Something I didn’t really see mentioned was the reasoning for *why* it’s still used officially.
Weight-for-weight, Willie Pete is the most effective smoke screening agent yet discovered. The vapor that comes off burning WP is phosphorus pentoxide, but on contact with the water in the air that immediately turns to phosphoric acid droplets. This means that whatever mass of vapor you produce is going to be tripled in mass by the water in the air, giving you insane amounts of smoke.
P4O10 + 6 H2O → 4 H3PO4.
Also, this means WP smoke isn’t actually smoke at all, it’s an aerosol. This means it actually scatters light instead of just providing a screen, which makes it much more effective at blocking view. Basically, it works like privacy glass instead of a cloud of smoke. Importantly, it absorbs infrared, meaning it can be used to hide from thermal imaging cameras.
So officially, WP is a screening, signaling, and marking tool. It wouldn’t surprise me one bit though if those marking rockets get used for a different purpose.
In addition to the other good answers here, WP can survive (not burn) if it’s covered with dirt. As a Canadian infantryman, mid 1980s, I personally kicked over a clump of dirt and *poof*, a small bit of WP started on fire. Could be dangerous if combustibles nearby (grass, leaves, etc.). We knew to expect it. Didn’t happen often.
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