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Old May 17, 2009, 09:30 PM
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Joined Feb 2009
363 Posts
Question
Micro Pulse Jet Engine?

I thought pulse jet engines were really interesting. I was wondering if it was possible to make a micro one out of a metal pen and using lighter fluid. Is it possible? and if so how?
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Old May 26, 2009, 02:46 PM
Master Crasher
zultone's Avatar
Fletcher, NC
Joined Oct 2008
681 Posts
I don't see why it wouldn't work... You would need to make a few things though.

I made a very rough diagram - A venturi fed pulse jet with a reed valve.

You would need to get your proportions right, and there may be a size that's just tooo small. It'd be an interesting project though
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Old May 26, 2009, 02:57 PM
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Joined Feb 2009
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Thanks for the diagram. How do you think I could make the valve?
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Old May 27, 2009, 08:45 AM
Master Crasher
zultone's Avatar
Fletcher, NC
Joined Oct 2008
681 Posts
I would make it out of thin steel. Since the engine will get very hot, you will want something that won't melt.

If you have an old cox engine laying around, they have reed valves and a venturi, it would give you a good idea of how to make one. They have almost a plastic looking valves, very simple engines, but effective.


I imagine the engine could run off of lighter fluid, or spirits. You'd want a fuel that can really make a cloud of gas real easy, without being quite as dangerous as gasoline.


http://en.wikipedia.org/wiki/Pulse_jet_engine

You could make a valveless pulsejet, they're more based on resonance than a valved pulsejet.


EDIT: I'm actually very excited about this! Just found a link that will create your plans! Just type in required thrust.

http://www.aardvark.co.nz/pjet/pulsejetcalculator.shtml
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Old May 30, 2009, 04:39 PM
The reviewer
XJet's Avatar
Tokoroa
Joined Mar 2004
3,827 Posts
You'll find that very small pulsejets are not really *practical* for model propulsion.

In reality, engines the size of the Atom, Tiger and Dynajet are about as small as you can go and still expect to have fun.

One of the problems is that as you go down in size, the effect of the boundary layer inside the engine becomes increasingly problematic. The changing Reynolds number plays a big part.

In short, if you halve the diameter of a tube, you get *less* than half the amount of flow through it for a given pressure difference.

On the inside skin of every tube/pipe there is a boundary layer that flows more slowly than the gas closer to the center. That's caused by the drag of the gas where it touches the surface of the tube. This layer can be quite thick. As the tube gets smaller, the percentage of the total area that is subject to this boundary-layer flow increases.

Once you get below a certain size, the effect of this boundary layer becomes very apparent and effectively "chokes" the engine, reducing power and possibly even stopping it from running.

If you use very fast-burning fuels such as hydrogen or even an acetylene/oxygen mix you *can* make tiny pulsejets but if you stick with more convenient fuels such as gasoline or methanol then small means "gutless".

Then there's the issue of weight.

As they get smaller, pulsejets become less and less fuel-efficient. An engine the size of a dynajet as a TSFC (thrust-specific fuel consumption) of around 2.5-3. That means it will burn 2.5-3 units of fuel per hour per unit of thrust.

Simple math shows that if a dynajet puts out 4lbs of thrust then it would use 2.5x4 (10lbs) of fuel per hour -- or thereabouts.

When you get down to very small sizes that number goes up significantly and so you need to carry much more fuel (per unit of thrust) to get the same run-time. This makes your model either very heavy or leaves you with very short flight-times.

And as for valveless engines, they're even weaker than valved engines in small sizes. As far as I'm aware, there haven't been any RC models powered by valveless pulsejet engines -- because they simply don't have enough power to do the job in those smaller sizes. A valveless pulsejet with the same power as a Dynajet will be more than twice the size and weight.

Another problem with valveless engines in small sizes is that they're very difficult to make run on liquid fuel. Virtually all the small valveless engines you'll see running on YouTube and elsewhere are fueled with propane -- not the ideal choice for a flying model obviously.

By comparison, medium-sized valved pulsejets are becoming increasingly practical (noise issues not withstanding) for RC models. My YouTube channel has video of my Long-EZ and my BobCat flying with my PJ8C engines.

The beauty of a valved pulsejet is that it needs on fuel pump, is compact and streamlined, and with a ceramic coating, they can even be mounted inside the fuselage of a model, providing adequate airflow is provided.
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Old Jul 11, 2009, 12:51 AM
Registered User
Canberra, Australia
Joined Jan 2003
107 Posts
Acetylene

My friend made an acetylene powered micro pulse jet that runs well. It was about 7 inches long.
For flight use you could try using calcium carbide and water as a gas generator.
Would impress spectators when you say it runs on water and they see you fill it up from a glass you have just drunk from !!!
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Old Jul 11, 2009, 07:30 AM
Dave Segal
Philadelphia PA USA
Joined Jun 2001
545 Posts
If memory serves, there was a miniature engine advertised in the 1950s that comprised a CO2 cartridge shell attached to an exhaust tube which had a flared open mouth at the front. Beneath the cartridge a little tray was suspended which was supposed to be filled with fuel and ignited to heat the cartridge. Looked like a good way to burn down the house. The maker might have been M.E.W. It must have been intended to run as a ram-jet, I guess. Anyone else remember this little bomb?
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Old Aug 15, 2009, 08:22 PM
American MoonMan
Joe Bennett's Avatar
United States, CA, San Diego
Joined Jul 2003
2,722 Posts
I remember the "Ram Jet Engine" well, I bought one from the Johnson Smith Company for under $5 as I remember. This would have been in the very early 60's. Ordered it (and many other goodies) from an ad they had in a comic book. I do remember that you were supposed to fill the co2 cartridge with alcohol (it had a removable nozzle with a cotton wick that fit into the cartridge and then screwed back onto the end). Once filled, you would put the aft thrust tube back onto the nozzle. Then, you had to put some cotton balls into the tray hung from the bottom and soak them with alcohol. You needed to light the tray and let the cartridge heat up until the alcohol began spraying out vigorously from the nozzle. It would catch fire at some point and begin burning in the rear tube, drawing air in almost like an afterburner. I was able to get it to spin around on a wire a bit, but the tray would shed its cotton balls and go out which would then allow the co2 cartridge to cool down and then it would all lose thrust pretty quickly. It WAS pretty impressive, and dangerous, but hey, it was loads of fun for a kid at the time....

Joe
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Old Aug 15, 2009, 09:31 PM
Dave Segal
Philadelphia PA USA
Joined Jun 2001
545 Posts
Joe, thank you for that wonderful recollection of that little engine. I never had the nerve to light it up!
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Old Mar 18, 2011, 06:01 AM
Kyle Montanaro
Mosta - Malta - Southern Europe
Joined Apr 2008
14 Posts
Hey X-Jet, Love your website and your projects got me trying to make a few ones myself... however, knowing my luck, I never managed to get it started, the most I did was a few flames down a tube.. :/ (and secondly, I love your country, been there for a month and was never willing to come back to Malta..) now I've just attempted at making a really small one because my dad doesn't let me use the big propane bottles. It's made out of a small metal hollow tube which was a torch and has it's LED housing very resemblant to a jet intake. and I made the exhaust pipe out of another slightly smaller tube and made a valve for the intake using 1 small sheet of kitchen foil. Fortunately, using a small lighter and a pipe and a computer fan to get the air through, I got a few short bursts of blue flames down the tube but I can't get it to reciprocate. I'm sure the overall length is long enough, but is there a chance of it being too long?
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Old Aug 12, 2012, 01:57 AM
Registered User
Joined Aug 2012
1 Posts
Those COLD flight line days...

November 770 Zulu This is Freeway Airport VFR flight rules are in effect active runway 30 wind calm, you are cleared to land.

http://www.airfields-freeman.com/AZ/..._N.htm#freeway

I love Pulse Jets. Solid State Hiller-Lockwood is elegant and how come ancients treasured high-temperature ceramic potters & trumpet makers. It get's cold on the Flight Line of mornin' ! I love to fly control line and used to work Tower and AvGas refuel GA and flight control VFR at Freeway Airport decades ago as a teen. The beauty of Key Chain pocket-sized Hiller-Lockwood Pulse Jet resides in Flight Line ground crew support comfort and engine pre-heat start-up. Warm the crew and engine by Heat Pump Pulse Jet, walk the prop through, and start-up. Pulse Jet designs are needed. We must field test and deploy small Pulse Jet Heat Pumps as soon as possible. They should be portable, self contained, and fueled by stochiometric Santilli H+H+CO. We'll save the water exhaust, filter as needed, and recycle the green, sustainable H2O into the Heat Pump. Three (3) 12v batteries will provide our fuel. Series 12v = 36v DC 40-50 amps. Heat Pump steam driven alternator recharge would be nice on such a dolly hand-truck Flight Line mount. Strike plasma arc by gravity feeding rods through ceramic tubes into a rod-target in such a fashion as to insure 'instant plasma arc'. KISS-- Keep It Simple Sovereign. Any excess fuel could be used to fill silicon fuel bladders held in spring loaded fuel bays on flying jets. Fuel expense is always our worst nightmare. Let's give the fuel bill 'at 'em boys give 'em the gun. Up ! And up ! Nothing can stop the US Air Force !' Fuel generator design here:

VTS_03_1.flv (2 min 59 sec)


My contribution to Tucson area R&D here:

(Sales ads classified as a result of unknown origin enemy military & law enforcement provocateur action)

Pulse Jet, you are a GO at throttle-up !


MM 2 pulse jet with spark plug (2 min 39 sec)


Good pilots always take care of their Ground Crew and this will keep the coffee and Cocoa and melting marshmallows piping hot !

https://sites.google.com/site/pulsejetcapusafaux/

BTW, functional steel Hiller-Lockwood Pulse Jet will serve as a mold for ceramic 'block' Pulse Jets suitable for Flight Line ground crew-only non-flying duty. Lose the flight Line 'fire barrel', strike the arc, and light-off the Pulse Jet.


Quote:
Originally Posted by XJet View Post
You'll find that very small pulsejets are not really *practical* for model propulsion.

In reality, engines the size of the Atom, Tiger and Dynajet are about as small as you can go and still expect to have fun.

One of the problems is that as you go down in size, the effect of the boundary layer inside the engine becomes increasingly problematic. The changing Reynolds number plays a big part.

In short, if you halve the diameter of a tube, you get *less* than half the amount of flow through it for a given pressure difference.

On the inside skin of every tube/pipe there is a boundary layer that flows more slowly than the gas closer to the center. That's caused by the drag of the gas where it touches the surface of the tube. This layer can be quite thick. As the tube gets smaller, the percentage of the total area that is subject to this boundary-layer flow increases.

Once you get below a certain size, the effect of this boundary layer becomes very apparent and effectively "chokes" the engine, reducing power and possibly even stopping it from running.

If you use very fast-burning fuels such as hydrogen or even an acetylene/oxygen mix you *can* make tiny pulsejets but if you stick with more convenient fuels such as gasoline or methanol then small means "gutless".

Then there's the issue of weight.

As they get smaller, pulsejets become less and less fuel-efficient. An engine the size of a dynajet as a TSFC (thrust-specific fuel consumption) of around 2.5-3. That means it will burn 2.5-3 units of fuel per hour per unit of thrust.

Simple math shows that if a dynajet puts out 4lbs of thrust then it would use 2.5x4 (10lbs) of fuel per hour -- or thereabouts.

When you get down to very small sizes that number goes up significantly and so you need to carry much more fuel (per unit of thrust) to get the same run-time. This makes your model either very heavy or leaves you with very short flight-times.

And as for valveless engines, they're even weaker than valved engines in small sizes. As far as I'm aware, there haven't been any RC models powered by valveless pulsejet engines -- because they simply don't have enough power to do the job in those smaller sizes. A valveless pulsejet with the same power as a Dynajet will be more than twice the size and weight.

Another problem with valveless engines in small sizes is that they're very difficult to make run on liquid fuel. Virtually all the small valveless engines you'll see running on YouTube and elsewhere are fueled with propane -- not the ideal choice for a flying model obviously.

By comparison, medium-sized valved pulsejets are becoming increasingly practical (noise issues not withstanding) for RC models. My YouTube channel has video of my Long-EZ and my BobCat flying with my PJ8C engines.

The beauty of a valved pulsejet is that it needs on fuel pump, is compact and streamlined, and with a ceramic coating, they can even be mounted inside the fuselage of a model, providing adequate airflow is provided.
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Old Dec 16, 2012, 08:53 PM
cheapskate extroardinairé
United States, CA, Marin
Joined Jul 2011
1,073 Posts
hmm, this is interesting. i've been trying to make a "medium" pulse jet about 1 foot long from copper piping from my diy shop, but it refuses to start. any ideas?
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