FIREWORKS

It’s November, which means the skies are full of sound and colour. The large aerial fireworks are either rockets, like the one shown here, or mortars. Both are packed with exploding shells, known as ‘stars’, that are launched into the air using gunpowder. Mortars propel the payload upwards with a single explosion, whereas a rocket is propelled by a slower burning charge that provides gentler acceleration, for longer.

Electrical igniter Commercial fireworks are often synchronised with music, so the fireworks are triggered via signals sent through electrical cables, which in turn trigger a smaller igniter charge.

Igniter charge

This consists of a small electrical heating element made from nichrome wire that glows hot enough to ignite a mixture of magnesium powder and potassium nitrate, triggering both the lift charge and the timed fuse.

Lift charge

A large part of the rocket’s mass is the gunpowder that provides the upward thrust. This is usually 10 per cent sulphur and 15

per cent charcoal, with 75 per cent potassium nitrate to act as an oxidiser for the chemical reaction when the gunpowder burns. Commercial firework displays sometimes use sulphurless powder to reduce the smoke.

Clay nozzle

A plug at the bottom of the firework has a specially shaped aperture that channels the expanding gases from the lift charge to create thrust.

Timed fuse

This hollow wooden tube packed with gunpowder is designed so that it burns through just as the firework reaches the top of its arc, setting off the main display burst.

Scatter charge Another load of gunpowder is packed into a cardboard or plastic sphere in the centre of the firework. When the fuse reaches it, the explosion ruptures the firework’s outer casing and flings the surrounding ‘star’ charges in all directions.

Star charges These are pellets of different metal compounds which burn to produce the various colours and effects we see in the sky. For example, copper gives green patterns and strontium is used for red. The star charges may have their own smaller starbur sts, fountains or pinwheels inside.