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I’m writing a novel where people from our universe move to another.

The other universe is a fantasy one with medieval technology.

These people have the help of an advanced AI that contains the history of the last universe down to the specifics.

Now I was wondering, when these people inevitably develop aircraft, would it be possible for them to skip piston prop engines in favor of jets?

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    $\begingroup$ This is like asking whether Edison's D/C or Tesla's A/C is easier to produce. $\endgroup$ – user6760 Feb 12 at 13:02
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    $\begingroup$ my guess would be: neither. they would go electric right from the start. $\endgroup$ – Burki Feb 13 at 10:11
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    $\begingroup$ Have you considered electrical propulsion in the form of batteries and an electric motor? It's quite possible that we will stumble upon a easy to make and lightweight battery technology in the near future and building an electric motor is much, much easier than a turbine or something with pistons $\endgroup$ – Dragongeek Feb 13 at 10:38
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    $\begingroup$ @Dragongeek It's quite possible that we will stumble upon a easy to make and lightweight battery technology in the near future It's hypothetically possible, sure. But easy-to-make, lightweight and cheap - let's just say there's enough people have been looking at this for a long time that it's pretty unlikely. And that's with our levels of manufacturing, which would not be available with low-precision technology. $\endgroup$ – Graham Feb 13 at 11:58
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    $\begingroup$ @user72381 Regarding your book, I seriously suggest reading Charles Stross's Merchant Princes series for how things can go right (and wrong!) with technology transfer and bootstrapping. $\endgroup$ – Graham Feb 13 at 12:00
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If you already know how, and you just need to do fabrication from scratch: jet engines, no question.

Yes, building proper turbine blades is a bigger technological problem than building pistons, cylinders, and crankshafts... but jet engines don't need turbine blades.

Pulse jets are just about the simplest form of aircraft propulsion you could possibly devise. You can build one out of a single piece of sheet metal and a spark plug, or a single bent pipe, with zero moving parts. If you want slightly better reliability, you can build a more complicated one with some slightly more involved interior geometry and... one moving part.

Now, pulse jets have a lot of problems. They aren't the most efficient, they tend to get really hot, and they are really loud. There are good reasons we don't use them in manned aircraft in the real world. But if you just want to build the first thing you possibly can to get in the air, as quickly as possible, with a minimum of prerequisite infrastructure... a pulse jet engine is it.

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    $\begingroup$ ahhh the good ol' V1 Sounds en.wikipedia.org/wiki/V-1_flying_bomb $\endgroup$ – Serverfrog Feb 13 at 8:53
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    $\begingroup$ I like this answer. Yes, if a Victorian engineer had had the pulsejet idea, we'd have had powered flight a long time before the Wright brothers. $\endgroup$ – nigel222 Feb 13 at 9:42
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    $\begingroup$ Another big positive I just thought of, they don't need any advanced fuel distillation technology. I imagine a pulsejet can run on just about any crude hydrocarbon or (distilled) alcohol more or less interchangeably. In fact ... Rudolph Diesel patented a diesel that ran on pulverized coal. Coal dust powered pulsejet, anybody? $\endgroup$ – nigel222 Feb 13 at 10:01
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    $\begingroup$ +1 for pulse jet. However, it this is just about flying a bit, rocket engines might be even simpler. Fireworks have been using black powder to build solid fuel rocket engines for centuries. $\endgroup$ – Pere Feb 13 at 13:46
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With the all knowing AI at hand, anything would be possible, so you could handwave this as you see fit, surely?

The issues are probably ones of fabrication... people have made piston engines for centuries, because the key elements that need to be made with the highest precision are things that can be handily fabricated on a lathe, and lathes are the easiest precision machine tool to construct.

Turbine blades on the other hand are not nearly so simple, and tend to have inconvenient geometries and be made out of more inconvenient materials that need complex precision grinding tools. The earliest "modern" gas turbine designs dated back to 1791, but the problem was that engineering and manufacturing technology just wasn't up to the job of making a practical working device that for over a hundred years after that, and it was nearly another fifty years before one could fly. Compare that to the first use of a steam-driven propeller aircraft (a dirigible) in 1852, and a piston-engine-driven propeller aircraft (in 1903), with no shortage of commercial, military and academic interest in that time.

That's not to say that you couldn't do it. You don't need modern computer-driven machine tools to make turbines after all, but building up the necessary industrial base, high quality metallurgy, precision machine tools and competent operators and engineers is non-trivial, even if you know exactly what you want, and what you need, and have all the educational material required.

This is the problem that all time-travellers face, see many previous questions on this site on this very subject!

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    $\begingroup$ @user72381 there are lots of them, too many for me to remember off the top of my head. Browsing through older questions tagged time-travel is perhaps a good start. There's at least one good question about printing presses worth reading, which details one example of the whole social background to technological change. I'll see if I can remember any others (but printing presses often appear as "magic bullets" in time travelling questions). $\endgroup$ – Starfish Prime Feb 12 at 13:59
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    $\begingroup$ Actually, you can quite easily make a centrifugal turbine engine with just the precision needed to make a car turbocharger. The original Whittle turbines didn't need a lot of precision engineering but they kept failing due to metal fatigue. It was more the development of high temperature alloys that made jet engines practical $\endgroup$ – slebetman Feb 12 at 23:53
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    $\begingroup$ Here's someone who actually built a jet turbine with a car turbocharger: youtube.com/watch?v=83jH0sVCM9Q $\endgroup$ – slebetman Feb 13 at 0:48
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    $\begingroup$ @slebetman: JUST the precision needed to make a car turbocharger? That's good for a laugh. $\endgroup$ – jamesqf Feb 13 at 7:06
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    $\begingroup$ @Hobbamok A lot of the early RC scale turbojet engines was handmade with steel tubing and welded flat plate steel as the compressor impeller. The original RC size jet engine actually used plywood compressor blades but it had a maximum RPM before the compressor disintegrates. Jet engines can be handmade. The quality would not be good but it would work. Frank Whittle did not have computers or CNC when he invented aviation grade jet engines $\endgroup$ – slebetman Feb 14 at 21:09
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As noted in another answer, fabrication is the key. Piston driven steam engines predated working steam turbines by more than two centuries, because any competent blacksmith can make a simple piston and crank (and a rather skilled one might be able to build a multi-cylinder version), and machine tools for this kind of work existed by the late 18th century -- where turbine blades and the wheels that carry them are completely beyond hand forging and are a challenge for manual machining processes.

Now, step up to internal combustion. The crank, piston(s) and cylinder(s) of a steam engine are just waiting to be converted, as the same parts can both compress and expand the working fluid; only a slightly more complicated valve drive is required -- but for a turbine, there must be a separate compressor stage (even if mounted on the same shaft, as with simple early turbojets), and those blades require significantly different design than the ones in the hot section.

Presuming your crosstime colonists start by building machine tools (lathe and shaper, at the minimum), they can build piston engines starting the day after the machine tools are finished and their cutting bits made. Even with detailed design, they'll be decades developing the material infrastructure to support the metallurgy for a 1940s technology turbojet -- and still longer making one that's as efficient as even a 1970s turbofan (which requires still a third set of blades with a third design).

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  • $\begingroup$ Do they need to invent welding? $\endgroup$ – In the name of the story Feb 12 at 14:48
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    $\begingroup$ @user72381: Welding was known since the antiquity, at least since the 5th century BCE. Compared with the advanced metallurgy and tricky casting and machining required to make gas turbine blades, welding is children's play. $\endgroup$ – AlexP Feb 12 at 14:52
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    $\begingroup$ @user72381 Not necessarily. You can build a piston engine without a single weld -- even if you can't machine the crank in one piece, it can be press fit or soldered/brazed together, but welding is very easy; it was done in forges from the beginning of the Iron Age, and with torches as early as the 18th century (it's a little tricky to get welding heat with an alcohol or naptha lamp, but it can be done, especially with the latter, without oxygen, just air to blow into the flame). $\endgroup$ – Zeiss Ikon Feb 12 at 14:53
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    $\begingroup$ @ZeissIkon: Crankshafts were generally cast as a single piece in ye olde goode times, and in more modern times they are generally forged as a single piece (or machined as a single piece out of billets for very expensive crankshafts). (Here is a small one fresh from the forge.) While multi-piece welded crankshafts are possible they are not at all common. $\endgroup$ – AlexP Feb 12 at 15:05
  • $\begingroup$ Pressed together or brazed crankshafts are fairly common in engine models (running IC engines built by a home shop machinist) but, of course, those don't run at high power or speed. Especially a brazed crank can work in the lower end of piston aircraft engine power, though -- say, similar to the VW conversions, 30-70 HP or so -- because of the low RPM common to aircraft engines. Not common since WWII, because machines existed to do it the better way -- but it would work. $\endgroup$ – Zeiss Ikon Feb 12 at 15:24
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No. In addition to all the difficulties involved in fabricating the engine itself, which the other answers address, piston engines and jet engines serve different purposes. Basically, jet engines are for when you want to go fast, piston engines and propellors are for going slow*.

The question you don't ask is exactly why your medieval culture is "inevitably" developing airplanes. If they are planning on developing commercial air travel**, then they need to build not only the engines, but the whole infrastructure. They need aluminum and other lightweight alloys to build pressurized airframes, miles-long paved runways, radio communications and navigation, instrument landing systems for inclement weather, and much more.

On the other hand, if they just want a few airplanes for other purposes, given a halfway decent piston engine, you can build a decent airframe from wood and fabric. That plane can be built to take off and land on a few hundred yards/meters of reasonably-smooth grass or dirt, doesn't really need radios, and you just don't fly if you can't see. Even if you are intent on replicating the modern world, there are many practical applications for which jet engines simply don't work well. Visit any small airport (or even a medium-sized one) and see how many small planes there are with propellors on their noses.

PS: If you want jet engines powering commercial aircraft, we really need to think about developing the supply chains. I mentioned the airframe problems, but have you really thought about what goes into an efficient, modern jet engine? For the turbine blades, you have to figure out how to mine & refine things like nickel,chromium, cobalt, and rhenium: https://en.wikipedia.org/wiki/Turbine_blade#Materials Then for your hot section, you want things like titanium alloys: https://aviation.stackexchange.com/questions/25645/what-material-is-used-to-make-the-hot-sections-of-jet-engines Refining titanium is not an easy task: https://www.essentialchemicalindustry.org/metals/titanium.html

Then to fuel your airplanes, you really need to develop the whole fossil fuel industry. Once you've done that, you start putting megatons of CO2 into the atmosphere, which means that in a couple of centuries your new world has serious problems with global warming...

*I'll skip over turboprops, which basically use a jet engine to drive a propellor. But compare the cost of a present day small turboprop with an equivalent piston-engined plane, and you'll see why smaller ones are fairly uncommon.

**This would mean that, given that they have the AI with all this world's history, there's a strong presumption that these people are unable to learn from experience.

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    $\begingroup$ What would they develop instead of commercial air travel? $\endgroup$ – Matthew Feb 12 at 19:34
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    $\begingroup$ @Matthew, high-speed trains. If you don't need to deal with existing landowners, trains are cheaper and higher-capacity than airplanes, and fast enough for most purposes. $\endgroup$ – Mark Feb 12 at 21:43
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    $\begingroup$ @Mark, what happens when you need to cross an ocean? Or travel a long distance? Jets are about 3½ times faster than trains. Trains may be better for short distances, but I'm less convinced they can totally replace air travel. $\endgroup$ – Matthew Feb 12 at 21:48
  • $\begingroup$ To make a nation join your fold you have to intimidate them with a show of force. Imagine a massive Antonov landing in some medieval kingdom(with a runway of course). $\endgroup$ – In the name of the story Feb 12 at 22:55
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    $\begingroup$ @user72381 the pulsejet (different answer) would have some serious advantages for scaring the s**t out of mediaeval types! $\endgroup$ – nigel222 Feb 13 at 9:48
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As they are now, piston prop engines are much easier to construct and get working properly than jet engines. They are enclosed systems with very sturdy components in their most basic forms. Without prior in depth knowledge about how propellers work, inventing a jet engine from scratch would be extremely challenging. However you state you do have prior knowledge about this technology, so this would definitely not be impossible.

If you do have the full technical specifics of engine technology before developing it, it is a simple question of what you want to build. Obviously it would be better to skip inferior technologies in favour of more advanced ones, if this is an option. Even if it is harder to build these engines, it might be easier to spend time developing the base manufacturing processes a little longer so a step can be skipped, rather than develop a whole technology you know will be redundant as soon as you finish it.

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    $\begingroup$ Note that jet engines are not inherently superior to piston engines. At low speeds, pistons win, so if you're building a crop duster, an STOL bush plane, or something else that benefits from flying slow, the extra effort of making a jet engine is counterproductive. $\endgroup$ – Mark Feb 12 at 21:38
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    $\begingroup$ @Mark: Strictly speaking, it's the propellor that's best for low speeds, so you can have propellor planes driven by gas turbines ("turboprops"). Turboprops are considerably more expensive, though, which speaks to the difficulty of construction. $\endgroup$ – jamesqf Feb 13 at 2:28
  • $\begingroup$ I wouldn't agree that the piston engine is redundant. It has its niche: Intermediate power per unit of weight from liquid fuel. It's no accident that both cars and ships still use piston engines even though a turbine could be smaller, lighter, and more efficient. No need to use fancy high-tech when you can get the same power output for much less fabrication cost. Turbines, though being an established technology, are still only being used where either weight (aircraft) or efficiency (gas turbine power plant) are paramount. $\endgroup$ – cmaster - reinstate monica Feb 14 at 7:54
  • $\begingroup$ @jamesqf, one thing to consider is that turbines have a narrower efficiency band than piston engines. $\endgroup$ – raubvogel Feb 14 at 20:10
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Theoretically, a turbine is mechanically simpler than a piston engine, and the principle could be understood by anyone familiar with a windmill. Leonardo da Vinci's "roasting jack" could be understood as a simple turbine devised to harvest the heat energy of the fire to perform mechanical work.

enter image description here

Leonardo's roasting jack

However, the real stopping point is the supporting technologies that would allow the building of usable turbines, as many other posters have already noted. A gas turbine was described in 1791 and a working model actually built in 1903, and the pulse jet was also described in principle in 1867 and an early form built in 1906, but it took until the late 1930's before other technologies like machining, high temperature alloys and so on made these devices at all practical.

In order to move directly to turbine powered machinery, the use of rotating machinery, including water wheels and turbines, as well as steam turbines, will have to be "forced" so developers will continue working on these devices and a large enough pool of engineers, mechanics, metalworkers and so on are created to support the continuing development of turbine devices at the expense of piston devices. This would also extend to the cutting of gears and development of efficient transmissions, since turbine devices for stationary and other mobile applications generally turn far too fast to be harnessed directly to output shafts.

So, assuming the AI can provide the "steps" to instruct the civilization and the idea is planted and supported early enough, it may be possible to develop turbine engines early on and have them used to power aircraft.

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A critical design difference is that a piston engine works with a discrete cycle that can take any amount of time to complete (within reason) but a jet engine operates continuously.

If you look at the early history of the steam engine, you can make a useful practical device that only operates at one "rev per minute". Of course with an external combustion engine, the fuel is burned continuously, but you can stop and wait as long as you like until you have enough steam pressure to drive the engine through one cycle.

There was a long time interval between making an internal combustion engine and the original steam engines.

A steam turbine is a logical development of a steam piston engine, with external combustion to produce a continuous supply of high pressure steam.

To build a jet engine, you need both turbine technology and the internal combustion technology, plus a compressor that can get enough air into the combustor to produce a useful amount of power.

So to start by designing a jet engine, you would have to create several novel pieces of technology and get them all to work together, all at once. If your world had access to "the history of the last universe" and the ability to understand that history, they would probably realize themselves that progressing one step at a time was a better option.

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There is one type of jet engine that is easier to build than a piston engine and in an alt-history or alt-universe setting could be fun to contemplate. It is the engine used by the the original cruise missile - the V1 flying bomb: a pulsejet.

Pulsejets

Starting one is not easy but you can easily learn the skill with practice. People have built pulsejets in their back yards with just good welding skills and some metal pipes. Google "pulsejet" and search for it on youtube to see some examples of homemade pulsejets.

Pulsejets can generally be divided into two types: valved and valveless. The V1 pulsejet (and the type commonly used in RC airplanes) are valved. A lot of homemade pulsejets are valveless. The advantage of a valved pulsejet is for the same amount of power you can build a smaller/lighter engine. The advantage of a valveless pulsejet is there is no mechanical part to wear down.

Here's an example of a homemade pulsejet (valveless):

https://www.youtube.com/watch?v=zsXWspo5hrc

And here's a commercial pulsejet (valved):

https://www.youtube.com/watch?v=_h6D47Lkfcw

A side note: Pulsejets are very loud and gives you a stem punk vibe. They make a distinctive buzzing sound (a bit like an overworked truck engine) which gave the V1 the nickname buzz-bomb.


Turbojets

Turbojets are a bit more complicated to build but they don't have to be overly engineered like commercial or military jet engines. Early turbojets used centrifugal compressors and were relatively simple machines.

The main issue with early turbojets was that the metallurgy of the time was not up to scratch. For example, the German jet engines of WW2 required a total overhaul after just 50 hours of operation. But if you are willing to do extreme maintenance then you can do it (even if badly). Modern jet engines use fancy alloys.

As a bonus, here's the same "home made pulsejet" guy above building a "home made" (albeit using a commercial car turbocharger) turbojet:

https://www.youtube.com/watch?v=83jH0sVCM9Q

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Yes!

You can build just anything. You do not need much to start.

You need only 3 things

You have an advanced AI. You can build just anything:
Things that you know. Things that you can imagine.
Things that you can not imagine.

But to build something, you need these three:

The physical elements as raw material

Any other raw material substance can be made by chemical processing.
For good jet engines, tungsten is useful, so the blades do not melt.
But use iridium, if you can afford it.

Energy

Whatever you want to process to create useful substances, and whatever you do with them, you need at least some energy. And if you have lots of energy, you can take great shortcuts. Ask your AI for instructions. Solar cells are hard to build, use water power to start.

Time

When you need to build anything "from scratch", you may get ideas from your AI that are possible, but time consuming.

For example you want to build a tungsten turbine blade.
You need to find tungsten ore, and create a piece of tungsten of roughly the same shape as the blade will be, a little larger.
Your AI knows the geometry, but you do not have a CNC machine.
But you can build a very simple EDM machine, and use spark erosion to shape the part.

But you can ask your AI for a suitable tungsten alloy,
help her to build a 3D printer,
let her use it to print a good 3D printer for metal,
and print the blades. That has the advantage that you can make them hollow, saving much material, and making them lighter. (In general, make good use of the AI to simulate and optimize parts.)

Oh, and make sure that your AI does not kill you,
just as we should in real live.

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