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C:B?

A place to ask general spud cannon related questions.
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C:B?

Unread postAuthor: Ecstasy » Thu Mar 27, 2008 4:31 pm

So, I've heard that a C:B ratio of 1.5:1 is good but how come this article http://www.spudfiles.com/spud_wiki/inde ... rrel_ratio shows that a 0.6-0.8:1 ratio is better?
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Unread postAuthor: Hubb » Thu Mar 27, 2008 4:47 pm

I didn't look at the article, but the information probably came from Burnt Latke. In the top, left corner is a navigation bar. Scrolling all the way down, it will show tests that he conducted.
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Unread postAuthor: Ecstasy » Thu Mar 27, 2008 4:58 pm

Yea it shows .8:1 C:B as the best ratio. I guess I'm confused because on RatCalc it has 1 : x as a constant on the website.
Image

According to my calculations, the test they did was with a 1.5" barrel at 8 feet (96 inches).

All we're given is that it's a 159ci chamber which in turn would be a 4" diameter 12.75" length chamber... or 3" diameter 22" length chamber.

Now thats for 0.8 ratio as noted in the value box
C:B - calculated: 1 : 0.837

So I'm just wondering if its actually 1.5:1 or 0.8:1 is actually better.
(Although isn't that technically like... 1:0.8 chamber to barrel ratio on the tool? I don't know it seems as if it's backwards.)
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Unread postAuthor: Hubb » Thu Mar 27, 2008 5:06 pm

I've never tested a combustion launcher to see what ratio is the best but here is my theory: People come up with different ratios because the test under different conditions, use different materials, and have an idea of what they think is the best conditions.

It would seem to me that the best ratio is the one that you are the most comfortable with and the most perfect conditions are the ones that you set forth.
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Unread postAuthor: psycix » Thu Mar 27, 2008 5:11 pm

1.5:1 is loud and wasting energy, 0.5:1 is silent and more efficient, 0.8 is the most powerful.
A too large chamber takes too much time to burn.
A too small chamber is just... too small and doesnt give you enough power.
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Unread postAuthor: Ecstasy » Thu Mar 27, 2008 5:11 pm

The difference between the same test with variable barrels is that a .08:1 and a 1.5:1 ratio is the difference between a 96" barrel compared to a 54" barrel lol. But the .08:1 calculations seem accurate and reliable so I think I'll stick with those measurements unless anyone else has some input =)
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Unread postAuthor: Ecstasy » Thu Mar 27, 2008 5:12 pm

Ah okay, .08:1 it is. Still the RatCalc thing being backwards listing it as 1 : x instead of x : 1 is throwing me off, but I think I'm set.
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Unread postAuthor: Hubb » Thu Mar 27, 2008 5:19 pm

I do believe the Ratcalc is backwards.

It may not matter, but I've always built my combustions (and hybrid :wink: ) around a.8:1 ratio.
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Unread postAuthor: Ecstasy » Thu Mar 27, 2008 5:20 pm

Hey one more thing, for figuring the length of a propane meter on FuelTool, should I leave tank pressure as 90 psi or bring it down to 30 or 50 (figuring the pressure under regulated conditions)
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Unread postAuthor: Ecstasy » Thu Mar 27, 2008 5:23 pm

Actually thinking about it, the regulator will more than likely be able to handle 90+ psi so I'll configure it for 80psi or so to factor in some kind of pressure drop for cold conditions.
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Unread postAuthor: Hubb » Thu Mar 27, 2008 5:33 pm

I built my meter, then did the math. The reason was this, gas seems to fluctuate with temperature. One day, it could be 85psi (which is what you built it around) and the next, it could be 75psi (which throws off the whole thing.

I say, set it around 65 or 70psi, just to be sure it will get there.

My meter does not contain a regulator, although they are very convenient. I simply open the first ball valve until the pressure gets right. I have a popoff on the meter to release some propane if I overfill the meter.
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Unread postAuthor: Ecstasy » Thu Mar 27, 2008 5:44 pm

So you think I should just build a meter that will fit properly on the chamber and then figure out the pressures and all that? Right now I have figured that:

Chamber = 12.75 inches long, 4" diameter
Barrel = 96 inches long, 1.5" diameter
= 42 incher long, 2.5" diameter
Meter = 3/8 inch diameter = about 6.82 inches long which seems good

Not sure if I want to make this a straight through design or a bottom chamber looping to a top barrel design though.
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Unread postAuthor: Hubb » Thu Mar 27, 2008 6:57 pm

With those sizes, you may want to consider an over / under. In a straight through design, your cannon is going to be 11 and a half feet long!

As far as the meter, yes.
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Unread postAuthor: Hydra » Fri Mar 28, 2008 1:10 am

When you say 0.8:1 is the best, is the chamber 0.8 times smaller than the barrel? or visa versa...
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Unread postAuthor: Hubb » Fri Mar 28, 2008 1:23 am

C:B, or chamber to barrel, ratio. This means the chamber's volume (not length or diameter) is 8/10ths the size of the barrel's volume.

Example: The barrel volume is 10 cc. To obtain a .8:1 ratio, the chamber would need to be 8cc.
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