Building a rocket in sections

[ghostfather]

Most of the rockets I build are cylindrical, where most of the strength and stiffness is from the outer skin (airframe), whether it be cardboard, phenolic, fiberglass, or carbon fiber. Most of the kits are that way. I'll refer to it as a "skin-bearing" rocket.
I've seen some of the advanced builds that some of the University competitions come up with for IREC, and the most intriguing to me use an internal frame with a non load-bearing skin. The frame can be simple like a circular pattern of carbon fiber rods held together with fiberglass bulkheads, or more like a spaceframe. I'll refer to them as "skeleton" rockets.
Sort of like comparing insects to mammals: Insects have an exoskeleton, mammals have an internal skeleton.

One of the limitations of building cylindrical rockets is that its difficult to make a free-form shape other than those based on cylinders, conical transitions, or maybe cut-down portions of a commercially available nose cone (think boat tail). I have a pretty good 3D printer, so I have some freedom to make my own things, but most are mated to cylindrical "skin-bearing" components.
I was watching something on the Discovery Channel, of of those mega projects about how the big container ships are built. The sections are so big they have to be constructed in sections, and welded together in the ship yard.

They did the same when building the big NASA rockets.

I suppose I do the same when I connect sections of rockets together using couplers.

Ships are more of a free-form shape (the hull) that is not cylindrical. I could 3D print a rocket, but I'm limited by the build volume of my printer, which is about 20cm x 20cm by 30cm high (aprox 8"x 8"x 12"high). So what if I printed a rocket in sections, and stacked them together?

This is made of two stretched Von Karman profiles with a small cylindrical section about halfway where the e-bay electronics are kept. The entire rocket is an nose cone and boat tail. Each section is 30 cm tall, and 20cm at the thickest part of the rocket, and 5 meters long (8" diameter x 17 ft long). Internal is the "backbone", a 98mm fiberglass motor mount and airframe. In the nose area I'm planning on using some fiberglass or CF hollow rods to stiffen it.

Four fins and one long nose cone.

Returning to the way big ship hulls are built in sections - what if I add a layer of fiberglass to the top and botton of each printed section, and when I epoxy them together, I get a sort of fiberglass bulkhead. That seems like welding the sections together on a ship's hull. Run some fiberglass rods thru the middle, and I've got an internal skeleton structure.

3D printing considerations - make it as light as practically possible, very little infill. I'm playing with some foaming PLA filament (foams slightly when heated in the extruder head) that looks promising, and saves lot's of weight. I'll be adding a light layer of fiberglass to the skin to smooth things out and take paint, which may maybe be slightly structural as well, though I'm not counting on it in my calculations.

Therefore, by building in sections, I'm able to get away from the limitations of using just skin-bearing cylinders, achieved by and internal skeleton, and getting a free-form shape to boot. This one is symmetrical, but it should be possible to build an oval-shaped cross section, or maybe the X-15 with it's air intakes that are hard to model. Use your imagination.

Any ideas as to how to improve on my thinking on this are welcome. Maybe it will bring you up on a few ideas for projects that otherwise would be difficult to realize.

 

[Nytrunner]

This reminds me of a couple large scale builds I read of somewhere with a wood frame and foam chunks in the spaces. The foam was easily carved to the desired profile, like your 3d printed shells form the profile that your fiberglass skeleton completes.

I imagine it would be heavy for lowpower, but quite interesting for mid or high

 

[lakeroadster]

I basically use the "build it in sections" concept too.. lots of internal couplers. But that design inherently results in a heavier rocket. I'd rather have a more durable design, but for some folks that is verbotten... they do everything they can to minimize and eliminate weight. Different philosophy's and it's all good.

 

[jqavins]

I see three different concepts here. One is using 3D printing for more shapes than those with circular cross sections. Sure, of course.

One is building in multiple sections, joining them after each is constructed. Sure, of course.

And then there's building frame and skin. That's how lots of model airplanes are built. I did it for my elliptical cross section rocket a few years ago with balsa ribs and stringers.

In my case the skin was load bearing. It was paper, and after I had it attached attached to the frame I applied thin CA to harden, stiffen, and strengthen it. Even without the CA it served to tie the frame elements together which strengthened and greatly stiffened the structure.

I suspect that in your case, as you've described it, the fiber glass wrap over the printed parts would add more than just a little strength. I suspect you could think of it as a skin-bearing FG rocket with 3D printed leave-in forms.

All in all, the short answer is yes.

 

[ghostfather]

It is a combination of ideas, but conceptually I envision and calculate the loads as a circular cage of carbon fiber rods fixed every 30cm (12") by a circular fiberglass bulkhead where the sections are epoxied together. That's the skeleton. I'm ignoring any structural contribution of the skin as far as keeping the rocket from buckling. Non-load bearing skin and a strong skeleton.

The build I have in mind is using a 98mm motor, probably an N, will fly maybe mach 1.5. A paper skin wouldn't be enough , and a single layer of 80g fiberglass should be enough to protect and smooth it, but not to bear the entire load. I am calculating the bending (kip) forces based on an internal frame of hollow carbon fiber rods fixed by the fiberglassed bulkheads every 30cm. Even though the skin will stiffen it somewhat, I'm relying on the Moment of Inertia of a circular pattern of rods to carry the load.

I don't have the engineering know-how (anymore ) to calculate what the stiffened skin will contribute, so I'm probably being somewhat conservative. The skin does have to bring the drag forces via the 3D printed walls and internal support, and the "bulkheads" to the load bearing carbon fiber rods. The 3D-printed sections themselves are already fairly stiff, though also light in weight.

>> I did it for my elliptical cross section rocket a few years ago with balsa ribs and stringers.
In my case the skin was load bearing. It was paper, and after I had it attached attached to the frame I applied thin CA to harden, stiffen, and strengthen it

Would be interested in seeing pictures or drawings if your frame, sounds like something I might want to try as well.

 

[ghostfather]

BTW, with this design I'm concerned about heating at the tip of the nose. I suppose I could epoxy in an aluminium point, as just a layer of fiberglass over foamed PLA might not take the heat very well.

Any other ideas?

 

[Steve Shannon]

BTW, with this design I'm concerned about heating at the tip of the nose. I suppose I could epoxy in an aluminium point, as just a layer of fiberglass over foamed PLA might not take the heat very well.

Any other ideas?

Leave the nosecone blunt or radiused instead of pointy so the heat isn’t concentrated. At 1.5 Mach I don’t think you have to worry about heating anyway and you’re only at that velocity for a very brief time.

 

[jqavins]

Would be interested in seeing pictures or drawings if your frame, sounds like something I might want to try as well.

Here's the build thread:
https://www.rocketryforum.com/threads/trf-summer-build-off-the-ellipse.134439/