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There are two common quirks I keep hearing about dragon anatomy and biochemistry.

The first is that dragons use lifting gases to keep aloft and breathe fire, both of which are just plain dumb. First, the breath weapon would be extremely dangerous (to the dragon) and have a very short range because hydrogen is less dense than nitrogen.

Second, lifting force per unit volume in air is ~10N per cubic METER, IF you have a PERFECT VACUUM. Sure, using hydrogen instead still gets you very close to that value, but it's still insanely small.

The second is a bit more interesting. Bombardier beetles and their unique natural defense mechanism is often cited as a way for dragons to breathe fire (incorrect).

Yours truly also knows that this mechanism is seemingly an irreducible complexity, a fact I love to throw at the head of "No beast in nature has four legs and wings" gremlins who can't even finish a book, let alone know what the Dunning–Kruger effect is or how evolution actually works.

Anyway, you probably know how the mechanism works, you mix hydroquinone, hydrogen peroxide, and some enzymes together, then the generated gas propels a ~100 °C hot liquid forward and also closes off the valves to the reservoirs of the beetle to prevent an unwanted explosion.

Here's Sam O'Nella and here's where his watchers then went.

Now, the problem is that dragons are bigger, at around horse-sized, and use their mouths. So, I doubt you could just copy-paste the original design and expect it to work. Especially since the distances involved are different as well.

But how would the defense mechanism work to accommodate these changes?

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    $\begingroup$ See, the dragon carries with it a pouch of soda bicarb. It fills his mouth with some and then regurgitates "a mixture of sulfuric acid and bleached water", essentially acting as a fire extinguisher. What it loses from the temperature of the jet, it gains in safety and the ejection speed. (and so they lived merrily ever after) $\endgroup$ – Adrian Colomitchi Mar 30 at 14:43
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Same Idea, Slightly Different Chemical Combination

I actually wrote a bit about this topic in another question that I'll link down below.

In my answer I mentioned that hydrogen peroxide is a very strong oxidizer and that it would cause exothermic reactions with a variety of chemicals. Now supposing that you could justify swapping out hydrochloric stomach acid for sulfuric acid, (something I'm not aware of existing, but not entirely implausible), you could get something called Piranha Solution. It is a powerful acid that can break down carbon bonds, leaving a burnt looking blackening and liquefaction of tissue or organic materials. It's nasty stuff, and probably an even more fearsome weapon than boiling liquid.

Since one part is just the stomach acid, the dragon is justified in having it, and a regurgitation/spitting mechanism is not unusual in the animal world. You would probably want to have an acid resistant esophagus for the sulfuric acid, but still mix in the Hydrogen Peroxide as it's leaving the mouth. Possibly squirting saliva glands or like a spitting cobra? You wouldn't want those chemicals to mix inside the dragon, because I'm not sure what organic material could resist it for very long.

As for the mechanism to throw the liquid forward, I see no problem with just spitting. There are animals that can pressurize liquid with muscle contractions and project it a fair ways.

Anyways, that's my two cents. Also, I agree with your assessment of the features of biologically plausible dragons.

Here's the link: if there was a dragon that spits pyrophoric liquids like a spitting cobra

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