Custom coupler/MMT for 38mm (1.525 ID) to 40mm ID composite tube?

[SolarYellow]

I have some carbon tube that's 40mm ID. Wanting to shim it down to a 38mm motor mount ID. Seems that BT-60 slit lengthwise with a wrap of a sheet of thick paper or cardstock ought to work well. I had the idea of maybe just using a thin layer of fiberglass on the outside of the cardboard. Use the lengthwise split to wrap it down to a smaller ID, slide it into the carbon tube, and then expand it. To make sure it cures with a nice bond against the ID of the tube, I was thinking of pressurizing the inside with an inflatable bladder. Reading indicates epoxy is likely to stick at least semi-well to butyl rubber, so a bicycle inner tube is out.

Does anyone know of a good source for silicone tubing that would be thin-walled enough to expand for this purpose with an initial diameter of somewhat but not too much less than 1.5 inch?

 

[kalsow]

I'd use a balloon (260 size == 2" wide, 60" long) with parchment paper to separate the balloon from the epoxy.

 

[watheyak]

I used a clown balloon and compressor for a molded nosecone. It generates a huge amount of pressure.

Details here-

https://www.rocketryforum.com/threads/molding-a-nosecone-using-a-pressure-bladder.159820/

 

[Steve Shannon]

I have some carbon tube that's 40mm ID. Wanting to shim it down to a 38mm motor mount ID. Seems that BT-60 slit lengthwise with a wrap of a sheet of thick paper or cardstock ought to work well. I had the idea of maybe just using a thin layer of fiberglass on the outside of the cardboard. Use the lengthwise split to wrap it down to a smaller ID, slide it into the carbon tube, and then expand it. To make sure it cures with a nice bond against the ID of the tube, I was thinking of pressurizing the inside with an inflatable bladder. Reading indicates epoxy is likely to stick at least semi-well to butyl rubber, so a bicycle inner tube is out.

Does anyone know of a good source for silicone tubing that would be thin-walled enough to expand for this purpose with an initial diameter of somewhat but not too much less than 1.5 inch?

Just get a piece of Loc Precision cardboard 38mm motor mount tubing and epoxy it in place. Shim it with fiberglass tape if needed or peel off a layer on the outside, but it should be pretty close. Leave enough sticking out to attach an Aeropack retainer.

 

[SolarYellow]

Both 38MMT and BT-60 coupler are too large OD to fit in the tube. I should look at peeling layers off them and see how clean and consistent I can get that.

 

[SolarYellow]

Tried some peeling action this evening. My instinct was that it wouldn't be awesome. The first layer on a chunk of BMS C60 went OK, but on the second layer, I found it inconsistent and difficult to control or predict. So I'm back to additive approaches.

An alternative approach would be to wrap a 1.5-in mandrel to make it the appropriate diameter with release plies on it, then wrap the BT-60 over it and from there, essentially treat it as if making a body tube from scratch. Could go a couple ways. One might be dilute Titebond on either nice office bond or virgin kraft. Another would be pre-gummed paper tape. I just need to end up with an outer surface that epoxy will bond to, without significant aliphatic resin saturation on the OD. I suppose I could just build up the whole thing as my own tube and forget the BT-60, even.

That's starting to make sense. Will have to refresh on some threads like this one: https://www.rocketryforum.com/threads/making-spiral-wound-paper-tubing.178953/

ETA: Changed the thread title accordingly.

 

[tsmith1315]

I have some carbon tube that's 40mm ID.

Like 40.00mm?

 

[SolarYellow]

Like 40.00mm?

Nominally 42 OD, 40 ID. From a Chinese tube maker, not specialized for rocketry, so not a standard rocketry size. Close to 38MMT & BT-60, but not a direct replacement for either. But dang cheap for c.f. and nice quality for what it costs.

 

[tsmith1315]

Nominally 42 OD, 40 ID. From a Chinese tube maker, not specialized for rocketry, so not a standard rocketry size. Close to 38MMT & BT-60, but not a direct replacement for either.

Ok. FWIW, Composite Warehouse lists thin wall tubing with OD of 1.600 inches. That’s pretty close, depending on your actual measurements.

If interested, it’s probably best to call and see if they have product in stock.

 

[SolarYellow]

I already have this carbon tube in my rocket room.

Going forward, I can buy 1m of the carbon fiber shipped for about 2/3 the price of 3 ft of that G12 tube from CW shipped. Plus, CW is on my "do not deal" list based on all the threads around here discussing his business practices.

 

[tsmith1315]

I was thinking a short piece may work for the motor tube, given the OD is close to what you need to fit in the carbon tube.

Issues with cost and questionable service are understandable.

 

[SolarYellow]

1.6 * 25.4 = 40.64

Grinding down that OD in G12 would be a LOT more difficult than doing anything with paper.

I probably have a solution pending involving rolling my own tube/centering ring stock. Might try a couple different methods and see what I like best.