Frequently on an explosives range you will hear people comment that a “deflagration” occurred which stopped the expected effect, or a “partial high order” meant that some of the explosives went flying instead of detonating. Do people know what they’re talking about? Usually they could stand to be a bit more precise and this post is aimed to help with exactly that.
Detonation vs Deflagration
Detonations and deflagrations end up with the some products but how they get there is markedly different. Basically, a deflagration is a (usually very quick) burning; you get deflagration in an internal combustion engine car to move the piston or the barrel of a gun to fire a bullet. Often deflagration is associated with propellants and pyrotechnics (previously called “low explosives”) because they are designed to undergo that quick combustion but you can cause deflagration in a high explosive too. Deflagration is fundamentally a thermal reaction in that the decomposition of the material happens at the rate at which it heats up and decomposes which happens at the surface and below the speed of sound.
Detonation is quicker. Specifically, the rate of that decomposition is faster than the speed of sound and is associated with a shock wave.
When do people get confused?
With these definitions in mind, where do the lines get blurred? Most of the confusion happens when detonations or deflagrations do not go to plan and a deflagration-to-detonation or detonation-to-deflagration transition occurs. On either end of the spectrum, a full detonation is one where all the explosive is consumed by the detonation and a full deflagration is one where all the explosive is consumed by deflagration (burning). Whilst I have never heard the term used, if you fired an artillery shell and not all the propellant was consumed, you would have had a “partial deflagration”.
What I hear more frequently is people using the term “partial detonation” and deflagration interchangeably, and I understand why the two get confused: often when a shockwave does not have enough energy to continue the chain reaction and consume all the explosive, the remaining explosive will be thrown by the blast wave, some of which will in turn burn due to the heat of the product gases. So when a partial detonation occurs, some deflagration usually occurs too. But deflagration is just burning, it is not the process of flinging around explosive materials.
To emphasise: the event that has occurred in a partial detonation is a detonation-to-deflagration transition because some explosives were burnt up after the shock wave failed to continue the chain reaction, but the thrown around explosive material that remains at the end has neither detonated (been decomposed by the shock wave) nor deflagrated (been decomposed by burning).
Burning to Bang: the Deflagration to Detonation Transition
If you have followed to this point you know that deflagration is just burning, so this is the same as “burning to detonation.” To understand when this occurs we need to recall that deflagration happens subsonically at the speed of burning at the surface and detonation occurs supersonically at the speed of the shock wave. So how can we go from the rate of burning to a shock wave?
The answer is that the rate of burning changes depending on the environment, especially the pressure. If you were to take gunpowder and light it in the ground, it would burn far slower than confined in the barrel of a gun. At some point of confinement, the rate of burning will hit a point at which it will exceed the speed of sound and a shock wave will be generated. That is how explosives burn to detonation.
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