In my view, space is either going to go a certain way or we are not going.
The first deciding factor is whether getting human beings off-world into independent colonies is going to be made the goal. This is actually how it all must start- by everyone agreeing that these survival colonies are the ultimate goal. An insurance policy for our species is the only way we are going to end up in space. These independent colonies are going to be artificial spinning hollow moons made of lunar material, at least for the next couple centuries. How big they are going to be is the interesting question. There is going to be a magic number based on available material strength and the most efficient construction technique. Diameter will be in miles anyway- whether a few or many is not clear. The fanboys do not get it and think we are going to Mars.
The second deciding factor is how to first create that Cislunar infrastructure that will turn the Moon into a factory manufacturing artificial worlds. The critical enabler is going to be a Nova-class Super Heavy Lift Vehicle (SHLV) with a “Fat Workshop.” Exactly how big the vehicle has to be likely depends on how large the wet workshop is. The Saturn V was 33 feet in diameter and since a cosmic ray water shield is going to be 15 feet or so thick then a double envelope may be up to 60 feet in diameter. One option might be the smaller diameter upper stage being placed inside the large diameter lower stage to effect the double envelope. The final design is going to be larger than any presently planned iteration of the SLS. The minimum acceptable amount of living space for long duration missions is the psychological requirement that will determine the size of the SHLV.
These workshops, when attached to each other with tether systems, will provide that vital prerequisite of a near-sea-level-radiation and one-gravity environment.
The third determining factor after how to get humans in the vicinity of the Moon for long periods are the Landers that will take workers down to the factories under the lunar surface. If there is ice at the poles in solid sheets and it has volatiles trapped in it then Robot Landers will be able to exploit these resources and make methane propellent, which is much easier to store and transfer than liquid hydrogen. The semi-expendable Landers will be able to intercept wet workshops and insert them into frozen lunar orbits and then fill their cosmic ray shields with water in preparation for astronauts. Landers can also dock with these crew sections and boost them into Lunar Cycler orbits around the Earth and Moon. Once the Lunar Cycler fleet and a ring of human-crewed GEO telecom platforms is in place then the first true spaceships can be assembled.
The variables are the resources on the Moon- such as whether there are comet volatiles trapped in ice and whether there are suitable large lava tubes that can be used as ready made factory sites.
The Narrow Path
The problem is radiation and zero G. Dosing and debilitation. The solution is the ice on the Moon used to fill a cosmic ray water shield. And of course a Lander to get the water and bring it up to the crew compartment. That habitat is going to have to be some kind of Skylab wet-workshop and only the SLS, likely a larger iteration with much bigger boosters, can place something big enough to hold that much water in the vicinity of the Moon. For every ton of tap water brought from Earth twenty-two tons can be brought up from the lunar poles. And water is by far the most utilitarian shielding material.
These are the hard facts, the elephant in the room. The article by Eugene Parker in Scientific American from 2006, “Shielding Space Travelers” makes it clear these plans for some kind of Lunar ISS are not going to work. It is not just radiation, artificial gravity to prevent debilitation is also required for long duration missions and a tether system is the only practical way to do that. A near-sea-level-radiation and one-gravity environment is the prerequisite and there is no way around that. It is long past time to end permanent damage to astronauts, especially young females.
The narrow path, not the flexible path, is the only path- and it means abandoning LEO and funding more SLS production to enable a shuttle era cadence. That is really the only way a “sustainable” human presence Beyond Earth Orbit (BEO) is possible.
Ice on the Moon 
I think the problem ultimately ends up being is that you have such radically different views from most other people on what short term and long term human spaceflight should look like and it ultimately makes attempting to come to any agreements an exercise in futility. The arguments the commenters present are valid; you don’t need 5m of water to shield from radiation if you’re only going for 90 day stays to the moon. But since you’re based in the idea that long term huge spinning space stations are the goal, the argument that intermediary steps of LEO and all that is pointless is valid in that context.
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Hi Barry and Hug. Or Hug if that is your sock puppet. I managed to post about a dozen comments on Ars Technica before being banned again. The problem the spacex fans do not see is that the “intermediary steps” have all been taken and are now a waste of time and resources. They are simply the “pork” the NewSpace crowd is always wailing and gnashing their teeth over. Of course NewSpace is in reality a far worse and profoundly ruinous ideology and has done more damage to space exploration than any “pork project” has ever done. But sadly, like neoliberalism, they are the dominant force in regards to space exploration right now. 5m of water is what is needed and your condition of “short term” is nonsensical considering the distances to any destination beyond the Moon worth going to (Ceres is the closest). It is the basic required amount of shielding for anyone in space. Even traveling to the Moon will require a Lunar Cycler fleet because it is just too unhealthy in an unshielded spacecraft. The same for “huge” spinning space stations. The minimum is going to that 5m water shield, a certain amount of living space to meet a minimum psychological limit, and the structure and systems that go with it- which for even a couple astronauts will be well over a thousands tons. Well over that. And at the end of the several thousand foot tether system is going to be an equal mass. You might consider this “huge” but it is simply the bare minimum necessary if human beings are not going to suffer permanent damage. Like the Saturn V was the bare minimum necessary to get to the Moon and only Lunar Orbit Rendezvous (LOR) made it possible without a larger Nova rocket. Thanks for the comment. I would ask you to try and herd some other people here and get some kind of discussion going that is NOT simply a spacex infomercial.
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