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Let's assume that some wealthy characters somehow find the plans for a valveless pulsejet. How is not important. Maybe there was a time travel incident, or some Atlantean book was unearthed, or someone had an intuitive leap after accidentally making a jam-jar valveless pulsejet (before it turned into hot shrapnel). So knowledge of the design is not a problem, nor is budget.

What is the earliest time period when artisans could conceivably have built a workable valveless pulsejet? The main consideration is for metallurgy and metal quality, though feel free to point out more limitations.

The goal is to have something powerful and light enough for heavier-than-air craft (the aircraft itself being out of scope here - see it more as a measure for the desired performance).

Pulsejets can run quite hot, so if some cooling system becomes necessary, it is fine as long as it can be reasonably built at the time and doesn't make the whole thing too heavy to work.

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    $\begingroup$ I'm not sure what is going to be the limiting factor, the metallurgy or the fuel. We could build quite intricate (for the time) metal objects like steam powered trains before liquid/gas fuels started taking over. $\endgroup$ – Demigan Mar 19 at 13:10
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    $\begingroup$ By the mid-1700 they could make fairly consistent small batches of reasonably good steel, at steep price. By the early 1800s they could make fairly consistent medium-sized batches of reasonable quality steel, at a price. By the mid 1800s they could make reasonably pure (albeit extremely expensive) aluminium. I have no idea what you mean by "too heavy to work"; you did not specify any application. $\endgroup$ – AlexP Mar 19 at 13:26
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    $\begingroup$ @Demigan It seems that pulsejet can run on crude fuel, like vegetable oil or even powdered coal. Not necessarily cheap at the time, but fuel should be available, which is why I suspect metallurgy is the limiting factor. $\endgroup$ – Eth Mar 19 at 15:53
  • $\begingroup$ @AlexP Oops, I forgot that pulsejets can be used for heating purposes. Or, for that matter, for propelling a surface vehicle. I'll edit that in. $\endgroup$ – Eth Mar 19 at 16:12
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Modern builder build valveless pulse jets from materials as flimsy as EMT conduit, even from high temperature composite. These are low pressure devices; vibration is more of a limit than pressure. Further, if everything is heavy, it matters little unless you're trying to fly.

The bigger issue is fuel availability -- but if the builder knows what he's doing, a pulse jet can run on natural naphtha, which was available to the Romans, and it might be possible to make it run on olive oil (possibly mixed with naphtha for starting). Alcohol of high enough proof will also work, though its lower heat content and combustion temperature (relative to naphta or olive oil) will make it harder to get the jet to run well.

Then, of course, one would have to know what's needed -- which isn't something likely in the Bronze Age or early Iron Age, unless you have a time traveler available.

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    $\begingroup$ You can just use distilled alcohol. That should be volatile enough, and doesn't require any specific resources. All societies have produced alcohol in one way or another, and a distillery is not exactly high-tech. $\endgroup$ – cmaster Mar 20 at 10:19
  • $\begingroup$ @cmaster I didn't mention that because it was my understanding that distillation wasn't invented until medieval times. Freeze jacking is of limited used, because seasonal before refrigeration. $\endgroup$ – Zeiss Ikon Mar 20 at 11:07
  • $\begingroup$ Well, if the plans for the pulsejet are no problem, the plans for a distillery should be either ;-) $\endgroup$ – cmaster Mar 20 at 11:24
  • $\begingroup$ @cmaster A quick online search reveals that the ancient Greeks had mastered alcohol distillation - of course, usual caveats for "quick online search" apply! And later, anyone that could come up with Greek fire would have found something. $\endgroup$ – Eth Mar 20 at 15:03
  • $\begingroup$ Okay, I yield -- alcohol added. $\endgroup$ – Zeiss Ikon Mar 20 at 15:05
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The Romans could probably build one if the plans were complete enough

Relevant questions (that I'm very familiar with because I answered the questions):

Technological development is a pyramid: a specific technology stands atop a mountain of experience, education, and invention. The problem is that the plans you're sending back likely don't include plans for manufacturing facilities, chemical processing, specialized tools...

As an example, I could hand you a schematic for a 1980s computer, but if you didn't know how to build the manufacturing plant for integrated circuits, or the manufacturing plant for silicon slugs to make silicon wafers, or the understanding of what the symbol for a transistor even meant.... (And let's not talk about glass processing and high-voltage controls for the monitor.) There is a massive amount of secondary and tertiary information that isn't listed in any schematic or set of plans.

A silly but obvious example: the plans for a bridge don't explain how to build a hammer.

Consequently, the plans must be comprehensible to the target audience. If you give the Romans (for example) a working mechanical device, they'd be able to duplicate it. They may not have the metals to make one as nice and efficient as the original, but they could duplicate the functions nonetheless. But hand them the schematic to an Intel 8080-based computer and it's just another meaningless foreign language — a mystery to build religions on.

So, how far back can you go with a pulsejet? How do you explain petroleum to people whose basic concept of oil has more to do with olives or whales? Even if you go to the Chinese, their gunpowder and canon tech is far too heavy to allow a working pulsejet to move, well, a canon.

Conclusion

Like @Demigan, I believe the problem is fuel (although metallurgy shouldn't be ignored, simple gasoline could be used if all you wanted to move was a go-kart, but if that go-kart weighs a half-ton it isn't moving). But for anything more complex, you're stuck with developing big-bang chemistry. Even with those instructions, you'd have to give them the instructions for extracting petroleum and refining it or complex chemical processing. I doubt you could build effective pulsejets much more than 50 years ahead of time simply due to the other dependencies on technologies that simply didn't exist yet.

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    $\begingroup$ Pulsejets can run on vegetable oil or coal powder, so fuel should be available to the Romans. But would Roman bronze or iron be strong enough? $\endgroup$ – Eth Mar 19 at 16:22
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    $\begingroup$ @Eth, you can get a plane in the air using a pulsejet running on vegetable oil or coal powder? I could easily be wrong, aerospace isn't my expertise, but I'd like a citation demonstrating that claim. $\endgroup$ – JBH Mar 19 at 16:41
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    $\begingroup$ Distilled alcohol would seem to be a better fuel than vegetable oil or coal powder: It's quite volatile, and can easily produce combustible mixtures with air. And a distillery is not exactly high-tech... $\endgroup$ – cmaster Mar 20 at 10:15
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    $\begingroup$ From a manufacturing standpoint, a valveless pulsejet is extremely simple to build: it's just a correctly-shaped bent pipe. The hard part is working out the theory that lets you design one; I suspect even an ancient Hittite bronze-smith would be able to build one from suitable plans. $\endgroup$ – Mark Mar 22 at 1:45
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    $\begingroup$ @JBH, make a lot of noise? A pulsejet isn't much use without either low-friction bearings or lightweight wings, but that doesn't mean you couldn't make one. $\endgroup$ – Mark Mar 25 at 19:56
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Hero of Alexandria was doing some remarkable things in the first century AD. He made a primitive jet engine (it wasn't capable of flight just spinning and making a racket)

enter image description here https://upload.wikimedia.org/wikipedia/commons/b/b8/Aeolipile_illustration.png

This time period is also when distillation started to seriously take off. The creation of high volatility fuels either petroleum or alcohol based would have been well within the reach of the early Roman empire.

Copper while not ideal can easily be worked into thin sheets and beaten into complex shapes over wooden forms which are then removed.

Theoretically, the Romans could have used primitive V1 buzz bombs (without warheads) at the siege of Masada.

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  • $\begingroup$ Would pure copper be strong enough for this? Does it mean the ancient Egyptians could have made it? (Though they may not have available fuel...) $\endgroup$ – Eth Mar 20 at 15:19
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    $\begingroup$ (1) It was much more like a steam turbine than a jet engine, with ridiculously low efficiency. (2) Crude petroleum is no way shape or form a "high volatility" fuel; it is actually quite hard to ignite. (3) I would not want to be near a pressure vessel made of thin sheets of copper. (4) The Romans had no idea of aerodynamics, and no idea of dynamics in general... $\endgroup$ – AlexP Mar 20 at 17:18
  • $\begingroup$ @AlexP 1) true. 2) I mentioned distillation as an available tech, which should allow for extraction of lighter alkanes from naphtha 3) pulse jets are relatively low pressure 4) aerodynamics were explicitly excluded in the question. $\endgroup$ – MongoTheGeek Mar 21 at 1:35
  • $\begingroup$ Roman bronze and steel were both very high quality. They may have been an ancient civilization, but their metallurgy techniques were on part with the industrial revolution era. $\endgroup$ – Nosajimiki Apr 25 at 16:27
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Recreating the pulse jet itself could have probably been been done as early as the Bronze Age. Bronze is about as heavy as steel, and is not as hard making it worse for making most tools and bladed weapons, but it is actually very good for its weight at resisting explosive forces, meaning it could be used to make a fairly light weight pulsejet engine.

Fueling it is another problem. The ancient Mesopotamians knew about and harvested petroleum oil since ~6000BCE, but its potential as an incendiary agent did not become known until the Chinese started burning it for light and heat around 0 CE, and advanced refining and manipulation took until ~700 CE when the Byzantines discovered Greek Fire. So whether your instructions include information on how to fuel it could make a huge difference.

The other big caveat here is that your ancient civilization would not go straight from pulsejet to plane. They did not know enough about lift, lightweight construction, etc. Instead, they'd need to see a similar pattern of evolution as the steam engine. It's likely to assume that one person would 1st use it to turn a mill, then the next guy would make a speed boat, the next would find a way to better contain and control the energy with an internal combustion, so on and so forth it would transform their society. Then after about 200 years of refining this tech, some guy would realize that engines are finely strong enough to fly with, if he could just figure out how birds do it; so, like the wright brothers, he or they would study birds much more obsessive that a world without engines would warrant until he cracks the lift problem, and makes his first plane.

This means you need to target a culture similar to the renaissance where a large portion of the population was big into science and experimentation for 2 centuries or longer to make this possible. If you add instructions for making fuel, giving this to ancient Greeks in 500 BCE could result in flight by ~300 BCE. If you don't include instructions for fuel, the Byzantine Empire could probably receive it in 700 CE and have planes by 900 CE.

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I'm not going to discuss fuel, since you seem to be sure enough about it beeing a non-issue.

Since metallurgy seems to be the main concern I can tell you exactly where, but not precisely when.

You want to go to southern India

Rome has been mentioned in other answers but I don't get why. After all, the Romans did import Indian steel for is superior quality. This Wikipedia artice [1] discusses the history of steel production in Inda. Artisans capable of producing the steels you want could be around as early as 300 BCE or 200 CE as Wootz steel, better known as Damascus steel was developed at that time. You project would find a fertile and innovative environment if timed right. While I'm not sure if Wootz steel would suffice for your purposes, the skilled artisans of what would be and already was the industrial heartland of the world stand the best chances of figuring the engine out.

[1] https://en.m.wikipedia.org/wiki/Iron_and_steel_industry_in_India/ (History Section)

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  • $\begingroup$ Overkill. You can make valveless pulsejets out of sheet metal, so there's no particular need for high-quality metal except to make it lighter. $\endgroup$ – Keith Morrison Apr 26 at 3:43

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