While couplers tubes can be purchased, they tend be expensive, especially for high power rocketry. The method describe here can be used to created coupler tubes of all lengths, for any size body tube, and for low, mid and high power flying model rockets. If you like to scratch build with non-standard size body tubes (such as mailer tubes), couplers (also known as cores) can be hard to find. While they can be found, it will likely be only in larger quantities. To make your own, all you need is a piece of tubing with the same diameter as the body tube for which you need a coupler.

In other words, if you need a coupler for a BT80 body tube, you need a spare piece of BT80 tubing. If you're using a 3" mailing tube, you need a spare piece of 3" tube.

Please note that the tube used here to create the coupler has a glassine layer on it. That layer was left intact as it makes the lines show up better in photographs. I would suggest removing the glassine layer, or simply use a bare tube. This will improve adhesion between the coupler and the model rocket's body tube.

Make your own coupler tube

Start by deciding how long you want your coupler to be. Mark the body tube at this lenght. To easily draw a line around the tube, wrap a piece of paper around the tube. Make sure the paper the line where the paper overlaps is nice and straight. Put some masking tape on the paper to keep it in place. Slide the paper to the desired lenght on the tube.

For example, for a 5" coupler, you'd slide the paper at 5" from one end of the tube. Once the paper is in the correct location, put some masking tape where the tube and paper meet (on the side where you will not be drawing the line).

Using the paper as a guide, draw a line across the tube. This should result in a clean and straight line, evenly spaced from the end of the tube. To make drawing the line even easier, you could use light cardstock instead of paper. Once the line is drawn, remove the paper. This is where you will be making your cut.

For Estes style types, use a hobby/exacto knife to make your cut. Simply move the knife over the line and only score the tube at first. At the next pass, cut a little deeper and repeat until the cut is complete. Do not try to make the cut in a single pass and use a fresh/sharp blade for the cleanest possible cut. For thicker tubes (such as mailing tubes), a hobby knife could still be used. However, using a Dremel tool with a cutting disc will be much faster and easier.

The coupler will have a diameter that is smaller than the original tube. To create a functional coupler, a section of the tube will have to be removed. Draw a line on the piece of tubing, from one end to the other, as shown in the picture. This marks the beginning of the section to be removed.

The amount of tube to be removed is equal to the outside circumference minus the inside circumference of the original tube. To avoid the math, enter the tube's diameters in this tool :

If you do not know the tube's diameters, ask the company you purchased the tube from, or make your own measurements.

Once you know that value/distance, make a mark at this distance from the line you drew in the previous step. Draw the second line on the tube starting at this mark, paralel to the first line. The area between the lines is the section that will be removed.

First make a cut on both lines, without completing the cut. Simple score it with your hobby knife or Dremel tool. Use several passes to complete the cuts. Continue to do so until the cut is complete and the section between the lines is removed. Push the lines you cut next to each other and see if you have a nice fit. If not, sand off any areas which may need adjusting. You could insert the coupler into a body tube at this time. However, it will tend to shift and not stay even. To avoid this, use the procedure detailed in the next step.

Put the lines on which you cut together and place some masking tape on the inside of the coupler. This will make the coupler keep its shape while you glue it into your model rocket's airframe. For high power rocketry, the coupler can be reinforced by putting a layer of fiberglass on the inside of the coupler. In this case, you'd put the masking tape on the outside of the coupler and remove it after fiberglassing. This step is optional and depends solely on the needs of the rocketeer.