What is important is what's under the skin, so let's have a look at what's under the LM's skin:
This is why any LM fan needs to make a pilgrimage to Hutchinson, Kansas. They have a LM ascent stage pressure hull you can inspect relatively up close.
Sadly none of these photos reveal the
real structural design of the ascent stage. This page is instructive.
http://heroicrelics.org/info/lm/lm-structural.html Scroll to the Ascent Stage Midsection drawings, SK 17-31-18 (3 sheets). These describe the central structural core of the ascent stage. It's built very much like the wing box of a large airliner's airframe -- the thing that takes the brunt of aerodynamic loads in several directions. The fore and aft frames of this box are machined frames. You start with a
slab of aluminum and machine away everything that's not a structural load path, leaving behind only a thin webbing between the bosses. This is why aerospace is so expensive. You start with a chunk of rather expensive metal and throw away 80 percent of it. But the result is as light and strong as a frame can be. The fore and aft frames are connected with a ventral beam assembly
See figure R-123 in the ANR:
https://www.hq.nasa.gov/alsj/LM19_LM_Manufacturing_ppB10-17.pdfWhat's in this part of the LM? Mostly the ascent engine and the environment equipment and controls. The crew cabin is cantilevered out front of this structure, so that the crew and flight instruments balance the AEB to provide inherent pitch stability.
To the left is the Descent Stage. Note that it is not actually octagonal, but rather is five box structures welded together, with vertical reinforcements rather like a wine box.
We still build spacecraft chassis like this, where possible. That is, we design them presuming they'll be made of a honeycomb sheet or other such material. This is the aerospace equivalent of corrugated cardboard. We can cut it into shapes, then we weld, glue, or bolt the shapes together, oriented in different cardinal directions, to build up the desired shape. Not the outer boundary of the shape, but an internal wine-box arrangement. The electronics, propulsion, tankage, and what-not to support the spacecraft's mission are attached to these pieces. Then the outer skin goes over that, which may or may not contribute to the overall structural design.
Grumman was well ahead of its time.
To the right is the Ascent Stage. You can see that the inner skin of the pressurized crew cabin is supported by closely-spaced ribs for maximum strength at minimum weight.
The skin-and-stringer method of building lightweight structures is no secret. Everyone uses it. The corrugated parts of the Saturn V show the interstage portions, where the skin is not being stiffened by pressurized propellant. The stringers there were on the outside. In the airplane designs of the time (and earlier), the stringers were on the inside. Normally you assemble these out of sheet metal. You cut strips of metal, bend them in a press brake, and then weld, rivet, or glue them to the skin (often made from the same material as the stringers). It behaves like corrugated cardboard with one of the facing sheets removed. it will bend very easily at right angles to the stringers, but remains exceptionally strong in the other direction.
For airplanes, the stringers are attached to the formers first, and then the skin is riveted or screwed on, usually from the outside. In advanced expressions of this standard method, the skin takes a lot of the structural load. So temporary fasteners called clecos are inserted in the rivet holes until the permanent rivets are installed. Building an airliner is surprisingly like building a ship. You lay a ship's keel. You also lay an airliner's keelspar. To the ship's keel you attach frames at intervals along the keel's length These frames describe the shape of the ship as if it had been sliced at intervals moving fore and aft. The shapers in an airliner's structure perform the same function. Then between the frames/shapers you attach the stringers at intervals, and onto this, the skin. The lunar module's longerons and stringers go every which way because there is very little other structure besides the skin and stringers at that point. Internal structure of the cabin was more concerned with those blasted windows.
When you see the complicated shapes that the ascent stage had to achieve, you see that the typical skin-and-stringer assembly methods wouldn't really work anyway. The stringers would have to attach at odd angles There would be tight corners where rivet guns couldn't reach. You could weld the skin together, but aluminum sheets that are thick enough to weld would be too heavy. Enter chem-milling. That's a process whereby you mask off certain parts of a sheet of material, then dip it in acid that eats away the unmasked portions to a carefully controlled depth. So you cut a sheet of aluminum into the right shape, like a dress pattern. Then you form if (if necessary) by rolling. Then you chem-mill away the parts that don't need to be thick enough to weld, or to attach stringers to. What you have is a thin plate of aluminum that is thick enough to weld or rivet, but only in exactly the places where it needs to be that thick. Elsewhere it's only as thick as it needs to be to hold cabin pressure.
You can see the thin stringers from which the outer skin will hang...
Minor nitpick: I will argue those are struts, not stringers. Yes, the thermal skin attaches to it, but its structural role here has it acting in longitudinal compression. The thermal skin is about as thick as the sheet metal used in the U.S. to make HVAC ducts.
The triangular sections between the outer boxes were storage areas for auxiliary tanks and equipment the astronauts would need on the Moon, including tools, science packages and, on the later missions, the folded-up lunar rover.
The end caps on the cruciform structure were for structural strength, not necessarily to protect (as with a skin) what was contained therein. Because the struts for the landing gear attach there, the end caps resist the torsion load that would occur if the LM landed at a more acute angle than strictly planned.
Grumman, whose proud engineers built the Lunar Module, also built the best and most durable naval aircraft ever...
I had the pleasure of helping to restore an F-14 Tomcat when I was a volunteer at a small air museum in Oakland, California back in the 1990s. Tom Kelly's team built that too.
When I first saw pictures like this, I thought that they had formed the skin to the right shape, and then added the ribs...
The reality is far more interesting and clever (and stronger): The skin & ribs are a single block of aluminum! They milled-down the sections between the ribs to the desired skin thickness. They did this for each section of hull, then welded the sections together at strong edges.
I've been saying that for years, so you're to be forgiven for repeating it. But I've lately found out (after having a good close look at the construction) that the integrated skin-and-stringer method was used only for very small elements. (I also asked some people, to confirm it.) The larger ones, which comprise what you can see in the photos, were made in a semi-traditional way. Which is to say, the skin and stringers were separate pieces.
As I said above, the only milling done on the skin was done chemically, and it was done to selectively reduce the thickness of the skin where not required. The skin was 0.055 to 0.065 inch thick under the stringers, and somewhat thicker (I don't have an exact figure) at the edges where welding might occur. The spaces between the stringers were only 0.015 to 0.025 inch thick. (For comparison, a carbonated beverage can in the U.S. is 0.015 inch thick.) The variable thickness was achieved by chem-milling. The stringers were either formed or milled, depending on whether they were for flat or curved sections, and attached by various means.
The confusion is natural, even among engineers, because that's exactly how we make some of the formers. As I said above, you start off with a slab and then mill it down to have just a very thin web between the thicker remaining parts that bear most of the load. If you wanted to go extremely thin, past the point where mechanical milling is no longer feasible, you could conceivably use chemical processes to erode the material further, leaving essentially a large integrated skin and stringer component. I don't know of anyone who does that, though. Such a thing would be strong, though, that one in which the stringers are attached later. This is because the loads passed between skin and stringer wouldn't be concentrated at attachments or weldments that would need to be thicker than the surrounding material to bear them.
I said semi-traditional methods. The skin panels were typically welded together first, then the stringers were attached. This is the reverse of the typical process. In aircraft the stringers are longerons too, and are attached first to the formers. In the Saturn V, the skin and stringers were assembled first as a large sheet, then formed into the shape of the rocket.
Grumman went onto use the same technique to build the F-14 Tomcat.
It definitely shows. Crawling around in the guts of one of those, you definitely see echoes of the LM.