Falling Leaf Shell (1" Sphere)Edit
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 or in this case, falling leaf fuse. At effect time the small pieces of fuse ignite inside the aerial shell and the shell breaks open. The fuse slowly burns as it falls towards the ground, giving the effect of falling leaves. |- | valign="top"|
Casing For a casing you will need a 1" 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 Standard black powder is recommended to burst open the shell. Using energetic compositions such as flash powder, H3, whistle mix etc are too reactive and burst the shell open before the small pieces of fuse have had time to ignite. Therefore less energetic powders such as meal powder are well suited in this application.
Fuse You will need about 15 cm of falling leaf fuse which will be cut into small pieces. Falling leaf fuse comes in a variety of colours and in this example we will be using blue.
Other You will also need scissors, eye dropper, razor blade knife and a hot melt glue gun. |- | valign="top"|
Click for larger image 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. |- | valign="top"|Click for larger imageCut the falling leaf fuse into 1/2 cm or 1 cm lengths, again on a 45 degree angle. The longer the fuse the more burn time you will have, so you need to cut your fuse to suit the height of your aerial shell. If it the fuse is too long it will fall to the ground when it is still alight and this must be avoided for obvious safety reasons. |- | valign="top"|Click for larger imageAdd the small pieces of falling leaf fuse to the bottom half of the shell. Only fill the half shell to the height of the rim because we need to have room for the black powder and expansion of the hot gases. |- | valign="top"|Click for larger imagePlace a very small amount of black powder into the top half of the shell. Basically we want the shell to break open gently so the small pieces of fuse have enough time to ignite. Depending on the strength of your black powder you may need to experiment. One way of doing this is cutting a longer time fuse and when the shell is fully assembled place it on the ground (in a safe location away from any potential fire hazards), ignite the fuse and swiftly move to a safe distance. When the shells bursts open you will be able to determine if your pieces of falling leaf fuse have ignited and adjust the amount of black powder accordingly. |- | valign="top"|Click for larger imageTake 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. |- | valign="top"|Click for larger imageUsing 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. Once the solvent has dried, gently jiggle the aerial shell to work the black powder around the small pieces of cut fuse inside the shell. This will help ensure all the pieces of fuse are ignited at effect time. |- | valign="top"|Click for larger imageThat was the final step in creating a 1" 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. Place a small amount of lifting charge at the bottom of the mortar tube and place the aerial shell carefully positioned on top, ensuring the time fuse is not damaged in the process. |- | valign="top"| 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. |- | valign="top"|
There are varying uses for the round shell i.e. mortar, payload for black powder rocket etc. |- | valign="top"| 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. |- | valign="top"| 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. |}