Concept diagram of command module structure... | Command module design | Forum

…wherein I reveal that I can't draw in 3D very well at all.

There are basically 3 parts to this imagining of the CM structure, which comes in two options, A and B.

It's easiest to begin with the skeleton, which is idenatical for both options.  This is the crissy-crossy looking thing that's mostly red with some blue parts.  First thing to note: the red and blue parts are all part of the same structure, made from the same material, etc.  The colour differences correspond to differences in the function of parts of the skeleton.  The red rounded part of the skeleton is conical in shape and supports the outer conical shell, which acts as a heat and other shields.  The blue straight parts are to support the main pressure vessel – the big blue box that's on the left of both diagrams.

The main pressure vessel is simply pushed into the skeleton from behind, positioned in the "cradle" formed by the blue parts of the skeleton and then bolted (or whatever) in place.  The pressure vessel and the skeleton can be manufactured completely separately as long as the sizes and shapes are made accurately enough that they mate well later on.

There are two options I can think of for the outer shell.  One is to build it up by attaching a large number of individual plates to the skeleton (option A above), the other is to create the entire shell as a single solid piece and them simply slide the skeleton into it from behind (option B).  Which option is best will probably depend on (i) which is structurally stronger and (ii) what we make the shell out of and how hard it is to create a single solid shell.

Structurally speaking, the skeleton is (I think) the easiest thing to make.  It just requires a whole bunch of flat strips of an appropriate metal (something very strong – light too would be nice but since the skeleton is relatively small compared to other components it's not as important.  Titanium might be a good choice?) and the facilities to bend and weld those strips.  It feels like we could make this in a well equipped, large-enough backyard workshop.

The main pressure vessel needs to be strong too, but unlike the skeleton there's a lot of whatever we make it out of, so weight is more important.  Something like aluminium or an aluminium-magnesium alloy might be a nice choice.  I don't know how easy it would be to make – the walls may need to be relatively thick (I think the pressure vessel structure will end up contributing a lot to radiation shielding for the astronaut and main computers).

I've mentioned the possibility of a carbon composite material for the shell.  We would probably want to have a sandwich kind of structure for the shell, with CC on the outside, some thin fabricy stuff in the middle to help with heat shielding, radiation shielding, etc and maybe some very thin, light metal of fibreglass on the inside.

The usual disclaimers apply: I am not any kind of engineer and this design is based largely on what feels to me like common sense.

With regards to figuring out a scale for all this – the pressure vessel and the supply tanks are the key here.  The cone should have the smallest dimensions possible (minimising surface area, not volume, would be best I think, from the point of view of making reentry easier) consistent with the pressure vessel and the supply tanks fitting inside of it (there should be enough room for everything else if those two constraints are met), so our first port of call should be to estimate the size of these two things.

I think the limiting factor in pressure vessel size will be having enough room for the astronaut to suit up and suit down before and after EVA.  We could perhaps approach this by making roughly correctly shaped vessels of different sizes out of plywood, sticking a car seat inside and getting someone to get into/outoff some bulky overalls, boots and a bike helmet.

Getting tank dimension estimates for our needs shouldn't be too hard.

I agree that ideally we probably want the pressure vessel to be as close to completely thermally insulated from the outer shell as possible.  This stops precious life support heat radiating into space during the mission and keeps the crew maximally safe during reentry.  Rather than making the skeleton out of something with a poor thermal conductivity (which may severely limit the range of materials) maybe we should design the way that the pressure vessel attaches to the skeleton so that there is a layer of something highly insulating between the two materials?

Any electronics which are newly inside the pressure vessel in this plan would have been on the outside of the vessel in a vacuum in the previous plan, so the vacuum safe requirement is nothing new.  However I guess the point is that we would still have to worry about their ability to endure radiative cooling, just not for quite as long.

I did read somewhere that some of the equipment on the outside of the Gemini pressure vessel were stored in their own small pressurised compartments, but that sounds like a lot of bother.

Mass and reusability are valid points, though.

Well, why not a hybrid of the two approaches?  Have a non-conical pressure vessel as in the diagrams above, to save on mass and allow reusability, but stick the electronics inside it anyway (possibly in their own pressurised compartments), and leave consumable tanks on the outside.