Two Stage Coupler

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Thanks! Nothing extraordinarily brilliant, but it solves the issue with this build. The biggest problem I'm having with assembly at this point is lining everything up axially. I did the best I could to attach the 8mm carbon tubes to the sustainer motor mount as straight as possible and at 120 degrees and in fact the end result was pretty accurate. The coupler bulkheads for the booster will need to be drilled to accept the 6mm carbon tubes which slide into the 8mm sustainer tubes. I'm am still brainstorming the process for this. Math is not my forte and my motor mount carbon tubes are not exact enough to use any triangular incenter formulas to mark the bulkheads, though I could very well be wrong about this.

I was elated, however, to discover that the sustainer airframe and motor mount dimensions were a perfect match leaving an exact amount of space between them to accept the 8mm carbon tubes snugly. No centering rings necessary. I was just as excited after receiving them to learn that the tolerances of the 6mm carbon tubes inside the 8mm tubes were tight yet just so to allow them to slide freely within the 8mm tubes with no binding. I will lubricate these with graphite just prior to flights.

I also need to reinforce the plastic transition with scrap carbon. That flimsy piece of crap is junk but I will use it. It is the Apogee product I bought with the idea that I would use it alone as the structural component. A true mistake. What a joke. I wouldn't even trust that thing with LPR. I was expecting something more robust than party cup plastic. Tim, if you're reading this, have your manufacturer bump up the wall thickness of this thing to 3/32 or something and offer that as well. I'd wager you would sell more of them. The benefit of 3/32 (or some comparable wall thickness, such as that of BT80 paper tubing "hint hint") would be that OD's would match more closely when mated to BT80 tubing as well as provide structural integrity sufficient for MPR. Even with such a product though, I would still be inclined to build an internal structure such as you see in the photos. I simply cannot tolerate a wobbly booster/sustainer joint. I cringe every time i watch aftward video from a sustainer and see the booster dancing underneath it.
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It looks like what you're doing is along the lines of the ASP WAC Corporal w/ Tiny Tim booster for staging. Even with graphite lube though I would be concerned with binding in your tubes. I have no experience... Just a gut feeling.