Adapt a Cardboard BT to use it as a Coupler

[ThisGamerAlex]

Hi fellow rocketeers, I hope you're doing great out there. I'm having trouble finding different sizes of cardboard tubes to use them as BT but I have found some. The problem comes when, because of the limited variety of diameters, I can't buy cardboard tubes that fit neatly into the BT and serve as a couple. So I started thinking of a solution for that (making my own fiberglass/cardboard tubes isn't in my plans, but if there is an easy method for it, please share it!) and I thought I could make a cut across the cardboard tube and make it fit into the right diameter, then glue it somehow, maybe with wood glue and done.

When searching a bit on the internet for someone that had the same problem as me, I found this video that does exactly the same as I thought but he uses fiberglass to secure it. Do you guys think that step is absolutely necessary or I could go with my main idea of using wood glue, and maybe a piece of thin paper or something similar to act as the fiberglass he used?

Also, if you have any other reliable method for this problem with bodytubes, couplers and diameters, please share it with everyone else! Images of the process are welcomed.

Have a great day,

Alex

 

[cerving]

One thing you need to be aware of is that cardboard couplers are usually a heavier gauge material than the body tube, since it's a natural stress point. I've made tube-joining couplers out of a piece of body tube by slitting them and taking a tiny bit of material out of them (maybe 1/16"), and fitting them into the tubes to be joined. Then, I take a second piece and do the same with it, taking out just a bit more and setting it inside the first coupler, with the slit 180 degrees away. This gives you a double-thickness coupler. I wouldn't do this for a removable coupler, however... just not strong enough.

 

[kuririn]

See post #15 on this thread:
https://www.rocketryforum.com/threads/body-tube-diameters.144677/#post-1765474

 

[waltr]

I've also done what Cris explained.

What size and weight is the rocket? This matters on methods. LPR to Smaller MPR Cris' method works fine.
For larger then start considering fiberglass/epoxy. Or use Epoxy to double up the tubes instead of wood glue.

 

[Kelly]

It depends on the size of, and use for, the coupler. I've done John's method for HPR stuff and it makes a great coupler, and can be used where the joint needs to come apart. It's possibly a bit overkill for LPR - you could get by with paper or a second tube. If the joint you're making doesn't need to come apart (you're permanently joining two sections) you don't even need that, just slit and trim the tube, and insert.

 

[ThisGamerAlex]

First of all, thanks guys for all your fast answers! The rocket I'm designing will have a BT of 50mm OD (or 1,96'' I think, I use metric) and 47mm or 46mm ID (1,85'' / 1,81'') wall thickness of 1,5mm/2mm, haven't bought it yet and the thickness varies. Then, inside will go a motor mount tube of 40mm OD (1,57'') and I still don't know the wall thickness of this one. The motor will probably be around an H more or less, depending on the needs but I'll always try to be more conservative.

The thing is that I want to use an Eggtimer Apogee for the apogee chute deployment and I'm still debating wether I should fit it into the nose cone or make an avionics bay. In case that an avionics bay is needed, that's when I'd need to make a tube coupler.
Also, if I mounted the avionics in the nose cone, the deploy charge would be "upside down", and I don't really know if that's a problem when organizing the parachute, cord, etc...

I know there are some extra questions that are a bit off-topic but I think that helps to understand my situation.

 

[Lord Rory Gin]

One thing you need to be aware of is that cardboard couplers are usually a heavier gauge material than the body tube, since it's a natural stress point. I've made tube-joining couplers out of a piece of body tube by slitting them and taking a tiny bit of material out of them (maybe 1/16"), and fitting them into the tubes to be joined. Then, I take a second piece and do the same with it, taking out just a bit more and setting it inside the first coupler, with the slit 180 degrees away. This gives you a double-thickness coupler. I wouldn't do this for a removable coupler, however... just not strong enough.

I find this method works well for BT repairs as well. Sometimes only a single thickness of BT coupler is all that is needed.