Cylindrical shell

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Cylindrical shells are said to have been in invented in Italy, centuries ago. Hence, they are also known as 'Italian shells'. They are easier to make than round shells. The pattern of stars (or 'break') produced by a cylindrical shell tends to be somewhat less symmetrical than that of a round shells. Because of their simple construction and because there are no stars involved, salutes are often constructed as cylindrical shells. An interesting variety of the cylindrical shell is the multibreak shell, which consists of a number of cylindrical shells, stacked on top of each other and interconnected with time fuse. These produce a number of breaks at regularly timed intervals.

Please note that this tutorial does neither represent a description based on common shell-building techniques, nor assemble a cylinder shell in the proper meaning of the word. Instead it provides a technique to make a more or less successful bombette. Readers aiming at more established methods of shellmaking are referred to upcoming articles!

[edit] Materials

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For a casing you will need a cardboard tube, and two cardboard end discs. You can of course use pre made tubes, but it is easy enough to roll your own tubes (like in this example) and the advantage will be that you can make them any size you like. In this example we will be using plastering paper, which is used to join two sheets of plaster board together. It is thick, strong and an excellent size for making an effective small cylindrical shell. Gummed brown craft paper is also an excellent alternative.

Burst charge
Black powder works well if your casings are sufficiently strong. Though more energetic burst charges (flash powder, whistle mix and H3) are often recommended for small shells. It is possible to obtain very satisfactory results using just black powder, and it is much safer to work with. Also, flash powder can disturb colour perception if the flash is too bright. Of course, if you use stars that are based on a chlorate composition (which is strongly discouraged for beginners) black powder burst should never be used as it contains sulfur. Sulfur makes chlorate compositions very, very unstable and spontaneous ignition will most likely occur. In this example we will be using whistle mix.

Round or cylindrical (pumped/pressed) stars are best. Cut stars can be used just as well, but round or cylindrical stars will allow you to arrange the stars more evenly giving a more symmetrical break. They should be small enough to fit nicely between the inner casing wall and the 'core' of burst charge.

Visco fuse works well for small shells. For larger shells, proper time fuse is better. If you are unable to source time fuse or visco fuse then black match may be used.

White glue and hot glue are required for constructing the casing.

Hemp string is traditional. You can probably use any kind of strong, thin string. Nylon for instance works well too.

Rammer or wooden dowel
A rammer, wooden dowel or any other solid tool which will fit neatly inside the casing will be sufficient. The purpose of this tool is to assist in the construction of the cylindrical shell. It's use will be demonstrated later.

In this example a 2cm length of plastic drinking straw is used to form the 'core' of burst charge, of course you can use a paper core.

[edit] Construction

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First step is construction of the casing (if you already have a pre made casing then skip this step). Cut a length of paper about 20-25cm long and place this on a flat surface (the length of cut will depend greatly on the thickness of your paper). Now take your rammer or wooden dowel and roll the paper around it until you achieve 1 revolution. Place some white glue on the paper near the rammer (see picture) and continue rolling for another 1 revolutions. Your biggest problem will be telescoping. That is, as you continue to roll, the paper tape will start to run off to one side or the other. If this happens carefully unroll and start again. Add some more white glue to the paper in a zig zag shape and roll the paper until you have about 1 cm remaining. Place a small line of glue close to the edge and finish rolling the paper. Hold it in your hand tightly for 30 seconds or until the glue has set and the paper does not undo when you let go. You can add a small piece or paper tape to the edge if you wish to aid this process. Remove the casing from the rammer and set aside to dry.

Click for larger image
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Next we need to construct the 'core' of burst charge. As previously mentioned you can use a paper core however a plastic drinking straw is quick and easy. Cut a 1-2cm length of visco fuse and 3cm length of drinking straw. Place a very small amount of hot glue about 3/4 of the way along the visco fuse and insert inside the drinking straw, attaching it to the inner wall. Now seal this end of the straw with a blob of hot glue. Wait until it has cooled and then fill the remaining space of the straw 3/4 of the way with your burst charge and do not compact the powder. If the powder is loose it will burn much faster. Add a very small amount of hot glue to the top of the straw and squash it down on a solid surface to seal the top. Trim the top of the straw with a pair of scissors to obtain a neat finish.

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Slide the casing back onto your rammer until you have about a 5mm void at the top, as this will be where the disc will be inserted. To make your cylindrical disc, take some cardboard and place your rammer vertically on top and draw a circle line around the base. Cut out the circle with some scissors and repeat the process again until you have two round discs. The two end discs should be made from non corrugated cardboard and should not be too thin.

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Place a disc into the void and put a ring of glue (white glue or hot glue) on the surface of the disc. Fold over the sides of the casing and press the casing firmly on a hard flat surface for a few seconds until the glue holds well. Drill a hole in the end disc large enough to slide the fuse through.

Click for larger image
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Take your 'core' of burst charge and place this (fuse end first) into the casing and feed the fuse through the hole. To get a symmetrical break, arrange the stars evenly around the sides of the 'core' of burst charge. When full, sprinkle some black powder between the stars to fill up the voids between them. This will act as a sort of prime and help ignite the stars before the shell burst open throwing the stars into the night sky.

Click for larger image
Click for larger image

As we did previously, place the end disc inside the top of the shell and press it down gently until it comes into contact with the stars. Put a ring of glue (white glue or hot glue) on the surface of the disc and fold over the sides of the casing. Press the casing firmly on a hard flat surface for a few seconds until the glue holds well.

Click for larger image
Click for larger image

All that remains is strengthening the casing with string, called 'spiking'. With larger shells the string is often tied to the fuse to start, but this is not a good idea with these small shells since it may fold the fuse, reducing it's reliability. You can instead hold the string in place with your fingers during the first wrap, and wrap it over itself to hold it into place. Make sure the string goes at least 8 times over the length of the casing and then 4 - 6 times around the sides as tightly as possible. Wrap the string over itself again to finish, and secure the knot with a drop of hot melt glue.

[edit] Cylindrical shell uses

There are varying uses for the cylindrical shell i.e. mortar, payload for black powder rocket etc.

Black powder rocket
Drill a small hole (2mm in the example) into the end plug and slightly (just barely) into the black powder propellant. At burn-out, when the rocket has consumed all of its propellant, the last bits of propellant will ignite the effect charge through this hole. The high pressure inside the casing will push hot gas and sparks through this hole at exactly the right moment. The hole is best made near the side of the casing not in the centre since the flame front propagating through the propellant grain will reach this point last.

The fire transfer hole in the end plug is primed with a bit of loose black powder, and the shell is attached to the casing with tape or hot glue. The time fuse of the shell must be adjusted for this purpose. Normally, the fuse of shells is long enough to allow the shell to reach maximum altitude when shot from a mortar. When used as a heading for a rocket as shown here, we would like the shell to explode virtually immediately when ignited since the rocket itself already provides the delay. Therefore, a very short length of time fuse should be used in this case. For the example use 5 mm of visco, giving a delay of less than half a second.

[edit] References

Wouter's Practical Pyrotechnics


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