The recent Moon Landing Hoax thread
http://boards.straightdope.com/sdmb/showthread.php?threadid=82912
brings up a question. Did the LEM do the manuvering to rendevous with the CSM or did the CSM descend to a lower orbit to meet it?
The LM essentially came up to meet the CSM, but the CMP was always prepared to decend to capture a low flying LM if necessary. Also, although both crafts were manuverable in orbit via small thrusters, only the CSM had the throtleable engine. The LM rocket was a simple on or off deal. Any serious manuvering would have had to have been done by the CSM, though I can’t recall any instance when the LM ascent engine didn’t perform exactly as it was supposed to.
Except in Apollo 13, when it performed spectacularly as it wasn’t supposed to do.
They didn’t call it the LM Lifeboat for nothing…
Ahh, yes, but that was the LM decent engine. The Apollo 13 crew never even had the chance to light the LM ascent engine. In fact, as the LM ascent engine only produced 400lbs of total thrust, I wonder how useful it would have been in altering the Apollo 13 course back onto a free return trajectory.
More on the ascent engine and rendezvous, as per the OP…
The CSM normally orbited at an altitude of 60 miles, but could descend to 10,000 ft if necessary to make an emergency rendezvous with an ailing LM ascent stage. They timed the LM upper stage launch to coincide with the CSM flyover so they could perform this maneuver if it were warrented. But as the moon has more than a few 30,000 ft mountain peaks, no one was really interested in trying this (and of course, they never had to).
Once in orbit, it was up to the LM crew to catch up to the CSM. They were flying in a lower, and therefore faster orbit, so catching up meant firing the ascent engine again to raise themselves to a higher, slower orbit. Once the two spacecraft were within capture distance, it was the CMP who did the final maneuvering.