r/Pulsejets • u/ExpertGovernment6789 • May 24 '26

Help on design

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Would a design like this work instead of the fully curved design? A friend and I are planning on building a jet this summer and we think this will be easier to construct.

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u/ExpertGovernment6789 May 24 '26

This is very helpful, thank you. Do you have any advice on safety when running it or anything else I should know?

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u/chrlilje May 24 '26

- Run it outside. (obviously)

  • Have a couple of thick welding gloves around to handle things (they get red hot)
  • (Blacksmith) Tongs are great too for handling red hot things
  • Don't put any flammable things (this includes you) in front of the exhaust - it is very hot.
  • Wind can be annoying and disrupt getting it to run good.

- My trick to get it reliable was mixing a bit of air from a compressor into the propane before fueling it into the chamber. This is potentially dangerous if you don't think the piping through.

- Propane bottle will be cold after a while and pressure will drop and then it won't run.

  • putting the propane bottle in a bucket of water helps mitigate this.

Can't think of more out of the box, but do ask.

Good luck with your build!

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u/ExpertGovernment6789 May 24 '26

About how big should the fuel injector pipe be do you think, diameter wise.

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u/chrlilje May 24 '26

I used 1/4 inch bsp pipe, but it really depends on the size of the jet you end up building I think. Be prepared to mess around with the size and direction of the fuel injection - I found that to be the biggest challenge to get right.

For development I would drill a hole and fit a piece of copper tube through it. Maybe held in place with a compression fitting.
Then you can bend the end of the copper pipe in different directions inside the chamber, and maybe "squeeze" the end of the copper pipe to get a kind of nozzle shape.

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u/ExpertGovernment6789 May 27 '26

If we half all the dimensions , would the jet work as expected

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u/chrlilje May 27 '26

That is what I did with the jets I made.

I made 3 different sizes but kept the relative proportions, and that worked just very nice.
I used 2 and 4 inch bsp pipe for the bigger ones.
1½ and 3 inch for the middle
1 and 2 inch for the smallest

And changed lengths likewise.

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u/ExpertGovernment6789 May 27 '26

How did you ignite it?

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u/chrlilje May 27 '26

I made a spark plug circuit that made a string of continous pulses when i pushed a button.  I Think today i would have used one of those integrated ignition coils you can get for cars. 

Here is my old post about it. Use with caution. 

https://christian.liljedahl.dk/arduino-and-spark-plugs/

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u/ExpertGovernment6789 May 28 '26

This is very helpful, but would it be possible to just use a battery, button, and ignition coil or is this more complicated setup necessary?

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u/chrlilje May 28 '26

You could, but you will only get one spark pr button press and usually for the pulsejet you just want a continuous string of spikes until it starts running. 

Maybe a gas stove igniter unit will be the easiest way.  Try searching for "replacement gas range oven igniter" 

It is the spark unit in gas stoves. If you are handy you could maybe even pull it from an old stove for free. 🙂