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Ratios? Please help

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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Ratios? Please help

Unread postAuthor: Potato Head » Sat Sep 15, 2007 4:20 pm

Hi I am a rookie builder and I want to make the most powerful gun. I know that 1.5:1 is the best ratio. Can someone calculate the math for me so that I can see the equations to help better understand. I want to try different sizes. I would like to try a 4" ID and a 3" ID chambers. I would like to use a 2" ID and a 1.5" ID barrel. How long would both the chambers and barrels have to be for the guns. So if possible 4 different math problems.

4" w/ 2"
4" w/ 1.5"
3" w/ 2"
3" w/ 1.5"

What would be the best design in your opinion, for example most practical and best looking? I would like to make the barrels interchangable and thats why I am asking about different sizes. Thanks for all the help!
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Unread postAuthor: hi » Sat Sep 15, 2007 4:59 pm

http://www.online-calculators.co.uk/vol ... volume.php

all you need is a little can-do attitude. :wink:

you acually want more like a .7:1 ratio if you are going for power. .5:1 will work just fine, but thats nore of a noise maker than for power, though it will still be very powerful.

i persomally think that either a 4'' chamber and a 2'' barrel or a 3'' chamber and a 1.5'' barrel is best.

if you plan to just launch potatoes then go with the 1.5'', it tends to fit potatoes better than 2''.
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you know you are not an engineer if you have to remind yourself "left loosy righty tighty"
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Unread postAuthor: Potato Head » Sat Sep 15, 2007 6:06 pm

Thanks hi for the calculator!

Ok so now I have a rough outline of what I would like for dimensions.

4" chamber in diameter, that is 18" long is 226.28 inches squared.

Now if I put a 2" diameter barrel that is 36" long I have 113.14 inches sqaured.

So in order for my launcher to have a 7:1 ratio I need my 2" barrel to be 32" long? The volume of that is 100.57. I divided 226 by 7. Is my math correct and if not how do I figure out my ratio. Thanks for all the help!
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Unread postAuthor: SpudBlaster15 » Sat Sep 15, 2007 6:21 pm

If you are going to be using aerosol fuels, use a ratio of 1:1. .7:1 (not 7:1) is used for combustion launchers that use a metered fuel, and 1.5:1 will be loud and inefficient.

In order to calculate the required length of your barrel from a known volume and diameter, simply use basic algebraic laws to rearrange the equation that is used to calculate the volume of a cylinder, V=pi*r<sup>2</sup>*L, where V = volume, r = radius, and L = length. Your new equation is L=V/pi/sqrt(r).

Using this equation, we find that a barrel with a volume of 226 cubic inches and a diameter of 2" has a length of 226/pi/sqrt(1) = 72".
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Unread postAuthor: boilingleadbath » Sun Sep 16, 2007 3:12 pm

If you want the most powerful gun possible, you build the largest one you can and feed it heavy ammo.

Large, because that gives it more chamber volume (for less pressure loss during launch), heavy ammo because it gives the fuel more time to burn.

Oh, and putting a fan in the chamber works wonders.
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Re: Ratios? Please help

Unread postAuthor: Velocity » Sun Sep 16, 2007 5:50 pm

Potato Head wrote:Hi I am a rookie builder and I want to make the most powerful gun . I know that 1.5:1 is the best ratio. Can someone calculate the math for me so that I can see the equations to help better understand. I want to try different sizes. I would like to try a 4" ID and a 3" ID chambers. I would like to use a 2" ID and a 1.5" ID barrel. How long would both the chambers and barrels have to be for the guns. So if possible 4 different math problems.

4" w/ 2"
4" w/ 1.5"
3" w/ 2"
3" w/ 1.5"

What would be the best design in your opinion, for example most practical and best looking? I would like to make the barrels interchangable and thats why I am asking about different sizes. Thanks for all the help!


1) Your not gonna be able to make the most powerful gun as a regular combustion.
2) 1.5:1 is not the best ratio; usually, people think that 0.7:1 is the best ratio.
3) We need to know more about how big you want your launcher to be able to do the math.
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Unread postAuthor: Potato Head » Sun Sep 16, 2007 11:19 pm

I would like my combustion chamber to 4" in circumference. How long would my barrel have to be for .7:1. I would like two diameters for the barrel to be 2" and 1.5"

I will attempt to do the equation, please correct me if I am wrong.

sch. 40

4" diameter chamber at 7" long = 87.8 ci
2" diameter barrel at 36" long = 118 ci

so my chamber to barrel ratio is .74:1 therefore is relatively the most powerful it could be at that size?

1.5" diameter barrel at 62" long= .71 This equals .71:1. Right ratio?


Thanks for all the help!
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Unread postAuthor: octane89 » Tue Sep 18, 2007 12:26 am

The basic formula is---

3.14 (pie) x radius(squared) x length (in inches)

For example a 2" barrel, at 25" long is....
3.14 x 1 x 1 x 25= 78.5

A 4" chamber at 15" is....
3.14 x 2 x 2 x 15= 188.4

That's 188.4/78.5. That is equal to 2.4 to 1 ratio.

From my expirence, that combination makes a great combustion cannon.
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Unread postAuthor: SpudBlaster15 » Tue Sep 18, 2007 1:04 am

octane89 wrote:From my expirence, that combination makes a great combustion cannon.


That combination will produce a cannon that is extremely loud and inefficient. If power is what you desire, a longer barrel is a must.
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Unread postAuthor: octane89 » Tue Sep 18, 2007 1:29 am

I have one here, fires over 300 yards. But I have seen a 1.5" barrel do a bit better.
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Unread postAuthor: markieclarkie » Fri Sep 21, 2007 1:49 am

I have a combustion with a 3"X18"chamber A 1.5"x48" barrel. (rifled) it is a breech loader with the inside of the barrel knife slightly chamfered. it is running a 1.5 to one ratio and has dual ignition. with the spark screws located exactly 6" from either end of the chamber, and with the gap at the exact center of the chamber. All those extra goodies, resulted in a distance increase of about 50 yards. before I made that one the best distance I ever got was 200yds.
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Unread postAuthor: SpudBlaster15 » Fri Sep 21, 2007 1:53 am

octane89 wrote:I have one here, fires over 300 yards. But I have seen a 1.5" barrel do a bit better.


There is no way you are achieving 300+ yards using a spray and pray launcher with a 25" barrel. I am sorry, but you have severely overestimated your range. With such a configuration, you would be lucky to get 100 yards of distance.
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Unread postAuthor: markieclarkie » Fri Sep 21, 2007 2:07 am

I would tend to agree with Spudblaster15.
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Unread postAuthor: Guest » Fri Sep 21, 2007 9:18 am

Hi I'm new and I already got 2 combustion spud gun !

Now i want to make one bigger and stronger ...

i have a 4'' chamber 18'' long = 226 ci
with a 2'' barrel 25'' long = 157 ci

226/157 = .69

Is That ok ?? or i'm really out of track ?

And i want to know if i cant put a 4'' chamber with a 1.5'' barrel without the whole explosion and suffering thing !!!
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Unread postAuthor: Hubb » Fri Sep 21, 2007 9:37 am

That is a pretty good ratio. Should have plenty of power.

As long as you are not using solid propellants, you should not experience the "whole explosion and suffering thing."
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