Explosions 5 Flashcards
What are 6 types of stimuli that can detonate explosives? Categorise these into 2 categories.
Direct: electrical, mechanical (percussion), deflagration/detonation
Indirect: magnetism, acoustic, radar
How does a percussive stimuli work?
Where are these common?
Explain the process.
What happens if air is compressed to a high extent?
Why can this be a problem?
Percussive force leads to direct explosion of low explosive propellants
Common in blasting caps
- pulling the trigger moves a hammer which hits cartridge case. the hammer compresses the voids of air which causes gases to get hot. this causes localised heating upon decreased volume.
- this causes percussion which sets off low explosive e.g. gun cotton (nitrocellulose)
- this compresses the anvil that sets off the powder explosive in the power chamber
- the flash hole allows gas to escape
Can compress gas to extent where generate sufficient heat to cause cracks in explosive material
We want to efficiently channel explosive power but won’t be detonated in controlled and focussed manner that we want - detonation can be non-uniform
How does a stabbing detonator work?
What is a problem with this type of detonator?
How is this problem overcome?
What is this type of detonator used in?
Needle stabs explosive with small surface area so get localised friction-based heating at hotspots
Needle can get bent causing a misfire
Slightly blunt the end of the needle to get higher reliability
Used in small arms handguns and machine guns and larger explosive devices (mortar)
How do we choose type of detonator for explosive?
Depending on explosive, some will be more sensitive to percussive and some more sensitive to stabbing.
Can also combine the two in a stabbing detonator
What principle do electrical detonators use?
Current passes through filament that gets hot (through resistive heating), which heats up the explosive which then detonates
Describe how low power detonators work.
What is used as initial primary explosive and why?
What is this process referred to and explain this term.
Device with contact pins - to wrap wire around to enable current to be passed through.
Insulated plug within wiring which goes to bridge wire
Bridge wire - fine bit of metal that sits within or on end of lead styphnate (PE).
Pass electrical current through bridge wire, quickly heats lead styphnate, this detonates lead azide (another PE with slightly more explosive power), this detonates PETN/RDX (main explosive)
Lead styphnate - it is highly sensitive to heating and will initiate very quickly
Explosive train - sequential increase in explosive power and decrease in sensitivity
Describe how high power detonators work.
substantial wire embedded within high explosive material itself
uses current around 2000 amps
What is the name for an indirect electrical detonator?
What principle is this based on?
Describe the process.
Is this method preferred or not compared to other electrical detonators? Why?
What are applications of this indirect electrical detector?
Slapper detonator
Based on principle of using electrical current to make percussive force to set off primary explosive (indirectly set off PE)
Electrical wires are connected to metal plate.
Current (1000A) is applied which vaporises thin metal piece to propel slapper plate into explosive at high velocity.
This initiates an explosive shock.
Yes this is preferred for two reasons:
1. safer as avoids putting electrodes directly into explosive material.
2. gives us more control. electrical detonators are less likely to be set off accidentally (as need to put current through it) compared to percussive detonators (just need to drop it)
Precision applications e.g. three dimensional compression of nuclear weapons required for their initiation
Define a blasting cap.
Give 5 types and define each.
What are the applications of blasting caps?
A small explosive device used to detonate a more powerful explosive
- pyrotechnic fuse type blasting cap - traditional (cartoon like) where light fuse, burns away, reaches explosive and detonates. contains crimping point to prevent fuse falling out of detonator as need good contact between fuse and pyrotechnic ignition mix
- solid pack electric type blasting cap - wires embedded in insulating material to hold them in place. a current applied through it, wire heats up, primary explosive detonates
- match/fusehead electric type blasting cap - wire used to heat up electric match containing pyrotechnic material. can control this and get a controlled time delay (useful for getting to safe distance, controlled demolition)
- exploding bridgewire type blasting cap - wire heats up rapidly and vaporises
- slapper type blasting cap - wire heats up and vaporises and plug sent at high velocity
usually used in mining, demolition, quarrying and military purposes. not illicit/clandestine purposes
What is the chemical composition of blasting caps? Give any reasoning for their compositions.
pyrotechnic ignition mix - electric match redox mixture (uncommon metal (so not commonly found in background environment) e.g. Ti/Zr/B and salt KClO4/KNO3)
primary explosive - ASA (lead styphnate + lead azide (Pb(N3)2) + Al) - nice blend of initiation sensitivity and enough explosive power and Al has efficient oxidation so gives energy.
output explosive - small amount of high explosive e.g. TNT, PETN
Draw and label the 6 stages on an explosive train.
Explain the explosive train process.
- primary explosive
- pyrotechnic
- propellant
- primary explosive
- booster
- main charge (secondary)
On left got primary explosive (1), which is set off to get artillery shell to be propelled where we want it to go. Burns some kind of pyrotechnic mix with some propellant packed around it. This is what is going to move artillery shell where we want it to go. Need bit of primary explosive to get this going. When get impact of artillery shell on impact, get compression of nose cone. As nose cone comes in, get percussive force to initiate primary explosive (4) which then in turn sets of booster (5 – slightly more potent primary explosive with higher explosive power). This is embedded within main charge (secondary explosive). SE is in much larger relative amounts.
What 3 ways can fuse types be set off electronically?
Give an example for each and explain how they work.
How is third one overcome?
What one is most effective?
Acoustic (acoustic sensor in underwater mines)
- acoustic (sound) wave propagated through water is detected by mine when above certain threshold. Sends electrical signal which then initiates our primary explosive
Proximity/radar (anti-aircraft missile)
- if you have an anti-aircraft missile that has radar, when this comes within certain distance of something, it will initiate explosion of device within it
Magnetism (magnetic sensor in underwater mines)
- as cruisers etc. move through water, they build up electrical charge. as vessel moves through water get a change in induced magnetic field within proximity of vessel. underwater mines would detect change in MF and blow up
Overcome by degaussing (wires wrapped around) vessel to pull off all extra static charge so doesn’t set off mines.
Magnetism is more effective as a lot of other things can cause acoustic and radar but not much can induce change in MF
What is incredibly important in terms of effectiveness of explosive devices?
Detonators e.g. quality of fuses
How are explosives and detonators controlled?
Give an example of this.
Control of explosive materials is the control of detonators - detonators require primary explosive to set off main charge.
Even if someone is in possession off explosive, needs a detonator
Even if someone has secondary explosive e.g. ANFO, nitroglycerine, they often cannot set them off easily as they need detonator (containing primary explosive) which is much less available and far less easy to synthesise
What part of IED contains most metal?
Detonator does.
Detonator contains primary explosive which doesn’t have same explosive power as main explosive (primary explosive doesn’t vaporises, it cracks open and fragments) so can look for tracers of detonators at explosive scenes rather than main explosive itself
