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When I was young a zillion years ago I visited the Smithsonian's Air and Space museum, and admired what I think must have been a mercury capsule. Upon peering through the window, what struck me first was the construction. Thick plates of aluminum, large electrical components, etc. These were the materials and components of the era.

Fast forward to this comment speculating about the BFR's capability of putting an Apollo stack on its way to the Moon (it seems not to be possible).

I'm wondering, roughly, hypothetically, if one wanted to build a working replica of an Apollo Moon landing mission from current aerospace materials being used in spacecraft and land on the Moon with a crew as a sort of homage, how much lighter would it be?

One third lighter maybe? Could it be half the weight?

Would modern engines, electronics, or electromechanics (motors, pumps, etc) help a lot, or would it be mostly structural weight?

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    $\begingroup$ With modern engineering techniques you could probably get it down to 180% of the weight of the original. $\endgroup$
    – Russell Borogove
    Mar 22, 2018 at 23:56
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    $\begingroup$ That's what I mean, yeah. $\endgroup$
    – Russell Borogove
    Mar 23, 2018 at 0:57
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    $\begingroup$ @RussellBorogove is referring, I think with his tongue firmly in his cheek, to current trends in government contractor aerospace design. +1 $\endgroup$
    – Organic Marble
    Mar 23, 2018 at 1:02
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    $\begingroup$ I'm not an aerospace engineer, so this is a comment instead of an answer, but I would guess that if you were fanatical about not allowing any new features to creep into the design, you could get everything down by 20-30%. Airframes haven't gotten hugely lighter over the last 50 years. Pure digital control systems are much smaller, but you still need power electronics to drive mechanical actuators, and those won't be much different. You still need consumables for 3 men x 2 weeks, etc. Figure on sending 35 tons into TLI, rather than 47 tons. You'll need a methane upper stage for Falcon Heavy. $\endgroup$
    – Russell Borogove
    Mar 23, 2018 at 21:07
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    $\begingroup$ Even without new features, just complying with current NASA safety standards would give you the 180% $\endgroup$
    – pericynthion
    Mar 24, 2018 at 1:34

2 Answers 2

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CSM/LM weight is dominated by propellants. CAD might allow you to reduce structural weight a bit, although from the description, they were already using e.g. milled skin panels to minimize structural weight. Electronics is where the biggest gains will be, but the stack has a limited amount of computers.

The biggest gain would come from replacing the engines with higher-Isp ones, which would drop the propellant weight.

What's needed for a better answer is a weight breakdown by subsystem of the CSM and LM, but I haven't found that yet.

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  • $\begingroup$ A low pressure tank could not be used if very simple and reliable pressure feed should be used again. $\endgroup$
    – Uwe
    Mar 31, 2018 at 12:09
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    $\begingroup$ The ISP of the Apollo engines is quite good for storable hypergolics; pump-fed engines would be heavier for only a small Isp gain. You'd only get appreciable gains by going to hydrogen-oxygen, which means you need a different thermal solution, which brings you dangerously close to "complete redesign" territory. There's some breakdown of the CM mass in the Wikipedia article: en.wikipedia.org/wiki/Apollo_Command/Service_Module if you want to take a stab at it. ntrs and history.nasa.gov will have all the numbers but they're going to be spread across dozens of docs... $\endgroup$
    – Russell Borogove
    Mar 31, 2018 at 12:13
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    $\begingroup$ Note also that while the mass is dominated by propellant, the required propellant mass should drop about linearly as you take mass off other parts. $\endgroup$
    – Russell Borogove
    Mar 31, 2018 at 12:17
  • $\begingroup$ Why does hypergolics imply low pressure tanks? Most hypergolic upper stage engine systems are pressure-fed, as was the Service Propulsion System. I would think the engine design drives the tank design, not the propellant. $\endgroup$
    – Organic Marble
    Mar 31, 2018 at 12:46
  • $\begingroup$ (at)RussellBorogove has an excellent point! That the weight might be dominated by propellants is irrelevant. Lower the collective weight of everything which is non-propellant bye 30%, and the required propellent will also scale by 30%. "Tryanny" of the Rocket Equation (that's what the title and the url say!) $\endgroup$
    – uhoh
    Mar 31, 2018 at 16:43
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I reckon it would be a quarter of the weight. The computers alone in the original Apollo landers must've weighed 100kg. Think now that we have the same computing power in a mobile phone! if not more. Yes they'd have redundant triple back up systems with data storage that is RAID'd but even then if they had 10kg of computer equipment just to match what they did in the Apollo days I'd be surprised. Light composites and incredibly tiny gyroscopes (Ring-laser gyro's now weight next to nothing by comparison). However the ISS still carries incredibly heavy gyro's for station stabilisation so that might be an inaccurate assumption upon my part.
All I can think is that whatever weight they'd be saving with new model landers would be made up for with the increased weights in many other areas (e.g. more fuel). Be fascinating to see what they come up with. I used to remember a TV show way back when, called 'Space 1999', and the Lunar transporters they had on that show were really cool. Hope someone designs something like that in the future! :)

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    $\begingroup$ In order to support an estimate of "one quarter" of the original weight, it might be better to focus on components that represent a large fraction of the weight, rather than a few specific items that are only a small part of it. $\endgroup$
    – uhoh
    Mar 31, 2018 at 4:44

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