T-19 Days to STS-135 - Why is there a Lunar Module?

" Landing a man on the moon and returning him safely to the earth." is a pretty daunting challenge.  Moving people and equipment in space requires lots of rocket power.  The more the equipment weighs, the more rocket power is required.  More rocket power, means more fuel, which is more weight.  A huge, vicious circle that resulted in the huge Saturn V and also the Lunar Module. 

LEM Diagram

The  LEM was created as an alternative to landing the command module on the moon.  A number of approaches had been considered.  One had been to separate the Command Module (CM) from the Service Module (SM) and land the command module.  Although adding another spacecraft to the equipment added weight, creating a separate landing craft meant a lighter module could be created to land on the moon.  That meant smaller rockets and less fuel to land.  It also meant a smaller module to take off from the moon, so less fuel there, too.  After analyzing the alternatives, the LEM approach was the winner.

LEM Controls in Capsule

Every pound counted.  The LEM would not be flying through any atmosphere, so a streamlined shape was not required.  The angular, irregular shape was the result of removing all the streamlining and just leaving what is necessary.  Since it wasn't returning to Earth, the heavy heat shield for reentry was not needed.  Pound by pound this approach won over the CM landing approach.  It could be counter intuitive, that we saved overall weight by adding the weight of the LEM. The paradox was repeated within the LEM, by equipping it with a second rocket for ascent rather than to reuse the landing engine.

Unused LEM on Display at Smithsonian

The LEM was made as a two stage spacecraft.  The Descent stage housed a more powerful rocket to land on the moon.  It also contained the heavier landing gear, supplies, and the oxygen and other life support consumables required for the lunar visit.  The Descent module, at about 23,000 pounds, became the launch pad for the Ascent stage. This permitted the Ascent stage to be reduced to 10,300 pounds plus the men and the moon rocks they collected.  A much smaller rocket and fuel was required for the Ascent stage to get back to orbiting SCM.

Meanwhile, unmanned Apollo launches are wrapping up. The first manned Apollo is set for February 1967, we are getting ready to go to the moon

http://en.wikipedia.org/wiki/Apollo_Lunar_Module