The making of a 2" (50mm) aerial shell

Transcription

1 The making of a 2" (50mm) aerial shell Aerial shells are designed and made to be launched into the air as the heading for a rocket or from a mortar tube. The tube is securely plugged at the lower end and often supported vertically in a frame or rack. The shells can be either spherical or cylindrical, the latter being particularly popular in Italy and other Mediterranean countries. Sizes can be from as little as 1" (25mm) diameter, up to the massive World Record size of 48" (1220mm). The current legal maximum in most of Europe for consumer fireworks is 2". In the UK, even these are banned, which is in itself a good enough reason for not living there. For 'normal' professional displays, 3, 4 and 6" devices are the most common. Very large shells may contain not only stars and other effects but also a number of smaller shells... The process of making a small spherical aerial shell begins with a pair of moulded paper hemispheres. These can be made at home, but they are usually purchased ready-made. Plastic equivalents are available, but are not very eco-friendly. The hemis are matched and marked for the best-fit alignment. A small 'passfire' tube is rolled from glued paper, dried and cut to a length such that it will extend from the centre of the shell to about 5mm outside. The inside diameter of the tube must be a close fit around the time fuse. One of the hemis is drilled so that the passfire tube fits snugly into the hole.

2 A piece of special time fuse is carefully measured, marked and cut to length. This time fuse burns at about 10mm per second and provides the delay time between the shell launch and the moment that the shell bursts in the sky. For a 50mm shell, the delay should be around 2 seconds but the fuse is cut a little longer, to allow for a later process. Hot-melt glue is used to fix the passfire tube into the drilled shell half. It must be glued both inside and out, to make sure that there is no possibility of hot gas from the launch (or 'lift') charge reaching the shell interior. If this were to happen, the shell would explode inside the mortar tube - a potentially dangerous event. The time fuse is glued inside the passfire tube and twisted around to ensure a good gas-tight seal. The outer end is cut down the middle with a razor blade as far as the mark indicating the required delay time, split gently apart then wrapped with tape for protection during later handling.

3 The two shell hemis ar then supported on scraps of tube and filled with stars. As can be seen, it is not possible to fit many stars into a 2" shell. The stars shown below were 'pumped' at 10mm diameter then coated with a 'prime' to ensure ignition. Consider that the stars must light at the instant the shell explodes in the air. Because this is such a quick event and some star compositions are quite difficult to light, at least one fast-lighting priming layer is usually needed as a coating on the outside. In larger shells that burst higher in the sky, larger stars are used and it is often possible to arrange them in patterns or even in several layers, to give special effects. The limited space inside a 2" shell does not allow for this sophistication! A piece of tissue paper is gently pressed down over the stars and a few pieces of blackmatch fuse are pushed into the passfire tube until they touch the end of the timefuse. When the time fuse burns through, the blackmatch ignites and very quickly passes the fire to the centre of the shell. Granulated black powder burst charge is placed onto the tissue paper and around the blackmatch. The burst charge is used to explode the shell and spread the stars across the sky. For a 2" shell, about a teaspoonful is used. In larger shells, it is much easier to arrange the stars evenly into a sphere lining the paper hemis and then fill the centre with the burst charge. This gives the familiar round shaped pattern in the sky. Because there is a sensible minimum size for stars, with shells as small as 2", this spherical break shape is usually not possible.

4 This is the tricky bit! The tissue paper must be trimmed, folded around the burst charge and held in place as the two hemispheres are aligned, brought quickly together and taped around the equator. It's a process much easier to describe than do! The outside of the shell is now covered in several overlapping layers of glued paper tape strips (this process is called 'pasting'), done in such a way that all parts are evenly pasted, to avoid weak spots. For a 2" shell, only about 4 layers are used. In larger shells, many more may be required. The power of the burst explosion -and thus the spread of stars across the sky - depends on the gas pressure developed inside the shell when the burst charge ignites. For bigger shells that need to 'paint' more sky, this gas pressure must be higher, so the shell container must be stronger. A loop of string is added, at a point opposite the time fuse. The loop is used to support the shell upright as it is lowered into the mortar tube.

5 The part of the shell that must be lifted into the air is nearly complete, so it is carefully weighed. This is necessary to determine the amount of powder needed for the lift charge. For small shells such as this, about 8-10% of the shell weight is required, depending on the quality of the lift powder. The protective tape is now removed from the time fuse, two short lengths of blackmatch are pushed down into the cut and the outer end of the timefuse is tied tightly. This blackmatch ensures that fire from the lift charge is reliably transferred to the core of the time fuse at exactly the point giving the required delay time.

6 Quickmatch is made from (usually) 5 strands of blackmatch inside a loose paper tube. Blackmatch burns at around 25mm per second in the open air. The tube confines the hot gases generated by the burning blackmatch, increasing the burn speed to several meters per second. A piece of quickmatch is prepared that will reach the full length of the mortar tube, plus about 100mm. The paper covering is removed for about 10mm at one end and the blackmatch is bent over. The required amount of granulated black powder is weighed out and placed in the corner of a sandwich bag. The stripped end of the quickmatch is pushed into the powder, the bag is tied firmly around the quickmatch and the excess bag material is cut off. This is the shell lift charge.

7 A small paper cup is roughly made to protect the lift charge bag. The lift charge and the end of the quickmatch are placed into the cup, pushed carefully onto the end of the time fuse and the cup is secured to the outside of the shell with tape. More tape is used to fix the quickmatch to the outside of the shell, after the fuse is passed through the string loop. 10mm of a short length (50-70mm) of safety fuse is pushed into the free end of the quickmatch and secured with string or tape, making sure that it is against the blackmatch and not between layers of paper. The end of the safety fuse is protected from moisture and sparks by a small piece of tape. The shell is now ready to be placed into a mortar and fired... :-)