sandwich airframe construction

[verex]

Hi all,
I'm considering nomex honeycomb CFRP tubing as airframe for my next rocket. What concerns me is the joining methods between particular tubes and internal structure. Because of the honeycomb structure with lots of empty spaces between the carbon wrapping I have doubts on using bolts to make joints. Do you have any recommendations of how to get strong joint on this kind of material?

 

[kalsow]

If you're making you own tubes (and are great at pre-planning) you can fill the cells that contain or abut through holes with epoxy. If the tubes already exist you can drill tiny holes and inject epoxy. I drilled the actual bolt holes I wanted, then used an L-shaped wire to bust out nearby honeycomb walls and filled the resulting cavity with epoxy. Then re-drilled the holes. Messy, but it worked.

I used the same L-shaped "tool" to reinforce the edges of fin slots.

 

[verex]

Filling the honeycomb cells did come to my mind, but it appears to me as quite difficult – the epoxy is going to spill out of the cells when the tube is rotated for layup. Not quite sure if using some fast bonding epoxy glue would affect hardening and curing. Seems to me like easiest way is to go with structural frame transferring all the forces and use of 1 long sandwich CFRP tube for airframe thus limiting the need of bolt connections. Probably also better for overall vehicle stiffness. Although still, if there are any other suggestions or solutions out there - much appreciated.

 

[kalsow]

Filling the honeycomb cells did come to my mind, but it appears to me as quite difficult – the epoxy is going to spill out of the cells when the tube is rotated for layup.

Thicken the epoxy. It won't go anywhere.

 

[verex]

Ok, I did my research and discovered industry uses various inserts. https://www.sciencedirect.com/science/article/pii/S1359835X1100114X
https://books.google.co.uk/books?id...e&q=honeycomb cells potting mass tube&f=false
Turns out it can be pretty time consuming task to manufacture such tubing with reinforcements in the bolted joint areas. And it's not that quick to prepare proper FEA analysis for this as well. But great solution for large fuselages.

 

[OverTheTop]

Someone (Australian I think) showed a sandwich tube with cork as the inner material. Light, permeable, flexible till locked in with epoxy.

I think it was one of Cryoscum's (Nic's) builds.

 

[PromptSupercritical]

I think a sourdough hoagie roll would make good sandwich airframe...

 

[Ronz Rocketz]

Now I'm hungry for a sandwich...

 

[thom9894]

If you're making you own tubes (and are great at pre-planning) you can fill the cells that contain or abut through holes with epoxy. If the tubes already exist you can drill tiny holes and inject epoxy. I drilled the actual bolt holes I wanted, then used an L-shaped wire to bust out nearby honeycomb walls and filled the resulting cavity with epoxy. Then re-drilled the holes. Messy, but it worked.

I used the same L-shaped "tool" to reinforce the edges of fin slots.

In aerospace, this is nearly the method to install Hi-loks for repair. Sharpen an Allen wrench, chuck it up in a drill, give it a few spins. Instead of epoxy, we use potting compound. Expanding foam, essentially. Then drill out just as you describe.

I really wish I had the pockets to be able to do the things the layup mechanics at work do...

In a honeycomb panel, the facesheets take in-plane loads and bending, the core takes the shear loads. 3 pcf 3/8" cell Nomex should be pretty good for subsonic flights. I don't deal with supersonic flight, wonder what kind of pressure loading is involved...

 

[shockie]

does anybody vac bag tubes? is it even possible? how do you get the vacuum pulled on all 360 outside of the tube? do you use some kind of bladder ?

 

[JohnCoker]

does anybody vac bag tubes? is it even possible? how do you get the vacuum pulled on all 360 outside of the tube? do you use some kind of bladder ?

It's hard, especially with smaller-diameter tubes, but can certainly be done. Key things:

• don't do too many layers at once
• get the cloth tight
• remove as much excess epoxy as possible

Here's one of my first how-to pages: https://jcrocket.com/kitchenbagging.shtml

 

[shockie]

It's hard, especially with smaller-diameter tubes, but can certainly be done. Key things:

• don't do too many layers at once
• get the cloth tight
• remove as much excess epoxy as possible

Here's one of my first how-to pages: https://jcrocket.com/kitchenbagging.shtml

thanks John.