• Video
  • 5 mins
  • Level 1: Introductory

Explosive accuracy

Updated Monday 11th October 2010

Explosives expert Dr Sidney Alford joins Jem to explain the art of effective explosions.

Love science? There's something for you in The Open University's range of science short courses

Dr Sidney Alford demonstrates how accurate metal cutting can be achieved using a minimum of explosive.

Watch

Copyright BBC

Read

 

Jem Stansfield
Dr Sidney Alford designs ways of channelling the power of an explosion to achieve very accurate results with the minimum amount of explosive.

Dr Sidney Alford

A simple crude way of severing steel is just to put a ribbon of explosive across it, but you need a relatively massive amount of explosive and it deforms the steel and it blows a bit off the back.  It’s a very ugly and inefficient way of doing it. 

I came up, quite some years ago now, more than 20 years ago, with an idea for shaping the shockwave.  What I do is put explosive on the back of one of these elements.  It’s magnetic rubber, which is a great aid to placement and the explosive went immediately on the back and the shockwave from this side went down that way and the shockwave from this side went down that way, so not surprisingly they crossed over.  Now, if you put it here and put explosive on the back they cross over in the steel, and what happens is you have a shockwave going down that way and a shockwave that way and where they cross, they’re already zones of extremely high pressure, you’ve got high pressure times two which suddenly relaxes.  Now, under those circumstances, steel cracks.  I would crack, you would crack!  Then the crack actually proceeds down from the surface to the back of the steel and if it reaches the back of the steel and it compresses it and then stretches it hard enough that will just fall in two parts.

Jem Stansfield
Wow!  I’ll be astonished to see that crack a piece of steel like that.

Dr Sidney Alford

I’ll be pleased because I haven’t done it for a few years, but we’ll give it a whirl.  That is very little explosive to cut that.

Jem Stansfield
Yeah.

Dr Sidney Alford

So, what I’ll do is take the end of a detonator in there and retire and we’ll press the button.

Jem Stansfield
Fabulous, I can’t wait to see it.

Dr Sidney Alford

Okay. Firing; four, three, two, one. 

Jem Stansfield
Detonation of high explosives like the sort Sidney is using creates a shockwave that travels faster than the speed of sound through the block of explosive and then through the material next to it.  Using high speed photography to slow the process down 250 times you can see the shockwave travelling through the air above the explosive distorting it like a mirage and radiating out like a bubble.  It is the power of this that Sidney’s rubber prism will hopefully have focused into a thin line along the block of steel. 

Look at that!

 

Dr Sidney Alford

Oh well, that shows you the principle.

Jem Stansfield
Wow, you can see there the crack’s quite clean, it’s really quite clean.

Dr Sidney Alford

Yeah.  You can see at the initiation end it didn’t go.

Jem Stansfield
Why does it become more powerful as it moves away from the detonator?

Dr Sidney Alford

It takes a definite distance for the detonation wave to accelerate up to a maximum value.  It probably would have severed completely if we’d initiated in the middle but in a long piece or let’s say you’re cutting up the side of an oil tanker, if an inch or two isn’t severed that’s neither here nor there.

Jem Stansfield
Yeah.

Dr Sidney Alford

And very little deformation of the plate.

Jem Stansfield
Yeah, it’s just a crack.

Dr Sidney Alford

Hmm that’s right.  This is in fact, when you get it right, it is the most efficient way I know of severing steel.  It is not cutting steel but it’s severing it.  That suffices to give a dead straight cut, so if you cut the wrong bit of steel it makes it easier to weld up again.

Jem Stansfield
I like that.

3’59”

 

 

For further information, take a look at our frequently asked questions which may give you the support you need.

Have a question?

Other content you may like

Child of Our Time: Try our surveys Copyrighted image Icon Copyright: BBC activity icon

Education & Development 

Child of Our Time: Try our surveys

How does your sense of self compare with other people? How good are you at performing complex tasks? How do you and your children cope with stress? Try our surveys to find out

Activity
Theo's Adventure Capitalists: Six Months On Copyrighted image Icon Copyright: The Open University article icon

TV, Radio & Events 

Theo's Adventure Capitalists: Six Months On

How did our companies get on? Six months after their meetings with Theo, we catch up with the exporters.

Article
The Educators: The episodes article icon

TV, Radio & Events 

The Educators: The episodes

Find out more about each episode in the BBC Radio 4 series, The Educators.

Article
Ever Wondered About... Sugar? Creative commons image Icon Com Salud under Creative Commons BY 4.0 license audio icon

Science, Maths & Technology 

Ever Wondered About... Sugar?

It might help the medicine go down - indeed, in the most delightful way - but that's only one of the amazing qualities of the sweet stuff.

Audio
15 mins
OU on the BBC: In Their Own Words - Great Thinkers Copyrighted image Icon Copyright: Marco Richter | Dreamstime.com article icon

TV, Radio & Events 

OU on the BBC: In Their Own Words - Great Thinkers

This series looks at important thinkers through the TV and radio broadcasts they made for the BBC

Article
Keeping Britain Alive: The NHS in a Day - Episode 4 Copyrighted image Icon Copyright: Viktor Levi | Dreamstime.com article icon

TV, Radio & Events 

Keeping Britain Alive: The NHS in a Day - Episode 4

Episode four shows the different ways in which the NHS helps people live as normal a life as possible, from rehab to nutrition appointments, gender reassignment to nipple tattooing.

Article
article icon

TV, Radio & Events 

Rough Science 3 New Zealand: Mike Leahy's diaries: Gold rush

Rough Science took Mike Leahy to New Zealand - and then asked him to find gold

Article
Rough Science 4 Death Valley: Mike Bullivant's diary: Rover Copyrighted image Icon Copyright: Production team article icon

TV, Radio & Events 

Rough Science 4 Death Valley: Mike Bullivant's diary: Rover

Charcoal and pumping under extreme conditions - Mike Bullivant is back facing some Rough Science.

Article
OU on the BBC: Civil War Copyrighted image Icon Copyright: Wark Clements article icon

TV, Radio & Events 

OU on the BBC: Civil War

Tristram Hunt tells the not-too-distant story of how the King was toppled and a Commonwealth established - only for Monarchy to return.

Article