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More distance

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More distance

Unread postAuthor: Mattmp36 » Sun Jun 17, 2007 11:49 am

Hows it goin? I'm new to the potato cannon world and I love it! I wanted some more distance with my gun. I was able to fire a potato 540 feet. I saw a kid fire one 740 feet. How can I get more distance? I have a PVC cannon: 4"diameter combustion chamber, 18"long and a 2"diameter barrel, 44"long. I have to wear ear plugs when using it! At night, it shoots out a foot of fire! I love that thing! I want more distance with my potato though. Should I make it a 1.5" barrel instead of the 2"? What could do? Thanks.
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Unread postAuthor: lukemc » Sun Jun 17, 2007 11:57 am

add a computer fan in the chamber it significantly increases performance because it mixes the fuel. also if you have the suppplys / money build a propane meter and that will also significantly increase your distance
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Unread postAuthor: SpudBlaster15 » Sun Jun 17, 2007 12:13 pm

A 1.5" diameter barrel of equivalent length will yield a higher muzzle velocity, and thus greater range. Extending the barrel will also increase muzzle velocity. Given your chamber size, I suggest a 1.5" barrel with a length on the order of 10 feet.

However, increasing muzzle velocity does not have as great an effect on range as you would expect, especially with a projectile with such a high drag coefficient as a potato. Don't expect to exceed 250 yards or so.
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Unread postAuthor: Kenny_McCormic » Sun Jun 17, 2007 12:39 pm

what fuel are you using?
if yopur cannon is all pressure rated then try some starting fluid
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Unread postAuthor: Mattmp36 » Sun Jun 17, 2007 12:40 pm

That sounds cool, except the 10 foot barrel. I would like to keep the barrel the same length as the one I have now, just a smaller diameter, so how small would I need to make the combustion chamber?

Merged:

Kenny_McCormic wrote:what fuel are you using?
if yopur cannon is all pressure rated then try some starting fluid


I'm using Oust Airsanitizer. I know it sounds funny, but that is the only thing so far I have found to work very well.
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Unread postAuthor: SpudBlaster15 » Sun Jun 17, 2007 1:12 pm

Mattmp36 wrote:That sounds cool, except the 10 foot barrel. I would like to keep the barrel the same length as the one I have now, just a smaller diameter, so how small would I need to make the combustion chamber?


By switching to a 1.5" diameter, 44" long barrel, you will not boost your range much at all. Based on running some numbers through the launcher range calculator, you could expect to achieve a range of about 150 - 180 yards or so, if you achieved stoichiometry in your air/fuel mixture, which is unlikely to occur when using aerosol fuels.

However, with a 10 foot barrel, you may be able to achieve a range of close to 250 yards.
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Unread postAuthor: Modderxtrordanare » Sun Jun 17, 2007 1:28 pm

If I did my math correctly, then your gun as it stands has a 1.6:1 C:B ratio.

If you cut your chamber down to 12" that should increase your preformance. Also, as it's been suggested, try building a metered propane system, that'll help you alot.

(I don't know if I did my C:B ratios right, since I'm more of a pneumatic person)
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Unread postAuthor: rednecktatertosser » Sun Jun 17, 2007 2:11 pm

I believe the ideal C:B ratio is around .7:1 so if you shorten your chamber to 7 or 8 inches you should be right on the money and you should see a boost in perfomance

and modder, yes your calculations were correct he is currently at a 1.6:1 ratio, which is huge.

also if you shorten your chamber you should notice that it wont be as noisy.
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Unread postAuthor: SpudBlaster15 » Sun Jun 17, 2007 2:46 pm

Reducing the volume of the chamber will not make a noticeable difference in performance. You may even notice a decrease in performance.
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Unread postAuthor: rednecktatertosser » Sun Jun 17, 2007 2:48 pm

I thougt that if he changed it to the 'ideal' C:B ratio it would increse performance.
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Unread postAuthor: SpudBlaster15 » Sun Jun 17, 2007 3:10 pm

That is only the case is the chamber is oversize by a considerable degree. Research has not been done to establish the exact ratio at which this occurs in a conventional cannon.
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Unread postAuthor: rednecktatertosser » Sun Jun 17, 2007 3:13 pm

Ok, well I was passing along info I have read here and on spudtech, I assumed it was correct and havent really done anything with combustions personally, so i dont really know for sure.
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Unread postAuthor: jimmy101 » Sun Jun 17, 2007 4:33 pm

SpudBlaster15 wrote:That is only the case is the chamber is oversize by a considerable degree. Research has not been done to establish the exact ratio at which this occurs in a conventional cannon.


Spudbalster's second sentence is correct, nobody has ever measured (TIKO) variable chamber on a constant barrel.

My combustion model suggests that C:B ratios in the range of 0.6 to about 1 are all essentially equivalent on a constant barrel. (Similar to Latke's results with constant chamber; relatively little difference in the range of about 0.6 to 1.0.) "Essentially equivalent" means the muzzle velocity changes by less than ~5% over that CB range.

At a C:B of 1.5:1 the muzzle velocity drops by ~10%.
At a C:B of 3.0:1 the muzzle velocity drops by ~30%.

So, by my model, on my standard gun, up to a ratio of about 1.5 there is minimal difference (though 1.0 is better than 1.5). Above 1.5 the performance starts to drop off pretty quickly.
<hr>
Back to Mattmp36 post. A chamber fan is probably the easiest way to get the maximum performance out of a particular combustion gun.

A fuel meter would be second most important. (A syringe is a cheap way to measure fuel, less than $2 total cost, and is at least as accurate as a much more expensive meter setup.)

The actual fuel makes relatively little difference as long as you are using the correct amount and are mixing it well.

A double bevel spud cutter might give you a little more range.
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Unread postAuthor: boilingleadbath » Sun Jun 17, 2007 8:46 pm

Your chamber is what many would consider oversize, and the flame emerging from your barrel suggests:
1) an overly rich fuel mix (get a metering system)
2) incomplete combustion (get chamber fan and/or multiple ignition points)

#2 will help alot; latke noted much benefit from a chamber fan, and multiple ignition points will almost certainly help (especially with an oversized chamber).
***********

As a quip for contention:

"there are no oversized chambers - only poor ignitions."
Even with the slow combustion model.
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Unread postAuthor: roughboy » Sun Jun 17, 2007 9:30 pm

Or build a pneumatic cannon coz it's more more powerful and not as loud as a combustion.
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