Critical Diameter
Definition
The minimum charge diameter below which a detonation wave cannot sustain itself, because energy losses to the sides of the charge exceed the chemical energy input. ANFO has a larger critical diameter than RDX, explaining why ANFO requires a booster rather than a bare blasting cap.
- Definition
- Minimum charge diameter to sustain detonation
- Cause of failure below it
- Side energy losses exceed chemical energy input
- ANFO
- Larger critical diameter than RDX
- Consequence
- ANFO needs a booster, not a bare blasting cap
Common questions
Why does a larger critical diameter mean ANFO cannot be initiated with a plain blasting cap?+
A bare blasting cap delivers a small-diameter, low-energy shock, and if the charge diameter is below ANFO's critical diameter the reaction cannot outrun its own side losses, so a booster charge is needed to build the detonation up to a self-sustaining diameter.
How does critical diameter help investigators identify what explosive was used?+
The presence or absence of a booster, along with damage patterns consistent with a low-order or high-order detonation, can indicate whether the charge diameter was near or below the explosive's critical diameter, which narrows down the likely explosive type.
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