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What dry mass could have soft-landed on the Moon if a Saturn V had been reconfigured to launch a robotic mission going entirely to the surface? Obviously, that wasn't possible at the time, but for a return to the Moon it might make a lot of sense.

Though there are no more Saturn Vs, if SpaceX produces their Mars Colonial Transporter design, Elon says it'll 'make the Apollo moon rocket look small'. So to mock-up large-scale missions to the Moon based on robotics, I'd like to base the payload on what a Saturn V could have done if it had been reconfigured with that in mind.

Here's what there is to work with:

  • Lunar Modules - Ascent - 4800 kg (can add all that, nothing is ascending)
  • Descent Module - 2000 kg dry mass, 8212 kg propellant,
  • Command and Service Module - 11,900 kg dry mass, 16,900 kg fuel (these masses need to be combined to calculate what mass could be landed if there was only one engine system)
  • Launch Escape System - 3600 kg (discarded soon after launch, so some portion of that can be added to payload to the Moon's surface)

So there are a few wrinkles here. What kind of mass savings could be gotten by combining the engines of the LEM and CSM and resizing for something that would land the resultant configuration on the surface? How much of the launch escape system can be regarded as extra payload mass to the Moon? I haven't found a figure for its mass, but would it be reasonable to tack on the mass of the Lunar Module Adapter?

A note - as this is about construction, I'm not really making a distinction between the spacecraft themselves and their payloads. In the context of ongoing missions directed towards settlement, spacecraft are useful materials that can be reused - even leftover fuel. So if it lands, it counts.

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  • $\begingroup$ Any reason you want the mass to be dry?! ;-) $\endgroup$
    – user2705196
    Dec 3, 2015 at 13:50
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    $\begingroup$ @user2705196 "Dry" isn't referring to water, but rather fuel and the like. The dry mass of a rocket is what it weighs before you load fuel, pressurization gas and the like. $\endgroup$
    – Loren Pechtel
    Sep 25, 2018 at 15:06

2 Answers 2

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According to Wikipedia, Saturn V could launch 48600 kg to translunar injection.

From there, you need about 2410m/s of ∆v to soft-land on the moon. Let's take a little additional fuel for safety margin and call it 2700m/s.

Per the rocket equation, assuming you're using a rocket that uses storable hypergolic propellants with a specific impulse of 312s (like the AJ-10 used on the Apollo Service Module), you need a mass ratio of 2.42 to provide that amount of ∆v.

Thus you can soft-land a total of 48600/2.42 = ~20100kg dry mass of ship.

NASA has considered the CECE, a deep-throttling evolution of the RL-10 hydrogen-fueled engine, for future lunar landing missions. This would provide much better specific impulse, but the associated fuel tankage would be much larger in volume and boil-off of cryogenic propellants would be a big concern. With a specific impulse of 455s, the landed dry mass would be about 26200kg.

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  • $\begingroup$ Heh, that was more straightforward than I thought. I'm wondering now if I should add to the question a little to make it more interesting. Are hypergolics still what would be the logical option? What would the mass of the ship be? I'm thinking it doesn't really make senses to ask follow-ups, but instead include that here... $\endgroup$
    – kim holder
    Dec 3, 2015 at 0:01
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    $\begingroup$ Fueled mass of the ship is the 48600kg TLI mass (i.e. same as Apollo CSM+LM) - it's a little more than half fuel at injection. I don't know what the state of the art is for keeping cryo fuels cold on a three-day flight; kerosene-LOX wouldn't give you much improvement in specific impulse, and hydrogen-LOX would mean a much larger volume in tankage, so hypergolics seem the most straightforward option (particularly since we don't need a very large ∆v). $\endgroup$
    – Russell Borogove
    Dec 3, 2015 at 0:07
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    $\begingroup$ Also, that means you can crash-land 48,600kg of e.g. raw materials. Although $0.5 \cdot 48600 \cdot 2410^2$ = 141 gigajoules of impact... that's 33 tons of TNT equivalent. It would need to be a very strong block of raw materials :) $\endgroup$
    – SF.
    Dec 3, 2015 at 8:42
  • $\begingroup$ Simple and elegant answer. I'm curious as to where the deltaV map information was calculated from though? $\endgroup$
    – Sarah Bailey
    Dec 3, 2015 at 14:08
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    $\begingroup$ @SarahBourt - I actually have found a bunch of different numbers and am not sure what's most accurate. en.wikipedia.org/wiki/… gives 5930 m/s from LEO to lunar surface; TLI is 3050-3250 m/s of that according to en.wikipedia.org/wiki/Trans-lunar_injection, so that leaves 2700+ m/s, a little higher than I got from the chart, so these numbers may not be conservative enough. There's also at least 400m/s ∆v difference between a perfect robotic suicide-burn and a wishy-washy Apollo commander's manual lunar landing. $\endgroup$
    – Russell Borogove
    Dec 3, 2015 at 19:53
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And here is the answer from the Apollo era perspective: "Popular Science" text from 1966: "What We'll Do on the Moon", written by dr. Wernher von Braun himself! - Google Books link

Unmanned cargo landers, launched by Saturn V rockets, could soft-land 30,000 pounds apiece on moon - opening way to large stationary and mobile lunar labs and, ultimately, permanent manned bases.

1966 Popular Science illustration, soft landing 30000 pounds cargo on the moon

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  • $\begingroup$ For comparison to the other answer, 30,000 pounds is 13,600 kg, so von Braun's estimate is more conservative. $\endgroup$
    – zwol
    Oct 30, 2018 at 13:13
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    $\begingroup$ @zwol Very similar actually. This figure is for cargo only. The answer of 20100kg is for cargo + structure + engines + avionics + residual fuel. $\endgroup$
    – CuteKItty_pleaseStopBArking
    Feb 22, 2021 at 6:47

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