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  Why was weight a factor in landing on the moon?

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Author Topic:   Why was weight a factor in landing on the moon?
Sam
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From: South Africa
Registered: Jun 2014

posted 07-08-2014 02:53 AM     Click Here to See the Profile for Sam   Click Here to Email Sam     Edit/Delete Message   Reply w/Quote
Why was weight so important for landing on the moon?

I heard that the lunar module had been redesigned a few times in order to save on on weight.

spaced out
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From: Paris, France
Registered: Aug 2003

posted 07-08-2014 06:33 AM     Click Here to See the Profile for spaced out   Click Here to Email spaced out     Edit/Delete Message   Reply w/Quote
Other people can probably explain better than me but here's some very rough calculations (which I'm sure someone will point out are wrong) to give a general idea...

Let's say you design a piece of the Lunar Module and you find it weighs 1lb more than expected.

What difference does 1lb make?

Well, according to wikipedia a typical Lunar Module weighed 14.8 tons, of which 4.6 tons were in the ascent stage and 10.2 tons in the descent stage.

Adding 1lb to the weight of the ascent stage means that descent stage needs to be proportionally heavier. The extra weight of propellant etc required in the descent stage would be roughly 10.15/4.55 x 1lb = 2.23lbs.

So overall the Lunar Module now weighs 1 + 2.23 = 3.23lbs more than before.

Also according to wikipedia, a typical Saturn V rocket could carry a payload of 45 tons to the moon (TLI) based on an overall weight of the launcher of 3,000 tons.

The ratio of overall launcher weight to payload is therefore roughly 3000/45 = 66.67:1.

This means that to carry the extra 3.23lbs of LM mass to the moon adds 3.23 * 66.67 = 215lbs to the overall weight of the Saturn V launcher.

Basically adding 1lb to the mass of the LM equates to an additional 215lbs of mass at launch based on this rough calculation. An extra 9.5lbs equates to roughly a ton of extra launch weight.

That's why every weight saving that could be made during the design of the lunar module was so important.

rjurek349
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posted 07-08-2014 09:49 AM     Click Here to See the Profile for rjurek349   Click Here to Email rjurek349     Edit/Delete Message   Reply w/Quote
In a word: fuel.

Captain Apollo
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From: UK
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posted 07-08-2014 11:04 AM     Click Here to See the Profile for Captain Apollo   Click Here to Email Captain Apollo     Edit/Delete Message   Reply w/Quote
Given this, is their any data on the weight of Apollo astronauts? Did NASA care, try to ensure that slimline persons went to the moon?

Sam
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From: South Africa
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posted 07-08-2014 11:08 AM     Click Here to See the Profile for Sam   Click Here to Email Sam     Edit/Delete Message   Reply w/Quote
Thanks very much.

Also, what was the reasoning behind the lunar orbit rendezvous? Why didn't they create a lunar module and command module in one. So that all three astronauts could walk on the moon.

Jim Behling
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posted 07-08-2014 12:24 PM     Click Here to See the Profile for Jim Behling   Click Here to Email Jim Behling     Edit/Delete Message   Reply w/Quote
The lunar orbit rendezvous would require less spacecraft mass (CSM and LM) than the direct approach of one spacecraft. There was no necessity for all three astronauts to walk on the moon.

Less mass is required for the separate spacecraft because propellent is not needed to land a heat shield, parachutes, etc. on the moon. Nor is the propellant needed to return to earth landed on the moon.

mikej
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From: Germantown, WI USA
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posted 07-08-2014 07:37 PM     Click Here to See the Profile for mikej   Click Here to Email mikej     Edit/Delete Message   Reply w/Quote
The NASA Technical Reports Server used to have a series of LEM Mass Property Reports, dating back to July 1963. (They haven't been re-added since the NTRS was down in early May 2013.)

There are numerous discussions regarding the impact of the LM's weight. Early on, Grumman formulated growth factors:

  • All items carried from separation to touchdown (staged) must be multiplied by the Descent Growth Factor 2.824.
  • All items carried from Separation to Burnout (unstaged) [i.e., back to lunar orbit] must be multiplied by the Round Trip Growth Factor 6.682.
  • To obtain the affect of items carried from lift-off to burnout in the Ascent Stage multiply the item by the Ascent Growth Factor 2.366.
  • The penalty for lunar liftoff capability differs from the Round Trip Growth Factor. The lunar lift-off payload is only carried
    from the lunar surface to Burnout, however, the Descent stage must carry an increase in propellant to land the Ascent stage propellant required to carry the lunar payload. The lunar lift-off capability factor is 4.285.
This included things like the additional propellant, increases to the propellant tanks to hold the additional propellant, and increases to the landing legs.

At one point, NASA decided to increase the theoretical size of the astronauts from 65 to 75 percentile men (which added 13.2 pounds). I seem to recall reading somewhere (although apparently not in the Mass Properties Reports) that they were considering reducing rations to a lower calorie count, again to shave weight.

As noted above, the weight of the LM also influenced the size of the Saturn V. Stages to Saturn takes a slightly different look at it:

To get one more kilogram of payload, the laws of orbital mechanics required that 14 kilograms be cut from the S-IC; or four to five kilograms from the S-II; but only one from the S-IVB.
As noted in the Mass Properties Reports, consideration to the size of the astronauts was given.

As to the choice of lunar orbit rendezvous, there were several advantages. As noted in another post, less spacecraft mass was required. This implies that a single Saturn V, rather than some newer and larger booster (or multiple Saturn Vs), was sufficient to launch the lunar mission.

Developing a newer, larger booster would have taken additional time, making it unlikely to land a man on the moon before the end of the decade.

A direct ascent would have required landing a rocket approximately the size of an Atlas on the lunar surface. It took a small army to launch an Atlas, but launching from the lunar surface would have to be done by only the crew, with little to no access to the booster during the countdown.

It would have been much more challenging to land such a large vehicle on the lunar surface. Additionally, most studies had the astronauts in their couches -- attempting to land this large spacecraft on their backs, with no direct viewing the landing area.

You can read more about LOR in Enchanted Rendezvous: John C. Houbolt and the Genesis of the Lunar-Orbit Rendezvous Concept [direct link to 1.3M PDF].

MCroft04
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From: Smithfield, Me, USA
Registered: Mar 2005

posted 07-08-2014 08:45 PM     Click Here to See the Profile for MCroft04   Click Here to Email MCroft04     Edit/Delete Message   Reply w/Quote
The book Moon Lander by Tom Kelly discusses the weight issue in Chapter 8.

All times are CT (US)

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