Theoretically scaling pulsejets UP

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Theoretically scaling pulsejets UP

Post by Jonny69 » Wed Aug 09, 2006 1:48 pm

This is a purely theoretical thread as while a lot of us have scaled jets down and had them run, not many of us have been able to scale them up higher than a few hundred pounds of thrust. The highest documented thrust I have encountered in my research has been about 900lbs in a NACA paper which was, IIRC, an 18" diameter exhaust tube, a bit of a monster I'm sure you'll agree and it must have been frightening to operate.

Lets not concern ourselves too much with pulsejets of that order, lets take it one step further into ground that has not been visited, to pulsejets that might operate at just a few Hertz and have a look at what happens inside. Perhaps some of you with U-flow or similar will be able to model a giant combustion tube and have a look at the characteristics.

My take on this subject is that in a much larger tube operating at lower frequencies we may be able to make a much more efficient engine due to the slower speed at which events occur. The time period between combustion cycles is a lot higher leaving us with more time to mix and inject fuel at the right moment and the added size of the engine allows us to inject the fuel at a more accurately placed position. Engine dimensions become less critical as an added millimetre is unlikely to have any effect whatsoever and we may able to design in direction rectification or pressure/speed alterations along its length.

We are also talking about much higher masses of fluid in the tube and much higher volumes flowing in and out during the cycle which may result in much higher compression at the combustion point. Could this ultimately lead to a detonation if the compression rose high enough?

Lets take a tube operating at 1Hz and have a look at it. The wavelength of 1Hz in air is 340 metres so we have a duct that is some 85 metres long to sustain resonance at 1Hz. From the point of ignition to a point in time one second afterwards quite a significant set of tasks have to occur. A deflagration of immense proportions is triggered, an earth shattering shockwave travelling at the speed of sound heads towards the exhaust end, an incredible vacuum occurs in the combustion area causing the intake duct to suck in gallons and gallons of fresh air, kilos of fuel have to be pumped in, the shockwave returns down the exhaust massively compressing the giant fuel/air mixture, it ignites and the process repeats.

Now this has been incredibly fun typing this and I'm almost certainly wrong so is there an upper size limit to pulsejets? Surely an engine of this magnitude wouldn't operate and if it did what would the thrust/time curve look like? Each second the thrust would leap from a building-shattering figure to zero, probably destroying its mountings in the process.

So lets have some some with this one, lets discuss what happens at the the upper limits of pulsejet operation and what happens when you reach and breach those limits.

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re: Theoretically scaling pulsejets UP

Post by larry cottrill » Wed Aug 09, 2006 2:21 pm

Jonny -

Well, that's certainly fun to think about. I don't know if UFLOW1D can be pushed that far. To get 1 hz, though, your engine would have to be even longer, since the speed of sound is WAY faster in hot, thin air than in normal air. You would have to "shock mount" such an engine, with something like automotive suspension springs and motion dampers, but acting horizontally rather than vertically.

I personally doubt that internal pressure swings would be much better than what we get in small engines. It can be shown that the Argus engine, in terms of actual performance, wasn't as good as a Dynajet, in terms of pressure development (of course, engineers after the war were able to make enormous improvements - but not anything like an order of magnitude over the way the Dynajet operates, just somewhat better). Large size means large power, not because of higher and lower pressures, but because large gas volumes are moving, and because speeds can be significantly higher in large ducts than in small ones (all other factors being equal). The alternating structural FORCES would be horrendous, of course.

Reminds me of Jules Verne's From the Earth to the Moon, where a big cannon dug into lower Florida provided all the impulse needed to get you there and back. A heck of a bang at launch.

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re: Theoretically scaling pulsejets UP

Post by vhautaka » Wed Aug 09, 2006 6:40 pm

Purchase a missile silo complex and join a couple of the launch tubes at the bottom. Then some creative steel and concrete filling to form some cones.

I understand some of those places even have earthquake / nukewar protections in the form of giant shock absorbers, and are also supposedly designed to not excavate themselves in case of a rocket or fueling failure.

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