Terry Pratchett already sorted it out; it's a little bit of a creative interpretation I guess but my reading of "Men at Arms" was that it is hypergolic fuel and oxidizer in two separate belly chambers and then they spray the two together and it ignites immediately in air. Like fireflies or bombardier beetles they just make the right stuff that they need.

I always thought dragons would have two seperate pressurized liquids (maybe a fuel + oxidizer combo) that form a combustion reaction upon mixing similar to the bombardier beetle

Magic

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