Thrust per CC diameter

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Mike Everman
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Thrust per CC diameter

Post by Mike Everman » Wed Jun 01, 2011 7:18 pm

I attempted to put some of Kentfields graphs into a form that we could use to predict theoretical maximum thrust, given a CC diameter. (SLS: sea level standard)
I've been trying to analyze the 4 pocket jet scaling attempts to make sense of them, and this was a logical first step. Using the power fit and equation display of the curve gets you this (roughly) .62xCCdia^2.7 (lb.)

What I don't know is if his predictive method assumed some expansion on the tail; I should re-read that.

I then looked at my (straight pipe exhaust) pocket jets, and made some interesting conclusions:
The thrust per CC area hovers around 1.2 lb per in^2 on all.
As size increased, I see a steadily declining percentage of the theoretical max discussed above. So more experimentation required. Variables violated there are that the largest one had a non-uniform length scale (longer than the diameter scale. shorter than the CC area scale). the second unit was scaled 1.2x on everything, but still less percentage of theoretical max.

Either the theor. max is not applicable to straight pipers, or it's this is just a neat linear 1.2 lb/sqin CC in this format.

Variables I wish I did not have were related to available tube sizes, though CC's were custom diameter to the numbers, so confinement ratios (total exit area/cc area) were not constant, unfortunately. Also, my notions on mixed scaling at the time made for some odd mixed scaling I cannot defend anymore, if I could then!

The 1.2 lb/in^2 cc though is very interesting. Didn't I see that rule of thumb number in the valve types? that would be cool.

I think a caliber number of 15-16 is really good for overall length to CC diameter, on straight pipers at least.
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Re: Thrust per CC diameter

Post by PyroJoe » Thu Jun 02, 2011 1:11 pm

Mike,
If I am reading this correctly the overall length is 9 CC diameters, does this include the intake length?
Thanks, Joe

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Re: Thrust per CC diameter

Post by Mike Everman » Thu Jun 02, 2011 2:08 pm

Yep, that's right. None of my attempts to put tail expansion succeeded, though. So it applies just to straight pipers, but you can fold it into a Chinese layout, which I did and works great.
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Re: Thrust per CC diameter

Post by PyroJoe » Thu Jun 02, 2011 3:28 pm

Hmmm, maybe I'm missing something, but the pocket jets look to be longer than 9 CC's.
9 CCs is a real shorty.
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Re: Thrust per CC diameter

Post by Mike Everman » Thu Jun 02, 2011 4:24 pm

Hmm, gotta drag out that geometry and check. I thought the smallest one was 17.3 long. I may have looked at the wrong number. Got to do that later, be back to you tonight.
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Re: Thrust per CC diameter

Post by PyroJoe » Thu Jun 02, 2011 4:33 pm

No worries, I could very well be mistaken, but 9 CCs sounds pretty short.

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Re: Thrust per CC diameter

Post by Mike Everman » Thu Jun 02, 2011 7:58 pm

Thanks for catching that, Joe. I had a whole column listed as length, but the 17.3 was tail length. I fixed my post below to say length in calibers 15-16, but more precisely the best one was 15.72 on the acoustically corrected length, and 15.4 on the physical length. I do not yet know how sensitive it is up or down from there.
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Re: Thrust per CC diameter

Post by PyroJoe » Thu Jun 02, 2011 9:14 pm

No worries,
The caliber/recipe method produces near 14, I think Bill H. is/was building near 21. I just couldn't figure out how you got down to 9, ha

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Re: Thrust per CC diameter

Post by Mike Everman » Fri Jun 03, 2011 12:47 am

Much like the Thermojet, these short straight pipers must have horrible fuel consumption.
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Re: Thrust per CC diameter

Post by hinote » Fri Jun 03, 2011 2:42 am

PyroJoe wrote:No worries,
The caliber/recipe method produces near 14, I think Bill H. is/was building near 21. I just couldn't figure out how you got down to 9, ha
I wasn't gonna reply to this thread--but I did so much research on this and found considerable success in my own little corner of the pulsejet world.

I determined that the period allowed for combustion was limited, as well as for several other significant events during the pulsejet cycle; due to this time constraint the diameter of the CC became directly "coupled" to the frequency--and thus also to the duct length (directly related as you should know) My belief continues to be that each of these events has to be allowed its own time period--and "crowding" any one of them significantly with another can only lead to reduced performance in terms of power and/or efficiency.

I can't comment on other PJ types than the "escopeta" since I concentrated on that design exclusively; nonetheless, I believe a similar scenario may be appropriate for other types.

It was certainly a fascinating time for me, and my little successes were a worthwhile fruit to savor. I won't be able to respond to specific questions about my efforts at this point due to the passage of time and the receding lack of familiarity with the variables involved.

I wish the best of luck to all PJ experimenters!!

Bill

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Re: Thrust per CC diameter

Post by PyroJoe » Fri Jun 03, 2011 1:33 pm

15.4 looks good for power to package size. My chamber diameters are a little fat, and the intakes are short so if I corrected, it would be close to 15.4

If the stationing is scaled on Bills 21 then it looks to be some what a fraction body of what we are using.

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Re: Thrust per CC diameter

Post by PyroJoe » Fri Jun 10, 2011 2:36 pm

FWIW
Looks like a 1/3 body from a Dynajet would be hitting about 14.75 with a fat CC akin to what I've been using. I think this one is a little bit better/more accurate interpretation than the last one I tried. Its hell trying to figure out how much CC cylinder to cone ratio there is.
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