Lecture 5 Flashcards
Primary Explosives
- Easily initiated
- Detonation velocities of 3500 to 5500 m/s (Lower detonation velocities)
- Different degrees of sensitivity
- Copper Azide, Nitrogen Trihalides, Lead Azide, Silver Azide, Mercury Fulminate, Lead Styphnate
Secondary Explosives
- Difficult to initiate
- Detonation velocities of 5500 to 9000 m/s
- TNT, RDX/TNT (60/40), Amatol, RDX/Grease
Tertiary Explosives
- Very difficult to initiate
- Ammonium Nitrate
- They require a secondary explosive to initiate
What is the problem with azide?
Needs to be stabilised by some form of cation as it has an overall negative charge
What is lead syphnate commonly used?
It is commonly used primary explosive as it is highly sensitive
How are explosives detonated?
Through a direct or indirect response to stimuli
What are the categories of direct stimuli?
- Electrical
- Mechanical (percussion)
- Deflagration/detonation of another explosive
What are the categories of indirect stimuli?
- Magnetism - response to a change in the magnetic field
- Acoustic - sound wave
- Radar - proximity
What does percussion lead to?
Percussion leads to direct explosion of low explosive propellants
How does percussion lead to direct explosion?
- The trigger moves a hammer which hits the cartridge case and compresses the voids of air
- This compresses the anvil which sets off the primary explosive in the powder chamber due to the gases getting hot
- The gases expand and escape through the flash hole
What is percussion caused by?
Percussion is caused by localised heating upon decreased volume
What happens if you compress air too much in an explosive?
- Can compress air to such an extent to generate sufficient heat to cause cracks in explosive material
- This can be problematic as the detonation can be non-uniform
How do stabbing-based detonation systems work?
- It is friction based which causes hotspots in the explosive which then leads to initiation
What is a stabbing system?
Slide 6
What is the problem with stabbing detonators?
In certain types of detonators, particularly when you need to rapidly set off the explosives (e.g semi-automatic weapons), the need can get bent which can lead to a misfire.
Where would you expect to see a stabbing system implemented?
- Small handguns and machine guns
How can you fix the issue that stabbing systems encounter?
If you blunt the end of the needle it can reduce this and produce higher reliability
How does a striker spring work?
It is used in larger explosives and strikes down on the primary explosive which causes the initiation
How does an electrical detonator work?
- An electrical current is passed through a filament which gets hot and causes resistive heating.
- This heats up the explosive which then detonates
What do lower powder devices often use to start detonation sequence?
Low-power devices often use lead styphnate to start the detonation sequence as it is highly sensitive
How does a low-power detonator work?
- There is a fine bit of metal that sits within or on the end of the primary explosive so when we pass the current through this it heats up the lead styphnate which sets off the lead azide
- This is known as an explosive train
Why is lead azide needed in a low-power detonator?
Lead Azide has a more explosive power which is needed to set off the secondary explosive
What is an explosive train?
A sequential increase in explosive power and decrease in sensitivity
What is the difference between high-power detonators and low-power detonators?
Much higher currents
Is a slapper detonator direct or indirect?
A ‘slapper detonator’ uses a high electrical current to indirectly detonate an explosive
What are the advantage of the slapper detonator?
- Safer method as it avoids putting electrodes directly into an explosive material
- It is more controlled
How does a slapper detonator work?
- Electrical wires connected to a metal plate
- A current is applied (1000A) which vaporises a thin piece of metal (the slapper plate)
- The current creates a percussive force which propels the slapper plate into the explosive material at a high velocity and sets it off.
When is a slapper detonator used?
It is used for precision applications e.g very controlled explosive compression in nuclear weapons
What are the advantages of using electrical detonators compared to slapper detonators?
Electrical detonators are harder to accidentally detonate and easier to set up
What is the advantage of using a slapper detonator over an electrical detonator?
You can have multiple slappers in different directions to detonate the explosive in different directions
Blasting Caps
A small explosive device used to detonate a more powerful explosive
What are the different types of blasting caps?
- Pyrotechnic Fuse Type
- Solid Pack Electric Type
- Match or Fusehead Electric Type
- Exploding Bridgewire Type
- Slapper Type
What is the traditional type of blasting cap?
Pyrotechnic Fuse
What is the purpose of the crimping point in a pyrotechnic fuse?
It stops the fuse from falling out
How does a pyrotechnic fuse work?
- You light a fuse and it burns away and eventually reaches the explosive
- An explosive train happens
How does a solid-pack electric type blasting cap work?
- The wires embedded with insulating material to hold it in place
- A current is run through the wire which heats it up and detonates the primary explosive
How does a Match/Fusehead Electric Type blasing cap work?
- A wire is used to heat up an electric match and we get a pyrotechnic burn
- This provides a time delay
What is an electric match?
This is a pyrotechnic material that burns efficiently
What is an advantage of Fusehead/Match Electric Type Blasing cap?
A known controlled timed delay
How does an exploding bridge wire type blasting cap work?
- The wire is embedded within and gets rapidly heated up
- This vaporises the material and causes initiation
How does the slapper type blasting cap?
- The wire gets heated by an electrical current
- The heated wire vaporises a thin piece of metal
- The thin piece of metal acts as a percussive force and gets propelled at a high velocity into the explosive material
- This force initiates the explosive material
What is the chemical composition of blasting caps?
- Pyrotechnic ignition mix
- Primary explosive
- Output explosive
What is the pyrotechnic ignition mix?
- Redox mixture
- Electric match
- A metal mixed with a salt
What are the features of the pyrotechnic ignition mix?
- Very efficient redox mixture that burns efficiently
- A metal mixed with a salt
- Metals are used bc they are traceable and we don’t want to use things that have a common use
What is the primary explosives in blasting caps made out of?
- Lead Styphnate
- Lead Azide
- Aluminium
- This is known as ASA
What is the purpose of the mixture in the primary explosives within blasting caps?
The mixture gives a nice blend of explosive power and initiation sensitivity
What is the output explosive in blasting caps?
This is a small amount of high explosive for example TNT or PETN
What are the features of the output explosive within a blasting cap?
- Main explosive
- Most explosive power
- May be used to detonate a much larger charge
What are the different stages of blasting caps?
- Primary explosive
- Pyrotechnic
- Propellant
- Primary Explosive
- Booster
- Main Charge (secondary)
How does an explosive train work?
- A primary explosive gets set off and the shell is partially propelled
- This burns some pyrotechnic with some propellant that’s packed around it
- This moves the shell where we want it to go
- When we get impact we get compression of nose cone a the front
- Progressive force initiates primary explosive
- What happens after??
How can explosives be detonated through acoustic stimuli?
A sound wave propagating through water gets detected and sends an electric signal which initiates an electric signal
How can explosives be detonated through Radar stimuli?
- When an aircraft gets to a certain distance it has a radar that initiates the explosion of the device within
- Usually one aircraft with an electric signal closes the switch and sets off the explosive
How can explosives be detonated through Magnetism stimuli?
- As ships travel through the water they build up an electrical charge so we get a change in the magnetic field
- The change is detected and the explosion is initiated
- This is harder to achieve compared to other techniques
- Degaussing the vessel to remove the charge so when it travels so it doesn’t accidentally set it off
What gas do azides generate?
Azides generate nitrogen gas however they don’t have explosive power.
What is important for the effectiveness of explosives?
Detonators
How are explosive materials controlled?
The control of explosive materials is the control of detonators
What do detonators usually require and what is the exception?
- Detonators usually require primary explosives
- The exception is for tertiary explosives (ANFO) which needs a secondary detonator
What part of the explosive contains the most metal?
- Often it is the detonator that is the part of an explosive that contains the most metal
- They generate the most metal signature
Why have many bombs failed?
Many bombings have failed because of poor fuses
Why is primary explosives helpful in a detonator?
- Primary explosives are quite unstable and usually generate unusual signatures in terms of the metallic components and other components
- This can help to detect the, it makes them traceable combined with the fact they are unusual materials
What do we look for at the scene of an explosive?
We look for tracers of detonators at scenes rather than the explosives themselves
What is needed to set of main charges?
We need a primary explosive to set of main charges
What is the problem with improvised explosive detonations?
The detonation is not very reliable or controlled compared to other detonators
How is the fuzee match detonated?
It is detonated by acid and a condom is in place to delay the action as it requires the acid to dissolve it
What is the primary disadvantage of percussion detonation?
It can compress the air to such an extent that generates sufficient heat and causes cracks in the explosives which can make the detonation non-uniform which is problematic.
