Making Custom Couplers

Long before rocket kits and rocket vendors appeared on the hobby landscape, innovative enthusiasts have been adapting tubes of various kinds for use as rocket airframes. Weve seen concrete forms, carpet rolls, bollard covers, and a whole host of sundry cylinders adopted into use. There are exceptions depending on the design, but nearly every airframe will need a coupler or two to join these tubes together. And chances are likely that a perfect fit for your unconventional airframe will not be readily available, so youll need to construct a set of custom-fit couplers for yourself.

Quite simply, a coupler is a short piece of tubing with an outside diameter just slightly smaller than the inside diameter of the airframe tube. A couplers length is typically twice its diameter. Half of the couplers length is inserted into each of the two tubes it is intended to join.

Before we move on to the actual instructions for making a coupler, I have a small confession. The first draft of this article contained some highly technical instructions, with over 300 words about taking precise measurements and multiplying by pi. I showed it to my brother who responded with a short paragraph explaining how to complete the same tasks without taking any measurements or performing a single calculation. The benefit is that readers are much more likely to actually try his method without being coerced or heavily medicated. The down side is that the edited passages also contained my best jokes. So if you arent entertained, please blame Rick.

Cut a piece of tubing twice as long as it is wide. Take a sheet of paper and roll it up inside of the tube. Square the top and bottom edges of the paper, then mark the paper precisely where the edge overlaps. Remove the sheet and wrap it around the outside of the tube. There will be a gap between the mark on the page and sheet edge. Mark the tube with both the location of original mark and the edge of the paper. Draw lines through the two marks parallel to the vertical axis. Cut the tube along those two lines. Save the strip you cut from the tubing. Join the edges of the cut and test fit the coupler into the airframe tube. Sand the edges of the cuts if the fit is too tight. Wrap the coupler in waxed paper and reinsert it inside the airframe tube. Apply epoxy to the seam. Apply epoxy to the strip you removed in step 5) above, and use it to reinforce the seam from the inside. If your rocket design is large or fast or both, you may want to reinforce your coupler with fiberglass. If this is the case, skip step 8) and proceed to step 9). This can be a very sloppy operation because you are working inside a tight space, so use some care not to get resin all over you and your airframe. Cut three squares of 6 ounce fiberglass cloth. Each side should be slightly longer than the length of the coupler. Use a cheap brush to paint the inside of the coupler with resin/hardener mixture. Apply the first square of fiberglass cloth to the inside of the coupler. Line up one edge of the cloth to be even with the tube seam. The square should cover about two thirds of the inside of the coupler. Soak the cloth well with resin. Apply the second square; slightly overlap one edge with the edge of the first square at the tube seam. The other edge should overlap the first square by about 50%. Soak the cloth well with resin. Apply the third square. Locate this square so it is centered over the tube seam. There should now be two layers of cloth all around the inside of the coupler. Soak the cloth well with resin. Inflate a balloon inside the coupler to keep the cloth pressed against the inside the coupler. I prefer to use a compressor to inflate the balloon, because like I said, this is messy. You dont want epoxy in your mouth. If you dont have a compressor, ask my know-it-all brother Rick to do it. The pressure from the inflated balloon should force out air bubbles as well as excess resin. Position your tube so this resin wont run down inside your airframe. Allow the epoxy to cure. Pop and remove the balloon. Remove the coupler from the tube. Remove and discard the waxed paper. Remove the excess resin and glass from the edges of your coupler tube and youre done!