Bulkhead method for attaching headers to motor
Attaching a canister shell to the top of a rocket motor can at times prove problematic. I dislike just hot gluing the canister to the top of the motor. This is my solution to the problem.
Basically what you will be doing is creating an adapter/bulkhead. For purposes of this document I will use dimensions for my 8 oz. rockets. The basic design can be adapted to fit just about any size rocket or header. I also use the same design on my 2 oz. rockets with good success.
To make the bulkheads there are a couple of things you need to know. You will need to know the inside diameter (ID) of your header canister, the outside diameter (OD) of your rocket motor, and the desired length of your finished canister shell.
In my case I use rocket motors with an OD of 7/8 inches, my canister shells have an ID of 1 ½ inches, and the finished length of the canister is 2 ¼ inches. (Because the bulkhead uses up ½ inch of the canister, the usable volume of the canister is actually 1 ¾ inches in length x 1 ½ inches ID).
The materials I use to make the bulkheads are poster board, white glue thinned with water, and a cardboard disc.
Construction: Using a 7/8 inch former (or whatever size your rocket motor casing is), begin by rolling a tube that has an ID the same as your rocket motors OD, and an OD the same size as the ID as your canister header. In my case that translates to a tube 7/8 ID x 1 ½ OD (the wall thickness of the tube is 5/8 inches). I roll mine about 8 inches long. Because the wall thickness is so large you will need to use multiple strips of poster board to build up the thickness of the tube. You want to roll a tube that is good and stout without any gaps between the layers. It takes a small amount of practice to get it correct and to calculate how long to make the poster board strips to achieve the correct dimensions. Don’t worry if the tube comes out with a slightly larger OD than you want. Later on in the process it’s easy enough to peel a couple of wraps off the bulkhead to achieve a snug fit for any given canister. After rolling the tube you should have a large tube 7/8 x 1 ½ x 8 inches long. At this point you need to let the tube dry. This can take a bit of time. Don’t rush it by baking in an oven. You want the tube to dry completely without gaps between the layers. I find that mine dry in about 4 days in a dry warm environment.
Next, you want to cut off a section of the tube the desired thickness for your application. For my 8 oz. rockets I make mine ½ inch thick. I use the band saw to cut my bulkheads. If you don’t have a band saw, a hack saw or some other type of saw should do the trick. You want to try to make the cuts as square as possible. (See the picture below).
Note: that the canister shell has a “fill line” on the inside of it. When filling your canister with stars and burst you do not want to fill above the line. If you do, your bulkhead will not seat completely in the canister. To make the fill line on your canisters, simply draw a line on the paper or poster board that you are using prior to rolling the canisters
Now that you have a “ring”, you want to glue a cardboard disc (or two) to the ring to complete the bulkhead. When I make mine I punch a ¼ inch hole in the center of the disc to function as a passfire between the motor and the canister shell. I then glue a single piece of tissue paper over the hole to keep the contents of the shell from falling out during final assembly. (You can however use any method that you like). The device will function equally well with a spollette, time fuse, visco fuse, or black match. Once you understand the basics of how the bulkhead works, you can easily tailor whatever type of passfire you like.
Once the glue on all your component parts has dried you can proceed to final assembly. Fill your canister shell up to the fill line with your garniture and burst charge of choice. Coat the outside of the bulkhead with a layer of white glue and insert the bulkhead into the canister. Allow to dry. Once the unit is dry you can attach your motor to it. (I use white glue).
Before I attach my motors to my headers I place a small amount of BP in the top of the motor to transfer fire to the header. This easily blows thru the tissue paper glued onto the bulkhead and ignites the header. If you desire to use spiking on your header you can now spike the shell using the off center technique of spiking. After spiking I cover the shell with a wrap or two of light weight kraft paper, fuse the motor, use some colorful tissue paper for nosing, glue a stick to the rocket, and finally add a wrap of light weight kraft paper or whimsical label on the motor to dress it up a bit.
As stated above, this method works on just about any size rocket and header provided you calculate the proper size of the bulkhead. Below is a picture of some of my 2 oz. rockets with 7/8 inch headers. To make the bulkhead for these rockets I simply roll my motor tubes with a 3/8 inch ID, and 5/8 inch OD. I then cut up one of my 8 oz. rocket tubes into sections on the band saw to form the bulkhead. The 8 oz rockets have a 5/8 inch ID and a 7/8 inch OD. I roll the canisters for the headers with a 7/8 inch ID. I then proceed in the same manner as the larger rockets.
I hope this info helps. Your results may vary but it works for me. Experiment, have fun, and be safe.