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I am including some dragons in the universe I'm building. Now, I have already decided that only baby dragons will fly, but I am having some problems with breathing fire. My current design:

Dragons have lots of symbiotic anaerobic fungi in their alimentary canals. Breathing fire requires a diet heavy in sugar, and as a result, dragons will often make their homes in places with lots of carb-rich food. The symbiotic fungi will break down the sugar, producing alcohol which will be stored in special glands on either side of the end of the dragon's mouth. The dragon's tongue has a small, hard protrusion under its tip. When the dragon must breathe fire, its alcohol glands undergo violent convulsions, ejecting their load with great force. At the same time, the dragon strikes its tongue against one of its teeth, creating a spark that ignites the alcohol. For extended torrents, glands in its mouth secrete an oily, highly flammable solution. The dragon sets this on fire, and then proceeds to shoot a continuous barrage of alcohol. Fire-resistant mucus protects its mouth.

Now, I'm pretty sure there is a problem with the science somewhere. Can somebody help me find it, and/or suggest a better method?

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    $\begingroup$ Does this existing question about whether dragons can breathe alcohol-based fire answer your question? $\endgroup$ – Frostfyre Mar 14 '18 at 3:44
  • $\begingroup$ It answers a lot, but not the continuous stream. $\endgroup$ – Budhaditya Ghosh Mar 14 '18 at 4:13
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    $\begingroup$ Is there a reason you prefer this method, with the fungi? There are any number of explanations for how dragons breathe fire. $\endgroup$ – KernelOfChaos Mar 14 '18 at 4:56
  • $\begingroup$ Or possibly why don't dragons burn themselves $\endgroup$ – Separatrix Mar 14 '18 at 8:51
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    $\begingroup$ I proposed that the napalm breathing dragons here spray acetone. worldbuilding.stackexchange.com/questions/106623/… could have the dragons generate that themselves without the symbionts / carbs. Also if there is no flame inside the dragon it would not need protection - like a flamethrower. The flame should not get back into the flame thrower. $\endgroup$ – Willk Mar 14 '18 at 13:12
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The method seems a bit convoluted, not to mention that a dragon who eats a carb rich diet will not be capturing princesses, but attacking fruit orchards to get the materials needed for fermentation. (This also leads to the interesting question of why vegetarian dragons need to breath fire on the first place.....)

What may work better is to scale up the defence method of the Bombardier beetle:

enter image description here

Bombardier Beetle in action

The beetle essentially secretes two chemicals in glands in the abdomen, and squirts them out the back end. The chemicals are hypergolic (react in contact with each other), releasing a searing chemical spray against their enemies. For anatomical reasons this is going out the back, there are strong sphincter muscles to contain the chemicals and all the abdominal muscles can contract to mix and eject the chemical spray.

Dragons could do the same trick with glands full of hypergolic chemicals in the cheeks or throat, and coughing (or even vomiting if things are far enough back) to eject the chemicals, hopefully the mixing of the spray occurs far enough forward that the ignition point is past the dragon's face (breathing in fire is especially not recommended). Or the dragon could be like the Bombardier beetle and shoot fire out the cloaca. The warning sign for brave Sir Knight is when the dragon suddenly turns around......

enter image description here

Hypergolic reaction. Imagine a pressurized mixture

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    $\begingroup$ Thanks for the answer. I seriously think this is the best answer I've got so far. BTW, can the spray be powerful enough to propel the dragon forward, if it shoots out of the anus? $\endgroup$ – Budhaditya Ghosh Mar 14 '18 at 16:41
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    $\begingroup$ @BudhadityaGhosh thank you for the best mental image I've gotten from this site $\endgroup$ – bendl Mar 14 '18 at 19:21
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    $\begingroup$ Looking at the picture of the beetle, the spray is aimed forward by the little tail-like projection... I wonder if it might be more plausible to have the dragon shoot said spray out of an opening in their tail, which would give them a lot more control and the ability to aim around themselves. Tail snakes around in preparation, dragon roars and sprays at same time, dragon "breath" is assumed instead of dragon tail-spray. $\endgroup$ – Megha Mar 15 '18 at 0:27
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    $\begingroup$ @Budhaditya Ghosh. You owe me a new keyboard! $\endgroup$ – Thucydides Mar 15 '18 at 3:42
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    $\begingroup$ @Budhaditya Ghosh. Re read your comment. The idea of a rocket powered dragon shooting fire out its a** made me spit up my coffee..... $\endgroup$ – Thucydides Mar 15 '18 at 19:39
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For sustained fire breathing, you need an insulator (A thick mucus or secretion) If you atomize it using air (violent convulsions will lead to spurts) than you can extend the duration of fire at the cost of heat; this is what we do in combustion engines. Also atomized ethanol with salt (to prevent the colorless flame) it will look brighter, be hotter, and last a good while (4 hr per liter) longer

https://www.hemmings.com/magazine/hcc/2012/08/Fuel-Atomization/3715001.html http://www.greenethanolfireplaces.com/how-long-does-bioethanol-fireplaces-burn https://www.youtube.com/watch?v=x_eK9K6UQoY

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  • $\begingroup$ Why an insulator if it ignites in the air beyond his body? $\endgroup$ – RonJohn Mar 14 '18 at 5:15
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    $\begingroup$ @RonJohn likely because the question does state otherwise? $\endgroup$ – dot_Sp0T Mar 14 '18 at 5:51
  • $\begingroup$ @dot_Sp0T that's Bad Biology, I think. The dragon that sparked the flame at the tip of his mouth (like an aerosol can) would fare much better. $\endgroup$ – RonJohn Mar 14 '18 at 5:55
  • $\begingroup$ A common issue with anything made to toss out flame is back burning. A hazard with the nature of fire and flammable things mixing. $\endgroup$ – Belqinor Mar 14 '18 at 8:58
  • $\begingroup$ The dragon has flane resistant mucus throughout the entirety of its alimentary canal. And the dragon ignites the flame just as it is leaving the mouth. $\endgroup$ – Budhaditya Ghosh Mar 14 '18 at 10:34
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Ok, to me the easiest way would be to create a dragon that is either ruminant OR that have his diet strictly made of ruminants because his digestive system is made for that. You know, the sheeps, and the deers and the cows.

Now, as ruminant (or ruminant eater specialist) he have few stomachs and one of them is made for that grass and methane. Or to be exact the absorption of methane. It then can be stored in bags near the spine that are pressed when dragon take a deep breath. So he have oxygen in lungs which he can exhaust with the gas and crate flammable mixture.

Kind like humans have one opening for air and food but still can have food in mouth and air in sinuses. Or how you laugh so much you spit milk out of your nose.

Now the problem is how to create a spark to light that. Again, easiest option would be teeth with traces of flint. Somehow, like we human produce stones in kidneys and have tartar, dragons can have flint. It can be created within dragon itself. Back to digestive system. As silicon is present in many plants (see typical dragon diet) and thermal waters it could be paired with oxygen. Thermal waters would also mark natural occurrence of dragons or dragons lairs.

It would also create a natural diversity among dragons as those who eat moose don't have such sustained fire breaths (more meat less grass so fever meals) or dragons that can't breath fire (as not everybody have kidney stones) but still make people die in their vicinity (as methane is odourless but people suffocate with the lack of oxygen).

With your design I would go with fully vegetarian dragons. Why? have you tried ever to squirt an orange skin into the lighter? Yeah, the juice in citruses skin is alcohol and flammable (also it smells nice).
So your world could have a lot of different citruses, dragons eat them, and again digestive system, are able to store Limonene and later use it.
And that also could serve as world building as in warm climate there would be a lot of dragons but very calm as there is a lot of food. While in colder climates the food would be scare the dragons would be angry and more prone to attack settlement and drink human stored mead.

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I know that this question's a bit old, but I happened to know something on the matter, so I thought I'd add my grain of salt.

So, there are two ways I know of of doing this:

  1. The evolutionarily plausible one, but perhaps a bit of a copout
  2. Actual fire possible, but evolutionary background questionable

So, let's look at number 1 first. This, while not actually fire, could achieve the effect of fire, while retaining the plausibility of them looking so much like real squamates. So, a poison gland in the jaw produces a very strong zytotoxin that burns skin, causes paralysis and induces fever. Upon contact, the skin of animal would blister, and the venom would flow through the victim's veins, feeling very much like fire.

This could potentially liquefy the inside of smaller prey, and do so partially for larger quarries. But, there are a couple of problems with this so far. The first one being that dragons are very big, and the range might be unsatisfactory for a "cool" plot. So, I suggest adding muscles around the poison glands which contract and pump the glands, shooting the poison several metres. These muscles would also allow the dragon to regulate the amount of venom they squirt. The second problem is that reptiles shed their teeth very often - and the teeth are the best body part I can think of for the poison to go out of. But, there is a solution at hand. If the venom glands only connected to a few permanent fangs, the rest could be shed as usual.

But, you asked for fire, and that's not really fire. So, if you're not really that pushed on explaining the evolutionary background of dragons (Convergent evolution could potentially do it, anyway.), there's another method that could achieve real fire-breathing.

Suppose you had a group of animals that had formed a symbiosis with microbes in their digestive system that produced hydrogen as a metabolic waste product. Some of these animals could store and compress the hydrogen in organs rather similar to lungs, and expel it through a tube leading to their mouths. The hydrogen would be ignited through bioelectricity, giving them the ability to spew flame.

But, there are obvious dangers when it comes to breathing fire. Perhaps your dragons could have a "false pallate" like crocodilians, which would prevent fire backfiring and barbecuing the dragon's insides. To protect their eyes, they could have a transparent membrane like seals and other animals.

There are some possibilities of fire-breathing aside the classic "Ima burn you" purpose. For example, dragons with less powerful flames could use it to start bushfires instead - igniting dry vegetation to create an inferno. To take a leaf out of the whales' book, each species could have unique flame shapes that would distinguish them from different animals.

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There are several problems with this means of generating flammable compounds:

  1. Alcohol-producing organisms are poisoned by the alcohol they produce before the concentration of alcohol increases sufficiently for it to be flammable. Flammable alcoholic beverages are distilled.

  2. The dragon must consume a sufficient quantity of whatever food is required to be fermented.

  3. Alcohol burns at a relatively low temperature, so as a means of producing a flame, it is not too effective.

You'd be better off having your dragon have a conventional dragonish meat diet, and have it produce flammable oils that it can ignite - such oils have a higher energy content, burn hotter, and may be produced by biochemical processes within the dragon's own tissues.

A better solution still would be for the dragon to obtain its fuel from an external source, but flammable or better yet, pyrophoric compounds, or even their precursors, are difficult to obtain in the natural environment.

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I think hypergolic fuel is the way to go. They can be mixed and stored separately, and combined in or slightly ahead of the mouth.

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    $\begingroup$ Please expand on this possibly excellent answer. (I think there's a beetle which shoots two chemical that get really hot when mixed.) $\endgroup$ – RonJohn Apr 11 '18 at 21:51
  • $\begingroup$ @RonJohn is right. One-line answers are discouraged here as the site seeks not just to answer questions, but to teach those who come later with similar questions. A paragraph explaining what hypergolic fuel is and how it answers the OP's question would make this a good answer. $\endgroup$ – JBH Apr 12 '18 at 5:11

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