Internal frame construction

Most rockets |'ve seen or flown rely on a cylindrical airframe to transport all forces from the motor (thrust plate and/or internal rings or bulkheads), and take any bending forces. These vary from cardboard to fiberglass and carbon fiber tubes, but they all carry the forces in the airframe tube (skin).

I know there are large commercial rockets using internal framing, and a thin skin to provide good aerodynamic form. Some larger HPR projects use internal frames, maybe because they can't easily find large tubes that are strong and light enough. A friend was telling me of a large project several years ago that was a frame built of wooden 2x3's, and skinned with thin plywood. Would love to see the build, if anyone has a link.

I'd like to explore some simple internal framing methods on a small scale, say a 3" or 4" diameter rocket on a J or K class motor. I'm thinking of a frame of pultruded carbon fiber rods/tubes held in place by several bulkheads (and centering rings by the motor mount), and a thin (non structural) skin of plastic. Think of how some older airplanes were made using wooden stringers covered with fabric skins.

I've been experimenting with 3D printed sections where the faces of the sections get a couple of layers of fiberglass that works like a thin bulkhead, and held together with threaded steel rods, and skinned with a layer of fiberglass, but the construction is a bit heavy, mostly from the steel rods, and the fairly robust 3D printing and infill. Built like a tank. Flies well enough, though.

I'm sure there are references to internal frame construction techniques, but are hard to find here on the forum. Just looking for links or your ideas and suggestions.

I have done 3d printed internal structures for mid power. It works, but unfortunately, it crashed and I no longer have it to take pictures.

The massive Mercury Redstone launched at MDRA was built in a way similar to what you describe. See the photos and a video here: https://mdrocketry.org/mercury-redstone/

mbeels said:

The massive Mercury Redstone launched at MDRA was built in a way similar to what you describe. See the photos and a video here: https://mdrocketry.org/mercury-redstone/

Click to expand...

That's an impressive flight, and the rocket seems to be mostly air on the inside, so it can be fairly light. I'm curious what they used to skin it with.

I built a frame and skin low power once (when I wanted an odd shaped body tube that I couldn't buy). It took me a lot of time and no small amount of frustration. It worked, and the crash that destroyed it was due to an entirely different design flaw. When I do it again it will go better.

That used laser cut rubs that included notches for the stringers. All balsa, with the stringer 1/16" squares and the ribs, if I recall, also 1/16" thick. I used posterboard for the skin; copy paper would have been plenty strong enough, but in posterboard I could get larger sheets, which let me skin the whole thing in one piece.

Here's the build thread.

The point is, yes, it can be done and has been successfully done. Go for it.

For mid power I might change it to 1/8" basswood, which would be a lot easier to deal with in any case, and for high power maybe 1/8" plywood for the ribs and 1/4" for the stringers, but that's just off the top of my head.

cwbullet said:

I have done 3d printed internal structures for mid power. It works, but unfortunately, it crashed and I no longer have it to take pictures.

Click to expand...

I've done lot's of 3D printed stuff for my rockets, starting with fin cans and nose cones, then motor mounts and electronics sleds. Have done several rockets, such as the one in my avatar - the nose cone and fins were 3D printed, the rest was phenolic tubing. Most of the internal stuff I use the 3D cubic infill from the Cura slicer, which is strong in all directions.
Just got a couple of spools of something called PLA-LW, which is a sort of PLA filament that foams at the high temperature of extruding, making a lightly foamed PLA that's about half the weight. Will be experimenting with that soon.

mbeels said:

The massive Mercury Redstone launched at MDRA was built in a way similar to what you describe. See the photos and a video here: https://mdrocketry.org/mercury-redstone/

Click to expand...

That's an impressive flight, and the rocket seems to be mostly air on the inside, so it can be fairly light. I'm curious what they used to skin it with.

jqavins said:

For mid power I might change it to 1/8" basswood, which would be a lot easier to deal with in any case, and for high power maybe 1/8" plywood for the ribs and 1/4" for the stringers, but that's just off the top of my head.

Click to expand...

That sounds about right. I was thinking pultruded carbon fiber tubes, like are used for kite sticks, as stringers. Could probably get away with pultruded fiberglass, but would be heavier and a bit more flexible. Need to do the numbers, as most of the load will be axial, but it has to be able to take bending forces, like kip force in a column under load. Problem is that parallel tubes have very limited torsion resistance. I'm not expecting a rocket to be subjected to much torque, but I'd rather not have it twist up under load at launch. Might have to thicken where the rods go through the bulkhead to get a stiff enough connection.

That comes from the skin. Little or no other structural strength comes from it, but if the skin has any strength at all and is well bonded to the ribs and stringers then it will provide a lot of torsion resistance. My Ellipse was twisty as heck when it was a naked frame, but rock solid once it was skinned. Think of cheap bookshelves; with no back they rack to one side and collapse, but even a flimsy cardboard back held on with a few tacks is enough to hold them up solidly.

The University High School Space Shuttle used the kind of construction commonly used for model aircraft. When I visited the teacher (Mr. Catto!) who helped the students build the rocket, he handed me one of the SRBs. Even though it was larger than an Estes Leviathan, it weighed about half as much.

https://blog.payloadbay.com/?p=23