Mortar tubes shells

Although the most common calibres of the mortar tubes associated with these fireworks are 50, 75 and 100 mm, the size of shells seems to know no limit - the record at the moment stands at a calibre in excess of 1 metre or 1000mm. Obviously, where such astonishingly large devices are concerned, the mortar tubes are no longer made from simple overlapping spirals of Kraft paper but preferably of stainless steel ... 

Figure 4.1 illustrates a range of shells and mortar tubes from 50 to 125 mm calibre, although these are no longer available to the general public. 

The gunpowder explodes in a further fraction of a second, the explosion blowing the shell out of the mortar tube in the manner of a shot from a cannon. At the same time a return section of piped match is lit by the explosion, transferring flame back to the top compartment where it reaches the quickmatch placed across the head of the 25 mm pressed delay fuse. 

For display shells of 200 mm and above, reinforced fibreglass mortar tubes are invariably used. Welded steel tubes have long been considered to be amongst the safest because, in the event of a pressure-burst, the tube will split in the vicinity of the weld, thus creating a predictable danger zone. With seamless tubes the fragmentation is completely random and creates a shrapnel hazard for the fireworks operator. 

Mines, are essentially shells that burst at ground level, the mortar tubes being used to give the projectiles height and direction. They are available in the common shell calibres such as 75 mm (Figure 4.9) and can therefore be hred from a common set of mortar tubes. Indeed, if a shell does, for any reason, explode prematurely in its mortar tube the effect is known as mining . 

If a mine or shell is too tight for the mortar tube (assuming that it is of the correct calibre), it should on no account be hammered or crushed to... 

Although professional operators tend to use the same set of mortar tubes on more than one occasion, it is safer to use them once only, particularly during a display because the act of reloading is hazardous if there is any smouldering debris or afterglow from the previous shots within the tube into which a fresh shell is inserted. 

In the absence of crates or sandbags, each mortar tube should be partially buried in the ground. In this event, care must be taken that there are no stones or other loose objects in the vicinity that might become missiles in the event of an unscheduled explosion. Having placed all mines and shells at the extreme rear of the firing area it is necessary to ensure that any racks or crates of mortar tubes are positioned perpendicularly or end-wise to the audience so that, in the event of a container tipping over, the contents are not left pointing towards the audience. 

AFTERGLOW The glowing remains produced by the firing of gunpowder-based products such as quickmatch. It is very important that any afterglow is extinguished, especially when reloading shells or mines into mortar tubes. 

SHELL A hrework designed to be projected from a mortar tube and containing propellant charge, delay fuse, burster and pyrotechnic units. 

Finally, an estimate can be made of the amount of energy expended by the gunpowder lifting charge in aspects such as warming up the mortar tube and overcoming the friction of the shell. 

An efficiency of 4.5% might seem surprisingly low but it should be remembered that the shell is never a good fit in the bore of the mortar tube, there are no gas-tight seals around the shell, and that the shell is not perfectly spherical (or cylindrical). 

BALLISTICS (INTERNAL) The science of internal ballistics is concerned with the propulsion of a projectile such as a shell along the tube of a mortar by gas pressure acting on the base of the shell, or, in the case of rockets, by the backward exhaust of the gas jet. 

MORTAR A tube from which a mine or shell may be fired. 

A 4.2 chemical mortar shell was also used for incendiary purposes. It contained a burster charge of Tetryl, placed in thin-walled tubing extending into a cavity in the phosphorous. The Tetryl broke the shell and simultaneously ignited and scattered the burning... 

Bofors NG NC proplnts, TNT, PETN, RDX, Hexotol(RDX/TNT), Hexotonal(RDX/TNT/Al), initiating expls, plastic expls, gaines, detonators, detonating fuses primers- electric and non-electric shells- HE, AP, star, smoke, mortar incendiary tracers, cartridge cases percussion tubes homhs-HE, flare photoflash rocket-propelled depth chartres- rnrbp/ i- air-to-air. air-to-vround. flare ... 

---