Anatomically Correct Jet Engine Capable Creature...

[Spacescifi]

So imagine that there are creatures anatomically designed for powered jet flight.

As such, they have mouths capable of sucking in large amounts of air at once, and have a pair of movable (made of bone) exhaust tubes that allow for steering and thrust through pressurized air exhaust (like octopuses have except with air instead of water). How one would design an organ or mouth capable of sucking in large amounts of air I do not know, but physics does not object to it as impossible.

They can store enough air for a burst lasting 30 seconds.... or just keep their mouth open while sucking air for infinite air thrust until they stop from exhaustion.

Where do they store the air? Bone tanks, which are designed sturdy enough that they don't leak air.

Factors: The more dense the atmosphere the easier it becomes from an anatomical engineering stand point to pull this off.

 

How possible is thus according to physics? And don't worry about evolution since this is basically god-mode engineering of new species to inhabit alien worlds (because what else do you do when get super advanced but play god?).

Edited September 4 by Spacescifi

[GluttonyReaper]

My immediate thought is: how you compressing the air enough to get any significant thrust? You want a light bone structure to store your air (more akin to a bird's than a human's) otherwise you're going to struggle to get off the ground: the human skull is a roughly spherical bone 'tank' around 5kg, which is pretty hefty, and I'm not sure even that's enough to store sufficient air for a reasonably 30 second burst.

Another option could be to compress the air just before release - i.e. something akin to sneezing, so instead of a continuous thrust you're doing much shorter bursts. This would get the weight down, but I think you'd still need some expansive wing structure to allow for gliding whilst your 'tanks' are being refilled.

[darthgently]

I'm not recommending the books as I found them overly contrived, unnecessary r*pe scenes, and oddball character arcs, but John Varley's  Gaea series, Titan, Wizard, and Demon, featured a genetically engineered "air shark bird" that had jet propulsion.  I can't recall the details of their anatomy or if rotating components were involved.  The gravity was lower than earth's as I recall so it was a bit more plausible.  I remember these creatures because, for me, it was the only thing memorable about the story

Edited September 4 by darthgently

[Vanamonde]

Why would they store air instead of venting it right away? Storing it would increase the mass they have to move and likely increase the size of the animal and therefore increase drag. 

[magnemoe]

14 minutes ago, Vanamonde said:

Why would they store air instead of venting it right away? Storing it would increase the mass they have to move and likely increase the size of the animal and therefore increase drag. 

I say going for something like an pulls jet perhaps the U tube rather than the V1 closing panels would be the best bet. Also have wings for flying and use the jet as fighters use afterburners, questionable for takeoff unless you store air? Else its used for intercepts there they grab or do damage in an flyby attack against larger enemies, as you already have something burning an fire attack like an dragon might also work well but you probably want to eject it more like an peeing with an flame thrower than breathing fire flying fast as you will fly trough your own fireball. 
 

[Terwin]

2 hours ago, magnemoe said:

I say going for something like an pulls jet perhaps the U tube rather than the V1 closing panels would be the best bet. Also have wings for flying and use the jet as fighters use afterburners, questionable for takeoff unless you store air? Else its used for intercepts there they grab or do damage in an flyby attack against larger enemies, as you already have something burning an fire attack like an dragon might also work well but you probably want to eject it more like an peeing with an flame thrower than breathing fire flying fast as you will fly trough your own fireball. 
 

You would need a reason that the much more flexible approach of storing that energy as gravitational potential(ie fly high and dive, like predatory birds on earth)  is not a viable option.

 

If you want a continuous flow(as opposed to a 'fart bird'), you will need continuous compression and combustion.  Those are not things that biological animals can manage very well.

How rich of a food source would the animal need to be able to afford the energy to fly?

 

A small jet(Eclipse 500) has a dry weight of 3,550 lb (1,610 kg) and consumes 130-220 gallons of jet fuel per hour of flight.   130-220 * 6.8 lbs/gallon = 884-1496  This is just under one quarter to just under one half of the dry-weight.  Gasoline which is less energy-dense than jet fuel, has ~5100 calories per gallon.

 

This suggests that a 2-lb jet-bird would need to eat an amount similar to the daily diet of Olympic swimmer Michael Phelps(10K calories) for every 2 hours of flight.

Assuming of course that there are no efficiency losses when reduced to such a small size, and that the conversion from calories eaten to combustion fluid is also free.

Jet propulsion is just too inefficient for biological animals that do not gargle with petrol.

[kerbiloid]

Also, Ph. J. Farmer's The Wind Whales of Ishmael.

[darthgently]

25 minutes ago, kerbiloid said:

Also, Ph. J. Farmer's The Wind Whales of Ishmael.

I like the Ph. J. instead of P. J. as it implies he almost completed a doctorate or something.  And he deserves that honor

[Kimera Industries]

This would only work on a planet with a dense atmosphere, high oxygen, and low gravity. Suppose this jet-bird can process a fuel from its food, like methane, to ignite in the oxygen-rich atmosphere for thrust. It could use its mouth as an air intake, with muscles to close off either the area leading to the thrust generation chamber (more on that in a second) or to the digestive system. The respiratory system could take a bit of air from the intake as it flies. Once it has processed enough fuel, which it stores in a gland, it can fly. Air from the intake is shoved into a small chamber which accelerates the air backwards. I can think of a few ways of doing this:

• Peristalsis (rapidly undulating muscles forcing the air in one direction)
• Flaps, kind of like wings on the inside of the creature, flapping the air.
• Gulping air in bursts, then compressing it with muscles and releasing it. This means there isn't a continuous stream, but is probably more energy efficient.

No propellers, though, for the same reason Vsauce explained why animals don't have wheels. (At least I think it was Vsauce.)

Anyway, through one of these methods or a combination of them, the jet-bird has accelerated air in a retrograde direction. This would result in thrust, but with a poor Isp. It sprays fuel (maybe through a gland) and ignites it with metal collected and concentrated in exposed lumps in the exhaust region. It can move these lumps with dedicated muscles, and makes a spark by striking them very fast. The exhaust nozzle is protected from the heat by tough, hardened skin, possibly bone, and also with un-ignited air or non-compressed air flowing around the sides similar to a turbofan. 

Why the jet-bird would need this adaptation, though, is a bit harder to guess. It probably flies like normal birds, but uses its jet engine for bursts of high speed, to gain altitude, escape predators, or catch prey. 

[darthgently]

1 hour ago, Kimera Industries said:

No propellers, though, for the same reason Vsauce explained why animals don't have wheels. (At least I think it was Vsauce.)

There are rotating flagella in many microorganisms, the exception to the rule.

 

[Kimera Industries]

5 minutes ago, darthgently said:

There are rotating flagella in many microorganisms, the exception to the rule.

As soon as you try scaling it up, though, it gets a lot harder. 

[darthgently]

50 minutes ago, Kimera Industries said:

As soon as you try scaling it up, though, it gets a lot harder. 

Maybe.  If the separate parts are separate organisms in a symbiotic relationship maybe not impossible. Or the rotating part could be a grown and shed, but mechanically retained, dead structure, like a shed deer antler is dead.  Very hard to imagine an evolutionary path, but perhaps a genetically engineered one is possible.  Who knows?

Edited September 5 by darthgently

[Kimera Industries]

21 hours ago, darthgently said:

Or the rotating part could be a grown and shed, but mechanically retained, dead structure, like a shed deer antler is dead.

I think a symbiotic organism would be more likely and easier to maintain, because replacing a dead part sounds quite complicated. I guess it could be done, but it's harder than other options that do similar things, which is why evolution hasn't gone down that path. Given enough time, maybe it will. Who knows.

[Superfluous J]

You can address the weight problem by lowering the gravity of the planet on which the creature evolved, down to 0 if you do something like Larry Niven's Smoke Ring.

A lifetime ago I wrote a story about a pair of worlds that are most easily described as Rask and Rusk but with an atmosphere, and on it I had flying creatures that didn't use jet propulsion (Frankly I feel the work involved in getting it to work would be better done having the creatures just have better regular wings) but did breathe in and out through different holes, and while flying they were just constantly breathing.

[Kimera Industries]

Here's another idea: taking jet propulsion to mean "shooting a fluid backwards for thrust" (which is really what it is) a jet-bird could store a flammable substance, like an organic solid rocket fuel, and ignite it when needed, much simplifying the process. It would mean only occasional bursts of speed, but in a microgravity environment (like the Smoke Ring) and a low-drag shape, it could keep the creature coasting for quite a while.

Edited September 6 by Kimera Industries

[PakledHostage]

Maybe hypergolics, as evolved in the Bombardier Beetle, but on a larger scale? Rocket propulsion more than jet propulsion?

[Spacescifi]

On 9/4/2024 at 9:53 AM, Vanamonde said:

Why would they store air instead of venting it right away? Storing it would increase the mass they have to move and likely increase the size of the animal and therefore increase drag. 

 

That is a valid point.

Possibilities: Smaller creature, likely winged for steering/gliding, whereas thrust is for takeoff. As for air storage, maybe a more exteme version of what dolphins do?

Storing oxygen in their muscles through a far greater amount of myoglobin than normal terrestrial animals have.

Then releasing it as a gas for thrust on demand.

[kerbiloid]

A hybrid scheme.

Spoiler

 

[Kimera Industries]

Wayne Barlowe's speculative biology book "Expedition" features a bird like creature that produces methane gas and combusts it for propulsion.

[Nuke]

im thinking some kind of pulse jet. you have bombardier beetles that produce hypergolic fuels. you can contain the explosion in kind of an inverted shell. perhaps excrete fluid into the chamber through pores in the shell wall that keep it from overheating via evaperative cooling and also to expand as steam. shoot in the hypergolics from other pores in the shell. the creature would use it to launch itself in the air and would use short bursts of thrust in addition to flapping and catching thermals to maintain altitude. making large quantities of the hyperbolic chemicals might be metabolically unsustainable unless they eat constantly sort of like humming birds. so they might prey on another flying creature and use the energy to gain more altitude and extend their glide. if prey is plentiful they could stay airborne indefinitely.

[Terwin]

Jet propulsion:

Simple, robust, easy to stumble upon(anything with any sort of moveable mantle or inflatable bladder can empty that bladder for thrust), low energy cost

real life examples: squid, many microorganisms, all finned animals(depending on your definition)

 

Rocket/explosive propulsion:

Simple, robust, very energy expensive, can be short bursts or extended, can be very damaging without proper preparations

real life examples: bombardier beetles

 

Jet Engine

very complex and delicate to air-flow disruptions, primarily extended uses (can take several seconds just to stabilize the air flows), more efficient than rockets but still very energy intensive, must be maintained long enough to destroy most biological materials before it becomes useful for propulsion. 

real life examples: None

 

While it is somewhat surprising that we have critter(s) that use rocket propulsion, jet engines require very smooth and consistent airflows for extended periods to be any better than rocket propulsion, and biological organisms just do not have the energy output to manage that.(assuming you do not get injury or inflammation near the path of the air-flow, which would make your jet non-functional)

 

TLDR;

No biological jet engine without both genetic and cybernetic engineering.

Jet/rocket propulsion, sure(see squids and bombardier beetles), but not jet engines.  Too fragile and complicated.

 

Edited September 13 by Terwin

[Kimera Industries]

4 hours ago, Terwin said:

Too fragile and complicated.

Ramjets are conceptually simple. You could use rockets to get up to speed and then switch on your biological ramjet. As far as no biological materials being able to handle it, a creature could evolve to eat lots of metal or something hard and durable, but non-organic, and build a thrust chamber out of it. Their outer surface could be made similarly, and all your squishy organic components would then be protected inside a shell, like real life airplanes.

4 hours ago, Terwin said:

No biological jet engine without both genetic and cybernetic engineering.

I think anything can evolve into anything, given the proper motivations and resources. Granted, finding those two requirements is often really, really hard.