Mike, Like with the Tip to Tip glassing, I'd be happy to help out with this project. The key issue is having a mandrel to wrap around.

1st step - take the Mandrel (in this case a piece of the airframe you want to "exo-couple" and polish it up and then wrap it with mylar release film - I have this if you need some. I'd probably do two or three full wraps of mylar to provide you some space when the coupler is removed from the mandrel

2nd step - start laying up your exo-coupler around the mandrel. How many layers is going to depend on how thick you want the thing to be. Also, consider the orientation of the fabric in your wraps. To emulate the thickness of filament wound tubing you may need 5 or 6 wraps of 6oz fabric. I'd also consider putting in a layer or two biased at 45 degrees. Finally I consider the top layer might best be done with aerosleeve type material to do away with the seam. You can do all this with or without vacuum bagging.

3rd step - sand it down and fill it, then remove it from the mandrel, trim the ends and final sand.

Warren

Mike, Like with the Tip to Tip glassing, I'd be happy to help out with this project. The key issue is having a mandrel to wrap around.
Warren

I plan on using a piece of Performanc Rocketry (PR) airframe for the mandrel, but having several layers of mylar will make the fit too sloppy on the exo-coupler. The difference in diameter between a PR airframe ID and coupler OD is only .002". Mylar is generally about .005" thick which would make the clearance at .010". The fit on this project will be critical. I'll just need to use a mold release and probably carbon fiber socks to make it.

I have very thin mylar release - I believe it is .002. You'll need a couple thousanths of slop in order to even fit the thing onto the airframe as the irregularities in the tubing will probably be that much or more. In any event, the offer stands - your judgement as project engineer is the key to what you want exactly. Using wax and release chemicals alone will almost guarantee you'll never get the exo-coupler off of the mandrel. Mylar is critical in these cases.

Warren

If you go with a little more space and it's too sloppy, you can always tighten it up again by putting a thin coat of epoxy/microspheres on the outside of the airframe, and sanding it to get just the fit you want.

I've been getting more comfortable with the idea of using the sustainers's motor as a coupler, if you can get one without a thrust ring. The extra fin area you need for compensate for the CG penalty of the motor sticking out the back is pretty minor. I've seen some pictures of recent big-time HPR multi-stage flights that used that technique also.

...I've been getting more comfortable with the idea of using the sustainers's motor as a coupler, if you can get one without a thrust ring.... .

Hmmm, Adrian, that is an intriguing idea. I don't want to say what I am doing exactly but you have cleverly read between the lines.

Does anyone have a lathe to knock down the knurl on an Aerotech aft closure?

If you go with a little more space and it's too sloppy, you can always tighten it up again by putting a thin coat of epoxy/microspheres on the outside of the airframe, and sanding it to get just the fit you want.

Another cool trick is to run thin CA (super glue) down inside. Let it dry thoroughly. It sands away quite nicely, and you can build up a fair amount. It doesn't mind heat, either - (THANKS to Tony2 in California for sharing this very cool trick)