14

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.

TildalWave
  • 75,950
  • 13
  • 274
  • 455
kim holder
  • 21,377
  • 9
  • 85
  • 188

2 Answers2

23

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.

Russell Borogove
  • 168,364
  • 13
  • 593
  • 699
  • 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... – kim holder Dec 03 '15 at 00:01
  • 1
    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). – Russell Borogove Dec 03 '15 at 00:07
  • 2
    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 :) – SF. Dec 03 '15 at 08:42
  • Simple and elegant answer. I'm curious as to where the deltaV map information was calculated from though? – Ezra Bailey Dec 03 '15 at 14:08
  • Ten tons - so maybe a small nuclear reactor and factory-in-a-box. – Chris B. Behrens Dec 03 '15 at 17:54
  • 3
    @SarahBourt - I actually have found a bunch of different numbers and am not sure what's most accurate. https://en.wikipedia.org/wiki/Delta-v_budget#Earth.E2.80.93Moon_space.E2.80.94high_thrust gives 5930 m/s from LEO to lunar surface; TLI is 3050-3250 m/s of that according to https://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. – Russell Borogove Dec 03 '15 at 19:53
  • Per this answer, Low Lunar Orbit to lunar surface is 1870 m/s one way. (Unsourced claim, however.) So that leaves 540 m/s delta-v for the TLI to LLO orbital injection based on your figure of 2410 m/s from TLI to lunar surface. I honestly can't really judge whether that figure is reasonable or not. – user Dec 03 '15 at 21:12
  • @MichaelKjörling: What about going from low lunar orbit to an elliptical "orbit" whose pericenter is below the lunar surface? One might end up with a pretty big trench by human standards, but I don't know that the effect of any man-made object would be significant compared with some of the other things that have hit the Moon. – supercat Feb 22 '16 at 01:29
9

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

szulat
  • 773
  • 7
  • 9