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I've seen Which is easier to develop: a piston prop engine or a jet engine? and my own Effects of ~10atm pressure on engine design neither of which answers this.

Would different atmospheric density, composition, etc. ever be able to achieve this? Are there conditions under which no practical aircraft could reach the speed at which jets are a significant advantage in the first place?

PS. And not conditions where rockets have an advantage over both.

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    $\begingroup$ Does the propeller have to be driven by a combustion engine, or can it be powered by some other energy source? $\endgroup$ – Ryan_L Apr 27 at 19:32
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    $\begingroup$ Propellers have the advantage on this planet for lower-speed applications. $\endgroup$ – user535733 Apr 28 at 1:39
  • $\begingroup$ I think it's primarily a question of super high density, which may well lead to viscosity effects that choke a jet or turbine. Consider looking into the wing-flap patterns of tiny insects, for whom our atmosphere is practically like molasses and redesign your propellor shape & aspect ratio accordingly. $\endgroup$ – Carl Witthoft Apr 28 at 15:41
  • $\begingroup$ @user535733 Yeah, but my question is "Can you have a planet where nobody is likely to bother using jets for any application?" $\endgroup$ – Tristan Klassen Apr 28 at 20:04
  • $\begingroup$ Why do propeller airliners such as the ATR 72, the Dash 8 or the Let L-410 exist? Why do propeller transport aircraft such as the Airbus A400M or the Alenia C-27J exist? There must must be something which makes them competitive with jet-propelled aircraft in their fields of application. (And what jet-propelled airliner do you know?) $\endgroup$ – AlexP Apr 28 at 20:35
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Low oxygen atmosphere or really rough terrain

1. Low oxygen atmosphere

Jet engines need at least some oxygen to operate. As the answers here illustrate, they don't need much oxygen, but there's a point where the atmosphere has so little oxygen that a jet engine can't operate efficiently. The "combustion" part of the diagram below will peter out.

Schematic of turbofan engine

On Earth, propeller-driven aircraft also operate using combustion engines, either a jet engine (turboprop) or traditional internal combustion. Obviously, they would also be affected by a lack of oxygen. You basically have two options: either bring your own oxygen (i.e. use a rocket) or find another way to power the propeller. There are projects at various stages of development to use electricity to power planes.

2. Really rough terrain

You don't see many jet aircraft in the backcountry of Alaska (though there are exceptions). If you want a reasonably priced plane that can carry passengers and freight through small dirt airstrips, you're going to want a prop. They're easier to repair, cheaper to operate, and less susceptible to foreign object debris. More broadly, you should think about the missions you want aircraft to accomplish in your world. If most aviation is regional and consists of ferrying goods and passengers over short distances in rough terrain, a prop is a natural fit. Supersonic travel across oceans would require more in-story explanation for why you're not using a jet.

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    $\begingroup$ you can add particulates, which will not have much effect on a prop, as long as the engine itself is sealed, but will destroy a jet engine. $\endgroup$ – John Apr 27 at 19:48
  • $\begingroup$ @John that's a really really good point! $\endgroup$ – Andrew Brēza Apr 27 at 20:05
  • $\begingroup$ I"m not sure this is accurate. You can easily build a jet-engine-powered aircraft with a very low stall speed. It's just not useful because the fuel cost per mile is wildly greater than that for a prop plane. $\endgroup$ – Carl Witthoft Apr 28 at 15:39
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    $\begingroup$ @CarlWitthoft I should have emphasized the "reasonably priced" part of the equation more. If you're running supplies between dirt strips, you could use a Harrier but you probably don't want to. To your second point, I did mention that you have the option to "bring your own oxygen." But just like with the jet, cost and practicality suggest a prop would be easier. $\endgroup$ – Andrew Brēza Apr 28 at 18:40
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    $\begingroup$ @TristanKlassen Earth underwent a few million years of nearly constant volcanic activity. Maybe its a fairly young planet. $\endgroup$ – TitaniumTurtle Apr 28 at 20:36
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The primary advantage of a propeller is that it is more efficient than a jet engine. Moving large masses of air slowly is simply more efficient than moving small masses of air quickly. In other applications we see helicopters are far more fuel efficient in hover than a Harrier "jump jet", or propeller driven ships more efficient than pump jets or other forms of water jet propulsion.

In fact, you might even wonder why use a jet engine at all, given these factors, but most users who have other considerations like speed, the need for a very compact powerplant, silence in operation (pump jets) and so on find these considerations have far greater priority than fuel efficiency.

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GE 36 Unducted Fan prototype

Unfortunatly I can no longer find the link outlining performance, but back in the 1990's there was experimentation with something called an "unducted fan", which was a form of turboprop which dispensed with the usual gearbox and used the turbine to drive the unducted fan blades with minimal speed reduction. While this was extremely noisy (the main reason that it was never adopted), there were great gains in fuel efficiency, and the large area that the unducted fan could "grab" when running gave the plane far superior performance in things like short field take off. The Soviet Union and now Russia have done more development work on unducted fan engines (they have far less stringent requirements for noise reduction, and most of their engines seem to be used on military transports)

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Antonov AN-70 prototype

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Progress D-27 Propfan engine

So the primary advantages of propellers is their efficiency (within the correct flight regime - no propeller can power a supersonic aircraft, for example) and greater performance in certain aspects, like short field take off. The power plant is actually irrelevant, model aircraft are powered by rubber bands, two stroke "glow plug" engines and electric motors, while most common propeller driven aircraft are powered by piston or turbine engines. A steam engine for propeller aircraft was developed in the 1930's, the Germans used diesel engines for a period during that time as well, and any other motive power source, like a Stirling engine could be used as well.

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