WEBVTT
Kind: captions
Language: en

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Have you ever realised that,
all your life,

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you've missed something
that's kind of obvious?

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It doesn't have to be something important.
It might just be really minor,

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but maybe someone says a word

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and your brain just kinda
shifts a little

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as you realise that:
you've never thought of that,

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and it sort of feels like
you should have.

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Last time I was making a video here
at this explosives test facility.

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I just casually used the
phrase "high explosives"

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because I think I assumed
that "high explosive"

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just meant really, really explosive.

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So I was surprised when Steve,
the explosives expert, pointed out

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that it is a technical term
and there is also "low explosive".

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So the obvious thing to do
was to demonstrate both.

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- Explosives are a mixture
of a fuel and an oxidizer.

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With low explosives, these
are roughly mixed together.

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So they're particles of the two,
in close proximity.

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When a low explosive reacts,
it's a thermal reaction.

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So it's heat travelling
through the low explosive

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causing the chemical reaction to occur.

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As it burns the fuel and the oxidizer
react together, producing lots of gas

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that will just bleed off
into the atmosphere

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as a big puff of smoke.

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In order to make a low explosive
have a more violent reaction,

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you need to contain it.

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So you need to hold those gases in
and allow the gas pressure to build up

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until it bursts through the container
that you've put it in.

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So you've got deflagration,

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which is when you hold all the
material in, and the gas pressure,

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so the pressure builds up,
which creates more heat

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which makes the low explosive
react even faster.

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It's a runaway reaction, and eventually

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the amount of gas produced overcomes
the container and bursts it open

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making a bang.

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- So a low explosive, just
like any other fuel,

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is a Thing that Burns.
It just burns quickly.

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The gas that it releases expands
and builds up pressure and then...

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- A high explosive is also a mixture
of a fuel and oxidizer 

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but it's not actually a mixture.

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It's down on the molecular level.

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So you've actually got
the atoms of material

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in such close proximity that they're
linked together within the molecule.

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And because they're so close together
they can react a whole lot faster.

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So rather than it being a thermal reaction
that propagates through,

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it's actually a shockwave reaction.

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It's so fast that the shockwave can
cause the chemical reaction to occur.

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The chemical reaction produces energy
which then sustains the shockwave

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driving it on even faster

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and that's why high explosives
can actually go

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off at between
6,000-9,000 metres per second

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which is pretty damn fast.

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Because high explosives require
a shockwave to detonate properly,

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they can be exposed to heat and flame
and will normally just burn away.

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In fact, you're actually allowed to have
50 kilos of high explosive in your car 

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without having to put a hazard symbol up
on your vehicle at all

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because it's considered not to be
that big of a problem.

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Obviously when the fire brigade
see you pinned

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behind the steering wheel on your car
at the side of the road, and it's on fire,

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they're going to stand back
and sell tickets

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rather than actually fighting the fire.

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So what you use is a detonator
to give it a shockwave, to get it going.

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A detonator itself is a little metal tube.

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Inside there, there's a small amount
of secondary high explosive,

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a tiny amount of primary explosive
which is set off very easily,

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and then there's some sort
of initiation system

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that will then set off
that primary explosive.

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With electrical detonators,
that's an electrical fuse head

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and you pass a current
through the fuse head

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that produces a flash as it burns out.

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That sets off your primary explosive

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which applies a shockwave
into your small amount

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of secondary explosive,
which amplifies the effect.

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That then bigger shockwave
then transfers into your main charge,

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which is then sufficient to set off
the main block of explosive.

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- Which means, weirdly, that high explosives
can be safer than low explosives.

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Despite the name, if there's an accident
around a load of low explosives

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they're all going to catch fire and go up.

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But around high explosives, as long
as you keep the primaries separate,

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like you're supposed to,

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it takes a lot to make a
high explosive detonate

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or sometimes even to burn.

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That is until you want them to.

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- Stand by for firing.

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Firing now.

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- One dead GoPro.
- Yep. Yep.