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cb ratio

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Unread postAuthor: Ragnarok » Fri Oct 30, 2009 8:13 pm

D_Hall wrote:LOL... No, just an inadvertent usage. The "pink elephant" became a bit of an inside joke amongst some friends of mine many years ago.

Not that it doesn't work here, given the forum has the now near two year old* joke about the Flaming Pink Elephant about claimed "subtlety" of the double entendre on the forums.

*It'll have its birthday on the 13th November - anyone want to make a cake?

Ever since, an ablaze and florescent pachyderm has been appearing in forum conversations ever since, usually around about the time anyone has made any lewd comments, but it can appear... well, any time.

And as I said a while back, it features in one of the best quotes of/about the forum yet:

jackssmirkingrevenge wrote:I love this forum, starts out as ideas for a new combustion, then in tramples a fluorescent pink elephant and before you know it, we're hunting raptors.
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Does that thing kinda look like a big cat to you?
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Unread postAuthor: D_Hall » Fri Oct 30, 2009 8:27 pm

Ragnarok wrote:Not that it doesn't work here, given the forum has the now near two year old* joke about the Flaming Pink Elephant about claimed "subtlety" of the double entendre on the forums.


In my case the term originates from a party we went to in college. One gal decides to strip to her underwear. Normally that'd be a good thing, but in this case she was... We'll be kind and say "a large girl." And it turned out that she was wearing a matching panty and camisole set that were... Yup, pink.

So we're sitting around BSing and we're all just trying not to stare (not the good kind of stare).

Thus, ignoring the pink elephant in the middle of the room. :D
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Simulation geek (GGDT / HGDT) and designer of Vera.
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Unread postAuthor: CS » Fri Oct 30, 2009 9:05 pm

Spud Wiki wrote:Experiments have shown that a C:B ratio of about 0.6-0.8:1 is the most efficient for a given chamber using potatoes as projectiles. The ratio will vary somewhat depending on the weight, friction, diameter and blow by of a particular projectile, as well as the fuel, ignition type and perhaps other variables. In most cases though, the optimal C:B is expected to be in the 0.6 to 0.8 range.


Chamber to barrel ratio - Spud Wiki

Please read Technician1002's post below. These tests are wacky.

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Last edited by CS on Fri Oct 30, 2009 11:55 pm, edited 1 time in total.
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Unread postAuthor: Technician1002 » Fri Oct 30, 2009 9:31 pm

pimpmann22 wrote:
Spud Wiki wrote:Experiments have shown that a C:B ratio of about 0.6-0.8:1 is the most efficient for a given chamber using potatoes as projectiles. The ratio will vary somewhat depending on the weight, friction, diameter and blow by of a particular projectile, as well as the fuel, ignition type and perhaps other variables. In most cases though, the optimal C:B is expected to be in the 0.6 to 0.8 range.


Burnt lake was a fantastic start, but didn't vary the other variables. They did fuel mix, mapp vs propane, and barrel lengths on a fixed chamber size. They didn't do variable chambers on fixed or variable barrel sizes. Some of the results are correct in C:B ratios, but some are simply barrel. A 10 foot GB barrel and a 100 foot GB barrel on a 500 gallon 100 PSI tank will show the 10 foot barrel is better. What kind of implied CB ratio does that give? 100 feet of 1.5 inch pipe has serious flow limitations.

Burnt Lake simply found the best barrel lengths for a couple sizes of chamber. A larger chamber would have produced another set of answers similar to the first, but with slightly differing C:B ratios.

Is the CB ratio independent of overall barrel length? Flow resistance becomes an issue in long barrels. We noticed this when selecting barrels and clocking them on the T shirt cannon (2 inch 3 gallon QDV) Longer barrels would have implied better performance with the 3 gallon volume, but we saw the effects of the barrel choking flow. In one case the 10 foot barrel and 7 foot barrel are almost exactly the same power.
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Unread postAuthor: jimmy101 » Sat Oct 31, 2009 2:09 pm

You must also remember that CB=0.8 is the most efficient. (Did I emphasize that enough?) Since 99% of combustion spudders don't give a damn about efficiency it is easy to think that 0.8 is important. It really isn't, it's just a reference point.

Who really gives a crap if it takes $0.005 or $0.006 worth of fuel for each shot?

If you double the chamber size (double the CB ratio) the efficiency of the gun will drop a lot, but the performance of the gun will increase.

You can't extract that observation from Latke's numbers since he never studied keeping everything constant except the chamber volume.
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Unread postAuthor: Ragnarok » Sat Oct 31, 2009 3:49 pm

D_Hall wrote:Thus, ignoring the pink elephant in the middle of the room.

That makes quite a horrifying, but amusing anecdote...
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Unread postAuthor: psycix » Sun Nov 01, 2009 7:38 am

Latke (or someone else) should study power VS chamber size with a fixed barrel.
With one spark gap, increasing the chamber too much will decrease power at some point: it takes too long for all the fuel to burn.
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Unread postAuthor: Technician1002 » Sun Nov 01, 2009 12:23 pm

psycix wrote:Latke (or someone else) should study power VS chamber size with a fixed barrel.
With one spark gap, increasing the chamber too much will decrease power at some point: it takes too long for all the fuel to burn.


Due to time constraints and the lack of a chronograph, I have toyed with progressively filling my 2 inch cannon with water to reduce the effective chamber size while using a fixed barrel length and pressure. It won't give the same results as a combustion, but the curves should have something in common.

Then repeat it with a shorter and longer barrel to see if there is an optimum barrel diameter/length ratio for large chambers. Is there an optimum barrel length? GGDT suggested the 2.5 inch barrel was too low of volume for the 3 gallon tank, but on the 2 inch valve, there was little performance gain from 7 foot to 10 foot. GGDT predicted best launches at about 15 feet. The valve and barrel flow limits the practical barrel length to less than the expected in GGDT.

The data would be used when I cut up the 8 gallon propane tanks. I intend to make a 3 inch and 4 inch valve. If the 3 inch is too restrictive, I may skip it and just make the 4 inch and use one as an air tank.
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Unread postAuthor: jimmy101 » Mon Nov 02, 2009 6:02 pm

Tech

The water is an interesting idea.

I suspect the main reason nobody hs done the variable chamber combustion study is that it's too much work. Latke's studies were a fair amount of work and he just chopped the barrel shorter and shorter.

To change the chamber is a lot harder to do. Water just might work.

Need to figure out how to protect the fan and perhaps the spark gap(s).
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Unread postAuthor: SpudBlaster15 » Tue Nov 03, 2009 12:36 am

Anyone know what happened to boilingleadbath? Back in 2007, he was in the process of building an adjustable volume chamber with movable spark gaps, and was planning to perform tests on both variable chamber size with a fixed barrel, and spark gap placement/quantity. However, it didn't materialize, and BLB has seemingly disappeared.
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