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I was reading more Laundry stories and just went through Equoid. The humorous "Procurement Specification" got me thinking. In a rational SF approach (as opposed to magic), imagine a civilization developing biological based technology. They don't build machines like we do, and never thought of a horseless carriage in the same terms we do; rather, designed an organism to carry loads.

Eventually they want to make powered flight. Given that some animals in the natural world do in fact fly, it was obvious to produce man-rated mounts and device cargo carrying solutions.

Given eventual technological advance, they can shape their planet's living things to their every will. Meanwhile, something like internal combustion engine is never invented.

How would they "make" a jet propelled aircraft?

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  • $\begingroup$ Seemingly related: Is a jet dragon possible?, Is a rocket dragon possible?, Jet Powered Animals, ... $\endgroup$ – a CVn Feb 10 '16 at 20:21
  • $\begingroup$ What about remote controlled rabbit-copters? $\endgroup$ – Xandar The Zenon Feb 10 '16 at 20:42
  • $\begingroup$ Probably not. Jet propulsion in the animal kingdom consists of filling sacks with the working medium (water) and forcefully ejecting it. This works as a "one of" escape mechanism but not for continuous jet thrust. "Jet engines" as used by human require rotating machinery (compressors & turbines). I've never seen anything in the animal kingdom that could do that. $\endgroup$ – Jim2B Feb 10 '16 at 20:52
  • $\begingroup$ They bred a horse that's prone to extreme flatulence. With the addition of a lit torch they were able to achieve short hops, but sadly despite experimenting with different feeds haven't been able to keep the flow going long enough for sustained flight. $\endgroup$ – AndyD273 Feb 11 '16 at 18:43
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My answer is going to go well beyond what we can ever expect from reality, so I hope your definition of "rational SF" is fairly liberal.

This is going to be a major undertaking in genetic engineering, probably requiring several generations. As you suggest, we will start with equine DNA- horses and mules- in order to get the load-carrying ability. We'll also keep those horse traits that make them good human companions.

Next, we mix in some cephalopod DNA- squids and nautilus- to get jet propulsion, or better yet salps. We will likely have to tinker around with this a lot to convert the jet-pumping organs from aquatic to aerial, but we have lots of time and research grants, right?

Ah, but we forgot the wings. Finally, let's find some amber-encased mosquitoes and pull in a little quetzalcoatlus DNA, so we have large wings that fold up when on land and still allow quadrapedal motion. We'll also take from quetzy the extremely beneficial bone-strength-to-weight ratio.

Notice that I excluded the use of birds. Birds tend to hop rather than walk when on land, probably not a good thing for riders or cargo. Besides, as I've stated here before, modern literature needs more quetzalcoatlus.

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Your genetic engineers are going to need to be very creative, and the horse is not going to be the basis of an organic jet engine.

In the natural world, jet propulsion does exist with squids, octopi and some other related creatures. Generally, they draw water into a sack and forcefully expel it though a siphon or similar tube. This might serve as a sort of JATO take off mechanism, but certainly not for continuing flight.

What will be needed is some sort of worm like predecessor. Air or water (for a water jet) is taken through the mouth/inlet, and would be compressed through a series of annular muscles in travelling waves through the "gut" until it has been compressed to sufficient degree, then released out the back end, which has been presumably engineered into a convergent/divergent nozzle to produce thrust.

This might work well on its own for a water jet powered boat, but compressing air enough to create usable thrust isn't going to be so easy. For a jet propelled aircraft, the creature will need some sort of mechanism to convert food into a fuel (some people's gut bacteria convert food into alcohol, so there is a precedent), and an ignition mechanism to create combustion in the "expansion chamber". An elaborate series of sphincters will have to be engineered in to meter the fuel, prevent the flame from moving up the tube to the "intake" and so on.

Essentially what you have here is a sort of organic pulse jet. Pulse jets were explored as aircraft propulsion units during WWII (especially by the Germans), but pulse jets tended to be much larger and less fuel efficient than even primitive 1940 era turbojets. On the other hand, they were much cheaper and did not require the scarce resources that turbojets did, so the were extensively used on V-1 missiles, and large numbers of cheap, semi expendable fighter designs were considered in the closing days of the war as well. The Luft 46 website (http://www.luft46.com) has lots of interesting conceptual designs to choose from.

How you work this into your jet propelled creature is up to you. Since this organic pulse jet is probably not going to have a lot of power, a single "pulse jet worm" will probably only be able to carry a small load a limited distance. You may consider something along the lines of a Remora, which can be attached in a parasitic relation to the actual flying creature. with multiple units attached for flight, and then detached and thrown in the feeding tank for "refuelling" after each use.

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