Fin slots are cut into airframe tubes so that the fins can be attached to the motor mount tube. This take the fins through the body tube wall. Attaching fins in this manners adds lots of strenght. This technique is most often used for high power rocket construction and can also be used for mid power (or even low power) rockets. The key to good fin slots is precision. This page will help you on your way and show a way by showing you how to mark and cut out the slots.

First, draw a line all the way across the body tube. This will be the line where launch lugs or rail buttons will be attached. When a vent hole is needed, I also put it on this line. Doing so keeps it on the back side of the rocket.

Now you must determine how far to cut the fin slots. This example keeps things simple and we assume slots will be cut all the way to the back of the airframe.

Say the fin tabs are 5" long and the aft centering ring is 1/8" thick. If you want the aft centering ring flush with the aft end of the body tube, simply add the fin tab lenght to the thickness of the aft centering ring. In other words, 5" plus 1/8" equals 5 1/8". Our fin slots will be cut 5 1/8" deep into the rocket's airframe.

Measure 5 1/8" from the end of the airframe and mark that distance on the line we created earlier. Wrap a piece of paper, or cardstock, around the airframe and line it up with the mark you just made. Use the paper wrap as a guide to draw a line around the airframe.

Remove the paper. You should now have a tube that is marked as show in the picture on the right.

The next step is to calculate the fin slot locations. The calculator tool below will help you with this part.

To measure the circumference of the tube, you can wrap a piece of paper around the tube (as shown above) and make a mark on the paper where it overlaps. Remove the paper from the tube and measure from the edge of the paper to the mark you just make. This will give you the airframe's circumference.

Measuring the fin thickness is easy, simply use a ruler and use it to take the measurement.

Enter the values in the form below and click the button to get the fin slow locations.

Mark a piece of paper (or cardstock) at the values the tool gave you. Wrap this paper around the body tube. Line up the beginning of the paper with the launch lug line (already on the tube). Make marks on the body tube by the marks you make on the paper. This is shown in a thich, red, line in the picture on the right. Obviously, you don't want a thick line, this was done only to better illustrate this step.

Once you have marked all the fin slot locations on the rocket's airframe, remove the paper. Draw lines on the body tubes on the marked areas. Extend the lines from the aft end of the airframe to the line around the tube which was drawn earlier.

Now make cuts on the lines that were created in the previous step. This should leave you with perfect fin slots. Remember, the quality of your fin slots will depend mostly on the accuracy of your own measurements and cuts.