Designing large flat but thin, lightweight and rigid panel

It seems to me, since you made the decision to employ a steel wing spar (box), you can have all the stiffness you desire, with a fairly simple laminate schedule, relying on the spar(s) to essentially hold everything up, including the cannons. I wouldn't over engineer this puppy, so much as insure it fits what you need, in terms of accuracy and demount-ability. If it was me, I'd concentrate on the disassembly aspect first, while working up a practical set of hinges, pins, connections, etc. for the welded assembly. Then just foam the crap out of it, gluing up blocks as required and shape as desired. A simple laminate schedule, just enough to insure the thinner stuff is stiff enough and fair your brains out.

On a engineering level, I've always found this particular "spacecraft" design a bit of a joke, though it might look good to some on the big screen. Guns (cannons, really?) wouldn't have a kink in them and a cannon certainly wouldn't, even if it was particle weapon technology. I also find the engine cant ridiculous, as vectored thrust currently exists and is a much more logical approuch than simply canting the burners. Don't crank me up on the number of wings or why they'd need to be stowed. Lastly, given unknown fuel storage, how does one expect this puppy to reach escape velocity, with the shapes employed, even on the lowest of gravity worlds?

Petty, I know, but engineers are an odd lot. We're the ones that point out that Cesar, as portrayed in the movie, likely didn't own the Converse sneakers, that were seen peaking out from under his robe (actual thing I saw a few decades ago). Or the onion shaped water tower or high tension power lines seen in the back ground of one of John Wayne's greats. I've screwed many a movie for the other half, but also created a monster, who now also sees these inconsistencies and improbabilities. Don't start me up on the catamaran rig used in Water World . . .

 

Great. You have decided to build it with frame and spar out of aluminum as I have earlier suggested. Over engineered maybe and not meant to "fly" but you can do the mock up skeleton and test all articulation and slap on the foam and fiberglass afterwards as PAR suggested.

Since you have the skeleton to handle all loads, there is no need for composite engineering. You can use the lightweight fiber you bought (2 layers) over the insulating foam you have chosen. Insulating foam do shrink in time but maybe 10-15 years after before your model becomes a sorry state.

You don't need a box beam. A "C" channel aluminum will work as a spar. If you are trying to visualize how a spar will work, try imagining a fishing rod. Thick at the base and thin at the tip. When extended, it has little or no droop at the tip. When you catch a fish, the weight of the fish will pull down the tip and the rod will bend. Same thing with "cannon" on wing.

The fuselage can be aluminum square tubing, built like a bridge. A "strongback" as we boatbuilders calls it. Wood is too flexible and its coefficient of thermal expansion is low. Aluminum is much closer to epoxy laminate. Just don't get it out in the sun too long as it will start pulling each other and microcrack might appear. Cover it with cheap thin aluminized mylar sheet or better yet, get the gold colored one for blockage of UV.

 

There are 100 different ways to succeed with this.
Get on with it, but figure out what the total weight is before you start.

 

Thanks again for the help and advice. I will post the update on this forum once things move forward - everything is taking long time and probably will not build the wings until January. Currently, working on the "cannons" to make sure we know the weight and then move on with the wings.