Pulse jet silencer?

[grant richardson]

Just a thought but it seems possible to silence a pj some using something similar to a silencer on a fire arm Like a 9mm silenced sound more like a 22. Of course its not like the movies and goes shew when they shoot. Guns are still loud when silenced but a 22 is quiet. with subsonic bullets all you hear on a automatic rifle is the bolt opening and closing.

Anyway a friend let us borrow his 22 rifle with a homemade silencer. it was a ruger 10 22 with 2 lawn mower mufflers taped on the barrel. the long thin mufflers. He drilled some holes in the barrel under the mufflers and packed the mufflers with steel wool and it was quiet.

Could this be done with a PJ or would it interrupt the pulsing? or either build a silencer over the end of the exhaust pipe to catch some of the noise? :wink:

[Mike Everman]

Tribal knowledge will tell you that it won't work on a conventional PJ. The exhaust pipe needs an open end to function; one that does not diminish the pressure pulse. That's not to say that you can't get some muffling before you move it too close and it kills.

[Stuart]

Tribal knowledge will tell you that it won't work on a conventional PJ. The exhaust pipe needs an open end to function; one that does not diminish the pressure pulse. That's not to say that you can't get some muffling before you move it too close and it kills.

Augmenters are the best you can do, they have muffling properties. I have a paper on a augmenter tested on a gulfstream in flight - both as thrust augmentation and more importantly sound abatement. With a pj, you'd have to probably include the augmenter in the length calculations to not wreck performance. But what do I know, I'm more a pressure jet kind of guy.

[Bruno Ogorelec]

You may have forgotten about it, but Conception GLC guys -- our Luc and Viv -- claim to have produced a quiet engine.

What I have been thinking about is a detail that usually escapes attention. OK, so a pulsejet needs a gap at the tailpipe end in order to generate a pressure signal. It also needs to be able to suck fresh air there in order to boost the amount of reaction mass. But, it doesn't say anywhere that both requirements could not perhaps be satisfied with a special duct meeting the tailpipe at right angles, in place of the conventional cut-off end of the pipe.

Imagine a side duct meeting with the tailpipe just before it broadens into a thrust augmenter. See the picture. As long as the side duct supplies fresh air to be sucked back, there is no reason why it couldn't be muffled. A simple tube lined with glass mat or rock wool would do fine. The lining would also kill resonance, so you shouldn't need any particular acoustic tuning.

As long as a gap is there at the end of the tailpipe to reverse the pressure signal, it hardly matters what's after the gap, right? It just mustn't generate yet another pressure signal that might skew the first one. Dunno; to me it sounds as if a muffled gap might work.

[Bo Danerius]

You may have forgotten about it, but Conception GLC guys -- our Luc and Viv -- claim to have produced a quiet engine.

What I have been thinking about is a detail that usually escapes attention. OK, so a pulsejet needs a gap at the tailpipe end in order to generate a pressure signal. It also needs to be able to suck fresh air there in order to boost the amount of reaction mass. But, it doesn't say anywhere that both requirements could not perhaps be satisfied with a special duct meeting the tailpipe at right angles, in place of the conventional cut-off end of the pipe.

Imagine a side duct meeting with the tailpipe just before it broadens into a thrust augmenter. See the picture. As long as the side duct supplies fresh air to be sucked back, there is no reason why it couldn't be muffled. A simple tube lined with glass mat or rock wool would do fine. The lining would also kill resonance, so you shouldn't need any particular acoustic tuning.

As long as a gap is there at the end of the tailpipe to reverse the pressure signal, it hardly matters what's after the gap, right? It just mustn't generate yet another pressure signal that might skew the first one. Dunno; to me it sounds as if a muffled gap might work.

Not knowing much about pulse jets but some on musical instruments. Is the majority of sound produced at the exhaust, how much noise does the vibrating structure produce?

/Bo

[Bruno Ogorelec]

Probably a small percentage overall. The main noise producer is the shockwave created as the hot exhaust gas spews into cool air and suddenly exceeds sonic speed. Think of it as the breaking of the sound barrier 200 times a second. The actual combustion noise is a minor contributor to the overall noise.

[Bo Danerius]

Probably a small percentage overall. The main noise producer is the shockwave created as the hot exhaust gas spews into cool air and suddenly exceeds sonic speed. Think of it as the breaking of the sound barrier 200 times a second. The actual combustion noise is a minor contributor to the overall noise.

Okay. Gotta build one just to hear it :D

[larry cottrill]

On my twin-FWE Water Purifier I proposed multiple Helmholtz chambers on each exhaust and intake. Here's an "improved" version, based on the tuned-augmentor approach of the SNECMA Turbine Pulsejet tailpipe. Note the assymetrical arrangement of outer shell and walls, separating the unit into four different sized chambers.

There has to be a lot wrong with this. Some experimentation required ...

L Cottrill

[dynajetjerry]

Bo and Bruno,

I have a comment on noise of pulsejets: When the Aeromarine crew were developing smoke-screen generators for the military, they were able to greatly lower the intensity of the exhaust noise. They incorporated the design of diesel "snubbers" into the housings. Further testing showed that as a result, the clatter of the reed valves became extremely noticeable, so much so that sound intensity remained above that permissable by the government. A door that contained sound-absorbent material was adopted and found to be acceptable. In fact, today's Army M157 SGS is a very close copy of our E19R1 generator, first designed and made in 1951, though Aeromarine Co. has never been credited with its creation.

Of course, such muffling steps greatly increased total weight and bulk and destroyed any pretense of generating useful thrust from such a device.

Jerry.

[Hank]

A far cry from my great -grandfather's use of vulcanized raincoats and barrels of bacon introduced into the firebox to create smoke running the blockade. Found the following on the unit mentioned by Jerry.

The M157A2 Smoke Generator Set (SGS) and M1059/M1059A3 Smoke Generator Carrier (SGC) produce large area visual smoke screens on the move. The M157A2 and M1059/M1059A3 use dual pulse jet engines, operating on standard Army fuels, to produce large white clouds of fog oil vapor which defeat visual-range observation and tracking methods, including lasers. The M157A2 and M1059/M1059A3 consist of 5 major components: Two (2) M54A2 Smoke Generators, an Air Compressor Assembly, a 120 Gallon Fog Oil Tank, a Fog Oil Pump Assembly, and a Control Panel. When mounted on an M1037/M1097 HMMWV with the M284A1 Mounting Kit, these components comprise the M157A2 SGS. When mounted on the M1059/M1059A3, variants of the M113A2/M113A3 Armored Personnel Carrier, these components comprise the M1059/M1059A3 SGC.

The M54A2 is a pulse jet engine which burns any mid-viscosity fuel (diesel, JP4, JP8, etc.) to vaporize fog oil which recondenses in the atmosphere to produce a thick, white cloud which provides visible obscuration. Each M54A2 is capable of vaporizing 40 gallons of fog oil in a one hour mission.

A rocket was launched today in Afghanistan that landed within fifty meters of its probable intended target after a flight of nine miles.

[Eric]

The human ear/brain is a tricky thing.

My linear augmenter pulsejet seems rather quiet. A normal pulsejet of the same size would scream rather loudly and be painful to listen to, though the noise would drop of with distance or with objects between you and the engine quite considerably.

Up close it is vastly quieter, much more of a bass noise due to the vastly lower frequency per length , but we all know bass travels better and goes through concrete walls with no problem, so there is no excaping the noise that it does produce, even though it sounds much more quiet.

A normal pulsejet is little better than a high velocity round being fired from a rifle at a high rate, where using multiple augmeters converts the beast into more of a gas powered air moving musical intrument.

So there is a difference between the actual "loudness" and the percieved "loudness" and how the various sounds travel.

The main thing that will "quiet" things is converting the rather triangle wave sound form of a normal pulsejet into a very smooth sine wave.

Changing the shape of the wave form without changing the energy at all goes a long way, since our brain has vastly different reactions to the different shapes.

Eric

[Bruno Ogorelec]

On my twin-FWE Water Purifier I proposed multiple Helmholtz chambers on each exhaust and intake. Here's an "improved" version, based on the tuned-augmentor approach of the SNECMA Turbine Pulsejet tailpipe. Note the assymetrical arrangement of outer shell and walls, separating the unit into four different sized chambers.

There has to be a lot wrong with this. Some experimentation required ...

L Cottrill

Larry, to me it looks as if we were thinking along similar lines.

[dynajetjerry]

Hank,

The image that you posted, of the smoke generator, is one I plan on including in my Dyna-Jet book. On the site where you found this picture you may have noticed the statement that this was the first such device ever used by the Army. "Hogwash" is an appropriate comment. As I mentioned before, Aeromarine created this machine and shipped 60 of them to the Army in 1952 then lost a promised contract to produce 600 more.

A Major conned Bill Tenney into supplying complete blueprints for this (E19R1,) for their "field maintenance". The Major then had all drawings copied onto official military forms and solicited bids from other manufacturers, in a successful effort to get the machines for less than Aeromarine's price of $1400 each. We were also rejected as a sub-contractor on this project, being told we were "not qualified!!!!!"

Bill sued that Major and the Dep't of the Army for lack of good faith and illegal use of propietary information. He won!! Bill told me the award was $100,000 but never said if he actually received it. That episode is what induced Bill to get out of the pulsejet business and he found Russell Curtis who bought the operation, in late 1952.

Curtis Dyna-Fog, Ltd. claims that Russell organized CDF in 1947 and his first product was the DFG, Jr. More hogwash!

Jerry

[Hank]

Thanks for the information, Jerry. I'd done research into military obfuscation devices prior to the 20th century. The army didn't have any projects related, North or South, in Lincoln's War.
I recall a Tenney Industries (Metal Supplier) in New Jersey.
Award of military contracts usually follow the axiom that, "Victory goes to the stronger." I don't know, but my guess is that with all of the knowledge to produce the system Tenney was weak in the manufacturing sector.

I am interested in your history of Dyna-Jet when completed.

Scott Crossfield was killed in a civil aviation accident in Georgia today. He spent part of his year in this Florida town, a genuinely nice guy.

[dynajetjerry]

Hank and all you others in this thread,

Insofar as I've been able to learn, Bill Tenney was NOT connected in any way, with Tenney Metals of New Jersey. His only businesses after selling Aeromarine/Shevlin was a second Aeromarine Co. which was devoted to his original pursuit of improvments in piston engines. He also organized an (I think,) insecticide application business and called it "The London Fog Co" of Minnesota. I don't know if he employed any Dyna-Fog equipment.

However, I believe he was pretty adept at running a successful company. His downfall came from relying too much on his "expert" advisers and, as a result, was cheated out of a big military contract. While he must have guided the directions of Aeromarine research and contributed to some of it, most of the truly innovative results were the work of his engineers, toolmakers, and other technicians, including, of course, Charles Marks.

I swap Christmas cards, annually, with Bill's widow, Patty. In 1996, it was she who invited me to her home, near Minneapolis, to carry away most of the Aeromarine material Bill had kept after selling his operation to Russell Curtis, in 1952.

Jerry