Lesson 3: Completing Construction

In this lesson, we're going to complete the construction of our kit.

Aft End

Finish off the aft end of your rocket by painting the aft centering ring with thin epoxy. If you are using hobby shop epoxy, the thin stuff is often sold as "finishing epoxy." If you are using an epoxy system, use the epoxy unthickened.

Note how I taped off the aft end of the motor mount tube. This keeps epoxy from it and allows a good bond for the motor retainer later. (I like to install the motor retainers after painting so that they don't need to be masked off.)

The aft CR on my Tomahawk was recessed into the body tube. So I also added a fillet around the edge of the CR to seal it up. If your aft CR is flush with the aft end of the body tube (the more common configuration), fill any gap between the edge of the CR and the BT with thickened epoxy. It's important to have a smooth and hard surface back here as soot from the motor will tend to collect and you want it to be easy to wipe off.

Once the epoxy has cured, sand the aft CR smooth. If you used a fillet, sand it to a smooth transition. If not, sand out any difference between the aft CR and the end of the BT.

Fin Fillets

The final fillets on the fins are the exterior fillets. For the internal fillets, strength is all that's important. For the external ones, looks also count. It's important to end up with a smooth even transition between the fin and the body tube. Not only does this look better when the rocket is painted, but it also produces less drag.

There are several ways to do fillets. Some people like to use thin epoxy and pour the fillets. To do this, use clay to make dams at both ends of the fin and turn the body tube so that the fin and body tube make a valley into which you can pour the epoxy. Fill up the valley between the dams with epoxy and let cure. Once the epoxy has cured, remove the clay dams and sand the fillet to shape.

I prefer to use thickened epoxy to make my fillets. If you thicken the epoxy with a light weight, sandable filler (such as micro balloons), it will be easy to sand the perfect radius. To do this, you need to thicken the epoxy to the point where it doesn't run, sometimes called a peanut butter consistency. Turn the BT so that one fin points straight up and use the mixing stick to pack epoxy into the valleys on either side of a fin. Then drag a gloved finger along the valleys to smooth out the fillet and remove the excess epoxy. Don't worry about getting it perfect now--sanding will make it perfect later.

Once you decide how you are going to make your fillets, it's time to do it. If your model has wooden fins, now is a good time to fill them as well. To fill plywood fins, paint thin epoxy on each one just before making the fillets for it. This way, the thin epoxy will soak into the plywood and provides an extra strong fillet bond. (Remember how we pre-painted the wood for the interior fillets?)

Once the epoxy has cured, sand a smooth radius on all your fillets. If you have wooden fins, lightly sand them to remove the epoxy bumps. Don't go finer than 100 grit at this stage; just remove any visible irregularities.

Electronics Bay

Read through the rest of your kit instructions again and see what is left to do. If your kit doesn't come with provisions for an electronics bay, consider adding one. Even if you plan to use motor ejection for your first flights, you still may be happy to have the bay later on.

I always build electronics bays into rockets and I always fly electronics on high-power rockets. Motor ejection is too inconsistent for larger rockets. I also find electronics easier to use as they avoid the guesswork on which delay to use.

In order to keep it simple, I purchased the Yank Enterprises "altitude package for 4" rocket." This provides an electronics bay plus an extra length of tubular Nylon® and a drogue parachute. The electronics bay is much like the ones made by Giant Leap Rocketry. Even if you don't intend to fly dual deployment, a bay like this gives you lots of flexibility in rigging your rocket.

My Tomahawk has two body tubes so this bay fits naturally between them, replacing the empty coupler. To use the bay for dual deployment, one would put the drogue in the booster body tube and the main in the payload tube (opening at the nose cone). And, if you use single deployment, this leaves a potential payload space in the forward tube.

Once it was assembled, I bonded my electronics bay halfway into the payload body tube with the end that opens sticking out. Remember to drill a hole for the ejection charge in the fixed end of the bay before you epoxy it into place.

Once the epoxy has cured, drill vent holes and switch holes as necessary. You can also do this after you paint, but by doing it now the insides of the holes will get painted to match the outside of the body. I generally drill as many vent holes as I have fins, for no good reason other than looks. You should have just one hole or three or more, never two. The holes should all be the same size, be evenly spaced and have clean edges. For a bay this size, one ¼" hole would be fine, although multiple holes reduce the chance of the hole getting blocked.

Finishing Up

At this point, you should decide whether or not to use the supplied launch lugs. If so, apply them per the kit instructions. For high-power kits, there should be two launch lugs and one should go near the aft end and one near the center of gravity (CG). I like to use blacksky ProRails instead of rods when I fly so I decided to install ProRail buttons instead of the launch lugs which came with the kit. However, I prefer to install the buttons after painting so I didn't do anything with them yet.

Do any other completion steps as described in your kit instructions. As always, be sure you understand the whole process before starting so that you don't work yourself into a corner.

As a good safety precaution, drill pressure relief holes in the airframe sections. Any sections which will slip apart for recovery should have a small hole to the outside to allow pressure to equalize in flight. Failure to do this can cause premature separation. I drilled a single 1/8" hole in the payload section and one in the booster section. Be careful not to drill them where they will be blocked by the nose cone or a coupler.