Worlds simplest valveless?

Bruce · Post by **Bruce** » Sun Dec 07, 2003 9:48 pm

> People somehow imagine that
> increased frequency will increase
> power. THIS IS NOT TRUE

Correct -- I believe that one of Tharrat's papers explains this also.

Bruce · Post by **Bruce** » Sun Dec 07, 2003 10:16 pm

Mike said:

> I was excited there for a minute,
> Bill. I'm not sure a straight
> pipe will get you a sine output.
> I'm looking at the Schmidt
> paper's pressure trace and it's
> ugly, and I believe it was a
> straight pipe. However, it is a
> valve type

Someone who's got more time than me could always analyse the audio from the video clips I posted on this page -- which are of a vavleless using a straight pipe.

http://aardvark.co.nz/pjet/pjet3.htm

Viv · Post by **Viv** » Sun Dec 07, 2003 10:33 pm

Bruce wrote:Mike said:

> I was excited there for a minute,
> Bill. I'm not sure a straight
> pipe will get you a sine output.
> I'm looking at the Schmidt
> paper's pressure trace and it's
> ugly, and I believe it was a
> straight pipe. However, it is a
> valve type

Someone who's got more time than me could always analyse the audio from the video clips I posted on this page -- which are of a vavleless using a straight pipe.

http://aardvark.co.nz/pjet/pjet3.htm

I wont put that reel stuff on my Mac (mac version is poison), if you want it done email me a sound file and I will run it through the FFT for you.

Viv

Mark · Post by **Mark** » Mon Dec 08, 2003 1:03 am

Bruce wrote:> People somehow imagine that
> increased frequency will increase
> power. THIS IS NOT TRUE

Correct -- I believe that one of Tharrat's papers explains this also.

I remember his quote of 2.2 pounds of static thrust/sq in. of mean cross-sectional area. Known ducts from the V-1 to the miniature Dyna-jets of 4 to 5 pounds of thrust confirmed the approximation to a remarkable degree he says.
For the higher frequency range, "is unattainable with chemical fuels, the requirement leads naturally to electric heating using spark-discharge techniques... (page 367)
Also he says, "It is this double independence of frequency which makes the exponential horn such a good reproducing device and this may be an extremely valuable property when banks of resonators are in close proximity to each other, as discussed in the next section."
"This also permits synchronous or programmed injection of heat to banks of ducts and elimintates the stoichastic combustion process associated with chemical heating in the higher frequency regimes."
He talks of using trigitrons. "We are now in a position to note an exciting possibility for, provided the necessary peak power, Pw can be passed, the pulse duration Te may be made very short and extremely high operating frequencies becoming possible, with a proportional reduction in chamber length and, consequently weight."
Lots of dreaming to be sure.
Mark

Mike Kirney · Post by **Mike Kirney** » Mon Dec 08, 2003 4:52 am

High-frequency engines are necessarily less powerful than low-frequency engines because they must be smaller in diameter as well as length. Since tailpipe diameter has a maximum size proportional to duct length, and thrust is proportional to tailpipe area, and frequency is proportional to duct length, you must make a longer, wider engine to gain more thrust, and in so doing also decrease the frequency of operation.

George · Post by **George** » Mon Dec 08, 2003 2:56 pm

Hello !

A quick prÃ©cision: the escopette is the first PJ done by SNECMA in 1950 and had just one evolution: the Tromblon in 1953 (for the name, it's like the Escopette an old rifle with a conical canon).

More efficient, the Ecrevisse series begins in 1953, last version in 1957.

I've read your topics about noise abattement, i would be possible to make an exhaust pipe like it's done for cars, no ?

I've done one very simple and efficient for my land cruiser: i drill the tube and put around a pot with glass wool...

I think that with modern matÃ©rials it would be possible to reduce sound with less weight: mixing the exhaust gasses with fresh air and soften the waves with an absorber around.

Before leaving you, an explanation for the photo down: it's the exhaust pipe of the fliying Ã©crevisse who controls the stability of the "bird cage".
You can see the system here with four spoilers used for jet deflection.

Post by **Mike Everman** » Mon Dec 08, 2003 9:14 pm

Looks like they're actually warping the end of the exhaust in a rudimentary thrust vectoring setup.

Faaaaascinating! (he said, stroking his goatee...)

Viv · Post by **Viv** » Mon Dec 08, 2003 9:43 pm

Mike Everman wrote:It does resonate, and true any one frequency component of the aggregate wave form is sinusoidal, but that does not make the output waveform sinusoidal. The addition of several frequencies make it anything but. Looks like it leans more toward a "sawtooth", which for any significant audible acoustic band cancellation is not good enough, but pressure waves will see the desired amplification/cancellation effect, just not 0x or 2x amplitude.

Chaos will lead to no two waveforms being exactly the same so cancelling will be average only.

Viv

Bruno Ogorelec · Post by **Bruno Ogorelec** » Mon Dec 08, 2003 10:31 pm

Viv wrote:Chaos will lead to no two waveforms being exactly the same so cancelling will be average only.

I agree about canceling. However, no one has answered my central question yet:

If the combustor output is not sinusoidal, what makes two of them synchronize so readily into out of phase operation?

Kentfield has proved in practice that you can make them do so by coupling their intakes, or exhausts, or both intakes and exhausts. It should not be so very easy with non-sine, semi-chaotic output. In fact, it should be damned difficult. But, it isn't.

Could it be the O-ray factor?

Bruce · Post by **Bruce** » Mon Dec 08, 2003 11:00 pm

For those who have trouble seeing how an apparently non-sinsoidal waveform can indeed contain a sinsodial component, this page is excellent:

http://www.clubi.ie/amhiggins/fourier1.html

Note how a sawtooth waveform is built from a fundemental (sine wave) plus harmonics.

What an FFT does is calculate the frequency/amplitude componsition of the (often) many individual frequencies that compose a complex regular waveform. I hope this page will make things a little more obvious.

Post by **Mike Everman** » Mon Dec 08, 2003 11:10 pm

the least energy condition would have to be one-sucks while the other blows. In-phase would be metastable, and you'd have to play games to get them in phase within any reasonable proximity to each other. Typical metastable behavior happens when there is a non-linear effect, here the exit pressure, which, if in phase would be greater than the sum of the two pressure waves in between the exits. It doesn't want to sit on a high energy condition, when chaos dictates they fall off the peak one way or the other.

Viv · Post by **Viv** » Mon Dec 08, 2003 11:11 pm

brunoogorelec wrote:
Viv wrote:Chaos will lead to no two waveforms being exactly the same so cancelling will be average only.
I agree about canceling. However, no one has answered my central question yet:

If the combustor output is not sinusoidal, what makes two of them synchronize so readily into out of phase operation?

Kentfield has proved in practice that you can make them do so by coupling their intakes, or exhausts, or both intakes and exhausts. It should not be so very easy with non-sine, semi-chaotic output. In fact, it should be damned difficult. But, it isn't.

Could it be the O-ray factor?

Ok can I answer a question with a question?

What do people mean about sinusoidal? to me it means the the positive and negative parts of the wave are the same, non sinusoidal meaning the positive and negative excursions of the wave do not match.

But they are still cyclic for this discussion, there is a positive part and a negative part of the wave, so when i say a pulse jet is not sinusoidal i mean the intake part of the cycle is not the same as the combustion part or the exhaust part.

That would be for pressure/vacuum etc, and yes i did throw you a curve with a third part of the cycle but as you know I can only be serius for one paragraph`;-`0

Now the second part, the Kentfield engine looks very similar to a patent on the USPTO site.

That motor was layed out the same and could support a number of engines in claimed antphase.

But the plenums linking the engines together were the important part as it relied on a transverse wave being setup in the plenums to carry out the timing of the whole assembly of engines.

So you did not get exact 180deg phase shifting so it was not a 1 2 beat but rather a 1 miss 2 beat.

The first Ska pulse jet?

Viv

Bruno Ogorelec · Post by **Bruno Ogorelec** » Mon Dec 08, 2003 11:21 pm

Bruce, I have no problem seeing that all cyclic phenomena boil down to an aggregate of various sinusoidal oscillations. My problem is with people who tell me that pulsejet output is not sinusoidal, the noise of two out-of-phase combustors will not cancel and the paired combustors will not work, despite the conviction of the best minds in the history of pulsejet research plus some experimental evidence.

I am aware that the theory behind pairing applies to ideal circumstances (never to be encountered in real engines) but that is the case for most natural occurences. I have never argued that the paired combustors will be quiet or that their output will have rock-steady pressure.

I have only argued that there should be some noise abatement and that the exhaust pressure will be much more even than in a single engine. If that means 80 percent of the noise and half the pressure amplitude at twice the frequency, I think I have won. To me, such a result does not look at all bad.

Bruno Ogorelec · Post by **Bruno Ogorelec** » Mon Dec 08, 2003 11:39 pm

Viv wrote:it relied on a transverse wave being setup in the plenums to carry out the timing of the whole assembly of engines.

After a curved ball, you are now pitching me a transverse one. Come on, Viv, you know as well as I do that there ain't no such thing. There's just a spherical wave squeezing into every nook and cranny it can find, up to and including those seemingly at 180 degrees to its perceived local direction of propagation. Give it a labyrinth, it will dutifuly negotiate a labyrinth. It will be a bit emaciated at the other end, but it will get there.

You won't get me into a ska rhythm. I don't dance. Two left feet.

Bruno Ogorelec · Post by **Bruno Ogorelec** » Mon Dec 08, 2003 11:55 pm

Viv wrote:So you did not get exact 180deg phase shifting so it was not a 1 2 beat but rather a 1 miss 2 beat.

Should be a loooooooooooong plenum to be able to do that, shouldn't it? Just count them wavelengths along the way. How many quarter waves can dance along the plenum length?

How come the waves in the very much longer exhaust don't skip the beat in the same way? Maybe the O-rays lubricate their way, so they just kinda glide out quickly.

When Kentfield was trying (on another design) to get the wave to skip a beat, he had to use ducts so long he had to wind them spirally around the engine.

Nope, I don't see what you are trying to describe. Doesn't make sense to me.

www.pulse-jets.com

Worlds simplest valveless?

Proportion, Frequency, and Power

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