I'm designing the couplings that will connect each of the propellant tanks to the rest of the structure for my rocket. The primary structure for the tanks is 5 inch OD, 0.125 wall 6061-T6 tubing. The question is: what wall thickness and length does the coupling tube need to be to connect two of the 5 inch OD tubes together? To simplify the discussion, ignore the tank bulkheads or any other structure for now. I don't have an estimate yet of the expected flight bending loads but for a first pass, one philosophy would be to make the joint at least as strong as the rest of the structure in bending so it won't fail at the joint. There would actually be 3 concentric elements: 1) the two primary structure elements butted up end-to-end, 2) the coupling sleeve inside, and 3) another sleeve inside the first one. NAS 623 fasteners, installed radially from the outside in are used to hold it all together, with the primary structure and coupling sleeve drilled to match the "grip" diameter of the fastener. The only purpose of the inner-most sleeve is to have something for the fastener to thread into. By using a second sleeve, there are no shear loads on the fastener threads, only tension. Even though the inner sleeve (with fastener threads in it) will provide additional stiffness, I wasn't going to count its contribution since it will be cut down as thin as possible. The material for the two sleeves would also be 6061-T6.
If the goal is to match the bending capability of the primary structure, then I would think the coupling sleeve only needs to be thick enough to match the moment of inertia of the primary structure. Then, the length of the coupling should only be a function of the shear tearout allowable for the joint. Am I on the right track here? Of all the allowables for the joint (fastener shear stress, bearing stress, shear tearout, net area tension), shear tearout and bearing stress appear to be the most critical for this design. One thing I'm not sure about is how to convert an arbitrary bending moment into the shear load per fastener.
See attached sketch for details. Any advice would be appreciated. -Bob
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coupler1.png
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