posted on Oct, 22 2008 @ 04:07 PM
Originally posted by sovietman
ngchunter, thanks for your time, but i think we didn't understand each other very well. For better understanding I made a (really) quick sketch of my
idea. It is located on this link. It's true that the heat shield and
larger (=heavier) living spaces would have to be brought to Moon surface and back to orbit, but isn't it still more fuel efficient than launching
from Earth with additional living spaces and two additional engines?
From your sketch I'm guessing that you're going for a design that basically brings a LEM minus the ascent stage with living area. That's kind of
what I assumed into my calculations later in my post. More on that in a second.
Maybe it isn't, I thought it was.
It isn't. Reason why - the command module weighs more than twice what the LEM's entire ascent stage (minus ascent fuel) weighed. For a trip to the
surface and back using apollo's engines you need a certain ratio of fuel to payload mass. 10,750kg fuel/2,000kg living space = 5.375. In other
words, you need more than 5 times as much mass in fuel than the mass of living space for a trip to the surface and back. This still assumes that you
leave your excess descent equipment behind on the lunar surface the way the real LEM did. It also assumes your ascent engine is significantly lighter
than your descent engine since you don't need as much thrust because you're not carrying descent fuel anymore - not the case in your example, so
we'll just be generous and pretend the engine size isn't a factor. Just for the sake of showing how generous this is, here's an image of a LEM
ascent engine model:
About the size of a watercooler. For comparison, here is the descent engine:
Ok, enough about that, let's just pretend for simplicity sake that that's not an issue and for some reason your universal descent/ascent engine is
lighter and smaller than the LEM's descent engine. We know that we need to bring at least 5 times as much fuel for the landing/takeoff as the mass
of our living space assuming we leave stuff behind on the lunar surface and discard the rest when we reach orbit again. 5 x 5,400kg = 27,000kg vs 5 x
2000kg = 10,000kg. That's 17,000kg extra mass you need to bring to lug that heavy command module to the surface and back - this is why they made an
entirely separate landing module - it needed to be optimized for mass in every possible way. Payload mass is an absolute premium in space travel,
every single kg counts big time. You'd save only 2,000kg if you dropped the redundant living area of the LEM, but it would cost you a whopping
17,000kg in extra fuel you would need. 17,000 minus the 2,000 you saved is 15,000kg. That would require a radical redesign of everything from the
Saturn V to the descent stage itself.
And another question . Why would you take additional crewman and all life support system for him with you if that man does nothing but sitting in
CSM when the other two are playing on the Moon? Again docking with an unmanned module was done before the first Apollo missions.
Good question. It is true that Gemini docked with an unmanned Agena as you wisely pointed out. In fact, as you know, that was done to practice for
Apollo. Apollo was somewhat of a unique case though. For apollo, and in particular the later missions, the slow rotation of the moon moved the
launch site of the LEM out of the plane of the Command Module's orbit during the time the astronauts were on the moon. On earth you can just wait
another day for a launch window, on the moon it takes a month to complete a single rotation and the astronauts didn't have nearly that long to wait.
That meant the command module (and service module with it of course) must complete a plane alignment to put it back in a place where the LEM's ascent
stage could reach it because the LEM didn't have the fuel for an alignment of that magnitude. Even in the earlier missions when the rendezvous
procedure was much more conservative I think it was important to have the redundant ability to use the command module pilot to finish the docking
should the LEM underperform during the proceedure (assuming it could still safely reach orbit close enough to be rescued).
[edit on 22-10-2008 by ngchunter]