Lecture 5 - Explosions 5

Question 1

how does an explosive detonate

Answer 1

in response to a stimuli these can be direct or indirect

Question 2

name three types of direct stimuli

Answer 2

electrical
mechanical (percussion)
deflagration into detonation

Question 3

name three types of indirect stimuli

Answer 3

magnetism
acoustic (sound waves)
radar

Question 4

what is percussion and
what is an example of percussion detonation in guns

Answer 4

striking the explosive leading to a direct explosion of low explosive propellants

pulling the trigger moves a hammer which hits a cartridge case setting off the propellant

the percussion results from a localised heating in a small volume and the hammer compressing gas voids

Question 5

what issues may arise in percussion detonations in guns

Answer 5

cracks of the cartridge casing due to air compression and heat generation

resulting in a non uniform detonation as not all the energy and gas is going where we want it to

Question 6

name another example of a percussive force detonation we looked at

Answer 6

‘stabbing’ based systems for example a needle stabbing the explosive to initiate it

Question 7

what issues can arise with using needles to initiate an explosive

Answer 7

dealing with a smaller area

needle can bend = a misfire (blunt the end of a needle to avoid this)

some material are sensitive to stabs and others are not

Question 8

in a stabbing detonator what two types of initiation tend to be combined

Answer 8

percussion and friction

Question 9

which detonation technique is used often in large explosive devices

Answer 9

stabbing

Question 10

compared to stabbing and percussion what is a more controlled method of detonation

Answer 10

use of electrical detonators

Question 11

how do electrical detonators works

Answer 11

they heat up the explosive so it detonates

an electrical current is passed through a tungsten filament which gets hot due to resistance

Question 12

in low power electrical detonations what type of primary explosive is commonly used to start the detonation sequence and why

Answer 12

lead styphnate

it is very sensitive so quick initiation

but it doesn’t have the highest explosive power

Question 13

explain the concept of an explosive train

Answer 13

there are multiple stages for detonation to occur

pyrotechnic mix leads to a more sensitive primary explosive initiating then a secondary explosive because of this and so on

the main charge is often hard to initiate so we use other more sensitive ones to get it going

Question 14

what is the difference between a low power and a high power electrical detonator

Answer 14

a low power uses lower currents than a high power

in higher power the wire goes into the actual explosive in lower power it is in the primary explosive used as a propellant

Question 15

what type of detonator uses electrical current to indirectly detonate an explosive

Answer 15

a slapper detonator

Question 16

how does a slapper detonator work

Answer 16

wires are connected to a thin metal plate which is propelled the slapper plate into the explosive material at high velocity

the current makes a percussive force driving the plates forward

this initiates an explosive shock

Question 17

is a slapper or electrical detonator safer and why

Answer 17

slapper

avoids putting electrodes directly in an explosive material

less prone to accidental detonation as harder to accidentally put an electrical current through the explosive

more controlled

Question 18

when are slapper detonators used over electrical ones

Answer 18

for precise applications e.g nuclear weapons for controlled compressions

as better than relying on a needle for example

can also have multiple slappers in different directions

Question 19

what is a blasting cap

Answer 19

a small explosive device used to detonate a more powerful explosive

Question 20

name the 5 types of blasting cap we looked at an how they work (brief)

in the answer BP = blasting cap

Answer 20

1 = pyrotechnic fuse type BP = there is a pyrotechnic ignition mix in front of the primary explosive which comes into contact with the fuse crimped before it

2 = solid pack electric type BP = a wire is embedded in the insulation header and just into the primary explosive, wire heats up at detonates primary explosive then secondary

3 = match or fusehead electric type BP = there is an electric match that contains pyrotechnic material also a wire in the insulation header - wires heats up and pyrotechnic burns then sets of primary explosive then secondary

4 = exploding bridgewire type BP = bridgewire or foil after insulation header, wires heats rapidly and vapurises

5 = slapper type BP = slapper foil after insulation header, wire heats and vapourises causing slapper to push forward with high velocity

Question 21

what are blasting caps made of chemically

Answer 21

the pyrotechnic ignition mix = metals mixed with salts

followed by the primary explosive = can be a mixture often see addition of Al (oxidiser)

then the secondary or output explosive = main charge

Question 21

what does the crimping of a fuse allow

Answer 21

the fuse to not fall out and allows good contact between fuse and pyrotechnic mix

Question 22

when are match or fusehead electric type blasting caps good

Answer 22

when you want to control the explosion and have a slight time delay - allow people to move away

Question 23

what component of the blasting caps allow these explosive to be traceable

Answer 23

the metals within the pyrotechnic mix e.g Ti, B, Zr

Question 24

what part of a blasting cap explodes and causes the main damage

Answer 24

the secondary or output explosive

Question 25

indirect initiation of explosives can be done using acoustic, radar and magnetism method - briefly edxplain how for each

Answer 25

acoustic = use of sound waves and sensors

radar = when something comes within close proximity to something

magnetism = something that has a charge can intercept a magnetic field causing initiation

Question 26

what are two issues seen when using acoustic, radar or magnetism as a way of initiating detonation

Answer 26

there are other things that can cause sound waves or be in close proximity or have a charge that is not the target you intend to explode

to prevent the effect of magnet field changes ships can be wrapped in wires and go undetected

Question 27

why are azides suggested as being ‘cleaner’ primary explosives

Answer 27

they generate nitrogen gas

Question 28

what does a detonator usually require to set off a secondary explosive

Answer 28

a primary explosive

Question 29

what does a detonator for a tertiary explosive require - give an example

Answer 29

a secondary detonator

e.g ANFO = tertiary explosive

Question 30

what is the part of an explosive that contains the most metal and why is this useful in the detection of explosives

Answer 30

the detonator so more easily detected than the actual explosive as these metals don’t vapourise

Question 31

what have many bombing failed due to

Answer 31

poor fuses

Question 32

how is the use of explosives illicitly controlled

Answer 32

preventing people accessing ways to detonate it

Question 33

give two examples of how improvised explosives have been seen to be detonated

Answer 33

1 = use of a fuse cross section (cross shape)

weedkiller + salt + car battery acid to form potassium chlorate + sugar + H2SO4

here KClO3 = oxidiser, sugar and H2SO4 = fuel

a fuse cross section can be used to delay the reaction - add fuel to the cross shape and add H2SO4 from the side

2 = use of a condom
condom delays the action as it dissolves