Explosives - Explosive Initiation

Question 1

What are the 4 mains stages of a chemical explosion?

Answer 1

1. Ignition
2. Growth of deflagration
3. Transition of deflagration to detonation
4. Propagation of detonation

Occasionally the process can move straight from 1 to 4 if initial stimuluse creates sufficient energy

Question 2

What is the stage of ignition?

Answer 2

• Occurs when a combustible material is heated to or above its ignition temperature. An external stimulus required.
• Pre-ignition reactions involve transitions in crystalline structure, liquid phase to gaseous phase transitions and thermal decomposition of one or more ingredients.
• This leads to self-sustaining combustion of material
• As temperature rises, rate of heat production increases exponentially whereas heat loss is linear.
• Occurs when heat production overcomes heat loss.

Question 3

What are the different types of ignition mechanisms?

Answer 3

• Heat - flame, hot gun barrel, laser, storage
• Mechanical: hit - percussion, stab, friction
• Electrical: electric wire, conductivity
• Spark: electrical discharge - lightning
• Chemical: reaction between compounds
• Shockwave (detonators): requires preceding device.

Question 4

What are the different classes of explosives?

Answer 4

• Primary Explosives: Easily initiated, detonation velocities 3500-5500 m/s e.g. Copper azide, nitrogen trihalides, lead azide, silver azide, lead styphnate etc.
• Secondary Explosives: difficult to initiate, detonation velocities 5500-9000 m/s, e.g. TNT, RDX/TNT (60/40), Amatol, RDX/Grease
• Tertiary Explosives: Very difficult to initiate, e.g. ammonium nitrate.

Question 5

What is the ‘stabbing’ initiation mechanism?

Answer 5

• No adiabatic heating of air gaps, works via friction.
• Issues - needle can get bent, misfiring can occur.
• Some materials are more sensitive to stab, others are more sensitive to percussion.
• Examples include small handguns and machine guns.
• To increase reliability, the stabbing needle should be blunted on the end.
• Works as a combination of percussion and friction.

Question 6

How are explosives initiated by a laser?

Answer 6

• Some munitions will only initiate from a laser.
• The laser heats up the explosive, causing it to ignite.
• Laser energy is not found in nature, so the chance of accidental initiation is minimised.
• Useful for very stable explosives.

Question 7

What is a slapper detonator?

Answer 7

• A slapper detonator is a recently developed detonator.
• It improves the explosive-bridgewire detonator
• Instead of directly coupling the shockwave from the exploding wire, expanding plasma from an explosion of a metal foil drives another thin plastic or metal foil called a “flyer” or a “slapper” across a gap, and its high-velocity impact on the explosive delivers the energy and shock needed to initiate detonation.

Question 8

How do blasting caps initate explosives?

Answer 8

• Pyrotechnic fuse blasting cap: Pyrotechnic burns and initiates explosive
• Solid pack electric type BC: wire heats and detonates primary explosive
• Match Electric Type BC: Wire heats up, causing pyrotechnic to burn which initiates primary explosive.
• Exploding Bridgewire BC: Wire heated up fast and vaporises
• Slapper BC: Wire heats and vaporises, sends plug at high velocity to impact detonate.

Question 9

What is the “explosives train” concept?

Answer 9

• Small quantity of very sensitive explosive receives an initial stimulus and detonates, serving as a stimulus to detonate a larger mass of less sensitive (secondary material).
• Initial detonation may be a blasting cap or detonator
• May be an intermediate booster between first detonation and the main charge.
• Initial stimulus may be electrical, mechanical or thermal.