Which hypergolic compounds are best suited for dragon fire?

Question

I'm looking for input from chemists, biochemists, and those familiar with rocket fuel propellants. I'm a fantasy world builder, but that doesn't mean everything in my fantasy world is excluded from scientific rigor (magic systems excluded, of course). I've been pondering the physiology of the dragons in one of my worlds, and would like to use hypergolic compounds to produce dragon fire. Hypergolic compounds are commonly thought of as propellants or rocket fuel. They are compounds that, when mixed with an oxidizing compound, ignite instantly. My fault is that these compounds are produced in a glandular system, conveyed by ducks to vents either under or on the dragons tongue, or located in the jaw structure at the forward end of the snout.

Hypergolic compounds are highly caustic and/or toxic to humans, but I see no reason why dragons could not have developed with the ability to create store and process such materials. I'd like to know if any of the known or theorized hypergolic combinations has greater viscosity than others. My research has indicated that some combinations produce more caloric energy than others, including the ability of some to melt stone and gravel. That seems to be the level of heat that would be about right for dragon fire, in my opinion.

From a biochemical standpoint, what types of organic compounds, rocks, or minerals would my dragons need to consume in order for their bodies to synthesize a hypergol and oxidizer?

I'd appreciate any and all feedback you can provide.

Answer 1

Hydrazine is Reasonable

Hydrazine is a pretty simple molecule (H2N−NH2), just Nitrogen and Hydrogen which a dragon could get from drinking water and a vegan diet, so not much difficulty there. There may (read: will) need to be some hand-waving in how the Dragon's internal organs create the chemical.

The Dragon will need a catalyst for the decomposition reaction. Perhaps the Dragons have a bit of pica and regularly chew/eat/consume rocks or dirt to get enough of this element. Rockets typically use iridium, but other catalysts have been shown to work, including ruthenium, rhodium, and rhenium, and (perhaps most promising for an evolved animal) iron-nickel-cobalt.

The dragon would need some sort of 'combustion chamber'. A hydrazine decomp reaction is very exothermic, and would likely burn soft tissue of the actual mouth very quickly. Perhaps the dragon actually shoots the flame from boney tubes near the front of the mouth (high in iron-nickel-cobalt). These boney tubes could be fed from hydrazine sacs that are squeezed by strong muscles to produce enough inlet pressure to get the hydrazine inside the chamber during a burn.

Since this is not a high-performance rocket, just a defensive tool, the catalyst and precise shape of the 'combustion chamber' don't need to be optimized.

Dealing with Biohazards

Hydrazine is a skin and eye irritant, damages the stomach and lungs when swallowed or inhaled, and can also cause tissue damage to other organs. It is "reasonably anticipated to be a human carcinogen", but there have only been animal studies, it has not been shown directly to be a human carcinogen.

I would expect that the Dragons would have to have special linings to their hydrazine sacs to prevent exposure damage. This could take two forms. One would be something like a mucosal stomach lining, where the sac walls secrete a protective liquid, however, I'm not sure what this would be that could protect without over-diluting the hydrazine. An alternate could be that the inner wall of the sac could be continuously shedding and regenerating cells that are damaged by the hydrazine. Perhaps the hydrazine is manufactured in these special cells, so the shedding is actually what refills the hydrazine, rather than diluting it? Note that some dilution wouldn't be an issue, for example, C-Stoff was a mixture of methanol, hydrazine and water used by the Germans in WWII.

Good luck, hope this helps!

Answer 2

This is more a frame challenge than an answer to your question: You don't need an hypergolic compound. Hypergolic compounds burn without need of oxidiser (oxygen) and that is useful because it simplifies rockets, compared with non hypergolic rocket engines which need to supply both fuel and oxidiser. However, dragons operate inside the atmosphere where is plenty of oxygen, so a dragon just need to throw fuel and ignite it to burn with oxygen in air.

In fact, a dragon is just a large animal with a built-in flamethrower, and flamethrowers just throw gas (nowadays propane) or liquid fuel (https://en.wikipedia.org/wiki/Flamethrower).

Then, the question is what liquid or gas fuel can be organically produced. I'm not a chemist, but actual living beings are known to produce methane and fatty acids, and producing hydrocarbons (including propane, butane or gasoline) doesn't seem far fetched.

Answer 3

I'll offer a feasible alternative to chemical fire. Plasma flame discharge. High frequency radio waves ionize air in a way to look like fire. Basically an electric eel on steroids.

A second alternative spiderman web shooter. Shoot an electrically conductive web that has streamers that sorta look like fire but not really. The conduction on whatever flesh the web touches will certainly burst into flame.

Best of both worlds is a web with a flammable mucus that rapidly vaporizes from the electric current going down the web, then ignites similar to a fuel air explosion.

If the above do not interest you, then hydrazine will work.

Answer 4

Hydrogen Peroxide

Used in rockets as a monopropellant, HTP (high-test peroxide, >80% concentration) will happily, reliably and violently decompose when it's blown across a suitable catalyst, fine silver particles for example.

It is a strong oxidiser but it's a molecule that is already produced in most living organisms during the elimination of superoxides. In our world peroxide is quickly broken down even further but the idea that some organism evolved a way to concentrate and contain it (like our stomachs contain hydrochloric acid), and then expel it as a weapon is not wildly implausible.

In rocketry long-term storage is a problem because it will slowly decompose over time but in a biological "rocket", where the gases created can be burped up and new H₂O₂ can be synthesised continuously, that shouldn't be too big a problem.

The temperature of combustion depends on the concentration but roughly 1000-1200K should be achievable, enough to melt some (but not all) silicate rocks, bronze and gold, but not iron or steel.