Advantages and Disadvantages
One advantage of lunar orbit rendezvous is that the propellant necessary to return from lunar orbit back to the Earth need not be carried down to the Moon, and back up from the Moon again. This saves the propellant needed to move that propellant, which is considerable; it also reduces tankage weight and greatly reduces the sizes of the engines for the lunar lander.
Disadvantages include the lunar lander needing a separate life-support system and requiring two sets of engines, one on the lunar lander and another attached to the command module.
Cutting systems back to the absolute minimum levels to achieve maximum fuel economy includes risks, also. Redundancy is often critical to survival. The LOR mode which was selected for Apollo offered some degree of redundancy for critical systems that gave the system more flexibility in handling unplanned events. For example, the separate systems enabled the survival of the Apollo 13 astronauts when the primary craft was disabled by an explosion. In that case, the LM's separate systems were pressed into service to save the lives of the astronauts, even though using the LM as a "lifeboat" was not part of its specifications (but was envisioned as a contingency prior to Apollo 13).
Read more about this topic: Lunar Orbit Rendezvous
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