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2nd Potato Gun - A Few Quick Questions for You Pros :)

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: cameronfield » Tue Dec 04, 2007 2:43 pm

So for me if I'm using a 4" chamber and a 2" barrel what should their lengths be? Sorry if I seem stupid...I tried to use the calculator on burnt latke but got confused.

Also do tennis balls travel as far as potatos? I wasn't sure since they're lighter and stuff. I cant wait to see flaming ones flying through the sky at night :).

Also are interchangeable barrels more trouble than they're worth??

Edit: Ok so I did some math and came up with a 10" chamber (4" diameter) and a 49" barrel (2" diameter) using the .8:1 ratio. Does this sound good??
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Unread postAuthor: psycix » Tue Dec 04, 2007 2:54 pm

Yup it sounds awesome!
My combustion cannon has something near those dimensions too.
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Unread postAuthor: cameronfield » Tue Dec 04, 2007 5:50 pm

Ok cool. What about my other 2 questions from above??
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Unread postAuthor: DYI » Tue Dec 04, 2007 7:19 pm

Well, tennis balls have a lower density than potatoes, but are more consistent. If you could find the drag coefficient of a tennis ball and a potato, you could figure it out for sure. Also, please refrain from discussing flaming or incendiary projectiles. They are banned by the forum rules, as well as being very illegal in most countries.

It's more trouble than it's worth to not have interchangeable barrels. They are a necessity on any decent launcher, and unless you want to get into unnecessarily complex breach loading systems, all you need is a male threaded adapter for the chamber and a female threaded adapter for the barrel (to enable breach loading). You really could have figured that out yourself.
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Unread postAuthor: cameronfield » Tue Dec 04, 2007 10:00 pm

Can someone explain how to breech load if it has an adapter? Dont you need that knife thing that sticks out? Or do you just shove it down on the potato?
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Unread postAuthor: octane89 » Tue Dec 04, 2007 10:08 pm

You can jush shove it upon the potato.
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Unread postAuthor: MikeNice » Wed Dec 05, 2007 1:05 am

1)What's the best chamber diameter to use? 3"? Is it practical to use 4" chamber?


4" works just fine, I've used it on all my guns.

2) What's the best location for the sparker? I've read in the middle or in the end cap, but I'm not sure which is right.


I use a piezo electric BBQ igniter wired for dual spark in about the middle of the chamber. I chose not to weaken the cleanout cap.

3) If I want the gun to be 5ft long maximum, what should my chamber/barrel length be? 2ft chamber 3 ft barrel? Or 1.5ft chamber 3.5ft barrel?


I use 18"x4" chambers with a 48"x2" barrel. This is 1.5:1 chamber to barrel ratio. This works very well for me. To keep the length down build an over under. With these chamber and barrel measurements the gun will be about 4' 6" long.

I'm using aerosol for fuel. I'm planning on adding a chamber fan. Any tips/advice on that?


I like Static Guard for fuel, butane refills work well too. Place the chamber fan close to the end, mount it in the cleanout / coupler part. Use a brushless computer fan.

By the way I'm going to do interchangeable barrels so that I can take it apart to transport it (and so I can have different barrel sizes).


Use a threaded union or cam locks to make your barrel detachable. This will also make it so you can have multiple diameter barrels.

Hope this all helps. I'll send you diagrams and parts lists if you want.




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Unread postAuthor: cameronfield » Wed Dec 05, 2007 2:02 am

Thanks for the advice. Diagrams and a parts list would be awesome.

How far do you guys think I could get a potato to shoot in this using aqua net or (old) right guard? Also is it worth upgrading to butane?? It just seems so much more dangerous..
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Unread postAuthor: cameronfield » Wed Dec 05, 2007 4:24 pm

Just went and shot my old gun. Calculated that it shoots at about 330mph, or 224 ft/sec. Not too bad since my friend and I just threw parts together.

Any ideas on the butane? Would it shoot further than aqua net or right guard would? I wanna be able to shoot as far as possible.
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Unread postAuthor: psycix » Thu Dec 06, 2007 8:05 am

Yes, metered fuel will shoot further and more powerfull then hairspray.
But no it wont be much more dangerous.

It will make your gun having way less missfires so its actually safer since the gun can be trusted more.
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Unread postAuthor: cameronfield » Thu Dec 06, 2007 10:56 am

Is propane pretty much the same? Like the little green canisters? It's super cheap at the Wal Mart here...$4 for 2 canisters.

And what exactly does metered fuel mean? And what is MAPP?

Also I've upped the CB ratio to .95:1. Is this a good balance between efficiency and power?

Lastly, will potatoes go further from a 1.5" barrel or from a 2" barrel? Using the same chamber both times with the same CB ratio. I would guess 2" cause it has more mass but I dunno.
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Unread postAuthor: jimmy101 » Fri Dec 07, 2007 11:59 am

cameronfield wrote:And what exactly does metered fuel mean? And what is MAPP?

"Metering" refers to any method that accurately measures the amount of fuel that is put in the gun. For maximum (and consistent) performance the amount of fuel needs to be fairly precisely controlled. With either too much or too little fuel in the chamber the gun won't fire. "Squirt and screw" (aka "spray and pray") fueling means the fuel canister is just pointed in the general direction of the opened chamber and the fuel is sprayed for a second or two. The actual amount of fuel that ends up in the chamber is highly variable and many people spend days trying to figure out the correct number of "one-potato, two-potato, ..." counting required to get the gun to fire at all. A gun fueled via "squirt and screw" will be inconsitent at best. The only advantage of "squirt and screw" is that it is cheap.

Meters can either be fairly complicated devices consisting of various plumbing parts, regulators, valves, gas cylinder etc., or it can be as simple as a $1 syringe. See the SpudWiki on fuel meters, or here or here for info on using a syringe. Both methods will reproducably put the correct amount of fuel in the chamber. This maximizes performance of the gun and, perhaps more importantly, makes the gun much more consistent and less prone to mis-firing.

MAPP is a variant on a welding gas, it has a small advantage over propane in terms of performance. See the SpudWiki on fuels or MAPP.

Also I've upped the CB ratio to .95:1. Is this a good balance between efficiency and power?

In a combustion spudgun there is really no tradeoff ("balance") between power and efficiency. The most efficent barrel for a given chamber will basically be the most powerful. The actual CB ratio does not have to be all that precise. Any CB between about 0.5 and 1.0 generally behaves about the same. See the SpudWiki on CB Ratio for some typical data. Outside the CB range of 0.5~1.0 the performance, measured as the muzzle velocity, starts to drop off pretty quickly. It is not unusual for a 1.5 CB gun to underperform an 0.8 gun by a factor of two in the muzzle velocity.

Lastly, will potatoes go further from a 1.5" barrel or from a 2" barrel? Using the same chamber both times with the same CB ratio. I would guess 2" cause it has more mass but I dunno.

Good question. This involves both the internal ballistics (behavior of the spud in the gun's barrel) and external ballistics (behavior of the spud after it has left the barrel).

A 2" barrel has 2<sup>2</sup>/1.5<sup>2</sup> = 4 times the area of a 1.5" barrel, for the combustion gases to push against. The mass of the ammo also scales roughly as the square of the barrel diameter. So a spud from a 4" barrel has 4x the area and 4x the mass of a spud from a 1.5" barrel. Since acceleration = Force/mass then the two barrels will give about the same acceleration to the spud. But a 1.5" barrel will be 2/1.5 = 1.33 times longer than the 2" barrel for a fixed CB ratio. The longer barrel should give higher muzzle velocities.

How far the spud carries depends on the muzzle velocity and the mass, frontal area and drag coeficient of the ammo.

I wouldn't worry about the range the gun'l shoot. There are more practical things to consider. In particular, spuds that'll fit tightly in a 2" barrel are expensive and not always easy to find. It is much easier and cheaper to get spuds that fit a 1.5" barrel.
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Unread postAuthor: Ragnarok » Fri Dec 07, 2007 1:57 pm

Few points to follow that up.

For a given chamber, the barrel length for the highest power is also the most efficient.
For a given barrel, then a larger chamber will add power - to an extent - but decrease efficiency.

In terms of the ballistics, it has a heavy dependency on the stability of the potato slug. If the round has low stability, there won't be a huge difference.
If they're both perfectly stable, then the 1.5" slug can theoretically go slightly further because for the same C:B ratio for both barrels, then the 1.5" will generate slightly higher velocities.

And jimmy, I'm seeing some mathematical errors in that post.
2<sup>2</sup>/1.5<sup>2</sup> is not ~4, it is 2.25.
You also started talking about 4" slugs?
Also, the ratio of barrel lengths needs to be done with squares as well - so it would be a barrel 2.25x as long as long for the same ratio, not just 33%
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Unread postAuthor: jimmy101 » Fri Dec 07, 2007 2:28 pm

Ragnarok wrote:Few points to follow that up.

For a given chamber, the barrel length for the highest power is also the most efficient.
For a given barrel, then a larger chamber will add power - to an extent - but decrease efficiency.

There is no data to back up this statement. An overly large chamber will decrease performance (measured as muzzle velocity) for a fixed barrel size. The bigger the chamber the slower it burns. There is a small possiblilty that a slightly oversized chamber (i.e., 10% or so) may slightly increase the muzzle velocity for the fixed barrel.

And jimmy, I'm seeing some mathematical errors in that post.
2<sup>2</sup>/1.5<sup>2</sup> is not ~4, it is 2.25.
You also started talking about 4" slugs?

Good catch, i totally hosed the math. The area of the barrel is 2.25 times bigger for the 2" versus the 1.5" barrel. The mass of the ammo is also in about a 2.25:1 ratio.
Also, the ratio of barrel lengths needs to be done with squares as well - so it would be a barrel 2.25x as long as long for the same ratio, not just 33%

Yep, you are right again. A 1.5" barrel would need to be 2.25 times longer than the 2" to get the same barrel volume.

The acceleration on the two barrels would be very similar, which is what I was trying to get at in my last post, the screwed up math still gave the correct answer in terms of the acceleration of the two barrels being about the same; acceleration = (force)/(mass) = (pressure)(area)/(mass) and the area/mass ratio is about the same for the two barrels.

The much longer barrel for the 1.5" barrel would give higher final velocity.
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Unread postAuthor: Ragnarok » Fri Dec 07, 2007 2:54 pm

jimmy101 wrote:There is no data to back up this statement. An overly large chamber will decrease performance (measured as muzzle velocity) for a fixed barrel size. The bigger the chamber the slower it burns. There is a small possiblilty that a slightly oversized chamber (i.e., 10% or so) may slightly increase the muzzle velocity for the fixed barrel.

Although there is no solid confirmed data, that's because doing an experiment like Burnt Latke did, sawing bits of a barrel is easy. One cut, then back to the chrono.
Who wants to cut the end of their chamber, then get a new end cap, put that on, wait for it to dry, work out a new mix, do some tests, and repeat until you're done?

However, there are calculations available that support the statement that scale Latke's results to give an idea of what a fixed barrel situation would be like.
For example, EVBEC can give a reasonable idea. Obviously, it's not ultimately conclusive data, but for a fixed barrel - if you increase the chamber size on EVBEC, the muzzle energy tends to increase. In some cases, especially when the C:B ratio is already extreme, power is lost, but that's sort of expected.
I believe that for a fixed barrel, 1:1 will yield a reasonable, but not huge velocity increase over 0.8:1 of maybe 5 to 10% - and some people like the extra bang that ratio would give.
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