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Crazy Differences

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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Crazy Differences

Unread postAuthor: noname » Tue May 22, 2007 7:52 pm

I made two basic combustions over the weekend, and decided to start making a few more.

The first one:
Chamber: 1 foot of 3" ABS
Barrel: 3.5 feet of 1 1/2" ABS
Ignition: Single Spark BBQ Igniter, spark at the middle of the chamber
No Chamber fan
No Burst Discs
Cool Black and Red paintjob
Really cool minigun style handle with the BBQ igniter mounted in it
Hairspray as fuel

The second one:
Chamber: 1 foot of 3" ABS
Barrel: 3.5 feet of 1 1/2" ABS
Ignition: Dual Spark BBQ Igniter, one spark near the back, the other near the frontish-middle section
No Chamber Fan
3 layers of aluminum foil as burst discs
Cooler Black and Red Painjob
Cooler handle with all wires hidden
Hairspray as fuel

The first one was a prototype, and has screws all over the chamber from trying to rig up ignition. The second was when I was more experienced and has no screws except for two holding the handle to the gun and creating spark gaps. The second was made extremely clean because I am now selling them, for $45-60.
The first one shoots a rifled potato bullet about 150 yards, while the second one shoots an amazing 250 (measured with Google Earth)!

Anyone else have performance differences similar to this?

I'll post pics of both of them as soon as my mom finds her camera. :lol: :roll:
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Unread postAuthor: paaiyan » Tue May 22, 2007 7:56 pm

You can measure that with Google Earth? For serious? Post pics of them, we can't tell how clean they are unless we can actually see them. And do you really need a burst disk when you're using hairspray?
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Re: Crazy Differences

Unread postAuthor: Velocity » Tue May 22, 2007 8:09 pm

noname wrote:Cooler Black and Red Painjob


I'm not sure I can believe that; remember the pink and brown paintjob on your coaxial :lol: . Just kidding, I am sure it is nice.

I'll comment more when I see the pictures.
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Unread postAuthor: spudthug » Tue May 22, 2007 8:19 pm

hehe rmich that made me laugh...but this is no laughing matter..this is super cerial...damn i forgot what i was going to say...

as my sig says...im not retarded, i jsut like licking windows ok?

btw in my mind i am imagining cannon 1 and cannon 2, canon one looks better than my firswt combustion although a bit ghetto with all of the crews...

number two lookstotally amazing and i bet if killa gorilla were still around his sig would come true...if everyone remembers waht his sig used to say...

invisible words coming up...

only man to ever habve an orgasm while looking at spudguns...


now that isnt exact but i dont feel like finding his sig again...
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4" piston valved cannon-half done..( i spilt my cement...)

Hybrid- 75% done. need to build propane holder and drill/tap sparkplug hole..
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Unread postAuthor: mopherman » Tue May 22, 2007 8:25 pm

Where did killa go? ive been wondering about his absence for a while now.
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Unread postAuthor: cash68 » Tue May 22, 2007 8:34 pm

I'd like pics too. This burst disk sounds interesting.
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Unread postAuthor: A-98 » Tue May 22, 2007 9:02 pm

i second that motion!
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Unread postAuthor: noname » Tue May 22, 2007 9:25 pm

Since all of you want pictures, I took some with my dad's camera, but I probably won't be able to put them up for a day or so.
Paaiyan, you can measure almost anything with Google Earth. I walk down to where the potato lands and find the splat, then use something as a reference point for Google Earth.
Apparently the burst discs help a lot, it also makes the cannon loads louder.
Rmich, it was only a brown paintjob. The pink was my sexy duct tape! :lol:

I just sandwich a few layers of aluminum foil or paper or whatever between the male and femal threads of the detachable barrel. It holds back the pressure until the combustion reaches its peak pressure, then releases it all at once. The disc alone isn't 100% airtight, but the threads make it airtight. You all know you can't shoot loose fitting objects out of a combustion without wadding because of fuel leakage, but with a burst disc, I ccan shoot anything.
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Unread postAuthor: SpudBlaster15 » Tue May 22, 2007 10:22 pm

There is no way a hairspray cannon with a barrel of that size is achieving 250 yards of range with a potato slug. That is the kind of range I get with my propane metered combustion with an 8 foot barrel. I am sorry, but I have to raise the BS flag on this one.

The (non exaggerated) difference you noticed was probably due to shot to shot variability, as multiple spark gaps do nothing for a launcher's performance, and burst disks help only marginally with high ratio cannons.
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Unread postAuthor: paaiyan » Tue May 22, 2007 10:37 pm

SpudBlaster15 wrote:There is no way a hairspray cannon with a barrel of that size is achieving 250 yards of range with a potato slug. That is the kind of range I get with my propane metered combustion with an 8 foot barrel. I am sorry, but I have to raise the BS flag on this one.

The (non exaggerated) difference you noticed was probably due to shot to shot variability, as multiple spark gaps do nothing for a launcher's performance, and burst disks help only marginally with high ratio cannons.


I do question that some. I have an 8 1/2 foot lonf combustion that I use Static Guard in and I top out at about 250. I don't have exact numbers, I just shoot at the lake and i can guestimate how far across it is at a certain point and I get almost all the way there.
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Unread postAuthor: jimmy101 » Wed May 23, 2007 12:03 pm

SpudBlaster15 wrote:...as multiple spark gaps do nothing for a launcher's performance, and burst disks help only marginally with high ratio cannons.


Really? I would expect multiple sparks to increase the perforamnce of a combustion cannon. Granted, no one has ever (as far as I know) compared one versus two versus... sparks using a consistent ammo and chrony. (Latke was using a spark strip which wouldn't be expected to behave the same as conventional spark gaps). So there is no proof but it would certainly be expected.

As to burst disks in large C:B guns (guns with chambers bigger than needed), I would expect this to be the type of gun that would benefit most from a burst disk. The larger the chamber the slower the burn and the slower the pressure rises. With a very large chamber the spud'll fart out of the barrel after being accelerated by chamber pressure just barely above the static friction threshold. Put a burst disk in so the slow combustion of the large chamber can get to serious pressure before the spud starts to move.

I do agree that the "250 yards" sounds a bit optimistic for this size and type of gun. Squirt-and-screw fueling without a chamber fan, heck the shot to shot variation is probably a hundred yards. (Variability in the muzzle inclination will also make a huge difference in the maximum range.)
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Unread postAuthor: noname » Wed May 23, 2007 10:50 pm

Thank what you will, either you're worng or Google Earth is.
If you don't believe it, it outshoots my biggest piston valve cannon with a 10 foot 1 1/2" barrel.
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Unread postAuthor: SpudBlaster15 » Wed May 23, 2007 11:39 pm

Noname, say no to hallucinogenic drugs. :lol:

jimmy101 wrote:Really? I would expect multiple sparks to increase the perforamnce of a combustion cannon. Granted, no one has ever (as far as I know) compared one versus two versus... sparks using a consistent ammo and chrony. (Latke was using a spark strip which wouldn't be expected to behave the same as conventional spark gaps). So there is no proof but it would certainly be expected.


My propane metered over/under combustion with an 8 foot long 1.5" diameter barrel originally had a 220ci chamber with a 4 gap, central mounted spark strip. I rebuilt the chamber, forming a new, 300ci chamber with a single spark gap mounted 2/5 from the rear of the chamber. The original chamber would splatter a potato on the surface of a refrigerator door, while the new one leaves fist sized dents in the same door. It also out shoots the original setup by a good 50 yards.

jimmy101 wrote:As to burst disks in large C:B guns (guns with chambers bigger than needed), I would expect this to be the type of gun that would benefit most from a burst disk. The larger the chamber the slower the burn and the slower the pressure rises. With a very large chamber the spud'll fart out of the barrel after being accelerated by chamber pressure just barely above the static friction threshold. Put a burst disk in so the slow combustion of the large chamber can get to serious pressure before the spud starts to move.


Yep, burst disks do work best on high ratio cannons (as I said), and have no effect on proper ratio cannons.
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Unread postAuthor: aturner » Thu May 24, 2007 8:00 am

jimmy101 wrote:Really? I would expect multiple sparks to increase the perforamnce of a combustion cannon. Granted, no one has ever (as far as I know) compared one versus two versus... sparks using a consistent ammo and chrony. (Latke was using a spark strip which wouldn't be expected to behave the same as conventional spark gaps). So there is no proof but it would certainly be expected.

Yes, we've expected that for years, but no one has actually observed it to be true. If there is any difference, the difference is too small to measure. The variation between shots is greater than the difference due to number of spark gaps.
jimmy101 wrote:As to burst disks in large C:B guns (guns with chambers bigger than needed), I would expect this to be the type of gun that would benefit most from a burst disk. The larger the chamber the slower the burn and the slower the pressure rises. With a very large chamber the spud'll fart out of the barrel after being accelerated by chamber pressure just barely above the static friction threshold. Put a burst disk in so the slow combustion of the large chamber can get to serious pressure before the spud starts to move.

Jimmy, so you have come around, finally? My first real topic on spudtech was about the potential drawbacks of an oversized chamber....and I was relentlessly attacked for suggesting there might be any drawback. So I'd love to know if you've got any data to back up your claims. Or is this just a theoretical guess, sorta like the spark gaps statements you made above?
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Unread postAuthor: jimmy101 » Thu May 24, 2007 11:22 am

Spudblaster: Sorry, when you said "burst disks help only marginally with high ratio cannons" (05/22/2007) I took that to mean they are not particularly effective on large CB ratio guns, not that "burst disks do work best on high ratio cannons" as you said in the later post (05/24/2007).

As to your spark gap results.... As I pointed out earlier, lots of theories, very little hard data. In your case, "splat" versus "dent" is not terribly quantitative. Is that a 100% difference in KE or 30%? Besides, spark strips would be expected to behave a lot differently than a basic "two-screws" gap. The problem with a spark strip is that the flame propagates from the strip as a ~hemisphere (at least immediately after ignition). With screws for the gap the flame propagates as a ~sphere. Rough estimate would say two gaps on a strip are about the same as a single gap made with screws. Of course, it also depends on where the strip is in the chamber (near the axis or the wall), how far apart the gaps are on the strip (two gaps separated by 1/4" are probably ~identical to a single gap), how wide the strip is etc. For best results the distance between the gaps should probably be about the diameter of the chamber. Separations that are significantly less than that will start to behave as a single gap. What were the dimensions of your spark strip; length, gap to gap separation, width? (The actual gap distance is probably not important.)

Besides, in the comparison you give you increased the chamber volume of the second gun by 36% (220 ci vs. 300ci). How much of the difference in performance is because of the larger chamber? Did you also resize the barrel? The 300ci should have had a longer barrel than the 220, enough longer to make a fair amount of difference in the velocity?

Aturner said:
"Yes, we've expected that for years, but no one has actually observed it to be true. If there is any difference, the difference is too small to measure. The variation between shots is greater than the difference due to number of spark gaps." Could be. But has anyone ever observed it to be not true? Has anyone ever compared multiple gaps, separated by ~chamber diameter distance, using a chrony? If so, I don't recall ever seeing it posted anywhere. All I've ever seen are things like varying number of gaps on a strip with the separation between gaps fairly small (generally an inch or less). Or, "I built a gun three years ago that shot 300 yards, last week I built another that shot 400 yards and the only difference was the number of spark gaps". Which is fine, but not exactly scientific or even particularly useful.

I'm not sure I follow your point about oversized chamber and burst disk. I believe I have always said<sup>*</sup> that a grossly oversized chamber is a bad thing in a combustion gun. ("grossly oversized" being C:B >1.5 or 2). I too have been flamed for saying that. People always come back with "but a larger chamber has more fuel, hence more energy, therefore it must outperform a smaller chamber". I suspect that is wrong. A large chamber burns slower than a smaller one. A properly sized chamber will outperform an overly large chamber. However, as with many things in combustion guns, the difference in performance is frequently pretty minor. If an 0.8 gun shoots at 400 FPS than a 2.0 gun might shoot at 350 FPS. The difference can be pretty hard to detect if you are measuring performance by range, or hang time, or by how big of a dent you can make in a refrigerator. And, the spud to spud variability in mass and friction can pretty easily obscure a 50 FPS difference. Bottom line though is that you don't gain anything with an grossly oversized chamber and, most likely, the performance actually drops a bit.

A burst disk gun is completely different than a conventional gun in terms of the affect of the C:B ratio. I don't believe I have ever posted that a burst disk gun can not utilize the greater energy in an oversized chamber. Isn't that partly the whole point of a burst disk? As SpudBlaster said, in a properly ratio'd cannon a burst disk doesn't really help much. (Not sure I completely believe that, would love to see chrony data. But I can believe that the benefit of a burst disk is not huge on a properly sized gun.)

Many of my statements are based on my combustion modeling. No doubt some of them are wrong. But they are actually based on something. If I've got anything wrong in the model I would love to know about it. That is the only way to make the model better.

Many things that you see posted on what makes a good combustion gun are based on "300 yards versus 200 yards range", or "Oh my god the gun was sooooo much louder...", or "my squirt-and-screw gun when done this way gave 250 yards but when done this way gave 300 yards", great except if you tried to do it three times in a row the same way that is probably about the shot to shot variability of squirt-and-screw. Heck, in cold weather I've gotten the same gun to fire 20 yards versus 100 yards depending on if the fan was used for mixing or not (that was using metered fuel).

<sup>*</sup> No fair pulling up a Spudtech post from more than a year ago since that predates my modeling!
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