LESSON SIX: Building the main parachute compartment

In our last installment, we completed the lower portion of the nose cone assembly, which houses the altimeter unit. In this installment, we will complete the upper portion of the nose cone assembly, which will contain the main parachute. Together, these two sub-assemblies allow us to house both the electronics and the main parachute inside the nose cone.

The main parachute compartment is a 7-1/2" length of 54mm body tubing with a solid bulkhead plate in the forward end supported by a pair of plywood centering rings at the base. The compartment is removable, to allow easy installation as well as to allow you to attach nose weight to the eyebolt mounted through the forward bulkhead if needed for flight stability.

To start construction, you need two 54mm centering rings: one is a standard 3/16" thick birch plywood 3" centering ring, the other is a 1/4" thick birch plywood centering ring 3-5/16" in diameter. The larger ring will be mounted permanently inside the nose cone, while the smaller ring will be mounted permanently to the 54mm main parachute compartment tube, so the larger ring should be loose enough on the 54mm tube to allow it to slip easily through without catching.

Before we mount them, we need to add two T-nuts to the larger ring to secure the main parachute recovery compartment to the nose cone. I used Dubro T-nuts because they have a smaller outer circumference to them than standard hardware store T-nuts, and space was a concern in this application. Dubro #6-32 T-nuts, available at most hobby shops that carry R/C supplies, were chosen to give us sufficient holding strength while still allowing for a small footprint on the installation.

I created another small jig, consisting of a small section of 54mm tube with a bulkhead plate inside of it, so I could place a bolt through the center. This would allow me to rotate the two centering rings once they were placed over the tube, and let me drill my holes in the same location radially from the centerline. After marking the centerline on the smaller ring, I drilled two holes large enough to allow the body of the T-nut to slide through.

Try to observe, marking if necessary, the orientation of the rings when you drill your holes — this side is up, that side is down, and use that orientation when you assemble the parts. It will assure that your holes line up later.

I use a vise to press the T-nuts in — I don't hammer them in, as that can split and splinter the wood, destroying your ring. Once they were in, I covered the back side with epoxy to deter them from pushing out prematurely.

To install the larger ring in the nose cone, locate the ridged lower marking on the nose cone, as indicated in the photo above with the red arrow, which indicates the widest portion of the nose cone. We are going to install the ring where the bottom of the ring is even with this ridged lower marking.

The yellow shaded area signifies the 1/4" wide location where we will apply the Testors plastic cement. To begin, take the larger ring and apply a light bead of Testors plastic cement around the outer circumference of the ring. With your finger smooth the cement evenly around the outer edge, removing the excess. The desired result is to wet the plywood and allow the cement to penetrate the grain. When done, clean your fingers well.

Next, take the plastic cement and apply a larger bead around the yellow shaded area, all the way around the inside of the nose cone, even with and above the ridged lower marking. Then, take your finger and spread the cement around smoothly. When done, clean your fingers well.

Now, using your fingers to squeeze the nose cone, forming an "O" with the nose cone's base, insert the centering ring sideways, and when you get it in as far as the location of the cement, rotate it into position. Then, by holding it up to a good light, use your fingers to pull the centering ring down flush with the ridged marking.

When it is in the proper location, it should look like the photo above, where the centering ring is sitting right at the edge of the ridged marking. Take your time to insure that it is lined up right, allowing the centering ring to be positioned squarely within the nose cone. Set aside to dry, preferably overnight.

Once the plastic cement has dried, take your 7-1/2" length of 54mm body tube, and slide the smaller centering ring over one end, placing the ring about 1" from the end. Do not use adhesive at this time.

Then, following the arrow's direction in the photo above, place the tube into the nose cone, through the centering ring we just installed, and slowly slide the tube forward. The loose centering ring will bottom out on the larger ring previously installed in the nose cone. Continue sliding the tube forward until it bottoms out in the forward end of the nose cone.

At this point, the loose centering ring should be sitting flush on the other centering ring. Slowly pull the tube back out, taking care not to disturb the loose centering ring. If your centering ring is too loose to pull out without being disturbed, you may need to tack it on with CA. Don't use too much!

Once you have the tube out and the centering ring is in its proper place, turn it over and apply thin CA around the entire circumference of the bottom side. Don't do it all at once. Start by tacking several places around the circumference and then fill in the remainder. Do not put so much on that it forms a noticeable fillet — it has to fit in the centering ring in the nose cone, and a large fillet will hold it away from the proper place.

After the CA has dried, turn it over and place a good epoxy fillet around the top side, but don't put so much on that it interferes with the two #6 screws that will be used to hold it in the nose cone.

To complete the main parachute recovery compartment, you will need to install a solid 54mm bulkhead plate in the other end, flush with the end. Apply the epoxy around the inside of the tube, about 1/4" up into the tube, so that when you insert the bulkhead plate, the excess epoxy will flow down and make a nice fillet.

As I mentioned earlier in the class, I like to insert my bulkheads, then place them on a flat surface covered with wax paper. This assures I get the bulkhead plate flush with the end, and all I have to do is reach through and tap the bulkhead plate flat after standing the tube on its end on the wax paper.

This assembly is now ready for the eye bolt to be installed, and using two #6-32 screws, 3/4" in length, with flat washers, the main recovery compartment will fit into the nose cone and the nose cone will slide onto the forward end of the electronics bay. I recommend leaving it out for now, at least until after the painting is done.