FIRE­WORKS

Focus-Science and Technology - - Q & A -

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.

Newspapers in English

Newspapers from UK

© PressReader. All rights reserved.