Something I learned about how spacecraft are built. | Spacecraft Engineering Workgroup | Forum

Well, flipping through the Gemini manuals I posted recently has already paid off for me, I learned something which has importantly changed my understanding about how Gemini was built (and I presume other craft) are built.  Spacecraft Engineering is the part of this project I know the least about, perhaps what I'm about to explain is common knowledge to everyone else, but in case it's not, here we go.

The conical structure of Gemini was not itself pressurised – that is, the walls of the truncated cone shape you see when looking at the spacecraft from the outside are not themselves holding in the cabin's atmosphere.  Rather, inside the cone is a pressurised box, roughly a rectangular prism in shape, doing that job – the cone is like a shield around the box.  Equipment that requires an atmosphere to work is inside the box, equipment that works fine in a vacuum is mounted to the outside of the box, in the depressurised space between it and the cone.

Those of you who have the Gemini familiarisation manual can see this arrangement very clearly in figure 3-1, on page 44 of the pdf.

In terms of important structural work, it is the outer cone's job to provide heat shielding during reentry and micrometeoroid and radiation shielding while in orbit.  It is the box's job to contain pressure and it is the box that will have to be kept at a comfortable temperature.

This arrangement has particular implications for heat issues during reentry.  It is the cone that heats up due to friction with the air, not the crew's pressurised box.  Heat gets from the cone to the box in two ways – via radiation, from the inner surface of the cone, and via conduction at those places where the box is attached to the cone.  Thus to protect the crew during reentry, we want the inner surface of the cone to have as low an emissivity as possible and the outer surface of the box to have as low an absorbtivity as possible, to minimise radiative heating (in contrast we want the outer surface of the cone to have as high an emissivity as possible), and we want the areas where the box is connected to the cone to have as low a surface area as possible and for the connecting materials to have as low thermal conductivities as possible.  These considerations will be very useful when it comes to choosing our materials.  They suggest to me (very much not a structural engineer) that our cone should have a three layered structure – a high emissivity, high melting point outer layer, a highly insulating middle layer and a low emissivity inner layer (perhaps the two innermost layers could be one if there is an appropriate material with regards to emissivity and conductivity).  Which layer will provide the structural strength to survive acceleration/impact I don't know enough about materials to answer.

Again, perhaps other people were already thinking this way, but this idea was new to me and really helped me clarify my understanding of what sorts of materials we will need and where certain things will go in the craft.

It also suggests an approach to reusabilty of craft – if we make the coupling between the box and the cone loose enough, it may be possible to simply remove the used cone from a returned craft and place a new cone over the same box and use it that way.  It seems likely that the cone is what will take the vast majority of the punishment involved in flying, whereas the box stays relatively safe.

Volume 1, I think.  The image I was talking about is this one:

I suppose I was idealising things a bit when I said the pressure vessel was roughly a rectangular prism, it's more of a wedge shape.  The equipment mounted to the sides of the pressure vessel aren't shown in this diagram, so it looks like a lot of empty wasted space, but you can see it in plenty of other diagrams in the manuals.

I'm also not sure if the box-in-a-cone approach or the cone-as-pressure-vessel approach would be best, it's something we would need to discuss, and research.

In addition to the prospect for higher reusability, box-in-a-cone also offers:

• Equipment mounted to the outside of the box can be easily accessed on the ground by removing panels from the cone.  If the cone was the pressure vessel, removable panels would be harder to implement without compromising air-tightness, so access to equipment could only be from inside the vehicle, which may be a pain, and would make last minute replacements before launch essentially impossible.
• Heat generated by equipment mounted to outside of box can radiate into space and be kept out of the cabin by good insulation.  If everything is inside a conical pressure vessel, everything heats up the atmosphere.
• The cone and the box could conceivably be built in different places which makes decentralisation easier.

Of course, there may be ways around these complications for the cone-only approach.

I haven't had a chance to take a good look through the Mercury manuals yet, but Mercury was small enough that I am willing to bet they had to go with cone-only, so that might be a good place to look for a comparison.