LESSON TWO: Assembling the fin can and airframe

In this lesson, we will assemble the airframe assembly from the two fin can halves and the motor mount that we constructed in the first lesson.

In the last lesson, we prepared the two fin can halves and test fit them to make sure they fit freely around the 29mm motor mount tube and specially sized centering rings. If your parts do not fit freely with room to rotate the motor mount tube inside the two fin can halves then you should sand the outer circumference of your centering rings and the inside edge of the fin can halves so that it can be every-so-slightly rotated in the tube. The fit should be snug, but not sloppy, nor too tight. Make sure there is no styrene flashing interfering with the fit of the two halves when they fit together.

Once you have the proper fit, it's time to glue the two fin can halves to the motor mount. In the photo above, the yellow shaded areas indicate where you should put a good bead of the Testors plastic cement. Do not put too much, as you don't want it to ooze out when the two fin can halves are places around the motor mount tube. You do what enough that there is no air gap between the parts.

Because plastic cement isn't like epoxy, you have plenty of time to position the parts before the cement sets. Once you have the cement in the proper places, place the motor mount tube in on fin can half and then place the other fin can half over it, and while holding the two parts together, wrap the parts with masking tape to hold the parts together. Wrap the tape snuggly, so the parts are compressed together, and then leave it this way for at least 24 hours. Failure to wait long enough could cause your parts to dry mis-aligned.

With the masking tape applied, I then applied a fillet of plastic cement around the upper joint where the motor mount tube meets the plastic fin can. This is the joint that is inside the airframe, hidden by the body tube, on the upper end. This is not the aft end joint. Set the motor mount assembly upright to dry.

We've still got some work to do while waiting for the fin can halves to dry on the motor mount tube, so grab the tubing coupler and the upper plastic centering ring. On the inside of the coupler, apply a bead of plastic cement such as indicated by the yellow shaded area above, and push the centering ring into the coupler, twisting it as you seat it against the built-in lip. This insures the cement is adequately distributed and sinks into the cardboard of the coupler.

If you didn't get too messy with the last step, your coupler should look like the one in the photo above. A light film of cement is all that you see. Place this aside to also dry for at least 24 hours.

The next modification I made was in case I needed to add nose weight to balance out the CP/CG relationship. I took advantage of the Tomahawk's nosecone design, in that it has a separate tip that is glued on. I was going to drill a hole into the tip of the cone under where the tip is attached, to insert a screw into. That way, if I wanted to add nose weight, I could pour BBs or lead shot into the nose cone, add epoxy, and the epoxy would grab the screw threads to hold the weight at the tip of the nosecone.

I started by taking a very small Phillips screw, about an inch long, and using my Dremel, I cut the head down as small as possible. I needed the head of the screw to fit inside of the glue-on tip, along with plastic cement. After I had the screw head the right size, I screwed it into a hole I drilled.

With the screw in the hole, I filled the tip with plastic cement and placed it over the screw head. Any plastic cement that oozed out was wiped away while holding the tip in place, and then as indicated in the photo above, retained with a wrap of masking tape. Because the cement in the tip is fully enclosed and out of the atmosphere, it will take longer to dry. Plan on leaving it at least a week to be absolutely sure it has fully dried.

Wrap things up for the day, because we are done with the gluing and need to allow for the previous steps to dry.

Now that the motor mount assembly has dried, we can remove the masking tape that was holding it together and prepare to complete the airframe assembly. Out next step is to "weld" the plastic fin halves along the seam where the two parts meet.

As indicated in the photo above, you want to apply the Tenax 7R Plastic Welder in a fashion where gravity works with you. Tenax is thinner than water and will run everywhere if you don't plan for this ahead of time. The photo above is for illustration purposes only and is not indicative of how I applied my Tenax. In real life, I was holding the motor mount tube at about a 45 degree angle, and slowly "sneaking up" on the joint with the Tenax. I suggest practicing on any other object than the actual styrene part until you get a comfortable handle on how fast it runs and how it reacts to being applied.

Tenax, being the solvent that it is, is forgiving if allowed to evaporate should you get too much on the part. It also easily sands once fully dried. It is not an adhesive in the traditional form of the word, but is a solvent that melts the plastic together. Because of the quick drying nature, if you get too much on the part, let it run off and dry. It will look like there is a slight film on the surface and a quick pass of 600 grit sandpaper will remove it. But whatever you do, don't touch the excess or it will "print" your fingerprints.

Likewise, on the upper seams, I turned the motor mount upside down to do the upper seams, again allowing gravity to flow the Tenax into the seams with a capillary action. Of particular note, there are two places on the bottom seams where two simulated recessed bolt holes are in the middle of the seam. An extra drop of Tenax into each hole will melt the plastic in the recessed hole and fill it in, making a very seamless seam. Fortunately, these two seams are underneath the fins when they are attached, so they hide the seams almost entirely, so this is a good place to get practice using Tenax.

While the Tenax is setting, I got the two airframe tubes ready to attached to the motor mount assembly. I started by wicking CA into one end of each tube. This "sets" the cardboard and gives a nice hard edge. This is a simple practice to insure your tubes have nice, square ends. When the CA dries, lightly sand it to remove any burrs.

In the photo above, the yellow shaded are indicates where you should sand to promote good adhesion of the airframe tube to the motor mount assembly. I used 600 grit wet-or-dry sandpaper, just knocking the gloss off and reducing the seam where the Tenax was applied.

When I had it sanded properly, I applied plastic cement onto the inside of the end of one of the body tubes, on the end that had the CA applied to it, and used a craft stick to smooth the cement around the tube evenly. I then placed the body tube over the top of the motor mount and slid the tube down over the upper fin can joint, twisting the body tube to distribute the plastic cement and seating the body tube.

On the other end of the body tube, I mixed some 12 minute epoxy and applied it with a craft stick as indicated in the photo above, going down to the top of where the motor mount tube stops.

Working quickly because we are working with epoxy now, I apply a good bead of plastic cement around the outside of the lip of the upper centering ring (that is attached to the tube coupler), and then insert the coupler, centering ring down, into the motor mount tube. When it gets to the bottom, make sure the notches you cut in the plastic centering ring clear the recovery mounting point.

Your assembly should look like the above photo, with the coupler and upper centering ring assembly clearing the recovery mounting point and fully seated against the motor mount tube. If you have followed the instructions properly, you should have 1/2 of your coupler still exposed, ready to receive the upper airframe tube.

At this point, it's time to attached your recovery harness, but all you need to do now is place one half on one side of the recovery mount, and the other half on the other side, letting it hang out the back of the rocket. Then ball up the recovery harness and stuff it into the bottom of the motor mount. This gets it out of the way for now, but at least it's already threaded over the mount.

All that's left to do is mix a small amount of two-part epoxy, apply it to the inside of the without CA on it and slip the upper body tube in place. Take care to put the CA'ed end up, mating the two ends that didn't have CA on them on the coupler. When the upper tube is on and lined up, you can run a bead of CA around the body tube joint to make sanding and filling easier.