whenever you want to, you have to launch at a time that will provide a rendezvous. $\endgroup$
    – PearsonArtPhoto
    Aug 4 '18 at 1:27

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