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Are there any known laws of nature, which don't allow the existence of an animal with following characteristics?

  1. Can fly to the altitudes of modern airliners (around 10 000 meters or 32808 ft).
  2. One of the sources of thrust are organic jets or rocket engines (details see below).
  3. The animal can carry up to three passengers (about 240 kilograms, 529 pounds).
  4. The animal can fly up to 1000 kilometers (621 miles) using his own thrust source (that is, not just gliding or using the wind).

Regarding organic jet engine: There is at least one animal, which produces organic, flammable material - cows generate flammable methane. So, theoretically, an animal could have a combustion chamber in his body, where the flammable gas could be concentrated.

The next thing it needs is some form of nozzle, from which the gas would escape and it must have varying shape, controllable by the animal. Several animals can control the shape of their bodily openings, so it could be possible.

Finally, it needs a spark so that the flammable gas actually starts to burn, but that's easy - as stated in the requirements there is at least one pilot, who could activate a contraption, which generates a spark close to the nozzle.

I imagine that the animal can gain low altitude using wings (one of potentially many thrust sources), then the animal starts to generate flammable gas at the command of the pilot (we train dogs and horses, why not this fictious animal?) and the moment it opens the nozzle, pilot fires the ignition.

Here we go: A VTOL-capable (vertical take-off and landing), jet-powered dragon with at least two power sources (wings, jets - i. e. it is fail-safe).

Notes:

  • I say animal on purpose - because of mental inertia I'm thinking about a dragon, but there may be other animals, which can fly with jets.
  • There are some military fighter jets with variable sweep wings that an animal could achieve easily (real birds do that all the time).
  • An interesting variation would be an organic liquid-fuel rocket engine. Unfortunately, I know too little about rocketry to imagine how it work in an animal. Ideas in this direction are welcome.
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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Monica Cellio Jan 25 '15 at 1:18
  • $\begingroup$ Regarding generating the spark - it may be better to give the animal a way to generate it itself (maybe "borrow" from the electric eel?) - this allows for the whole "jet" system to be a coherent, evolved set of organs, along with the required instincts and control abilities - might be easier to explain than jet beast which comes ready with enough gas to fly 1000kms, but requires a pilot to light its farts :p. $\endgroup$ – G0BLiN Sep 4 '17 at 14:51

15 Answers 15

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I'm going to assume you're not talking about something that is genetically engineered for this purpose (otherwise I think the answer is pretty straightforward).

The closest analogue to your dragon would be a jet-powered, pressurized Cessna 172.

You're going to have a few hurdles to overcome here.

Environment

A Cessna 172 has a normal service ceiling of approximately 4000m (13000ft). Much above that, and you'd need to pressurize the plane for the passengers. Even if we assumed we could create a creature that could operate at over twice this height, its passengers certainly would not.

Evolution

A system like this is really hard to explain evolutionarily. There are three pieces here: (1) the combustion chamber, (2) the fuel, and (3) the ignition source

A rudimentary combustion chamber could be a squirting sack of some sort. Our best example in real life is the octopus (and related animals). They use a powerful squirt of water to quickly move through the ocean. This gives us a possible explanation for how these muscles developed.

Once we get the sack, we need something inside it. Obviously, for a flying creature, filling a large air bladder, and then expelling it through a muscle controlled nozzle would give our create an evolutionary advantage in catching prey or escaping predators. And since we already have an evolutionary analog for this, I'm not going into the details here.

Like you said, a possible fuel source is methane. Conceivably, if our creature is an omnivore, it may produce methane through its diet. An evolutionary advantage may lead to this being stored in the air bladder instead of being directly expelled. Eventually, all the creature's produced methane would be in the air bladder.

At some point, once the creature can produce its own "air", it may evolve a storage capacity (basically, a second air bladder), which would allow it to refill the primary air bladder more quickly.

What I believe I have shown is an evolutionary pathway for a bird-like creature to develop multiple air bladders, of which at least one is used for supplemental propulsion, where the air is in part methane produced by the creature.

However, that doesn't quite answer the fuel part.

Fuel

Methane is approximately 55MJ/kg, or 36.4kJ/L. Contrast that to jet fuel, which, while only 46MJ/kg, is 37.4MJ/L (note the megajoule unit in the second one). It's got approximately 1000x more volumetric energy density.

A cow produces somewhere between 100 to 500L of methane a day. Assuming we could reliably produce 500L of methane, this is really only half a liter of jet fuel worth of energy. Assume that is proportional to the mass of the animal (an adult cow weights approx. 750kg), that's about 0.7 mL/kg. Even if you increased that to 10 mL/kg (assuming our creature is super efficient about producing methane) and you'd still need a creature that weighs 21 tonnes to fill a Cessna 172 with fuel every day. For comparison, elephants weigh 6 tonnes, and the largest flying creature to ever live only weighed 250kg - one third of a cow.

So, you'd need some other way of fueling this beast. If you want to use gases, you'd need to make your creature massive to handle the volume (or massive with muscles to create the needed pressure), or you'd have to use something else.

Enough jet fuel would require 171kg (half the weight of or weightiest flier). Even compressed hydrogen would require 55kg worth of fuel (to reach the same energy). But, of course, what good is fuel without...

Propulsion

Now we get to the crux of why this will never work. Since something as complicated as a highly balanced rotating shaft will never exist in the organic world, we need an alternative. The only two non-rotating jet engines are the pulsejet and the ramjet.

Ramjets are very efficient--but only at about mach 3 or above. At the very least, you need supersonic airflow to make them work. So, unless you have a way for your creature to flap its way past mach 1, a ramjet would never work.

Pulsejets give very poor compression, and are not very fuel efficient. This would require even more fuel. They are also very loud. However, I could see an evolutionary explanation (though we are now straining reality) for this.

Now, even for a pulsejet to work, we would need our creature to be able to carefully add in air and fuel, spark the mixture, and then withstand the intense heat of combustion.

The first step is not impossible, the second step is highly improbably, and the third step is almost impossible. How is a dragon supposed to be able to create a combustion chamber inside of their air bladder?


All in all, I think this is an impossible task. You are trying to make a natural creature that evolved both flight with wings, and with flaming jet powered farts. Not going to happen.

TL;DR - You can't power a dragon with fartjets.

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  • $\begingroup$ Hi @Nick2253, Nizidramanii'yt tells me that he has a low pressure compression chamber, and stores liquid methane in there to burn peasants. Would that help? $\endgroup$ – Serban Tanasa Jan 19 '15 at 23:49
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    $\begingroup$ I'm thinking about the properties of palladium, which can store surprising amounts of hydrogen. Somewhere along those lines might be a solution for fuel storage. Then again, when you're on the subject of dragons, you're dangerously close to magic hand-waving anyway, so... ;) $\endgroup$ – DevSolar Jan 20 '15 at 11:47
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    $\begingroup$ @DevSolar The palladium angle is interesting. Now we're talking an evolutionary pathway of first taking on metals for some purpose, using those metals to protect the combustion chamber, developing more sophisticated metal processing cells, eventually drawing in palladium, realizing an evolutionary advantage that the palladium stores hydrogen, which makes the fartjets just a bit more potent, eventually culminating with a complex palladium/hydrogen storage system. $\endgroup$ – Nick2253 Jan 20 '15 at 17:19
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    $\begingroup$ Sufficient quantities of palladium would require accumulating huge quantities of jewellery, most of which would end up in a pile of useless gold... $\endgroup$ – Brian Drummond Jan 20 '15 at 17:25
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    $\begingroup$ @raxacoricofallapatorius You obviously completely missed my point. I see no irreducible complexity here. But the fact that you're saying the brain, with a relatively simple evolutionary pathway, should be harder to explain than something as complex as an organic jet engine is simply ridiculous. We're talking about evolution here, in which there is obviously no irreducible complexity. If you can come up with a simple pathway for how something as complex as an organic jet engine can form, I'm all ears. But if you can't, you'd necessarily have a complicated one, which makes it hard. $\endgroup$ – Nick2253 Jan 21 '15 at 15:28
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Rylatar'ralah'tyma was giggling audibly.

That only made Nizidramanii'yt more furious. His tail was still coiled up to cover his rear, which, unsurprisingly, was still incredibly tender, despite the regenerative magic field he'd put in place.

"The indignity of it all!" he moaned, trying to set fire to a nearby copse of trees and three hares, out of sheer spite. His fireglands were still frosty from the cold flight, however, so only a puff of smoke came out.

Polishing her red scales, Rylatar'ralah'tyma asked, distractedly:
"I mean, I have only one question. What exactly did the mage make you eat before-hand? A small town's bean supply for the year? I mean, that much gas...

That turned his fire-glands back on, alright. Too late, Rylatar'ralah'tyma was already in flight, her glorious body contorting mid-flight from the heavy laughter.


Meanwhile, at the Unseen Academy in Halruua.
A large audience of mages, archmages, a few liches and a gorilla are gathered around a presentation board, upon which lies the magically animated chalk-outline of a very scary looking dragon. The title is "Dragon Tubing". A mage in red robes wearing a prominent nose-ring is strutting about proudly. Behind him, tapping her foot impatiently is a younger female mage dressed in all-pink robes.
Edwin Odesseiron: "Oh, it was a feat of pure bravery, a true battle of the wills between me and the great ancient dragon."
Imoen: "Guys. It was barely a centenarian. And it was drunk from drinking Duke Eltan's entire wine cellar fermentation vat. And asleep."
Edwin: "Sure, that might have helped a bit, but it is nonetheless..."
Imoen: "Listen, while I was all for traipsing in quietly to dislodge a few gems, nobody asked you to start shooting firebolts at it."
Edwin: "It attacked!"
Imoen: "It snored. And let me tell you, it is never a good idea to cast a fireball at a dragon, but it is a particularly terrible idea to do so while you're on the dragon."
Edwin: "Uh, regardless, a particularly well-placed Bigby's Crushing Hand and a Fireball from my Contingency trigger, had the effect of..."
Imoen: "Squeezing his buttocks while setting fire to its farts."
Edwin: "Of course, the dragon took off, with us on it..."
Imoen: "That was fun!"
Edwin: "It quickly got rather cold, but my brilliantly crafted Oiluke's sphere held fast."
Imoen: "After about half an hour, the fermentation vapors ran out, and the dragon was probably dazed from the cold and the lack of air. We got off as it glided to the ground."

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    $\begingroup$ This is brilliant but it's not really an answer... maybe you could add some actual analysis to the story? $\endgroup$ – ApproachingDarknessFish Jan 20 '15 at 6:22
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    $\begingroup$ @ApproachingDarknessFish, your wish is my command. $\endgroup$ – Serban Tanasa Jan 20 '15 at 6:46
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I want to address a number of inaccuracies in previous answers, so will collect them all in one place rather than scattering comments.

1) Cessna 172 service ceiling can be up to 14,000 ft, depending on model. This is really a function of it having a normally aspirated piston engine, though. A turbocharged model could fly much higher. A jet would be even better, as they tend to be more efficient at altitude. From personal experience, I've flown my Piper Cherokee (similar size & engine) at 14,000 ft a number of times.

2) Oxygen needed for survival above 12,000 ft. This is just plain wrong. There is an FAA requirement (FAR 135.89) for the pilot to have supplemental oxygen if flying above 12,000 ft for more than half an hour, but that's an issue of pilot acuity, not survival. (Edit: since the dragon is doing the flying, and has evolved for altitude, this is not a requirement.) I've also spent a good bit of time hiking & skiing above that altitude, and have flown sailplanes to about 17,000 ft, and am still here to tell about it. And that's not even considering the Tibetans :-)

3) The dragon needs to carry oxygen for its own survival at 30,000 ft. Hardly, as geese have been recorded flying at close to 24,000 ft (7290 m) http://rspb.royalsocietypublishing.org/content/early/2012/10/25/rspb.2012.2114

4) Methane storage/energy density: Yes, methane is not good in this respect, but if the creature produces methane, it's only a small evolutionary trick to add enzymes that convert it (or other parts of its food) to more complex hydrocarbons, e.g. jet fuel.

5) Temperature: Sure, normal flesh does not deal well with high temperatures, but there are numerous examples of carbon-based life forms which create specialized ceramic structures like shells and bones, or the SiO2 shells of diatoms, so it's not impossible to envisage the evolution of ceramic jet exhausts.

I think the biggest practical objection is to carrying people, which is likely to terminally mess up the dragon's weight & balance.

For another point, why should the dragon have jet power as its normal means of propulsion? As others have mentioned, flapping wings work well and have a lot of advantages in the low-speed realm. Maybe the jet is an adaptation for quick escape from predators - which is not all THAT hard to imagine, when you consider the skunk.

So did breathing flame evolve from the jet propulsion, or vice versa? Or is the firebreathing just a myth, caused by people seeing the "fierce dragon" doing a panic escape from a distance?

Edit: Thought of another possible use for a jet propulsion system. Given square-cube law & Earth-biology muscles, a large flying creature just can't take off under its own power. However, as sailplanes & hang gliders demonstrate, once aloft it can use thermals & air currents to stay up indefinitely. So your dragon would normally live on cliffs, but would have a jet/rocket assist takeoff system for launches from flat ground, for instance after killing prey.

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    $\begingroup$ +1: I grant you the Rylatar'ralah'tyma seal of approval. She dislikes the comparison with geese, though she claims they make for a delicious roast'n'dive kinda snack. $\endgroup$ – Serban Tanasa Jan 20 '15 at 6:13
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    $\begingroup$ Maybe the jet is an adaptation for quick escape from predators - which is not all THAT hard to imagine, when you consider the skunk. I would hate to meet the predator from which a jet-powered dragon feels the need to escape. What is it trying to run from? F-22s? $\endgroup$ – reirab Jan 21 '15 at 19:01
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    $\begingroup$ Unless, of course, by escape from predators, you meant escape from MQ-1 Predators. $\endgroup$ – reirab Jan 21 '15 at 20:43
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    $\begingroup$ @reirab: No, I'm thinking of a dragon as having a lifestyle similar to hawks & eagles (or maybe vultures/condors). It loafs around at altitude (using thermals) until it spots prey on the ground, then dives to kill it, and eats on the ground. While it's on the ground, it's vulnerable to predators unless it can take off quickly - and if large, probably can't take off from level ground at all. $\endgroup$ – jamesqf Jan 22 '15 at 3:34
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    $\begingroup$ Maybe it evolved from a smaller creature that did need a way to escape from predators. Shooting fire out of your backside would be an effective defensive mechanism in its own merit, and giving the creature a forward boost would only improve its effectiveness. Also, it's probably a lot easier to initially evolve a flight mechanism in a smaller creature than a larger one - square-cube law and all that. $\endgroup$ – IndigoFenix Feb 16 '16 at 9:49
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There is no reason why such a beast could not exist in theory. See this answer https://worldbuilding.stackexchange.com/a/345/75 for the arguments in favour of regular wing-propelled dragons.

Now, suppose that we have a being that, like a bombardier beetle, uses a flaming exhaust as a weapon. It is a logical extension for such a being to evolve in such a way that the exhaust can be used as a thrust source. The main issue would be waste heat, but that could be handled by passing the fuel over the combustion chamber to pre-heat it while cooling the combustion chamber and providing thermal isolation. Precipitated metals and carbon fibre could be used as a thermal liner. Since biological rotation in multicellular animals is unlikely to evolve naturally, thus precluding turbojets, this could be a pulsejet, pulse-detonation-engine (PDE) and/or a ramjet or scramjet. As a biological organism, the possibility exists that the one "engine" could be reconfigurable to operate in more than one of these modes, beginning in a pulsejet or PDE regime, and potentially changing configuration to a ramjet and then a scramjet.

The main constraints in this system would be those of size - too small and the creature would not be able to contain sufficient fuel to overcome its drag in order to generate useable lift for the required time, too large and it would not be able to support its own weight in the air.

Naturally, it would be in the interest of the animal to evolve an efficient fuel in terms of energy per volume and mass. A compound such as 2,5-Dimethylfuran could be a possible fuel, as it is potentially capable of being biosynthesised and has a high energy density, 42 MJ/kg and 37.8 MJ/l, though for PDE engines a lighter fuel such as methane or hydrogen would be preferable. If the being had a reconfigurable engine, there may be advantage in carrying multiple fuels.

As for high altitude flight, birds, with their more efficient lungs, have been recorded to fly at these altitudes. In fact, the higher the altitude, the more efficient the flight (due to lower air pressure reducing drag). The main trade-off is going to be lung efficiency decreasing as altitude increases versus flight efficiency increasing as altitude increases.

The range factor can be solved by carrying enough fuel. As manufactured aircraft can have the required performance, this is not beyond the bounds of possibility.

The most problematic factor is carrying passengers. Putting three lumps on the beast's skin is going to cause massive parasitic drag that will dramatically reduce the beast's speed and range, even if these three lumps lie down flat against its skin and cover themselves with a slick, streamlined shell. The most likely option is to carry the passengers internally, where their mass would lead only to increased induced drag. I can speculate that such a means of carrying passengers could evolve where the beast carries its young about in an internal pouch so as to both protect them and reduce parasitic drag. The pouch could be repurposed to carry other passengers.

It is theoretically possible that such an organism could evolve naturally, given the right selection pressures, but it is far more likely that such a beast would be a bioengineered organism. In the case of a bioengineered organism, a turbojet would be possible given that its biology would not be determined by the requirement of continuous evolutionary fitness, but by the requirements of a designer. However, a PDE/ramjet/scramjet system would probably be preferable.

The issue of obtaining fuel would require that these beings either ingest the fuels from an external source in their final state, or have the means to synthesise the fuel from their food. The latter would require that these beings have truly prodigious appetites and rapid digestive processes.

In the more likely bioengineered organism scenario, it would make sense to have something like "fuel trees" that use photosynthesis to produce the fuel the flying organism needs. It may even be that the fuel trees are one gender (probably male) and the flying form is the other gender (probably female), thus we have a system that provides both transport and fuel production.

Finally, as to what these beings would look like, the most likely answer is some combination of dragon and bird. If we are following the basic vertebrate body plan, the neck would be short and thick (to allow air intake), not long. The creature would likely be more batlike than birdlike, having membranous rather than feathered wings, as feathers could be torn loose by the high speeds. There would not be a single tail (which could be burned by the jet exhaust), but more likely there would be fins on the rear limbs, well to either side of the jet exhaust.

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  • $\begingroup$ Yup, I've got a dragon like this in a story of mine. She has pulsejets venting out her back, which is basically your standard venom-based dragon breath turned backward and run through rocket nozzles made of bone. She typically flies with her wings, but can use the jets for a boost of extra speed. Mix in some bird anatomy like hollow bones and you have a jet-powered dragon. She just can't do #1, 3 or 4 on that list, haha. $\endgroup$ – Graph Theory Jan 20 '15 at 18:34
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1.Can fly to the altitudes of modern airliners (around 10 000 meters or 32808 ft).

Take a look at this graph:

Atmospheric pressure graph

At 10,000 meters, the air pressure is about 25% of its value at sea level. Wikipedia notes that pressures this low and oxygen levels this low can cause some problems:

The lower partial pressure of oxygen at altitude reduces the alveolar oxygen tension in the lungs and subsequently in the brain, leading to sluggish thinking, dimmed vision, loss of consciousness, and ultimately death. In some individuals, particularly those with heart or lung disease, symptoms may begin as low as 5,000 feet (1,500 m), although most passengers can tolerate altitudes of 8,000 feet (2,400 m) without ill effect.

At altitudes above 12,000 meters, an oxygen mask is imperative for survival in a non-pressurized environment. So this fellow has to be able to survive without much oxygen, which could be hard because of his size. His body parts also have to withstand the low pressures. These are going to be problems.


2.One of the sources of thrust are organic jets or rocket engines (details see below).

Here's a diagram of a turbojet engine:

Turbojet engine

According to Wikipedia, the compressor blades may spin at rates of 2,500-50,000 RPM. The dragon is presumably quite large and so will have large engines, so this rate will be towards the lower end of the spectrum. I have a feeling the vibrations will pose problems for his general structure.

This notes that the temperature of gases in a turbojet engine may reach 2000 degrees Celsius. Any organic material at this temperature will be scorched and turned to ashes. There is absolutely no way any carbon-based life form can survive this temperature - especially not for the two or so hours it will take him to travel those 1,000 kilometers.


These are the two main problems I can find with the scenario: pressure/low oxygen and temperature. Because I'm feeling less pessimistic than usual, here are some solutions:

  1. Pressure/low oxygen: You can't really have the dragon hold his breath because of the long duration of flight. So how about having him carry extra oxygen with him? I wrote about fictional pufferpolyps in another answer set in a different context. The feature we can take form them is an easily expandable chamber that can store gas for a while, on tap. You've basically got an inflatable oxygen tank with you.

  2. Temperature: Take a look at Monty Wild's excellent answer to How could dragons be explained without magic? Monty suggests

    By squirting the fuel out fast enough from a duct venting into the mouth as the dragon exhales, it need not ever come into contact with the burning fuel. (Think spraying the flammable gas from a spray can over a cigarette lighter - it doesn't melt the plastic nozzle.) With a change in biology so that a dragon could precipitate metals such as aluminium or magnesium, a dragon might even be able to spit a liquid mixture akin to thermite that would spontaneously combust due to the presence of other reactants.

    And therein seems to be our solution. If you go for a traditional jet engine, your dragon is going to be blackened. The hot air will have to go through some sort of cavity, and there's a high risk of it burning up that cavity. So you'll want to go for the rocket approach: shoot out flames like a fire breather, in reality expelling some flammable substance and igniting it as it goes by.

    I'm not sure how good an answer this is. I hope it helps.

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  • $\begingroup$ The rocket equation really only applies when there is no drag on the rocket, and the rocket contains all it's own reaction mass. It doesn't apply to (for example) propellor aircraft, which must thrust forward constantly to counter drag, and contain none of their reaction mass (they use air). $\endgroup$ – 2012rcampion Jan 19 '15 at 22:50
  • $\begingroup$ @2012rcampion In this case, the dragon does contain all its own reaction mass, but I guess the drag issue is a problem. $\endgroup$ – HDE 226868 Jan 19 '15 at 22:52
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    $\begingroup$ Even in the case of a jet engine (this case), the aircraft contains almost none of its reaction mass. A tiny amount of the exhaust consists of the fuel, but mostly consists of the air that the engine took in. In fact, most of this air is not even used to burn the fuel, but merely accelerated. This is because engines are more efficient the cooler and slower their exhaust is. $\endgroup$ – 2012rcampion Jan 19 '15 at 23:05
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    $\begingroup$ If the dragon doesn't have some sort of rocket nozzel (i.e. the traditional rocket "bell") where the high pressure (and high temperature) combusted fuel is directed to the rear of the craft, then he's not going to get any propulsion from the combustion -- spitting out the fuel and igniting it in mid-air won't give any more propulsion than just spitting it out and not igniting it. $\endgroup$ – Johnny Jan 20 '15 at 16:53
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    $\begingroup$ @HDE226868 Many humans have climbed Everest without supplemental oxygen. It's over 30,000 ft. So, no, it's not wrong. I'm not saying it's advisable, but it's definitely possible. Even people who have been in aircraft cabins that decompressed at altitude have almost always survived the decompression, even if they didn't put on masks. The problem is not that they die from lack of oxygen, but that they blackout from quick pressure change and then die when the plane runs out of fuel and flies into the ground. $\endgroup$ – reirab Feb 6 '15 at 16:25
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A spontaneously evolving creature would likely fail more than one or two of your requirements.

It would be quite a stretch but here is the closest I can fit what you'd describe. I'd use a bombardier beetle as a template. Based off that, we have a winged organism that can produce a strong chemical reaction internally with the ability to direct the result.

If we assume that hydrogen peroxide (H2O2) is the desired propellant, we solve a few issues. Decomposition products are hydrogen and oxygen. Reserves of each gas can assist respiration and a fuel source. Bombardier beetles use hydrogen peroxide, hydroquinones, enzymes and water to produce their jet. A short stretch would be a creature that can focus on hydrogen peroxide.

Direct oxygen production would allow high altitude flight and allow a larger creature. Dragonflies have been show to increase size up to 20% in hyperoxic environment. http://www.wired.com/2010/11/huge-dragonflies-oxygen/ Canada Geese can fly up to 1000km in a day and have been spotted up to 9km altitude. http://en.wikipedia.org/wiki/Canada_goose

Fuel ignition could be started with compression, like a fire piston. Pistol shrimp can create enough force to cavitate water. It would be a small step to have a chitinous chamber to compress a gas mixture to autoignition temperature. Sustained firing would be extremely difficult due to the force and temperatures. Conceivably, the burst could be for takeoff or additional speed.

Size and energy conversion would be the limiting factor for this imaginary creature. Any creature with an exoskeleton has to molt to grow. The old chitin is shed and the new chitin has to harden. This would leave the creature completely defenseless while the process happens. Any interruption could lead to a deformation serious enough to cripple it until the next molting. Supporting it's own weight would be difficult, let alone passengers, barding and gear. Without gliding, flight would carry a huge energy load necessitating huge amounts of food before each flight.

With a stretch, a gigantic bug that can produce a hydrogen peroxide fuelled, organic jet burst could grow large enough to support passengers and be pushed to fly extreme altitudes and distances.

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  • $\begingroup$ What on earth is a "spontaneously evolving creature"?! $\endgroup$ – Lightness Races in Orbit Jan 21 '15 at 18:00
  • $\begingroup$ Naturally evolving would be a much better word. I was thinking opposed to a future/sci-fi level of genetic engineering. $\endgroup$ – Huninzero Jan 22 '15 at 14:12
  • $\begingroup$ Aha okay. The notion that anyone thinks creatures "spontaneously evolve" is a common misconception by misinformed creationists, so be careful :P $\endgroup$ – Lightness Races in Orbit Jan 22 '15 at 14:14
  • $\begingroup$ Definitely have to my watch wording in regard to evolution. $\endgroup$ – Huninzero Jan 22 '15 at 14:29
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I can't believe such a creature could evolve.

The problem is, that jets are a very inefficient means of locomotion, compared to flapping wings. While I could believe a flying creature with a jet engine (the evolutionary pathway is exceedingly complex, but I don't believe it contains any uncrossable gaps) I can't believe one that would evolve to cruise on a jet engine.

Having that kind of fuel reserve/ability to take the sustained heat would serve no evolutionary purpose. A jet would be either for evasion or pursuit (think: cheetah) only.

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  • $\begingroup$ At low speeds and relatively small sizes, sure, flapping wings are efficient. I have more trouble believing that they'd even be plausible, let alone more efficient than a jet-powered fixed wing, at Mach 0.85 and 30,000 ft., though. If it were plausible and more efficient, we'd probably already be designing airliners to do that. Moving a wing back and forth rapidly without breaking apart gets to be intractable pretty quickly as you scale up the size of the craft, let alone airspeeds of Mach 0.85. At those airspeeds, the wing would probably exceed Mach 1 as it moved through the forward stroke. $\endgroup$ – reirab Jan 21 '15 at 19:26
  • $\begingroup$ @reirab Of course they couldn't do Mach .85/30k ft. The point is the dragon has no evolutionary reason to be able to do that. Sprinting speed is valuable, sustained travel is normally at efficient speeds, not at peak speeds. $\endgroup$ – Loren Pechtel Jan 21 '15 at 20:27
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Let's ignore for a moment the requirement that it carry 3 people. To just say, could there be a living create with a jet engine?, I think the answer is obviously yes, because there really is something that resembles such a create: the bombardier beetle. It doesn't use it's "jet engine" for propulsion but rather as a weapon, but the principle is there and clearly works, because bombardier beetles really do exist. Note the bombardier beetle's body solves all the problems that others have discussed here on a small scale.

So the question is, could such a system "scale up" to be a viable method of propulsion for a create of any given size? And if so, would it scale up to the point of being viable for a creature capable of carrying 3 people?

I doubt that the specific chemicals used in the bombardier beetle would work in a larger animal: if they were powerful enough to propel something the size of horse or cow to significant speeds, they would likely be so powerful that they'd blow a beetle to smithereens. But could a create exist that used other chemicals?

It's difficult to do calculations because you have to make up the details as you go along. And if I could absolutely prove that, say, chemical X does not have sufficient energy, is there not some alternative chemical Y that might? You could point out that there is no animal in existence that has a body part that does Z, but so what? Proving that it is not possible for such a creature to exist gets tough. Clearly you cannot prove that there is no way possible for jet engines to operate, because of course they do operate. So the issue is whether it is possible for a living creature to have a jet engine, or, I presume, something analogous. Which I just don't know how you could ever prove that it isn't possible.

And by the way, a lot depends on whether you are assuming that such a creature had to evolve by chance process, was created by a creator God, or was built by people using bioengineering. Lots of things that would seem impossible in scenario 1 are much more plausible in scenarios 2 and 3.

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There are several issues with this that has to put your desired result into the realm of magical unfortunately...but I'll try to get as close to reality as possible.

Methane is not jet fuel. There is more potential energy in a kilogram of liquid methane than there is a kilogram of jet fuel, however the methane takes up a huge proportion of space (this is referred to as having a low volumetric energy density) compared to jet fuel. That said, there is a synthetic process called the Fischer–Tropsch_process (wiki here http://en.wikipedia.org/wiki/Fischer%E2%80%93Tropsch_process) that could theoretically be reproduced in a very flame resistance creature. With a few tweaks, this process could have a creature that collects carbon monoxide as it breathes and creates a bio-diesel fuel from it. Completely theoretical if such a process could exist naturally, but we're talking dragons, so why not?

Birds have some interesting lungs...so do reptiles for that matter...but birds in particular use pressure changes to regulate the flow of air from their lungs. Once again, way in the realms of theory, but if it was possible for a dragon to get a 40:1 compression ratio from a series of contracting lung muscles...

There are a couple heat issues, but there is possibly some balance to be found here. First and most obvious is the extreme heat of the combustion reaction. Your Dragon will need some heat resistance above and beyond what a 'natural creature' really could have to make this work. However, there is some offset to this...as really warm as that engine is, it is exceedingly cold at this level of the atmosphere. A cooling system cold be possible here...'blood' (quoted that because our blood would boil...you'd need some sort of supernatural 'blood' to work here) could be used to take the heat from the combustion region (a high tech term for 'ass' in this case) over to it's wings or perhaps another cooling apparatus. Not a first in nature, some theories on dinosaurs fins (stegosauruses plates as well) were used as cooling apparatuses. Just for visual effect...the 'jet engine' lights up and the dragon extends two 'fin's out of either side of it's body designed to quickly cool blood off to cool it mid flight.

I would suggest that this entire jet process would be intermittent or in bursts just to keep the total heat produced down...in a story this might work as feasible. I still think funny as hell, add in a dragon making a face a human on toilet would be as it shoots flames out of a misc hole on it's body, shooting it through the air.

You would need a few very special conditions on the dragon...in particular, how is it capable of extracting the oxygen it needs to survive at the altitude it is flying at

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As the other answers have calculated, have jet power be your primary propulsion is unlikely to be realistic. However many birds have been observed at altitudes exceeding 30000 feet: http://en.wikipedia.org/wiki/List_of_birds_by_flight_heights

You could allow for standard wing propulsion boosted or augmented by organic jet power on an as-needed basis.

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I'd like to present a different opinion: yes, it is possible, but it wouldn't be a 'jet' engine. I'm assuming the fuel is liquid (so not methane), which would be most logical for internal storage.

Fuel

A dragon has had an evolutionary advantage of creating organic fuels. Arguments that the amounts cows produce is extremely little, are silly because cows have no evolutionary advantage of creating methane; it's a byproduct. Dragons have an advantage by creating the fuel, so they will evolve to do so in copious amounts. It will evolve enzymes that break up organic compounds to highly flammable compounds, just like petrochemical plants do (and animals can probably do this better; compare the most advanced polymer science can make with infinitely more complex proteins in your eyes that makes you see kittens on the internet).

Now, imagine a dragon that is evolving to fire-breathing. It already has DNA present to create very hard, scaly bodyparts that are fireproof - I would think that some of the dragons teeth have evolved (grown bigger) to protect softer bits of dragon; think of elephant teeth.

Excess fuel...

Now it does have a fuel-producing organ, but this organ cannot be simply switched off. In early points of its evolution, it simply burnt off excess fuel by breathing fire, but this posed some other problems; for example, barbecueing the entire forest while sleeping or setting your dinner on fire. So, the dragon evolved ducts leading from the fuel-creating organs to its flanks (for example, under its wings), where excess fuel would be burnt off in a controlled manner. To prevent carbon monoxide asphyxiation due to incomplete combustion, air ducts were evolved as well, that would help combustion.

Now comes the interesting part - in flight, the dragon discovers that burning excess fuel not only protects its vulnerable belly from enemies, but also creates thrust, which is used to get away in emergency situations. From here, evolution of the side-burning accelerates, and the little patch of scaly cells around the fuel excess outlets swiftly evolves to nozzles.

This particular breed of dragon will soon split off evolutionary from 'normal' dragons, and very probably, its fire-breathing capabilities will soon diminish, since fire-breathing is effectively wasting fuel.

Differences from jet engines and other drawbacks

As others has pointed out, this would be a rocket engine and not a jet engine. Jet engines compress intake air to several hundred MPa, which organisms simply cannot do (in this extreme case, due to compression heating, cells performing the compression would be burnt, because fire-proof 'hard' cells cannot flex to accommodate compression). The dragon can probably not reach your quoted 10 000km, but may use high altitudes to migrate efficiently, since it will need a large foraging area to create large amounts of fuel.

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I think it would be possible if some mineral are abundant in the "concerned world" and the dragons have evolved from integrating them for heat resistance and fuel production... but from an evolutionary point of view, there must be a reason for them to evolve the jet engine and I think the reason should definitely be survivability in regards to either food or escape mechanisms from a predator. Come up with these reasons and using the excellent answers from the previous users you can get a "reasonable" Dragon.

Also, for carrying people, like how we use mounts for riding horses, a specially designed mount may be possible for the purpose.

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There are a few problems with organic jet engines.

  1. Jet engines require compressors. Otherwise, there is no way to maintain the flow of intake air into the combustion chamber. Unless your dragon is a hypersonic ramjet-powered dragon, that means rotating parts. I know of no animal (real or fictional) with precision-balanced thousand-rpm spinning organs.

  2. A dragon that breathes fire doesn't need to worry about burning, as long as it can manage the short bursts of heat. However, continuous combustion means that you have to deal with high temperatures all the time. Either the dragon's combustion chamber must be ablative (burns away slowly) in which case it probably ablates away much faster than it grows back, or must be made of a heat resistant material like typical jet engines. Again, I know of no animals with titanium organs.

  3. A typical small jet has a fuel consumption of around 6 gallons per minute, equivalent to around 20,000 L of methane gas per minute. At this point your dragon will probably look more like a blimp than a jet.

There are no laws of physics that say it's not possible, but they do say that it's not plausible.

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    $\begingroup$ For compression, an animal could compress in the manner of a pulse detonation engine. Essentially, it would breathe into its combustion chamber, close off the combustion chamber, pressurize through muscular action while ingesting fuel, ignite, and then open the rear nozzle of the combustion chamber to release the exhaust and generate thrust. $\endgroup$ – ckersch Jan 19 '15 at 23:31
  • $\begingroup$ @ckersch That's going to have to be some serious muscle contraction. $\endgroup$ – reirab Jan 21 '15 at 20:41
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No.

There are a couple of rather forbidding problems that this creature (even a genetically modified super-creature) would have to overcome, here are a few:

  • Methane and similar substances burn at roughly 1900-2000°C. This is rather incompatible with "coming out of an animal's belly". Temperatures of around 2000°C will cremate the animal and any other organic materials nearby.
  • Square-cube law. Such a feat as rocket propulsion might be imaginable for an insect-sized animal (with a somewhat less burning reaction, and at smaller scale), but not for something large enough to carry passengers. I believe there even exists a bug which does something similar.
  • In order to produce the necessary amount of methane (or similar), to lift off a not-insect-sized animal, the animal would have to feed and digest for months.
  • Laplace's formula. In order to store that amount of gas, the creature would need an immensely big belly under tremendous pressure. Laplace tells us that the belly's wall would thus have to be extremely thick, which means the animal would be forbiddingly heavy.
  • Lack of pressure/atmosphere at 10,000 meters. Low outside pressure amplifies the problem in the previous point, and lack of oxygen does not precisely help.
  • Temperatures below -50°C.
  • Noticeably elevated radiation (both gamma and ultraviolet), so the animal would need a metal coating of sorts.
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    $\begingroup$ I've addressed most of your objections in my answer, but you've added a couple of new ones. First, many earthly animals - anything living in the Arctic or Antarctic - regularly deal with temperatures of -50C. And of course our dragon could divert some waste head from its jet to keeping warm. As for radiation, even if the levels at 30,000 ft were a problem (which they aren't), it's perfectly possible for creatures to evolve radiation tolerance. See e.g. en.wikipedia.org/wiki/Tardigrade $\endgroup$ – jamesqf Jan 20 '15 at 17:54
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From an abstract point of view, I would say yes - a jet dragon could exist. Evolutionarily speaking, any creature is possible given a series of environments and mutations that would result in that creature. From the above posts, it seems highly unlikely that current earth conditions would ever allow for such a creature, but in the grander spectrum of evolution, a series of stranger environments could easily lead to such a dragon, or any physically possible structure.

There seems to be a heavy focus on designing a dragon that has the mechanical properties of a man made jet, and the material properties of carbon-based life. That is a difficult task, probably more or less impossible.

If you are talking about evolution, I think you'd be better off figuring out the series of environments that would cause natural jet-animals to exist. For example - why would flying at mach speeds be so much better than traditional flight?

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