Lunar Crew Sizing: The More The Merrier?

As I mentioned in my previous post, after actually reading the TeamVision paper as opposed to merely skimming someone else’s brief summary of their conclusion, I was a lot more impressed than I thought I would be. In spite of going off on what I think is a wrong direction later on in their paper, they had several insights in the first section of the paper that I found rather interesting (and not just because they presented some additional data to confirm my preconceived notions). The first one I want to briefly comment on revolves around the question of how big the crew compliment should be for a lunar surface mission.

The counterintuitive idea from TeamVision was that you could accomplish more exploration, for less money, sooner, and safer by using a two-man architecture instead of a four-man one.

This is an intriguing, and potentially controversial conclusion, and I’d like to give a few of my own thoughts. I’d also like to discuss a little bit about the implications of crew size on an architecture.

Why (Crew) Size Matters
The single biggest reason why crew size is important is how it impacts the overall transportation architecture. It’s fairly intuitive that the more people you have, the bigger your lander needs to be, the more mass you need to start out with in LEO. The more “Initial Mass in Low Earth Orbit” (IMLEO for those of you out there that love you some acronyms) required, the more and/or bigger launchers you need.

For instance, a bare-bones minimal one person lunar mission such as the one George Herbert suggested for his Lunar Millenium Project back in the day could be launched on a single Delta IV Heavy, Atlas 552, or Proton. An upgraded two-person version of the Apollo LEM that used LOX/LH2 for all of it’s interorbital transportation and LOX/Kero for the lander could be launched on two Delta IVH’s, or a Delta IVH and two Atlas V 401s. However for a four-person long-duration architecture like ESAS, you start needing either on-orbit propellant transfer, a large number of EELVs and a lot of on-orbit assembly, or some sort of HLV, and even then you need at least some on-orbit propellant transfer or assembly.

The more assembly, on-orbit tanking, or HLVs you need, the higher a bar you set for initial missions. If you can use existing vehicles, and don’t require a huge amount of up-front development, you can have boots on the ground doing exploration sooner.

Larger landing parties also make for less frequent sorties. HLVs have high fixed infrastructure costs and high marginal costs which tend to make it so you can only afford a couple of missions per year. On-orbit assembly or propellant aggregation from smaller launches also takes a set amount of time to get enough propellant for a mission. The larger the mission, the lower the flight rate. Higher flight rates typically end up making for lower-costs and higher reliability.

The Buddy System
The Moon as Robert Heinlein’s book was titled really is a harsh mistress. While there have never been any fatalities during lunar exploration so far, space is a very dangerous place, and being the first guy to die on the Moon is a rather lousy way of getting yourself into the history books. There’s a reason why out in the wilds the buddy system is often recommended. Many accidents or injuries that are quite survivable when you have a friend with you could end up being fatal if you are by yourself. You don’t want to be like the guy in Utah a few years back who’s arm got pinned under a boulder and then had to proceed to cut his own arm off with a pocketknife because he wasn’t with someone who could help or get help.

Many potentially fatal accidents require nearly immediate attention to have even a decent chance of survival, especially anything related to decompression. Having someone else, nearby greatly increases your odds of surviving such an incident.

What does this mean? It means first that solo missions are probably right out. Sure they’d be a lot easier and cheaper, and lower cost to develop, but the risks of fatal injury are just way too high.

Second, it also means that odd-numbered crews are less effective than even-numbered ones. With three people, you really need all three to go out on an EVA. If you leave one behind, and go on an EVA with the other two, the third man would be taking big risks if he headed out by himself to meet up with or help that initial EVA team, which means he couldn’t really do that much during an emergency. It also means that you’d probably need to size all rovers for three instead of two. While it is a marginal improvement over two, it probably isn’t worth it most of the time. The same ends up happening with five or seven person crews. At least one of the teams will end up a threesome, and while that isn’t the end of the world, and is probably at least a little better than a twosome, the productivity per person is probably a lot lower.

Teams with even numbers seem to the be the ideal size for optimal productivity. So the question boils down to which is better two or four?

    Pros of a Two-Man Architecture

  • Much higher transportation flexibility. A two-person architecture can be launched in a 2-3 launch mode using existing boosters (Atlas V or Delta IVH) with or without on-orbit assembly or propellant transfer. If a Shuttle Derived option is mandated by “political realities”, you could launch the mission on a single smaller SDLV like the DIRECT concept.
  • The lighter IMLEO requirements mean at most only one new booster is needed before lunar missions, which means that lunar missions can start at a much earlier date, and at a much lower cost, with more money available for actually developing hardware to use on the Moon.
  • The lower fixed and marginal costs per person delivered to the moon makes it so you can actually do more exploration on a given budget.
  • Higher flight rates mean more frequent resupply shipments which means lower probability that you’ll have to abandon an outpost due to some important piece of machinery breaking down that you can’t repair due to lack of spare parts. It’s a lot easier to hang on for a month or two than it is for having to wait 3-6 months for a replacement part (as can be seen from recent ISS experiences).
  • Vehicles like the planned LSAM that don’t have an airlock require depressurizing the vehicle before any sortie. This means that if one team is going out, the other team has to don it’s pressure suits as well. With a two-man mission, you don’t have to worry about this. If one is going out, the other one pretty much has to anyway.
  • Smaller, more frequent landings make it easier to explore more places on the lunar surface. This increases the odds that areas with useful resources can be explored much quicker than with a four-person two mission per year architecture.
  • You can still do four-person missions, it just requires a second lander to land at the same site. That way if any of the four gets injured or sick, and needs evacuation to earth, only one person needs to go with him instead of having to prematurely end the rest of the mission.
  • As ISRU technologies mature, a two-person lander can easily be upgraded to a four person lander.
  • A two-man lunar capable CEV could be easily launched on existing boosters. Possibly even ones that are almost man-rated like the Atlas V 401. This means that you can start developing and testing the CEV itself a lot sooner, as you don’t have to wait for the development, testing, and fielding of a completely new booster. This means a lunar capable CEV could be ready by 2010-2012 instead of having to wait until 2014-2016. This also means that an Apollo-8 style mission might be doable by about that time using a “1.5 launch architecture” with an Atlas V 401 and a Delta-IVH.
  • Since a two-man architecture can be done with either LOR or EOR using existing boosters, if the NASA SDLV based design for some reason fails to get developed (due to budget cuts, technical failures, etc), there is a workable backup plan that still will allow for lunar exploration. Backup plans are a very good idea.

    Pros of a Four-Person Architecture

  • If one person is mildly injured, only one EVA team is taken out of commission instead of both.
  • More ground at a given site can be covered more quickly without needing a follow-on mission.
  • Four man landers allow for much larger unitary cargoes to be landed if the payloads can’t be broken down into smaller chunks.
  • There is a psychological benefit to claiming that you’re doing more in a given mission, therefore aren’t just “repeating Apollo” in spite of the fact that you really are.

I could keep going on this discussion, and there are a lot more details on pages 29-33 of the TeamVision report, and I’ve been writing this for too long already.

My personal opinion is that if you really want to get more exploration per given amount of NASA funding, and if you want to begin exploring as soon as possible, that changing the space exploration architecture to a two-man per lander design would be a very good idea. If it’s cheaper, sooner, more productive per dollar, allows you to send more people to the Moon per year, allows you more flexibility, and allows you more programmatic redundancy, at what point do you have to admit that when it comes to crew size, bigger isn’t necessarily better?

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Jonathan Goff

Jonathan Goff

President/CEO at Altius Space Machines
Jonathan Goff is a space technologist, inventor, and serial space entrepreneur who created the Selenian Boondocks blog. Jon was a co-founder of Masten Space Systems, and the founder and CEO of Altius Space Machines, a space robotics startup that he sold to Voyager Space in 2019. Jonathan is currently the Product Strategy Lead for the space station startup Gravitics. His family includes his wife, Tiffany, and five boys: Jarom (deceased), Jonathan, James, Peter, and Andrew. Jon has a BS in Manufacturing Engineering (1999) and an MS in Mechanical Engineering (2007) from Brigham Young University, and served an LDS proselytizing mission in Olongapo, Philippines from 2000-2002.
Jonathan Goff

About Jonathan Goff

Jonathan Goff is a space technologist, inventor, and serial space entrepreneur who created the Selenian Boondocks blog. Jon was a co-founder of Masten Space Systems, and the founder and CEO of Altius Space Machines, a space robotics startup that he sold to Voyager Space in 2019. Jonathan is currently the Product Strategy Lead for the space station startup Gravitics. His family includes his wife, Tiffany, and five boys: Jarom (deceased), Jonathan, James, Peter, and Andrew. Jon has a BS in Manufacturing Engineering (1999) and an MS in Mechanical Engineering (2007) from Brigham Young University, and served an LDS proselytizing mission in Olongapo, Philippines from 2000-2002.
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11 Responses to Lunar Crew Sizing: The More The Merrier?

  1. Paul Breed says:

    Do you think the first non-government
    trip will be a well funded expidition, or a single person stunt?

    I can see someone like Steve Fawcett
    doing just to say they did.

    Could a single person mission be reduced in size to the point it would fit on the largest Falcon 9?

  2. murphydyne says:

    I personally would not work on such a project. I think Jon’s on the right track with the Buddy System. It’s used in so many other activities that it obviously makes sense for use on our Moon.

  3. Anonymous says:


    Four people on the Moon brings a lot of benefits that are not enumerated in your post.

    The formost benefit is that there are more people to actually do work. If all NASA was going to do is a little bit of science, pick up rocks, and other things, then two people would be fine. However, if we want to really do ISRU, technology development, and actually build out a robust outpost, then you really need four people.

    With a ~4380 hour crew rotation (6 months), and working the crew during daylight hours on outside task (illumination factor of 0.7 at the poles) you get a maximum number of work hours of ~5840 hours of crew time (4 crew x 8 hours per day). Also, with telepresence technology, the other time can be used productively by operating from the Earth.

    This shifts the crew work from being the primary workers to support and construction crew. If ISRU is the focus, then there is a heck of a lot of Oxygen that can be processed. We need people on the Moon as much as we need machines.


  4. Anonymous says:

    Jon, thanks for taking the time to read the paper. You may have noticed that once we are into Era 4 we are back to using ELV’s to get the crew to the EML1 Station for Lunar Crew rotations via a Re-useable ISRU fueled Mars Proto-type lander. The EML1 Station would be a central integration point for all space activities/methods joining all capabilities into a cohesive and synergistic effort; (Public/Private/International) Lift Vehicles/Space Programs, Mars, Moon, Lunar Surface ISRU, Manned Exploration, Robotic Exploration, Missions to the Gas Giants, Telescope Repair, Sun Shield, etc.

    The engineering/business model moved us towards ELV’s as soon as it could because continuing to use an HLV for crew rotations would have consumed the entire budget leaving little money to build additional pieces. Any good architecture needs to provide at least 25% surplus at all times over operations to continue building incrementally pieces or it will stop dead in its tracks. Not chucking billion dollar LSAM’s is a good place to start.

    We put up to four people on the Lunar Surface once we have Lunar habitats and six once we have the EML1 station up and running. Again not throwing away hardware is a good thing. Shoehorning four into a LSAM is not good thing either. It’s too small for a habitat and too big/$$$ to throw away after each mission.

    The method of Lunar Surface Rendezvous (LSR) is superior way of increasing mission scope provided the crew component can return independently of the stage elements. Direct Ascent Architectures have a definite advantage in mass to surface.

    Robot rovers will also have an important role exploring a vast area around the landing area and delivering return samples to the astronauts.

  5. tankmodeler says:


    Jon did leave the door open to 4 person teams on the moon, just not to 4-person LSAMs. That is where the real cost and architecture problems are. If you want 4 guys for a certain mission, put two 2-person crews on the ground. With the money you save from not developing the HLV (or the Stick, in a logical world) you could esily afford a two to one flight ratio to achive the same number of boots on the ground as the current plan. As Jon states, the incremental costs of the existing launchers should come down after boosting the flight rates.

    Plus, you add flexibility for the missions and add a lifeboat emergency mode that gets an injured crew home without scrubbing the entire mission. You can stagger launch and arrival dates to have your site continuously occupied and make exploration more efficient with one crew briefing and teaching the next.

    In addition, if you are in a higher rate production and launch mode, then sending cargo LSAMs to the site to deliver larger habitats, equipment, spares, vehicles and the like becomes much, much easier and cheaper. Apollo demonstrated that you could land within 300m of your desired point with no exterior landing aids. Imagine how close we could get them if an existing groundstation is providing a “fix” for the landing systems of the follow-on vehicles?

    High flight rates are no longer normal at NASA and they may have reservations on doing them, but Gemini proved it could be done. 12 mission in 24 months with several being multiple launches of Geminis and Agenas. It can be done off the same pads if we think it through up front.

    So, even if you decide you really have to have 4 people on-site to do a good job, the 2-man crew seems a much better vehicle size overall.


  6. Jon Goff says:

    Paul Breed,
    A one-person stunt would be really dangerous. And as Ken points out, there’s a reason why the Buddy System is used in so many places–it has a good record for increasing the odds of getting you back alive.

    That said, I think the first non-government trip may well be a stunt. While it may be possible to launch a single-man mission on a single Falcon IX S9 launch, I doubt it’s realistic. Falcon IX doesn’t exist yet period, and even if he does deliver it, he’s already stated that he’ll only do the S9 variant if someone pre-pays for the upgrades. Add to that the fact that a one-person mission isn’t safe, and that you’d have to be very agressive mass-wise to pull it off…

    I think the first private lunar mission is going to be two-person. One pilot, and one passenger.


  7. Jon Goff says:

    Paul beat me to the punch. By the time you’re doing ISRU work or base development, you’re going to have a habitat on the ground. You’re going to be using Lunar Surface Rendezvous to place lots of cargo missions along with crew missions. The ability to have 4-people on the ground at one point doesn’t imply that they should all be launched at the same time.

    The ability to get there a lot sooner, cheaper, and probably at a lower cost per person by going with a two-man design seems to outweigh any benefits of doing a 4-person lander (at least as the first generation lander). You get a lot more flexibility by having two vehicles.

    I don’t doubt that in the long run, the more people there at a base, the more you can get accomplished. I’m just dubious of the importance of all of them arriving on the same vehicle.


  8. Brad says:

    The mission architecture advantages of a smaller 2-man crew spacecraft are important, and something I’ve contemplated myself in a non-lunar mission context.

    What I don’t understand from my cursory scan of the Team Vision plan, is why their two-man lunar lander doesn’t take full advantage of the two-man mission size? The 7.1 tonne capsule for the two-man lunar crew is the same behemoth as the 8 tonne capsule for six-man ISS crew transfers. So a giant Jupiter II launch vehicle is needed with it’s 80 tonne TLI capacity to launch a two-man mission.

  9. Anonymous says:

    We kept it at a similar size to be less disruptive of the current approach. Our new capsule design is six places for ISS/Emergency, four for EML1, and two for Lunar surface. All this fits in a 4.5m Diameter and has a mass reduction of about 20% over the 5.0m. With this reduction we can perform the mission in the AIAA paper with a 2-4-Seg SRB, 4 RS-68B, under a 8.4m tank with 4-J2X for the second stage. This is our minimum cost/time point design at present. The other important modification was going from Methane to LOX/LH2.

  10. Brad says:

    hey anon

    Thanks for the updated capsule and Jupiter plan info.

    I have always thought the 5m Orion capsule was oversized. It seemed to me that 4.5m was the largest size needed to accomplish the NASA requirements, and a capsule even smaller such as the old BAE study for a 4m ‘multi-role capsule’ could do the job.

    But to followup on your two man lunar surface mission design, I still have some questions. If you are using a capsule which can carry a crew of four (whether it is a 5m or 4.5m capsule) why send only two crew? I thought the whole point of a two crew mission (as in the old Gemini direct scheme) was to use a small, lightweight two man capsule and thereby permit a much smaller launch vehicle.

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