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Dragons commonly breathe fire. They can do so over considerable distances, often many times their body length. But this needs to be powered somehow, by a substance that the dragon can generate on its own. Flamethrowers in real life can be powered by either a gas or a liquid.

However, gas based flamethrowers are pretty lame compared to their superior liquid brothers (insert MGS joke here) because they generate what is nothing more than a long flame. This is what flamethrowers look like in the movies, mainly out of safety concerns. Liquid based flamethrowers are a lot more dangerous, firing streams of burning gasoline at targets.

Given that I want my dragon's flamethrower be spelled with an F instead of without, I want to go with a liquid-based flamethrower. However, this means that a dragon will have to generate the stuff somehow. A methane powered one would be relatively simple given that a dragon's digestive system would produce this on its own, but to me creating the equivalent of dragon napalm seems a lot more complex. And needs to be able to combust somehow, preferably not inside of the dragon. They wouldn't want to end like draco vulgaris, after all.

So how would the biology of a creature like a dragon create a flammable liquid? Would it require a special diet, or dedicated organs? Or perhaps something else entirely? I am looking to make it a continuous stream akin to a real-life flamethrower rather than a short spray created by the equivalent of a bombardier beetle.

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The Discovery Channel asked this same question about 12 years ago, in Dragons: A Fantasy Made Real. They came up withe some interesting (and surprisingly plausible) hypothesis. Look it up on your favorite streaming service if you haven't already seen it. (Disclaimer: This is not a promo and I am not involved with any company related to this production. Just trying to help the OP.) – cobaltduck Mar 14 at 14:19
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@cobaltduck If you think it answers the question, make it an answer with link instead of a comment. – Thomas Jacobs Mar 14 at 14:21
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In order to make an answer, I would have to re-watch the show myself and draw out the details from it. You can do that just as easily yourself. I don't have the information- but I was aware of a resource that does, and so provided that. – cobaltduck Mar 14 at 14:29
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Possible duplicate: How could dragons be explained without magic?. – Frostfyre Mar 14 at 16:08
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"Lamethrower" adequately describes my jokes and their delivery method. – corsiKa Mar 14 at 20:52
up vote 16 down vote accepted

A liquid based approach could be done.
There are a number of biologically produced substances that are flammable.
The dragon would just need to produce a quantity of it and store it in a gland of some kind, similar to how a snake produces venom and stores in a venom gland until it's ready to use. How big the gland will be, and where in the dragon it's located will depend a bit on how you want them laid out and how much fire breathing you want them to be able to do before recharging.

So what about lighting it? Well we can look at nature for this too.
There is a bug called a bombardier beetle, which produces two different chemicals in glands in it's thorax.
When it is threatened it sprays the chemicals, which mix and cause an explosion to scare off attackers.

So the dragon could create a catalyst that is sprayed out in smaller quantities that reacts with the fuel to cause combustion shortly after leaving the dragons mouth, reducing the risk of burns.

Another idea would be something like potassium, which in it's pure form reacts violently with water, generating a large amount of heat and hydrogen gas, and can cause an explosion.
One neat thing is that potassium was first isolated in potash (the ashes of plants, from which its name is derived), and so a dragon burning plants and eating the ashes would be able to collect potassium, and then could process it into an oil fuel mixture which would be exposed to to water/saliva when sprayed out generating enough heat and hydrogen to ignite the oil part of the mixture.
The oil would also keep the chemical reaction from happening inside of the dragon.

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I'll also include a previous answer to how something like that could develop: worldbuilding.stackexchange.com/a/11689/6751 – AndyD273 Mar 14 at 15:41
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Biological systems tend to be quite wet on the inside; that doesn't sound like a good place to purify or store pure materials that react violently with water! – Mason Wheeler Mar 14 at 18:25
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@MasonWheeler It's pretty common for these materials to be stored in oil. Luckily for the question we need something flammable, and so the oil can be duel purpose, both to keep the material suspended and away from water, and to burn when it's sprayed. And Oil is something that biological systems make pretty commonly. Palmitic acid from Evilscary's answer is basically fatty oil, and one of the main components of palm oil, though a lot of life forms make it. – AndyD273 Mar 14 at 19:05
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This is almost exactly how the dragons from Reign of Fire (2002) work. Two glands shoot different chemicals that combust when mixed. – Michael Frank Mar 14 at 22:35
    
@MichaelFrank Huh, it has been a long long, time since I saw that movie. I'm actually curious now how much my ideas of dragons got formed by watching that at an impressionable age. I should rewatch it sometime. – AndyD273 Mar 15 at 14:37

I thought about this for a high fantasy RPG I worked on a while back.

Liquid dragon fire could be formed from palmitic acid and naphthenic acid (the base ingredients of Napalm).

Palmitic acid can be found in nature in our bodies. Excess carbohydrates in the body are converted to palmitic acid. As a consequence, palmitic acid is a major body component of animals, so your dragons just need to have a gland that stores and can excrete this when needed.

The second ingredient is naphthenic acid. This is a bit trickier to synthesis in a living creature. Feasibly a dragon could ingest peat, coal or similar substances and crack them in a special stomach. The problem is naphthenic acids can be toxic, but we’ll just assume the dragon is immune (and as a bonus this makes the dragon poisonous to would-be predators).

So now your dragon just needs some special glands in their mouth to spray the Palmitic and naphthenic acids at their target and some kind of ignitier to light the spray as it leaves the dragon's mouth. Borrowing from Flight of Dragons, perhaps the dragon has a small patch of cells in the roof of their mouth that can create a charge in the manner of an electric eel. As a bonus this mixture will probably be quite clinging due to the fatty base.

There you have it.

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Storing the toxic byproduct could just be another element of the dragon's devistation. – corsiKa Mar 14 at 20:53
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The dragon could have a specialised tooth made of ignitable material, and a different specialised tooth in the opposing jaw to scrape it. Kind of like a giant reptilian Zippo. They would grow constantly like rabbits so the dragon would have to burn things a lot. – Whelkaholism Mar 15 at 10:29
    
@Whelkaholism Yeah that's a possibility. Beavers have iron deposits in their teeth, so presumably the dragon could have something similar. Dental flint and steel! – evilscary Mar 15 at 11:23

For the dragons' fuel storage and pumping system, liquid is the way to go.

  • More storage: Flammable liquids have more energy per volume than gas, just because density.
  • More range: A liquid stream can be pushed farther than a gas one, as you point in the question text.
  • More Power: the flow of matter is greater with a liquid. Again density.
  • Less liability: In order to store lots of gas, the dragon would need a pressurized organ, that if punctured could cause internal damage.

Modified glans would produce the flammable liquids. Possibly as a binary fuel, to be combined directly in the muzzle. It can have two glans in opposite sides of the body, each producing one part of the binary fuel.

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How To Train Your Dragon didn't go into specifics, but the "monstrous nightmare" breed used a liquid as its fuel, so this is all quite plausible (if, y'know, you suspend disbelief that fire-breathing dragons exist). As for "how":

The dragon has a rudimentary biological refinery; bacteria in its gut ferments any cellulose it eats into ethanol or methanol. The dragon's intestines absorb these alcohols as well as the fats from its diet, and transport both to a gallbladder like organ above the stomach. When the fat and alcohol react here, the product is very similar to diesel fuel. The dragon can essentially vomit this cocktail of fuel oil and half-reacted fat and alcohol, which self-ignites under such pressures, at its enemies.

You don't need a Ph.D in fantasy biology to come up with something plausible.

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How would liquid diesel flying through the air be pressurized in order to self-ignite? – Dan Mar 14 at 21:41
    
It would be at max pressure at the point where it exits the dragon. – KeithS Mar 15 at 14:58
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Max pressure would still be inside some part of the dragon. As soon as it is released into the atmosphere, the pressure is zero. And diesel only ignites under pressure when vaporized and mixed with the correct amount of air (like in the cylinder of an engine). The dragon would have to atomize, mix it with air (in an exact stoichiometric ratio) and compress it to over 700psi (in a pre-heated chamber) to ignite it. Then the pressure of the explosion could be vented outside, but all that would be pretty unspectacular to an observer other than the loud noise. – JPhi1618 Mar 15 at 15:13

Butane is an interesting possibility. While it is usually a gas at room temperature/pressure, it only requires moderate pressure (about 40psi) to keep it liquid at typical body temperatures. This would require a reasonably sturdy system for compressing it after production, but that wouldn't be outside of the plausible range that could be achieved with a muscle diaphragm (Sauroposeidon's heart must have been able to manage at least 20psi in order to get its blood to its head). It would begin the process of evaporation immediately on release, which would assist with the process of throwing it a long way.

Butane isn't produced by any natural biological system that I'm aware of, but it has been demonstrated in a relatively simple change to the fatty-acid production systems of E. coli.

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The human gut has a complex ecology of microorganisms, and in some cases people are being naturally intoxicated becsue the various gut bacteria are producing ethanol: http://www.vice.com/read/my-gut-turns-yeast-into-alcohol

Now a drunken dragon is not going to be successful in evolutionary terms (being likely to fly into mountains or rashly challenge knights to a fight), so evolutionary pressures will develop for dragons to sequester the alcohol in their bodies. This creates a bit of an issue, since a high concentration of alcohol will be toxic, and needs to be excreted from the dragon's body somehow.

Sadly, the most plausible mechanisms would involve a kidney like organ filtering alcohol from the bloodstream and a bladder like organ which exits via the dragons cloaca rather than the mouth. This means the dragon will have a means of attacking its enemies while in flight, but in a much nastier form than "just" fire.

For a proper alcohol based flamethrower to evolve, there will have to be some sort of enzyme ignition system in the cloaca (something similar to how Bombardier beetles eject steam), and a powerful set of sphincters to propel the noxious stream at targets. (If mixed with the dragons fecal matter, you will have a very nasty stream of burning you know what, which should make all enemies beware of dragons suddenly turning away from them and raising their tails....)

While sequestering alcohol and ejecting a stream of alcohol laced dragon poop at high velocity at potential foes does seem like a reasonable evolutionary development, it is difficult to see where the flame part comes from. It seems horrible enough without the flame.......

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So … dragons light their farts is what you're saying? – Jörg W Mittag Mar 15 at 12:50
    
Actually, from that last paragraph, it's their sharts. – Vogie Mar 15 at 15:49

All oil in common use is ultimately biologically based (even so-called "mineral oil"), so its certainly feasible.

If it was me, I'd assume the creature was creating something closely akin to whale oil, since that was the most commercially useful animal-based oil, up until the whales dwindled and hunting them was banned.

Blubber is one of the two main sources of whale oil. Its used for fat storage, of course, but blubber in marine animals also serves a couple of purposes that would also seem useful in any large hairless creature that might fly at altitude: heat regulation and resistance to pressure changes. It also appears that marine animals bodies will adjust the chemical composition of their blubber based on need. So getting to a useful "FT" composition wouldn't be a big genetic stretch.

The drawback to blubber for this purpose is that humans typically had to "cook" it to get the oil out. One might imagine a scheme where its used as the reserve, while the creature's body has an organ that naturally does this over time and keeps as smaller supply of ready oil in a more liquid form. Which brings us to...

Sperm oil is the other natural form of whale oil. Its actually found in a waxy liquid form in the heads of Sperm Whales (and some other related toothed whales). Their nose is full of it. If only they had a natural ignition source, Moby Dick would have been far more interesting. enter image description here

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Something like a dragon, with a really nasty flame breath, doesn't work with real-world biology.

So, "magic" is the answer: you can wrap that magic up in biological technobabble if you like.

External Magic

The Dragon consumes some substance which it processes into flame-fuel. Maybe some kind of rock, or some plant, or astral ley line energy.

This moves the magic outside the dragon, which lets you be all realistic in how the dragon itself works.

Internal Magic

The dragon has an enzyme, a gland, a sack, a second stomach. In that, it stores its dragon-napalm. How that works you hand-wave or obscure away with technobabble. It expels it in a jet, and sets it on fire. The fire could be from exposure to air, mixing chemicals that cause an exothermic reaction, an electrical shock caused by bio-capacitors in the dragon, etc.

Environmental Magic

The laws of reality are different in some way that makes Dragons work. Maybe in your world there are N elements from which all reality is made, one of which is Fire, and the Dragons use their access to the element of Fire to produce flame.

Science Magic

Modern industrial technology uses ridiculous energy budgets to build things like flamethrowers and flying airplanes. Biological creatures matching them requires energy budgets far beyond the reasonable. You'd have to be in a preternaturally energy-rich ecology to have anything at all dragon-like.

So stick the Dragon on some world where blasts of flame like the Dragon's actions happen all the time, where there is a ridiculously high biological energy budget. Explaining why entropy doesn't cause that steep energy gradient (the ease of fire) to collapse is tricky, but that can be explained by the situation being relatively short-term. Maybe a Type 2 civilization engineered a habitat so they can play with Dragons. The Dragon's breath is actually caused by an AI watching the dragons spitting some mundane saliva: the energy projectors embedded all over the place then concentrate beams and create plasma.

After all, a sufficiently advanced technology is indistinguishable from Magic.

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