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N33b who needs some pro 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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N33b who needs some pro help

Unread postAuthor: shmoesus » Wed Feb 09, 2011 2:32 am

Hello all, im new to this place and I have many questions but I will ask this, I have made a few potato guns made of PVC and the grill ignition and have had great amounts of fun with it but I have herd that people have made 500 yard shots with the basic potato gun like I have, If true can anyone provide the best fuel and techniques for these Potato guns, Im basically a n33b who wants a potato gun that will fire past 200 -300 yards each shot. I'll take a pic of my Potato gun later and post it!
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Unread postAuthor: jackssmirkingrevenge » Wed Feb 09, 2011 3:34 am

Welcome to spudfiles :)

You seem to have made combustions which work, so you're past the basics, now you just need to optimise certain parameters to get the most out of it.

Have a browse through the wiki page on combustions, especially the parameters under the "Advanced improvements" section.
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Unread postAuthor: Technician1002 » Wed Feb 09, 2011 6:16 am

500 yard shots with a basic combustion with spuds is mostly tall tales. Getting a couple hundred yards to maybe 300 yards is more realistic.

200-300 yards per shot is in the ball park of what these will do.
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Unread postAuthor: jackssmirkingrevenge » Wed Feb 09, 2011 7:02 am

Technician1002 wrote:500 yard shots with a basic combustion with spuds is mostly tall tales.


Agreed, the crucial point being that if you're launching something the mass and shape of a potato fast enough to go that far, it will likely disintegrate well before it exits the muzzle.

I do however see it being more than possible to go further than 500 yards with an optimised (read saboted dart) projectile from an advanced combustion. As previously stated on this forum it's a shame sometimes that people put so much effort into their launchers while neglecting the projectile itself - ie if its performance you seek, it's no use building the best combustion ever if you still insist on firing potatos...

some wag once wrote:The weapon of artillery is the shell, not the gun ;)
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Unread postAuthor: omniscient » Wed Feb 09, 2011 1:45 pm

jackssmirkingrevenge wrote: As previously stated on this forum it's a shame sometimes that people put so much effort into their launchers while neglecting the projectile itself - ie if its performance you seek, it's no use building the best combustion ever if you still insist on firing potatos...


I also agree that people who claim to launch a potato 500 yards seem to have a pretty tough time differentiating between "yards" and "feet".

The Punkin' Chunkin' competitions show people launching pumpkins well over 1000 yards, so I'll have to disagree (to an extant) to jsr's ending statements,

If you want to launch a potato over 500 yards, then the best place to start is with your barrel size/length. 1-1/2" and under for the diameter, and a full length of pipe (at least 10 feet).

Pre-cut your potato in a short section of the same diameter pipe as your barrel and freeze them overnight.
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Unread postAuthor: Technician1002 » Wed Feb 09, 2011 3:26 pm

Nice theory, but the longer barrel won't give you the range. I've measured in barrel acceleration and velocity. I even shortened a barrel to increase muzzle velocity.

Subsonic and transonic all fall prey to wind resistance. Instead of spoon feeding, I'll send you on a mission to find the relationship of speed vs air drag.

Low performance launchers have muzzle velocities between 150 and 250 FPS.

Decent ones outperform paintball markers exceeding 300 FPS to about 450 FPS.

Great spudguns exceed 500-700 FPS.

If you manage to double the muzzle velocity of a great spudgun, you won't double your range, even though the muzzle energy is 4 times greater

Now back to the issue. What rate does the wind resistance increase?

A pumpkin flies far due to it's sectional density. Double the diameter of a sphere and it's cross sectional area increases 4 times. At the same time it's volume increases 9 times.
Area increases as a square. Volume increases as a cube.

I can shoot a bowling ball much further than a bouncy ball due to this.

An easier sample size to see this effect is to throw a rock, then throw a hand full of sand. Which goes far? Think of a spud as sand next to a pumpkin.
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Unread postAuthor: omniscient » Wed Feb 09, 2011 7:28 pm

Cut a full-grown pumpkin open and you'll find the center mostly hollow, so your comparison of throwing a rock as opposed to throwing a handful of sand, is not even close to comparing a spud next to a pumpkin.

I had mentioned freezing a pre-cut potato slug, but only to help prevent the potato from shredding, before exiting the barrel... just to be clear that it had nothing to do with increasing its sectional density.

You were on the right path when you stated that;
"If you manage to double the muzzle velocity of a great spudgun, you won't double your range, even though the muzzle energy is 4 times greater "... but why do you think artillery engineers design barrels that are so long?

They're ridiculously long to give the projectile a chance to reach the same velocity as the flame speed of the expanding gases. Same powder charge, with half the barrel length, results in a shorter distance.

So, since we're on the easier example bandwagon: A .22LR bullet fired out of handgun, will not travel as far as when fired out of a rifle.

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Unread postAuthor: DYI » Wed Feb 09, 2011 7:33 pm

How much does a chunk of potato cut to fit in a 2" barrel normally weigh?

From what I see playing around with HGDT's range calculator, it seems unlikely that such a projectile is going to reach 500 yard range without assistance from wind or topography. Even with a fairly optimistic drag coefficient of 0.5, it doesn't look like a potato is heavy enough. It might be possible with a fairly long-barreled hybrid, but I can't see it happening under "fair" conditions with an "advanced combustion".

EDIT: As to the pumpkin-potato comparison: a simple calculation will show that the pumpkins used in chunkin contests, despite being hollow, have sectional densities twice to three times as high as those of a 2" potato section being fired from a spudgun. Chunkers, by virtue of their ridiculous dimensions, also tend to approach the speed of sound in the propellant gas, meaning that they tend to shoot about 50% faster than a typical good quality advanced combustion.
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Unread postAuthor: shmoesus » Wed Feb 09, 2011 8:04 pm

Wow, guess im dealing with the pro's!
Thanks for all the Info.
OK so I just finished the blue prints to my new potato gun and it looks good on paper but I want to to know 2 things:
1) people use fan's in there potato guns and I have no electronics background really only a little to no experience so I want to know how to make the fan turn on with a battery or some simple rig or even a on/off switch if some one can provide me the plans and step by step procedures.
2) As im new here I would like to share the blue prints and get some feed back from anyone because I have read threw some threads and it seems that everyone here is always happy to help and knows vast amounts on these subjects.
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Unread postAuthor: Technician1002 » Wed Feb 09, 2011 8:09 pm

It is true that a pumpkin is hollow. This leads to many pie in the sky shots as they attempt to go the distance. If a pumpkin was solid, it would fly much better.
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Unread postAuthor: omniscient » Wed Feb 09, 2011 9:35 pm

DYI wrote:From what I see playing around with HGDT's range calculator, it seems unlikely that such a projectile is going to reach 500 yard range without assistance from wind or topography. Even with a fairly optimistic drag coefficient of 0.5, it doesn't look like a potato is heavy enough. It might be possible with a fairly long-barreled hybrid, but I can't see it happening under "fair" conditions with an "advanced combustion".

EDIT: As to the pumpkin-potato comparison: a simple calculation will show that the pumpkins used in chunkin contests, despite being hollow, have sectional densities twice to three times as high as those of a 2" potato section being fired from a spudgun. Chunkers, by virtue of their ridiculous dimensions, also tend to approach the speed of sound in the propellant gas, meaning that they tend to shoot about 50% faster than a typical good quality advanced combustion.


By virtue of the Chunkers "ridiculous dimensions", would you agree that they do not fall under the "fair" conditions with an "advanced combustion" category?

With only a <a href="http://www.portlandcompressor.com/store/images/Product/medium/KT0700.jpg">7 gallon air-carry tank</a> as a Combustion Chamber, venting into a 96 foot-long 1-1/2" SCH40 PVC Barrel, I would bet money that a frozen chunk of potato could easily outgun a similarly-dimensioned pneumatic.

However, building a 96-foot long barrel with no seams every 10 feet, would be ridiculous for either type of cannon. I still stand by my original opinion, that a 10-foot long barrel (on an advanced combustion) could possibly send a chunk of potato over the 500 yd. mark.
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Unread postAuthor: SpudBlaster15 » Wed Feb 09, 2011 10:41 pm

omniscient wrote:I still stand by my original opinion, that a 10-foot long barrel (on an advanced combustion) could possibly send a chunk of potato over the 500 yd. mark.


Nope. A potato's ballistic characteristics are simply too poor for it to achieve that kind of range.

I ran some rough figures through the projectile range calculator I've been working on, and in order to achieve 500 yards range using a 2" diameter 100g potato slug with a rather optimistic C<sub>d</sub> of 0.5, you'll need a muzzle velocity of approximately 1400fps at the optimum firing angle of 28 degrees above the horizontal. My program doesn't account for supersonic flow effects at this stage, so the required velocity will actually be significantly higher. This is not happening with any advanced combustion launcher on this site, and no current hybrid technology can propel a potato slug to such velocities without destroying the frangible structure.

Nobody has ever fired a potato past 500 yards with an ordinary spudgun.
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Unread postAuthor: jackssmirkingrevenge » Wed Feb 09, 2011 11:29 pm

SpudBlaster15 wrote:propel a potato slug to such velocities without destroying the frangible structure.


Jacketed bullets, jacketed potatos... I see the pattern, would it count if you could wrap a spud in copper alloy :D

The Punkin' Chunkin' competitions show people launching pumpkins well over 1000 yards, so I'll have to disagree (to an extent) to jsr's ending statements


That was my point - the fact that they can do that shows how awesome the launchers are in spite of the poor projectiles they are firing - were they not limited to pumpkins, I'm confident the range achieved would more than double.

As its name suggests, the Zalinski-Mefford Dynamite Gun was not concerned with throwing farm produce, rather explosive charges in streamlined projectiles, and reached some impressive range figures:

These guns had a range of 2,000 yards with 500 lbs dynamite, 3500 yards with 200 lb dynamite, and 5,000 yards with 50 lb dynamite.


Image

With only a 7 gallon air-carry tank as a Combustion Chamber, venting into a 96 foot-long 1-1/2" SCH40 PVC Barrel, I would bet money that a frozen chunk of potato could easily outgun a similarly-dimensioned pneumatic.


"pneumatic" is a "length of a piece of string" term though - it depends on the valve and the pressure used.

Anyway, lets not neglect the OP's questions:

1) people use fan's in there potato guns and I have no electronics background really only a little to no experience so I want to know how to make the fan turn on with a battery or some simple rig or even a on/off switch if some one can provide me the plans and step by step procedures


There really is nothing to it - there are 2 wires coming out of the fan, connect the red one to the <sup>+</sup>ve terminal of the battery and the black one to the <sup>-</sup>ve :D Add a switch in between and you're sailing. If you feel like a challenge, you can make a timer circuit.

Another option if your chamber opening is big enough is to just chuck one of these inside as a self contained unit.

Again thewikihas some good advice.

2) As im new here I would like to share the blue prints and get some feed back from anyone because I have read through some threads and it seems that everyone here is always happy to help and knows vast amounts on these subjects.


We are as happy to help as to argue amongst ourselves as you can see :) sadly we can't see your pictures though.
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Last edited by jackssmirkingrevenge on Thu Feb 10, 2011 12:58 am, edited 1 time in total.
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Unread postAuthor: shmoesus » Thu Feb 10, 2011 12:55 am

Ah I see, I have been a fan of any type of firearms since I was 5 years old so I can argue about anything to do with guns, but when I got into Potato guns 2 years ago I realized there is so much science to it that I basically have to start back at N33b level, so when I read this argument I have been slightly researching some of the info mentioned.

With a self contained unit, are you referring to putting the fan inside the combustion chamber itself? If so would that not ruin the fan, I would think it would hold up but then again there is a lot of power being exerted in that part. Also I have not gotten a chance to look at the wiki page yet because I have been studying for college but now that I have a little time to myself I will do so by tomorrow.

And Also Thanks again for helping a new comer, and to those debating weather or not a potato will fly 500 yards, you may proceed because it has been very interesting reading this debate, ... Im like a kid watching cartoons on Saturday morning reading that stuff lol.
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Unread postAuthor: jackssmirkingrevenge » Thu Feb 10, 2011 1:01 am

shmoesus wrote:With a self contained unit, are you referring to putting the fan inside the combustion chamber itself? If so would that not ruin the fan, I would think it would hold up but then again there is a lot of power being exerted in that part.


Image wrote:Will my fan survive the combustion?

If you think that your fan might get burned or melt, don't worry, it wont. Since the heat in the combustion chamber lasts no more then a few milliseconds the fan doesn't have time to heat up or melt. As stated above, your fan should not be too close to the barrel. If its more to the back, it wont get blown to pieces due to the extreme flow.

A fan may have a somewhat shortened lifespan inside a combustion chamber, but it still can survive for many years. So for combustion cannons: Yes, your fan will survive.


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