Aerial Shell (2" Sphere)

2" Plastic Aerial Shell

A plastic aerial shell or round shell is a spherical cartridge containing chemical composition, and a black powder propelling charge (lift charge). Shells are most commonly 50 mm to 150 mm diameter and are fired from high-density polyethylene (HDPE), fibreglass or heavy cardboard tubes. Upon firing, the lift charge is consumed and the shell is projected into the air. The pyrotechnic effect is produced near the highest point of flight when a burst charge within the shell explodes, dispersing effects such as stars and whistles.

[edit] Materials

Tools and casing

Casing
For a casing you will need a 2" plastic ball shell and these can be purchased from most online pyrotechnic supply stores. Make sure the two halves fit together nicely as there are some rare occasions when they do not. You don’t want to be in a position where you have gone to all the effort of preparing the shell and they won't snap together. A wide range of plastic shells can be purchased from Pyrocreations.

Burst charge
Energetic burst charges such as flash powder, whistle mix and H3 are recommended for small shells although it is possible to obtain very satisfactory results using just black powder, and it is much safer to work with. 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.

Meal coated rice hulls are used to help break the shell apart without shattering the stars to dust at effect time. In the core of this we will be adding a very small amount of whistle mix.

Stars
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 around the outer casing wall and the 'core' of burst charge.

Time fuse
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.

Glue
Xylol, methylene chloride or xylene all work very well to weld the plastic shell halves together.

Other
You will also need paper tape, scissors, eye dropper, tissue paper, empty toilet roll paper tube and a hot melt glue gun.

[edit] Construction

Preparing the fuse

First step is preparing the fuse which will ignite our shell. Cut a 5 cm length of visco fuse and using a razor blade cut one end of the fuse on a 45 degree angle. Cutting it on this angle exposes more of the black powder and increases the chances of ignition. Try not to use a scissors to cut the fuse as it crushes the inner core of the fuse.

Inserting fuse in shell

Cut a 2cm length of paper tape and wrap this around the middle of the visco fuse. This will act as a delay when the fuse burns inside the shell. To obtain a symmetrical burst the fuse needs to ignite the whistle mix in the dead centre of the shell. The paper tape will prevent premature ignition. Again cut the fuse with some scissors leaving about 5mm of visco fuse exposed. Feed the paper tape encased fuse through the fuse hole in the plastic shell, it should be a neat fit. If its too large or small simply adjust the amount of paper tape. Using your hot glue gun place a ring of glue around the base of the paper tape to finally secure the fuse in place.

Loading the stars

Take an old toilet roll cardboard tube and cut it neatly in half. Place both halves of the plastic shell on top of the cardboard tubes and arrange your stars around the wall of the plastic shells one layer deep only. Using cardboard tube supports simplifies the process and also protects your time fuse. Do not load the stars above the rim of the plastic shell, as this will prevent the two halves being snapped together later.

Preparing burst charge

Measure out a teaspoon full of meal coated rice hulls and place this in the centre of the tissue paper. Make a small well in the middle if the hulls and add a small amount of whistle mix. Close the tissue around the hulls and twist into a little ball. This step can be a little tricky, as the ball needs to fit in the centre space of the plastic shell so you may need to adjust the amount of hulls. Using the tip of your razor blade make a small hole into the tissue as this will slide over the fuse. Remove the excess tissue with some scissors.

Loading burst charge

Place the tissue ball of meal coated rice hulls and whistle mix over the fuse so that the exposed part of the fuse is in the centre of the ball (it should be in contact with the whistle mix). Half of the tissue ball should be protruding above the plastic shell rim, as this will fill the cavity in our other half. If you find that you are unable to join the halves together in the next step, as the tissue ball is too large, then simply remove some of the hulls and try again.

Closing shell

Take hold of one half of the plastic shell in each hand and carefully line them up. Rest the inner lip of the shell on the outer lip of the other shell and snap them closed. This is not as difficult as it sounds and the halves may not line up perfectly first go. If this is the case then keep applying gentle pressure and work the two halves together until they snap close.

Sealing shell shut

Using your thumbnail or the edge of your razor blade carefully pry open the shell halves about 1mm, just enough for the xylene solution to flow around. Using an eyedropper carefully run the solvent into the seam of the shell. Again close the plastic shell and the solvent will weld the plastic together. Try not to apply too much solvent as it does melt the plastic and will create a huge sticky mess. Typically 1 or 2 drops of solvent will be sufficient for this size aerial shell.

2" Plastic Aerial Shell

That was the final step in creating a 2" plastic aerial shell. All you need to do now is trim your time fuse for your intended use. Remember if you need to shorten the fuse cut it on a solid surface using a razor blade on a 45 degree angle. Don't cut the fuse too short as you don't want the shell exploding in the mortar tube, the aim is for the shell to explode at apogee. A lifting cup of black powder can be attached to the bottom of the shell if desired. Otherwise the lifting charge can be placed at the bottom of the mortar tube and the aerial shell carefully positioned on top, ensuring the time fuse is not damaged in the process.

An easy way of loading the aerial shell into the mortar tube is to first measure the depth of the mortar with a ruler and add 10cm to this. Cut a piece of string to this length and attach one end of the string to the crown of the aerial shell (opposite to the fuse end) with some paper tape or similar. Make sure the weight of the aerial shell can be held securely with the string. This will ensure when the aerial shell is lowered into the mortar tube the time fuse will be sitting in the lifting charge and pointing in the correct direction. When you have lowered the aerial shell into the mortar leave the excess amount of string hanging over the edge of the tube, this way if you need to remove the aerial shell for any reason it can be done easily. Remember, never ever pass any part of your body directly over the mouth of a charged mortar. A mirror should be used to inspect the interior of the mortar.

[edit] Round shell uses

There are varying uses for the round 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 burnout, 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.