Why Fin Slots?

Few components of a rocket can do more to strenghen a rocket than "through the wall" fins. Where instead of being adhered to the surface of the outer airframe, the fins actually protrude through the aiframe, and are adhered to the motor mount tube, and usually one or more centering ring. This accomplishes three primary goals:

1. This Stiffens Fin Joints - While it is typically done for only larger rockets, even on smaller rockets, this handy technique will make your fins much more solid. Instead of just gluing the root of the fin to the airframe, giving only a couple of inches of length, and sometimes as little as 1/8" of thickness, you have very little surface area for your fins to be held on by. Instead, with the fins extending to the motor mount, you not only get the length of the root (in the form of a fillet to the airframe on both sides of the fin), but you also get the same length on the motor mount, plus the length of the tab that comes in contact with a centering ring. plus, if you wish, and "inside fillet" between the tab and the inside of the airframe. All of these make for one very stout joint.

2. This Strengthens Fins - "You just covered that!?" Not really. For balsa fins, and even some plywoods, we arrange the strength to follow the leading edge of the fin. The grain runs parallel, or close to parallel to the leading edge of the fin. You'll notice on some rockets, you can actually pinch the tip of the fin between three fingers and bend it. This can happen naturally in flight, leading to fin flutter, and all kinds of other undesireable effect. By extending the fin to the motor mount, and adhering it to both the motor mount and the airframe, you are essentially clamping it flat in two places. There are certainly fin shapes on which this has little effect, but the second location of adhesion does it's best to resist that same three-finger clamping force on the tip of the fin, reducing or eliminating entirely undesired effects like fin flutter, etc.

3. This Strengthens Centering Rings - Centering rings are only as good as the material they're made of. They have two functions: keep the thrust-line constant in relation to the airframe (preominantly in center), and transfer the thrust of the motor to the airframe. The rest of the components are attahced to the airframe, or another part of the motor mount, so it's a pretty important task. I've seen centering rings fail, letting loose motors, recovery train and all. Not pretty. As you use motors with more and more thrust, you have to compensate in the thickness or material of your centering ring. The go from being cardstock or light balsa, or sections of tubing to plywood, or even multiple rings made of plywood. They have all of this thrust in their center, and they have to transfer it to the outside, and to the airframe. Some high power rockets even resort to stacking several rings, and connecting them with all-thread to help transfer the thrust to the airframe. By using fins with through the wall (TTW) tabs, extending on top and bottom to a centering ring, and adhering it to both centering rings, you have created a giant "I" beam from your airframe to your motor mount. You will have all the strength you need, and you haven't added much weight. In fact, in many cases, you can even reduce weight by using a thinner material for the centering rings, and get the same or better strength.

How do I make Fin Slots?

Fin slots are easy to make, but they require a little bit of planning. Often times, your assembly process has to be modified slightly to acommodate the use of fin slots. Instead of assembling your motor mount tube, and all of your centering rings, and inserting that into your airframe, it is usually a better idea to have at least three centering rings, and leave the aft ring for last. Assemble your motor mount, and fwd two centering rings and allow to cure. Refrain from adding a fillet, or getting any adhesive on the aft-side of the middle centering ring, because your fin tab has to seat all the way in the corner made by the motor mount and centering ring. Dry fit your motor mount/centering ring assembly inside of your airframe, locating it correctly. Measure from the aft end of the tube to the aft side of the middle centering ring. Transfer this measurement to the outside of the airframe to determine where your slot will start. Make another mark for the aft side of your fin tabs by measuring from the fwd mark. Find your array of fin center lines, and cut a slot.

One easy way to cut slots, is by cutting your center-line of your fins, and then offsetting a line to one side of that (the same side for every fin) by the thickness of your fins, however this will only work if your rocket has no other differentiating characteristics like some fantacy scale rockets like an Estes Interceptor, or asymetric fins.In this instance, it would be better to offset to both sides of your fin center lines. After you have your slots cut, it is a good idea to dry fit again, making sure that your motor mount assembly fits where you want it, and that all of your fins fit where you want them to, and your aft centering ring seats properly against your fin tab.

For final assembly, you adhere your motor mount assembly, and then as you add fillets to the aft side of your middle centering ring, you apply adhseive to every sufrace with which your fin comes in contact with a motor mount, centering ring, or airframe. Leave your aft centering ring off for now. After your assembly has cured, go back and add fillets to everything using the open aft-end of your assembly to access the joints you need to put fillets on. At this point, you apply adhesive to the aft side of your fin tabs, and to the motor mount and airframe where your aft centering rings goes, and seat your centering ring. The final step would be to apply fillets to the aft side of your centering ring at the motor mount and airframe. The resulting assembly will be very strong.

Submitted by Tim Scott