How to Design Build and Test Pulse Jet Engine.

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gone2fly
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How to Design Build and Test Pulse Jet Engine.

Post by gone2fly » Sat Sep 19, 2009 11:17 pm

I have been building pulse jet engines for 32 years. Now that I am retired I can build them full time. I have a book, english translation of the 10+ years of research the Germans did to learn how to build a working pulse jet engine. The engines are a lot more complicated than they seem. The entire engine is designed around the exhaust pipe or tail pipe. The air intake is designed for the speed that the engine travels through the air not the engine static thrust. The faster the engine travels the smaller the air intalk because ram air makes the engine run lean. A correctly designed engine start very easy at 10% throttle then they can be throttled up to full power. A pulse jet engine will run on anything that will burn, gasoline, methyl alcohol, ethyl alcohol, mixture of 75% gas 25% kerosene, and propane. The Germans modified their original design and increased the speed several times the last week of the war they had the speed up to 500 mph. After the war the USA did some research and got the speed up to 950 mph then later 1200 mph. If the engine is designed correctly the reed valves will last pretty much for ever. Most of the hobby engines have too high of a compression ratio that is why the valves don't last very long they basically hammer themself to death. If the combustion chamber pressure is too high it slams the valves shut too hard and the life if the valve is only a few minutes. I have built some engines that have run full power in a test stand for an hour and the valves are still in good condition. The Germans used a flame guard in the V1 engine to protect the valves from the heat but they also made 3 other improvements to extend the live of the valves. I have built a lot of pulse jet engines over the pass 31 years, in the past 2 weeks I have built 7 pulse jet engines. As soon as I get some good videos of my engines starting then throttle up to full power and get the information on paper it will be for sale on Ebay. I hope to have that done by Thanksgiving.

Mark
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Re: How to Design Build and Test Pulse Jet Engine.

Post by Mark » Sun Sep 20, 2009 12:04 pm

Welcome gone2fly,
I recall Tharratt mentioning an engine merrily bubbling along at ~7 pounds of thrust and throttling up to 100 lb instantly. And he had some reeds that lasted 24 hours at full thrust, some runs lasting 6 hours. He also used a dish flame guard and reeds with neoprene layers to soften the impact. I think he also mentioned forward air speed being effectively the same as increasing valve area.
As you have been building pulsejets for many years, have you built any straight Schmidt tube designs? I saw this little hobby engine on eBay and was wondering if you knew what kind of valve it had, if it was an "angled" pyramid design or petal valve like the Dynajet or something else? I just like the cylindrical shape for the simplicity, even though it never became the hobbyist's dream engine.
Have you made any engines with a new kind of valve? To me, inventing something new is the fun stuff.
Thanks,
Mark

Here's that little engine I saw on eBay if you might know of the valve design.
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Last edited by Mark on Sun Sep 20, 2009 3:02 pm, edited 1 time in total.
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gone2fly
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Re: How to Design Build and Test Pulse Jet Engine.

Post by gone2fly » Sun Sep 20, 2009 2:30 pm

That is a cool looking engine. If I had seen it on ebay I would have bid on it. I check ebay from time to time. I have not been on ebay all summer. It is hard to tell from the picture but it looks like the tip end of the exhaust is smaller. The Germans learned the combustion pressure has to be no less than 1.2 to 1 for an engine to pulse. They also learned the higher the combustion chamber pressure the more efficient the fuel burns and the more power the engine produces. I assume that is why most hobby engines have a compression ratio of about 3 to 1 so they will produce more power and use less fuel. I have not built a straight tube pulse jet engine a compression ratio of about 1.5 to 1 is as low as I have gone. I have one engine that I put a 2 cycle engine carburetor on the intake of the pulse jet engine. When I open the carburetor throttle the engine throttles up. It works great.

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Re: How to Design Build and Test Pulse Jet Engine.

Post by noclassmac1972 » Sun Sep 20, 2009 4:16 pm

Mark on the valve of the glowstick I have a picture of it somewhere! Which I will try and find for you. Its one very complicated piece of work and Its definately not something we could whip up in the shed... Its sort of like reed valve from a Cox engine (same principle) but much more complicated..

Robert Bolton

WebPilot
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Re: How to Design Build and Test Pulse Jet Engine.

Post by WebPilot » Sun Sep 20, 2009 5:16 pm

Hi gone2fly,
gone2fly wrote:... The Germans learned the combustion pressure has to be no less than 1.2 to 1 for an engine to pulse. ...
Let's see

0.2 × 14.7 = 2.94 psi

You didn't say what paper you're reading, but it's in the literature it can be lower.

Last year I posted some results of an independent theoretical analysis I did and here is one frame.

Did you ever make an engine that vibrated at either of 2 pressure amplitudes?
WebPilot wrote:... and I located this 2nd limit cycle, too.

Image

It is the 'big brother' to the little sister since he vibrates at the same frequency but at an amplitude of 1.25 psi(gauge).

Now the reader may think these amplitudes are small compared to a pulse jet and they are.

However, I am modelling a pulse combustion burner and the experimental data published reports amplitudes varying from 0.25 to 3.0 psi(gauge).

Pulsating combustion is responding to a mathematical treatment as I always knew it would.

The veil shrouding pulsating combustion in mystery now has a serious rip in its fabric.
Image

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Re: How to Design Build and Test Pulse Jet Engine.

Post by Mark » Sun Sep 20, 2009 7:09 pm

noclassmac1972 wrote:Mark on the valve of the glowstick I have a picture of it somewhere! Which I will try and find for you. Its one very complicated piece of work and Its definately not something we could whip up in the shed... Its sort of like reed valve from a Cox engine (same principle) but much more complicated..
Robert Bolton
I've a few old Cox engines and a McCoy. When I was about 12, I would fly a model such as this doing loops and having to fly up and over cars as they came down my street, my yard not big enough. ha
http://www.craftsmanshipmuseum.com/images/Cox10.jpg
I guess you are saying the reed is something like an oil can where when uplifted the fuel and air can skirt in all around the edges, instead of like a petal valve that is pinned down at one edge? The old Cox reeds were a copper-beryllium alloy shaped like an X but now they make them in mylar.
http://www.modelenginenews.org/cardfile ... 1_reed.jpg
I have an 18 inch plumbing pipe, one inch in diameter that I screwed a bell onto one end, the bell a one inch necking down to a half inch for the intake. And I used a single reed for this engine. I could never get enough compression to get it to start either by spark plugging it with a little 1/4 32 plug or by priming and lighting the tail end with a flame. All I got was a sick duck quacking sound. The only way I got it to start was by corking the tail after misting the insides with methanol and then like a pop gun, spark the cylinder, this gave it enough compression/impulse to sustain. It would run for a good five seconds but then die slowly or just stop, the fueling was just some very small diameter copper tubing cocked upright inside the intake of the bell, very crude. At some point I want to try making another, but I want to use my quartz tubing.
I bought an old Dynafog fogger and it too had a morass of parts for the valved intake.
Thanks for that tidbit Robert and I hope you can find your picture of that engine intake, I am curious what features it has. Funny, the literature says the P-80 Jet-Stick is made of long life "titanium - inconel X stainless steels." Mine will be made of waveguide tubing - flame-fused natural quartz crystals mined in Madagascar. ha

(I asked a branch of Heraeus in Georgia if they knew anything about my tubing I bought on eBay)
"Hello Mark,
We have found some old paperwork which I can with 100% accuracy say
that part#1565 was produced for Nortel and the material is Heralux WG material. That is Flame fused natural
quartz crystals. OH content 130-180ppm, <50ppm Cl This natural base material was taken from mines in Madagascar in Africa."

Another meager tidbit.
http://www.youtube.com/watch?v=llI8Mf_faVo
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Re: How to Design Build and Test Pulse Jet Engine.

Post by Mark » Mon Sep 21, 2009 12:06 am

I was noticing how perfectly similar this part is to the P-80 Jet-Stick head intake. You know those U-shaped flanges of metal that keep the needle valve from vibrationg to a different setting? It looks like the P-80 picture has one and the disassembled valve part jpg is missing its. But you can tell these parts are both made by the same company. Try blowing up the attachment picture and one can see it seems the P-80 borrows that part from some other company, if not Cox. Still, there may be more tricks to the P-80 but this tidbit is a good start of the puzzle.
I wonder if they made the finned head or if that came off some other engine as well? I wonder where they got the straight tube. ha
It's interesting to keep in mind the dimensions too, 1 3/8" X 14", I suppose the actual length to diameter ratio of the duct by itself is less. Might the head be 3 inches long or what? That would be a L/D ratio of 8 to 1 if the duct is 11 inches long. No telling how much of the finned head is hollow or adds on to the ratio.

"The reed design and its retention spring are again, distinctly Cox (of the "old" pattern). The R/C throttle plugs into the center of this assembly with yet another "O" ring and two cross-point screws. The throttle is quite conventional, being of the "non-compensating" type. It has a fixed air bleed hole and a screw sets the idle limit. Fuel inlet is provided by a neat nylon banjo. Not so neat is the throttle arm. As moulded, this would have interfered with the crankcase, so the tip has been circumcised with side-cutters. This, and the prop nut, are totally out of character with the quality exhibited by rest of the engine, but completely original."
http://www.modelenginenews.org/cardfile/gm12.html
Stick.jpg
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Last edited by Mark on Mon Sep 21, 2009 1:00 am, edited 5 times in total.
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vturbine
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Re: How to Design Build and Test Pulse Jet Engine.

Post by vturbine » Mon Sep 21, 2009 12:33 am

There's some info and small photos here Mark, on page 31:
http://www.ultraligero.net/Cursos/mecan ... iacion.pdf
No problem is too small or trivial if we can really do something about it.

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vturbine
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Re: How to Design Build and Test Pulse Jet Engine.

Post by vturbine » Mon Sep 21, 2009 2:40 am

P-80 (as I make it out)

Overall length: 14.125" (incl. mixture or idle? screw)
Overall Width: 2.5" (incl. needle valve and control arm)
Tail tube: 1" OD, no taper, 11" showing
Head: tail socket 1.25" OD, 0.5" long; fins 1.375" OD, 0.625" long; fwd taper section .125" long
Inlet tube: 0.625" OD, 0.625 long (taper to throttle body)
Carb: 0.75 long (incl. throttle body and fwd. end of venturi)
...more or less...

(The advertised overall dimensions (14" x 1.375" agree with the main components minus the needle valve, control arm and mixture or idle stop screw.)
No problem is too small or trivial if we can really do something about it.

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vturbine
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Re: How to Design Build and Test Pulse Jet Engine.

Post by vturbine » Mon Sep 21, 2009 2:29 pm

Gone2fly, how have you been measuring pressures? I imagine it has changed over the past 32 years. Thanks for starting a very interesting topic.
No problem is too small or trivial if we can really do something about it.

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Re: How to Design Build and Test Pulse Jet Engine.

Post by Eric » Tue Sep 22, 2009 2:33 am

They definitely can run at lower pressure like webpilot said. In fact just tonight at the shop I sneezed so hard that one of the 70 lbers 15 feet away started to resonate, was quite amusing that my sneeze matched its resonant frequency.
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Re: How to Design Build and Test Pulse Jet Engine.

Post by Mark » Tue Sep 22, 2009 11:52 am

I remember one time attaching a 4 foot long pipe to a Dynajet sized homemade pulsejet. I think it was some 3/4 inch plumbing pipe. The intentionally absurd jet ran but was quite muted and there was a hot spot somewhere along the exhaust tube. It kind of sounded like a muffled 049 Cox engine at idle. Then there was a foot long design I made with a single .003ths thick blue tempered spring steel reed. After priming it with methanol, I would light the tail and the the flame wouldn't advance but sit at the exhaust making a sound like an angry bee in a jar - the reed barely vibrating, just very unusual. Then after several seconds, the flame would decide to advance up the exhaust tube starting the engine with a terrific high-pitched advancing "doppler shift" effect. Often, this purring buzz would go on for at least 10 seconds, the bee sound was kind of a time delayed start, very comical to be sure, an odd flip flop feedback event. Less often the bee sound would run and run and then die out ever so softly. The reed was quite sensitive in that respect.
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Re: How to Design Build and Test Pulse Jet Engine.

Post by dynajetjerry » Tue Sep 29, 2009 8:34 pm

Hi, Gonefly & Mark,
As you've learned, the P-80 purports to be a "fully throttleable true jet engine" and was one of many p-j designs to come out of N. Carolina. Don Laird got some info on the man responsible but I can't recall his name.
The reed valve was circular and made from .005 in. thick brass or bronze, with zig-zag members extending from the outer rim to the center disc (about 1/2 in. diameter,) that was trapped in place by the inserted head assembly. It was so very sophisticated that I decided it was mfd. for another application and usurped by the P-80 mfr. The bottom line is that, insofar as I've been able to learn, none of the products of that company ever delivered what was claimed, some of them (such as the P-80,) not functioning at all!
I have another product from them, purporting to be a pulse-jet but appearing to be a questionable pressure jet. Made from material similar to the P-80, it has two main parts; the front is a pressurized tank (filled with acetylene or similar fuel,) and several nozzles that feed the fuel through ports into the rear pipe. In mine, the adjustable needle valve is so poor that it couldn't retain a few ounces of pressure, much less the hundreds of pounds from acetylene.
Anyone trying to operate any product from this company would be well advised to be careful and not get his hopes very high.
However, as a collector, I'm always on the lookout for such devices, whether or not they work.
Jerry
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Re: How to Design Build and Test Pulse Jet Engine.

Post by vturbine » Tue Sep 29, 2009 9:16 pm

Hello dynajetjerry,

very interesting -- we've been discussing this (p-80 and p-83) more extensively here;

viewtopic.php?f=2&t=5506

I have some questions about your p-83 if you have time to answer, and will ask them on that thread. :D
No problem is too small or trivial if we can really do something about it.

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Re: How to Design Build and Test Pulse Jet Engine.

Post by SR71Fan » Sat Oct 24, 2009 12:24 am

Hello gone2fly:

If you care to look at the "Rotary Valve Pulse-jets" topic, you will see my sort of
"outside the box" concept for a high performance pulse-jet, and if my bubble is
going to be burst, it may as well be from someone who has a lot of experience
building pulse-jets.

You also may have some quantitive data to address my quiry in the topic
"Pulse-Jet Operating Frequency Variations".

regards,

SR71Fan

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