The simplest pulsejet ever

[Bruno Ogorelec]

Given the great interest in pulsejets that would require no welding, here's a proposal. It has not been tested, but I can see no reason why it should not work. Everyone is invited to give comments and advice on practicality of the idea -- and other aspects, of course. Experienced builders are especially welcome.

The original idea comes from F. H. Reynst, but this is not really a Reynst combustor. It is an acoustical pulsejet, but the arrangement of its parts follows Reynst practice, inasmuch as the intake is also the main exhaust (not just auxiliary) and the fresh air pickup point is downstream from the combustion chamber.

You build it by finding a steel tube of relatively thin walls of (I’d say) at least 4 inches in diameter. Its walls should be thin enough so that you can bend them with hand tools. Length should be at least 12 diameters, preferably more.

If you hate welding, you will need to machine a cylindrical plug that is a very tight slip fit into one end of the engine. Put it into deep freeze to shrink. After a number of hours, heat one end of the tube with a blowtorch or a hot air blower. Even a hairdryer should do, I think. Slip the frozen cold plug into the hot end of the tube and let cool. Bang a little around the edges with a hammer to make sure the expansion fit is helped a little bit, though it should be perfectly adequate. The moment it is heated up by combustion it will hold very well indeed.

You might mount a spark plug on the end plug before fitting it to the tube, too.

About four diameters from the plugged end, saw the tube across to perhaps half diameter with a hacksaw or whatever.

Push the tube wall inwards on the shorter side, so that it buckles in, roughly as on the sketch. Voila! A combustion chamber, an intake/exhaust tract and a resonant exhaust.

Mount the propane gas jet wherever you like it. I’d try with the location shown on the sketch, but other people are sure to advocate other positions. Certainly, the gas must be blown into a place where a lot of air turbulence is expected.

The length of the exhaust part, downstream of the saw cut, is up to you. Theoretically it should be a bit longer than twice the shorter part (upstream of the cut). My guess would be to make it four times longer for starters and that you cut it down progressively, inch by inch, until you find a ‘sweet spot’. When you overshoot the sweet spot, note it down and build the second engine to the optimized dimensions.

The obvious disclaimer here is that novel pulsejets never work right away. Not even the well-proven designs built to spec will always work. So, much will be up to you. Just don’t pick small tubes. Never build a small pulsejet unless you have already built dozens and know the ups and downs of the trade inside and out.

[Ray(in England)]

Thanks for that simple description and drawing,Bruno.
Even a total novice like me can follow it, clear as a bell.
I don't understand why you warn folks like me off from building it.
It seems the only design I could attempt.
I have lots of empty food tins in my shed that I could use for it.
What do you think of them?.
Thanks, Ray.

[Bruno Ogorelec]

I don't understand why you warn folks like me off from building it.

Ray, it's just that I want to keep people away from great expectations. Pulsejets are pesky little bastards and frustrate people so often. I don't want the guys in the forum to think they can take a tube, follow instructions and have an engine. It almost never happens that way. Builders sometimes have problems even with jam jars, which are the simplest pulsatiing combustion devices you can imagine. Also, my knowledge of pulsejets is 95 percent theoretical. My first and last pulsejet was built in 1976 and worked for a few seconds before falling apart. (It was a weird design. I was going off the beaten path even then.)

[jmhdx]

I love an elegant solution, less is more, I hope it works and can see no reason why it couldn't be made to resonate, but you know how fiddely these things can be. You right about the minimum 4" tube as it would be difficult to get the air/feul mix right on a smaller model.
I've never seen a reyns't or closed end tube rock like a lockwood though.
Still I've never seen a large logan type either, any available specs for comparrison?
Mass flow should be large.
Should have tooling soon and can join the race.
Other projects have priority.
Mike.

"This doesn't stop people proposing motors which are driven only by magnetic fields. These motors have rotors which are pushed or pulled most of the way around a circle by some arrangement of magnets. There's nothing impossible about this, but the designers then expect the rotor to suddenly ignore the magnetic field and to complete the cycle. This gets the rotor back to its starting point after delivering a net output of energy. This is impossible."
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[Mike Everman]

So many designs, so little time! I love it of course. I'd be tempted to make the tail conical with a strategic squash. I may just make one, but with 3" tube, that's the biggest I have.

[Bruno Ogorelec]

I've never seen a reyns't or closed end tube rock like a lockwood though. Still I've never seen a large logan type either, any available specs for comparrison?

Mike, the Reynst-built meter-long combustor with a conical exhaust could be heard clearly over six miles. That's rocking in my book. See the pic showing two of the beasts in his shop, mounted vertically, with exhausts turned upwards.

I have no information on a really big Logan. I have very little information on any Logan, for that matter.

In my opinion, pulsating combustion is pulsating combustion. If you cram a lot of mixture in a chamber and provide a reasonable amount of confinement, a lot of energy will be released. What you do with that energy will determine whether your combustor is a good heater, a great noise maker, a good gas generator for a jet engine or whatever.

There is little doubt that the Reynst pot can cram a lot of mixture into a given volume and provide reasonable confinement. Some sources say that it may well be the best of them all. That remains to be proved. I have very little hard data. But, if it does pump a lot of mixture, well, that's the heart of good pulsejet. It is up to us to tell the resulting gases what to do.

Of course, this is a vastly oversimplified statement, but true enough to hold water, I think.

[Bruno Ogorelec]

I'd be tempted to make the tail conical with a strategic squash. I may just make one, but with 3" tube, that's the biggest I have.

Well, it's easy enough to press the tube wall inwards on the downstream side, too...

[tufty]

Well, it's easy enough to press the tube wall inwards on the downstream side, too...

That's what I thought on first seeing this post. And I note that Bill H has drawn exactly what I was thinking. I also seem to recall (on Larry's site, maybe) something very similar being proposed a while back.

That all said, I can't see any reason why this wouldn't work.

Nice photos of the Reynsts, also. They even _look_ loud.

Simon

[Viv]

Well bugger me swinging from the rafters as Gary would have said! how long you been sat on this one Bruno?

I like the idea, nice, damn well worth a go, when ever those arses in the US get there blasted fingers out of my container so i can have some bloody tools to start work I may get to build one for you:-)

And Mikes suggestion to Kazzoo it a bit is coolt too, thats a worker i recon

Oh and to add my ten peneth, make multiple incisions so the intakes are disposed symetrically round the tube so centralising the combustion chamber inlet outlet to the centre of the tube.

That way differant diameters can be tested easy

Viv

[Bruno Ogorelec]

Well bugger me swinging from the rafters as Gary would have said! how long you been sat on this one Bruno?

Um... a long time, Viv. Nick was making his first BCVP tubes when it occurred to me first, I think.

Well, some of them take time to grow; others go off quicker than a bucket of prawns, as Gary may have said for all I know. He didn't swear much when we were talking Unicones, but you could sometimes feel he was on the verge of saying something that would peel paint off his latticework. Pulsejets do that to you. They test your resolve.

Glad you like the idea. I think I have shown it to one or two people before, in some form. Originally it even had a BCVP version. Someone likened it to a whistle.

And Mike's suggestion to Kazzoo it a bit is cool too, that's a worker i recon.

Uh-huh. The whole idea is right up the kazoo alley, I guess. The same approach of utter simplicity. Minimum means for relatively sophisticated results.

Oh and to add my ten peneth, make multiple incisions so the intakes are disposed symetrically round the tube so centralising the combustion chamber inlet outlet to the centre of the tube.

Agreed. I'd try to make the thing axis-symmetric if possible. However, that will demand some planning of the pressing-in technology, to get the openings at least similar if not identical.

Good to hear from you, Viv. Get those Yanks to release the tools. How's technology to progress if a man is kept tool-less? You turn any wood these days? Did I ever tell you that I used to earn my money turning drumsticks?

[Viv]

Ah Bruno do i turn wood? I would like too!!

My container shipment has all my wordly goods in it and tools from a complete woodworking workshop plus the metalworking workshop I was building.

The whole shipment is a fraction under a ton including my DVD collection, and the arshole shippers sent it via new york to go to Montreal.

Can you imagine how US customs freaked when they found a pulse jetters workshop in a container?

Took the wankers a week to sort through all the nuts and bolts and engine sections:-)

I hope they cut there fingers on the chisels too

Viv

[Mark]

It's interesting to see the size of the bolts on the Reynst clam bakers. I guess they vibrate quite a bit too with all that sound. Those bolts look like the ones used on sky scrapers.
On another note, it's getting breezy in Pensacola today, I just took my weiner dog for a walk, because we will be staying in tonight. Guess the winds will be a good 120 mph this evening. I bet if I got my Dynajet out, I wouldn't need starting air from a tire pump or air compressor, I'd merely face the red head into the wind and turn my buzz coil on. I could be the first to start a Dynajet with hurricane force winds. Your man in paradise,
Mark

[Bruno Ogorelec]

Oh, my God, I can picture it -- Mark in his back yard, angled at 30 degrees to the ground, lighting a Dynajet, while a dachshund is flapping behind him wildly in the wind at the end of his leash, the poor thing. I am worried about the flying debris, though. Bound to do terrible things to the reeds.

Mark, take good care of yourself. Make a bong out of one of your weird vessels and contemplate pulsations until the wind abates. I am holding my fingers crossed for you. God bless you.

[Viv]

Ok Bruno

Here you go budy a nice 3D model to help with the design process, and interesting task tto draw, I did start to have a go so I could learn Solidworks but Luc was out the front and in the lead before I had a tube drawn on the screen.

We should get Kenneth to allow html file posting so we can post the vuiewable interactive file.

Viv

[Bruno Ogorelec]

Nice work, Viv, thanks. It will be an aesthetically pleasing engine, even if it doesn't work, he-he-he...