Coupler the same diameter as PML phenolic

[rocketcharlie]

Does anyone know of a fiberglass coupler that is the same diameter as a PML phenolic coupler tube? I know a Madcow coupler is too big. I need a fiberglass coupler that will fit inside a fiberglass airframe tube that has been layed up on a PML phenolic coupler tube.

Thanks-

 

[Steve Shannon]

I have tried in the past, but at least for three inch body tubes the available fiberglass couplers are just enough larger the forcing them in results in cracks in the PML body tubes.
The only ones that might work that I haven’t tried are the PML brand carbon fiber tubes.

 

[rocketcharlie]

Thanks Steve. I actually wasn't aware that PML made CF couplers. I will call them today to be sure they are the same OD as their phenolic couplers.

 

[Steve Shannon]

They recently rebuilt their website. It took me a little looking to find the right page. It’s not on the airframe page; it’s on the composites page:
https://publicmissiles.com/product/composites
Please let me know what they say. I’m sorry I didn’t even think of this when I was rebuilding a Sudden Rush that had a phenolic coupler failure just after burnout.

 

[Steve Shannon]

Of course the other thing you can do is reinforce the PML phenolic coupler tubes on the inside. We used to use balloons to expand the glass fabric against the inner surface while the epoxy cured. It’s messy, but it works.

 

[rocketcharlie]

Steve, I didnt get a chance to call PML yesterday but I will.

Could you walk me through glassing the inside of the coupler with an inflated balloon?

How much fabric do you apply at one time?

How many layers of fiberglass do you feel is adequate for an M motor flight?

Do you leave the balloon in place until the epoxy is cured or will it bond to the layup?

Will a round balloon work or do you need to use one that is already tube shaped?

Thanks, I've never tried this before.

Charlie

 

[Steve Shannon]

Steve, I didnt get a chance to call PML yesterday but I will.

Could you walk me through glassing the inside of the coupler with an inflated balloon?

How much fabric do you apply at one time?

How many layers of fiberglass do you feel is adequate for an M motor flight?

Do you leave the balloon in place until the epoxy is cured or will it bond to the layup?

Will a round balloon work or do you need to use one that is already tube shaped?

Thanks, I've never tried this before.

Charlie

I don’t think it would be easy to do more than a single layer each time internally. That’s all I ever did. You apply the fiberglass (which is a major pain all by itself because it will do everything it can to stick to itself). Then you put a balloon into the coupler and inflate it to keep the fiberglass firmly compressed against the inner surface. It’s even harder than it sounds, but it can be done. The balloon needs to be longer than the piece you’re glassing so a longer balloon is better. I think John Coker may have an article about it on his website. I don’t remember when I removed the balloon or what that process was. I don’t find remnants of it on the fiberglass though.

Edit: I am forgetting some details. That’s what happens when you haven’t done something frequently and not at all for a long time. Fortunately, John Coker came through again!
https://jcrocket.com/justmarried.shtml#couplers
Edit 2. I’ve been trying to remember how I might have prevented the balloon from sticking to the epoxy. At that time I had no peel ply, so I cannot claim that. But I was working with a bunch of drafters. I might have used a piece of Mylar drafting material, the kind that looks like vellum, but thicker. That would then have peeled right out after the epoxy cured. I’m sorry I’m not more help. Apparently I’m getting old.

 

[kalsow]

I reinforce couplers all the time. My 2 tips:

1. Don't try to get a single piece of fiberglass to cover the entire inner circumference. It's sure to grab itself and create a bubble between the glass and the tube. Use two pieces, each a bit over 1/2+ circumference wide. It's much easier to get two pieces down flat without having the glass grab itself. I've laid 3 layers of glass using this technique.

2. After the glass is in the tube, line it with parchment paper. It's slippery. Then use either some poster board or a piece of tubing with a wide enough longitudinal slot that it fits inside your glassed tube. Finally, add balloons. Whatever shape of balloon you have will work. Sometimes I use two spherical balloons, one on each end. The balloon presses the inner tube or poster board which distributes the pressure more or less uniformly. The parchement paper allows the inner tube to slip and prevents it from becoming a permanent part of the assembly.

 

[manixFan]

Another tip, use a can of compressed air to inflate the balloon.


Tony

 

[kalsow]

Another tip, use a can of compressed air to inflate the balloon.

Great idea! Epoxy on the lips is nasty.

 

[rocketcharlie]

Well. I've been experimenting with fiberglassing the inside of my phenolic coupler. Thank you Kaslow for suggesting using 1/2 circumference pieces, that worked 10 times better than a full piece.

As I was trying to inflate a balloon inside this layup I remembered something. I own an "inflatable test plug" to seal 3" or 4" drain pipe for pressure testing plumbing. It worked great. I inflated it to 20 psi without incident.

In the photo I have wraped the outside of the coupler with parchment paper to keep epoxy fingerprints off it. As well I applied peelply to the layup and then taped parchment paper to the test plug before inserting it to keep it from sticking to the epoxy.

 

[Steve Shannon]

Nice! Way to contribute!

 

[rocketcharlie]

Thanks Steve, it feels good to be GIVING advice for once.

Charlie

 

[Steve Shannon]

You did well!