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My friend and I are working on building a valveless pulsejet out of sheet metal. We initially built a small one (a few feet in length) using the below dimensions: Dimensions

The issue was that we were unable to get it running without compressed air. When we had a nice stream of compressed air running in the intake, it would fire right up, but it would die when the air was removed. We hypothesized that our tolerances on the engine weren't very accurate, and so decided to make a bigger one where the tolerances wouldn't matter as much. We scaled all the dimensions by two. With the new engine, we were able to get it running with compressed air just fine like the first engine. However, we had the same issue. When the compressed air was removed the engine would die. We now had a different theory. We wondered if the fuel wasn't mixing well when it was injected, and what the compressed air was doing was mixing the fuel really well. To remedy this, we took the fuel line and crimped it at one end and drilled small holes in it along its length to try to create more of an even distribution of fuel. This also didn't work, and the engine died when the compressed air was removed.

So, I am wondering if anyone here has suggestions for what we can do to get this engine running without needing to hold a compressor to it the whole time! Below is a link to a bunch of pictures of the engine: https://imgur.com/a/iCIfnIq

Any ideas are appreciated!

EDIT: We are using propane as fuel

EDIT 2: With compressed air, we are able to get it to pulse correctly. It is not just a continuous whoosh. We watched some videos of pulsejets, and we know the correct sound.

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  • $\begingroup$ Go ahead and migrate to Engineering (I'm one of the site mods.) The community will have some additional questions to pull out the details, but this should be fine on Engineering. For example, what is being used for fuel? $\endgroup$ – user16 Aug 22 '19 at 12:00
  • $\begingroup$ We are using propane as the fuel $\endgroup$ – andycate Aug 22 '19 at 16:37
  • $\begingroup$ This is Stackexchange; please respond to requests for clarification by editing your original question. The goal is to have a nice tidy stand-alone question, with answers. $\endgroup$ – TimWescott Aug 22 '19 at 19:36
  • $\begingroup$ Please edit your question: did the original author also use propane for fuel? The use of a venturi & needle valve suggests otherwise. If they were, at what pressure were they feeding the needle valve? At what pressure are you feeding the needle valve? I would be concerned that your metering isn't good enough to maintain a flammable mixture when the air comes off. $\endgroup$ – TimWescott Aug 22 '19 at 19:42
  • $\begingroup$ When your jet runs from compressed air, is it pulsing, or is it just a really hard-to-make propane burner? If all you're getting is the typical white-noise "whoosh" of a propane burner then it's not pulsing, so when the air comes off it's not going to maintain the "suck squeeze pop phooey" cycle that it needs to be an internal combustion engine. I don't think I could describe the sound you should get -- look to YouTube videos of similar jets and see if you can find one with sound. $\endgroup$ – TimWescott Aug 22 '19 at 19:46
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AIU pulse-jets, they require forward motion to keep supplying them with air (hence the need of the V1 to have starter rockets). In your case, the compressed air is supplying a flow of air which satisfies the engine's need for air. But when you stop the compressed air, then there is no air for the engine and it stops. What you need is something to supply the forward motion, which will keep the engine alive, which will supply the thrust to keep the forward motion alive . Try strapping it to a small airplane with a 1-tonne bomb in the front and see what happens .

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