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propane newbie

A place to ask general spud cannon related questions.
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propane newbie

Unread postAuthor: Chris spud » Sun Sep 14, 2014 2:45 pm

Hi all. I recently got into this, and love it. Thing is that I've heard that propane is a better fuel, compared to hair spray. However...showing my noobness, I though you could just use the propane as you do hairspray, spray some in the back. I've got the screw-on back type of spud gun. I tried it today, and it eventually did work, but what surprised me is how long it took to get enough gas in there. I've calculated that I need around 190ml of propane for my combustion chamber. I honestly don't wan't to do the whole metered propane thing, and I don't think my chamber is gas tight to boot. so, is it possible to use a propane torch as you do hair spray, spray some in the back and close it up? Is there a way to measure a certain amount of propane, like in a syringe, or something? Otherwise, I'll just stick with good ol hairspray..I've tried to bling my spud gun by using stungun with twin spark, and even a computer fan..but it just make it more complex, and more can go wrong. Struggling to get twin spark consistently. Thanks
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Re: propane newbie

Unread postAuthor: jrrdw » Tue Sep 16, 2014 5:04 am

Propane is heavier than air therefore have the cannon pointed down and let the fuel sink into the chamber. Do you have a chamber fan? If not use a pill bottle, small ball or just anything you can shake around to mix the fuel and air. Experiment with the amount of fuel, count 1-2-3 and if it works well go with it. Make sure you clear the chamber well before refueling.
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Re: propane newbie

Unread postAuthor: Chris spud » Wed Sep 17, 2014 6:48 am

jrrdw wrote:Propane is heavier than air therefore have the cannon pointed down and let the fuel sink into the chamber. Do you have a chamber fan? If not use a pill bottle, small ball or just anything you can shake around to mix the fuel and air. Experiment with the amount of fuel, count 1-2-3 and if it works well go with it. Make sure you clear the chamber well before refueling.


I realized that propane was heavier than air, and had the chamber pointing downward when filling it. I realized that if you ignite hairspray, it make a big ball of flame, whereas with the propane tank, it makes a much smaller focused flame - that might be because of pressure differences inside, but I think that the hairspray releases combustible fuel at a higher rate, compared to propane tank. At one time, I filled the tank for 10-15seconds (propane), and only then it shot the spud out, but not as far as with hair spray. I've got a fan that I used. I tried your method, but got nothing until I opened the propane tank for more than 10s, and like I said, it did not go as far compared to hairspray...I was seriously disappointed.

The hairspray has both butane and propane, propane releases more energy per volume gas, compared to propane. Hairspray might gum up your electrodes, which is not nice, though
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Re: propane newbie

Unread postAuthor: jrrdw » Wed Sep 17, 2014 2:27 pm

"10-15 second" how big is your chamber?
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Re: propane newbie

Unread postAuthor: mark.f » Wed Sep 17, 2014 3:11 pm

Butane has a lower heat of reaction than propane, so hairspray is most certainly not more powerful than propane itself. A propane "meter" can be as simple as a syringe to measure out the gas required, and will give you the mixture you need.
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Re: propane newbie

Unread postAuthor: Fnord » Wed Sep 17, 2014 7:48 pm

If you don't really want to build a meter but want a more reliable spray-n-pray, just pick up a can of starting fluid. It's designed to burn, whereas hair spray is designed to... well, I have no idea. /male
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Re: propane newbie

Unread postAuthor: Chris spud » Sat Sep 20, 2014 11:10 am

My chamber is roughly 284 cubic inches. How do I used a syringe to get my 200ml propane? Thanks

shot off the old spud canon today. Let me just come out and say this: I hate multiple spark because it doesn't work! Current flows through the path of least resistance, and air had quite high resistance, even with the fuel in there, so if your electrodes are not exactly as close to each other (close to impossible), then there is a path of less resistance, or the one electrode has a sharper point than the other - there's a path of less resistance. The charges will take the path of least resistance each time... wish I didn't try it, best is sparkgap in the middle of the chamber it seems.

I'm a science teacher, and my learners are now building spud guns... one grade against the other, and against me. Good to get them interested in science. The grade 12s built one today, barrel length: 2m... using the C:B ration of 1.5:1. I think they will beat me for distance

Stun gun ignition is the best, for me. It's very reliable. I need to up the ante....going to try and get some propane in a syringe!
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Re: propane newbie

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Re: propane newbie

Unread postAuthor: jimmy101 » Sat Sep 20, 2014 3:24 pm

mark.f wrote:Butane has a lower heat of reaction than propane
Nope. For all practical purposes the heat content in butane and propane are identical.
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Re: propane newbie

Unread postAuthor: Chris spud » Sun Sep 21, 2014 6:11 am

This is one thing I don't get...heat of combustion, the bigger the hydrocarbon molecule, the more energy can be released during combustion, so butane has a higher heat of combustion compared to propane, and pentane would be higher than butane (albeit liquid at room temp). In theory, pure butane should give you more bang for your buck. So...who came up with the idea that propane is better? And if it is, why?

For that matter, why is actylene so dangerous? is heat of combustion is not that impressive, because it's a small molecule. Oxy-propane should be more potent, than oxy-acetylene. But...burn rate might be higher for actylene which might make the difference
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Re: propane newbie

Unread postAuthor: DYI » Sun Sep 21, 2014 11:11 am

Let me just come out and say this: I hate multiple spark because it doesn't work! Current flows through the path of least resistance, and air had quite high resistance, even with the fuel in there, so if your electrodes are not exactly as close to each other (close to impossible), then there is a path of less resistance, or the one electrode has a sharper point than the other - there's a path of less resistance. The charges will take the path of least resistance each time... wish I didn't try it, best is sparkgap in the middle of the chamber it seems.


Are you wiring your gaps in parallel? :lol:
A multi-spark ignition system has the gaps wired in series - the ONLY available path in this case is to cross all the gaps.

This is one thing I don't get...heat of combustion, the bigger the hydrocarbon molecule, the more energy can be released during combustion, so butane has a higher heat of combustion compared to propane, and pentane would be higher than butane (albeit liquid at room temp). In theory, pure butane should give you more bang for your buck. So...who came up with the idea that propane is better? And if it is, why?


Yes, a molecule of butane reacting with air releases more energy than a molecule of propane. However, it takes more air to do this. As the length of the hydrocarbon chain increases, so does the proportional number of oxygen molecules required for full combustion. Note that methane burns best at around 34% concentration in oxygen, while propane burns best at roughly 17% concentration. This trend continues as you move up. Specifically, the ratio of oxygen to fuel goes up as roughly ((3*n)+1)/2 for alkanes. As a result of this relation, with a fixed volume and a close to ideal alkane/oxygen mix, you won't see much difference in heat of combustion as you move to longer chains.

Propane provides similar performance to butane while being generally cheaper, more available, and with a higher vapor pressure. You can reach higher mixes with propane, and it doesn't become useless on a cool day. You'll actually find that the heating value of alkane fuels increases slightly with shorter chains, but this does not necessarily translate into higher combustion pressure, which is of more interest to us. You can gain a much better intuitive understanding of the factors involved here by playing with Gaseq.

For that matter, why is actylene so dangerous? is heat of combustion is not that impressive, because it's a small molecule. Oxy-propane should be more potent, than oxy-acetylene. But...burn rate might be higher for actylene which might make the difference


Acetylene isn't an alkane - it has a single carbon-carbon triple bond with a high energy content. This is source of its higher combustion pressure, increased reactivity, and tendency to explosively polymerize. Oxygen/acetylene mixes have a high burn rate and are very prone to detonation, as a result of the factors listed above. I notice that you mentioned acetylene to have a "low" heat of combustion - its heat of combustion is higher than any alkane but methane. Acetylene is a relatively energy-dense molecule, as common fuels go.
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Re: propane newbie

Unread postAuthor: mark.f » Sun Sep 21, 2014 6:33 pm

jimmy101 wrote:
mark.f wrote:Butane has a lower heat of reaction than propane
Nope. For all practical purposes the heat content in butane and propane are identical.


Indeed, I even got it backwards. :oops: That's what I get for opening my mouth without checking my facts.

DYI nailed it, as usual.
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Re: propane newbie

Unread postAuthor: jimmy101 » Mon Sep 22, 2014 3:31 pm

DYI explained the reason why the various hydrocarbons all basically have the same energy. The amount of energy in the chamber is really set by the volume, which controls the amount of oxygen, which controls how much fuel you can burn. The number of moles of fuel in a fixed volume chamber will decrease as the molecular weight of the hydrocarbon goes up (methane, ethane, propane, butane, ...) but the mass of the fuel is basically the same. Heats of combustion are usually quoted per mole of fuel. If you convert the heat of combustion to grams of fuel you get basically the same heat of combustion for all the hydrocarbons.
DYI wrote:Acetylene isn't an alkane - it has a single carbon-carbon triple bond with a high energy content. This is source of its higher combustion pressure, increased reactivity, and tendency to explosively polymerize. Oxygen/acetylene mixes have a high burn rate and are very prone to detonation, as a result of the factors listed above. I notice that you mentioned acetylene to have a "low" heat of combustion - its heat of combustion is higher than any alkane but methane. Acetylene is a relatively energy-dense molecule, as common fuels go.
Acetylene has one other unique characteristic; it has an unusually wide range of combustibility. Unlike hydrocarbons (propane, butane, ...) acetylene will ignite at almost any fuel to air mixture. That's why Bang-site cannons work, they are very insensitive to the fuel to air ratio. Hydrogen also has a very wide range of combustibility.
http://www.inpharmix.com/jps/Combustion_fuels.html
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Re: propane newbie

Unread postAuthor: Chris spud » Tue Sep 23, 2014 2:29 pm

parallel!!! can't believe I missed this... going for triple spark again. Thanks for all the great info. Jes, of course, the bigger the molecule (hydrocarbon), the more oxygen is needed....so, though you get more energy per molecule, you need more oxygen. Bottom line: less butane (mol) can combust in a given chamber, compared to propane, because the chamber can hold a maximum amount of oxygen and no more (if unpressurized). In the end, these effects cancel out so propane and butane is no different in efficiency. That's also why higher percentage concentration of fuel is possible if the molecule is smaller, it's all based on the fact that equal amount of gases under standard conditions take up the same volume - cubic inch for cubic inch, there are similar amounts of methane, ethane, propane and butane..even oxygen molecules. So, in your combustion chamber there is an ideal ratio of fuel to oxygen. This ratio changes as the molecule size changes (bigger molecule need more oxygen).

With acetylene, a small molecule with a strong triple bond between two carbons, I find the heat of combustion to be 1300kJ/mol, about the same as ethanol, ethane. Propane is 2220, and butane 2877. http://en.wikipedia.org/wiki/Heat_of_combustion

However, since acetylene needs less oxygen to combust, more can be burnt for a given combustion chamber - hence the high concentrations that work. Is it possible, that the potency of acetylene is due to the burn rate mainly? Pressure builds quicker, so more usable work is done on the spud. Quite exited, the Grade 12s busy finishing a 10 plus foot spudgun...

just RE acetylene again, the strong bond does not mean it's energy rich really, it means more energy is required to break the bonds - the activation energy is more than with ethane, say. The products that form are presumably still the same, CO2 and water - this is where the energy is released. If during product bond formation more energy is released than the energy absorbed during reactant (Acetylene) bond breaking - it's exothermic reaction (all combustion reactions are such). I've heard of acetylene molecules polymerizing. The question remains - how many bonds break and form during the chemical reaction, and how much energy is absorbed and released respectively when this happens. At least, that's how I understand it
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Re: propane newbie

Unread postAuthor: Chris spud » Wed Sep 24, 2014 4:47 am

Thinking of the syringe method. My chamber is 49cm long, and 11cm in diameter. Got a volume of 4656 cubic cm, or 4.656 liters. So, 21 percent oxygen = 978 cubic cm O2 in chamber. According to balanced equation for combustion of propane, I need 5 times more oxygen than propane, so around 190 cubic cm of propane is needed - 195ml. I don't have a syringe that big, so I will have to fill up two 100ml syringes. Any comments?
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Re: propane newbie

Unread postAuthor: Chris spud » Sun Sep 28, 2014 12:08 pm

Got the triple spark gap to work. Wow..it makes a difference. And, I'm using metered propane with a syringe. I calculated that I need about 187ml, taking into account the air that is pushed out when the propane goes it...man, does this work. Honestly, I had shots where the spud went out of sight. We counted 18 seconds with a shot straight up into the air before it landed...However, the spud was put length-wise into the barrel. I did not do this, my friend loaded the gun. I've read that this is dangerous, because too much friction and the chamber can explode. The theory goes that if this is not the case, then the seal is much better with a significant increase in surface area - there's more friction due to this as well, so the spud does not start to move until sufficient pressure has been built up. When the spud does start to move, friction decreases (static friction is greater than kinetic friction), and now you have higher psi pushing out the spud, so acceleration increases (but greater mass would work against it)...all I can say is that this increased the power significantly (just loading the spud parallel to barrel)...is it too dangerous? any experience? The increase in recoil alone tells me that the force on the spud is a lot greater than before - Newton's third law
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