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my first spud gun!

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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my first spud gun!

Unread postAuthor: drisken » Tue May 27, 2008 12:58 pm

sorry i don't have any pictures of it, it's nothing special trust me! i do need some input, however. it has a 14" 4" chamber into a 2" 38" barrel. Which is almost a 1.5/1 volume ratio; i just used a lantern flint for an igniter and have been using aquanet. it is all made out of schedule 40 pvc. the gun shoots well, and yesterday i had it shoot approximately just over 200 yards into a slight wind. Do you think this is the max distance i'm going to get out of this? is there anything i can do to get more distance, better performance overall without sacrificing safety, etc? any input would be appreciated, thanks!!!

Steve
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Unread postAuthor: Carlman » Tue May 27, 2008 1:36 pm

only thing i can think of is a chamber fan (computer fan mounted at cap end of chamber) allows for beter mixing and faster ignition
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Unread postAuthor: drisken » Tue May 27, 2008 2:18 pm

other than that would you say i probably maxed out the distance on it then? at first i thought i didn't make it the appropriate size to shoot as far as i was wanting, but it seems as though people are reaching upwards of 350 yards with the same size spud gun.
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Unread postAuthor: Boom_erang » Tue May 27, 2008 3:03 pm

Hairspray is not a very good fuel. Try Static Guard - it takes much less and seems to have a very fast combustion. It won't gunk up your chamber like hairspray will, either. Also, you would get more power from a longer barrel - up to 6' probably. I made my first gun to a 1.5:1 ratio, but my second, more powerful gun is a .8:1 ratio and it's very potent.
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Unread postAuthor: Biopyro » Tue May 27, 2008 3:20 pm

Your best bet would be to switch to camping gas for fuel - get a camping cooker and unscrew the cooker bit so when you open it upside down it sprays out liquid propane. (This would also mean you could get away with a much lower ratio)
The next thing would be to get a dual ignition, this is a great improvement.
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Unread postAuthor: jimmy101 » Tue May 27, 2008 4:47 pm

Biopyro wrote:Your best bet would be to switch to camping gas for fuel - get a camping cooker and unscrew the cooker bit so when you open it upside down it sprays out liquid propane. (This would also mean you could get away with a much lower ratio)

What? The correct ratio is the correct ratio. Too much or too little fuel drops the gun's performance.

Back to the OP...
1. Mutliple sparks will help.
2. Shooting spuds (or other vegies)? then double bevel the muzzle.
3. Add a fan, it'll increase performance and reduce shot to shot variability.
4. Add a meter system (even a $2 syringe is much better than "spray-n-pray").
5. Quit using any fuel that has anything in it that doesn't burn. Get a $0.49 butane lighter and electrical wire (for hose), or a Bernzomatic torch, or any of a dozen other cheap sources of pure fuel.

To maximize range you need to launch at the optimal angle. For spuds that is probably about 36 degrees above horizontal.
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Unread postAuthor: Biopyro » Wed May 28, 2008 11:58 am

http://www.burntlatke.com/jpg600/25cb-graph.gif
Not so. A 0.7 ratio is by far the most effective with propane. It's higher purity means you need a lower volume of fuel compared to hairspray.
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Unread postAuthor: jimmy101 » Wed May 28, 2008 9:12 pm

Biopyro wrote:http://www.burntlatke.com/jpg600/25cb-graph.gif
Not so. A 0.7 ratio is by far the most effective with propane. It's higher purity means you need a lower volume of fuel compared to hairspray.

Nope. Nope and Nope.

Not sure what you mean by 0.7. Are switching back and forth between CB ratios and fueling ratios? (The fueling ratio is 4% for propane.)


1. Look at latke's numbers. There is no significant difference in the range of about CB 0.6 to 1.0. And, 0.7 sure as hell isn't "by far the most effective with propane" since the performance is constant in the range I just gave. To really compare Latke's numbers you need to take into account the shot to shot variability. For his standard slug it tends to be about 4%. For spuds about 7%. Any difference in performance between two CB ratios that is less than that is not statistically significant. And, probably more importantly, you wouldn't be able to measure the difference unless you had a chrony and were willing to shoot the gun at least five times at each CB.

2. Latke's numbers are how you maximize the efficience of a gun. To maximize the performance the CB ratio is different. Nobody understands exactly how to predict the optimal CB to maximize performance on anything other than a gun with a fixed chamber volume. Hopefully, HGDT will provide a method to predict optimal gun designs. EVBEC might be able to help design a gun for maximum performance.

3. Harispray is probably something like 95+% propane (+butane + isobutane + other possible flammable hydrocarbons) and 5% or less laquer etc. Ax, RightGaurd (the old kind) are probably similar. There is really very little in the can that won't burn and the non-flammables have little to do with the combustion energy. Since the inflamables make up only a small percentage of the spray volume they have no real affect on the fueling process. In squirt-n-screw you are lucky if you get +/- 20% or 30% reproducability. So, the couple precent of junk in the spray has little to do with the fueling process.The non-flamables can muck up the inside of the gun. Fans and spark gaps in particular don't like getting coated with laquer.

The reasons for avoiding sprays as fuels are;
1. It is basically impossible to fuel the gun accruately and consistently.
2. The spray fuels have crap in'm that muck up the inside of the gun.

In terms of the actual combustion process, and the combustion energy, there is little or no difference between say Ax and pure propane. If you could accurately meter a spray fuel it would combust just as well and generate just as much energy as propane.
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Unread postAuthor: Biopyro » Thu May 29, 2008 4:16 am

1. If you look at the rest of the site you'll see they did at least 5 shots for each, 10 for some, and took an average. This rules out shot -to-shot variation. Also, they did use a chronograph to measure speed...
2. Exactly what is the difference here? If a gun is more efficient, it uses all the fuel to propel the potato - increasing performance. Forgive me if I have misunderstood you on this one.
3. Quite right. By volume there is so much propellant that the gunk is insignificant. This 5% is a small difference bit it is a difference. If anything the real motivation to use pure fuel is that it can create short circuits. :)
Ax wouldn't generate just as much energy as propane, since the enthalpy change of combustion is measured in moles, not volume. There are more moles of a substance in a litre of liquid than there are in a litre of gas.
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Unread postAuthor: jimmy101 » Thu May 29, 2008 11:28 am

Biopyro wrote:1. If you look at the rest of the site you'll see they did at least 5 shots for each, 10 for some, and took an average. This rules out shot -to-shot variation. Also, they did use a chronograph to measure speed...

Yes, I've spend many hours reading latke's pages. You completely missed the point. With many shots for each CB Latke's data still cannot say 0.7 is better than 0.8. Furthermore, for a typical gunner, it would be impossible to measure the difference in performance within the CB range of about 0.6 to 1.0. Take a look at this graph with the velocities for each shot along with the group averages and standard deviations. Now tell me that "A 0.7 ratio is by far the most effective with propane".

2. Exactly what is the difference here? If a gun is more efficient, it uses all the fuel to propel the potato - increasing performance. Forgive me if I have misunderstood you on this one.

There is a big difference between maximizing efficiency and maximizing performance. The Latke studies are about maximizing the performance for a particular chamber size. That is the same as maximizing the efficiency since the fixed chamber size has a fixed amount of energy. If you want to maximize the performance of a gun you can decide that the chamber volume is not the limiting factor. If you use Latke's rule to design a CB 0.8 gun you get a particular barrel geometry for a starting chamber geometry. If you then take that barrel and ask what chamber volume maximizes the barrel's muzzle velocity you will end up with a different chamber size and a higher muzzle velocity. Spud gunners have kind of gotten into a mindset that maximizing efficiency is the way to go. But if maximum muzzle velocity is your goal then the rules are different (and pretty poorly understood). Most "real" guns are pretty inefficient and efficiency is rarely a big design factor. Other factors, like accuracy, ability to use standard ammo, overall size etc. are generally what drive the design of real guns.
3. Quite right. By volume there is so much propellant that the gunk is insignificant. This 5% is a small difference bit it is a difference. If anything the real motivation to use pure fuel is that it can create short circuits. :)

But you probably can't tell the difference between pure fuel and 95% fuel. And, you sure as heck can't tell the difference in a spray-n-pray gun when the fueling variability is probably more like 20 or 30% (that's on top of the 3~7% variability you get with metered fueling).

As to creating short circuits, it isn't so much a pure fuel can create the short circuit (the spark event) as it is that the impurities in some fuels (like hairspray) will coat the electrodes and act as electrical insulation.

Ax wouldn't generate just as much energy as propane, since the enthalpy change of combustion is measured in moles, not volume.

Actually, the energy change is not measured in the molar enthalpy change. By that logic butane (~690 Kcal/mol) would be 30% more energetic than propane (~530 Kcal/mol). Propane would be 140% more energetic than methane (~220 Kcal/mol). In fact, all three fuels are essentially identical. The proper way to measure the energy content is per mole of oxygen, not per mole of fuel. The chamber size defines how much oxygen is available for combustion. Fuel is added to match the available oxygen. On a molar basis, a methane fueled gun would be charged with about 2.4 times more moles of fuel than a propane charged gun. The 2.4 times more methane used offsets the differences in the molar enthalpy of combustion. You might want to read the SpudWiki page on <a href="http://www.spudfiles.com/spud_wiki/index.php?title=Common_Fuels_for_Combustion_Spudguns">combustion fuels</a>.
There are more moles of a substance in a litre of liquid than there are in a litre of gas

True, but completely irrelevant since (1) both fuels are gases and (2) presumably your are going for a stoichiometric mix anyway. Spudguns can, and have been used with liquid fuels. Liquid fuels work fine and should perform about the same as propane. I don't believe anyone has ever measured the actual performance of liquid versus gaseous fuels though. Thermodynamics, chemistry, physics etc. all predict that hexane (a volitile liquid) should be the same as propane as long as it is measured correctly and allowed to fully evaporate before ignition. The problem with liquid fuels is that it is a PITA to accurately measure the very small volumes required for a typical spudgun. (Usually, only a drop or two of liquid fuel is required per 100ci of chamber volume.)
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Unread postAuthor: Biopyro » Thu May 29, 2008 11:42 am

You make a very good argument - ever considered becoming a lawyer? I see what you're saying now youve explained it a little more, thanks!
Only thing is, on your last point, antiperspirant etc often comes out as liquid or solid and remains so at room temperature, does it not?
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Unread postAuthor: drisken » Thu May 29, 2008 1:23 pm

thanks for all the help guys! so general consensus is if i add a fan and something to more consistently monitor the input of the "fuel" i could attain the desired 300 yards with my current setup? thanks again for the help, as i am very new to all of this.
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Unread postAuthor: drisken » Thu May 29, 2008 2:37 pm

one more question, i've been trying to search for an answer and there is really no exact answer as to how to do it, just kind of what to use. how do i go about wiring the chamber fan? can i just take a 9v battery and a rocker switch; wire the positive and negatives of the 9v to the switch and fan and be done? or will this cause the battery battery to get hot and explode? what about wiring the negative to a ground and wiring the positive directly to the switch and battery? thanks again guys for all the help!!
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Unread postAuthor: Biopyro » Fri May 30, 2008 2:52 am

It's a DC low voltage current, so it doesn't need a ground connection.
It is exactly as you said, however I strongly reccomend you brush up your basic electronics knowledge.
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Unread postAuthor: jimmy101 » Fri May 30, 2008 1:12 pm

Biopyro wrote:You make a very good argument - ever considered becoming a lawyer? I see what you're saying now youve explained it a little more, thanks!
Only thing is, on your last point, antiperspirant etc often comes out as liquid or solid and remains so at room temperature, does it not?

Some of what comes out of the can is a liquid and/or solid. For anitperspirants the solid is probably talc (non-flammable). Any liquids will be listed on the label. The liquids are almost certainly going to be volatile (rubbing alchohol and the like probably) and should evaporate pretty quickly. These will contribute to the fuel in the chamber.
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