There are two major types of spud guns pneumatic spud guns and the conventional explosive propellant spudguns. A pneumatic spud gun uses compressed air that is stored in a separate container to force the projectile down the barrel. There is quite a bit of danger involved in using pneumatic spud guns because of the high pressures that can be rapidly forced into a firing chamber. Also the pipe used in Pneumatic spud guns is PVC not ABS. PVC is pressure rated and ABS is more impact resistant. These pages will deal with the explosive propellant type instead. While the pressure generated by the explosive propellant is substantial it is not nearly as high as that of the pneumatic guns. The selection of Propellants is quite possibly the most important part of spud gun performance. The propellant is the fuel burned during combustion. This combustion creates the pressure needed to force the projectile potato out of the barrel and down range. There are many fuels used by the spudguners of today but each has its advantages and disadvantages. The most common fuels used presently are Acetylene, Hydrogen, Ethanol alcohol, Butane, Propane, Spray Paint, Carburetor cleaners, and Hairsprays.
Hairsprays:
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Of the list of propellants the most common is of course hairspray due to its availability and because it is relative inexpensive. Hair spray is combination of chemicals but only two of these concern us. These are the hair spray itself and the cans propellant. One of the major disadvantages with hair spray is the its original intended use as a form of hair cement or adhesive. Because hair spray is not in itself flammable but rather the propellants in the hair spray can, they actual adhesive qualities of the spray are built up in the firing chamber of the spudgun. This gummy substance never burns and constantly builds up on the walls of the chamber and the igniter causing frequent misfires, it locks the threads of the end cap on the firing chamber, and eventually forces you to retire the Firing chamber altogether. Another draw back found in using Hair Spray propellants for combustion is its relatively low expansion when burned. The lower volume of gas produced during combustion produces a much lower level of pressure in the chamber therefore decreasing the range of the spudgun. This fuel is safer than the others are. |
Carburetor Cleaner:
As a result of the negative properties of the Hair spray the search for a better fuel turned to cleaning agents such as Carburetor and brake cleaners. These are superior to the hair spray in two ways. When the cleaners are sprayed into the chamber and burned there is no residue left over because the cleaners themselves are flammable and burn as well as the propellants. The explosive properties of this fuel are also superior to that of the Hair spray. A higher pressure is developed in the firing chamber and as a result a higher velocity and range can result. Unfortunately there is a major hazard involved with the use of this fuel. The fuel has a tendency to eat the materials used in construction of spudguns. That’s right the cleaners dissolve PVC ABS and the adhesives used to join them. This fuel is not recommended due to the risk of firing chamber failure.
Spray Paint:
This fuel is a favorite of spudguners for two reasons it combines the higher performance level of the Carburetor cleaners with the cheapness found in hair spray. The other major benefit is that although there is residue left by the paint it is solid not gummy and builds up much slower than hair spray. The paint is flammable by its self and set up very quickly with the heat generated in combustion. This prevents the paint form gumming up and inhibiting ignition. The propellants used in the paint are also more flammable with the atomized paint in it. This causes a larger volume of gas to be produced and consequently a higher velocity to be achieved. This fuel is the all around best choice for performance and safety.
Propane:
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The use of these fuels requires a basic understanding of chemistry. It becomes important to understand the chemical reaction that takes place when the fuel is burned in the chamber. There is a set volume that you are dealing with and that is the volume of the firing chamber. During combustion air is combining with the fuel to produce a higher volume product. Air is a mixture of 78% Nitrogen, 21% Oxygen and 1% Other elements and compounds mainly carbon dioxide. Air has a density of approximately 1.29 grams per litter. The density of propane is approximately 1.96 grams per liter or roughly 1.5 times heavier than air per volume. The ideal propane mix is 40 ml per liter of air. Propane is a hydrocarbon, which is to say it is composed of Hydrogen and Carbon. Propane belongs to the group of hydrocarbons known as alkanes and its chemical formula for propane is CH3CH2CH3. This is the chemical structure. |
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| | | H-C-C-C-H | | | H H H |
When Propane is burned it forms water and carbon dioxide. This relationship is represented by the formula. |
The products of the reaction are therefor non toxic and leave no gummy residue. This reaction is extremely violent and creates a much higher level of pressure in the firing chamber. The level of danger increases with this choice of fuel as well. The risk of firing chamber failure is greatly increased the more explosive the fuel used.
Butane:
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Butane is quite similar to propane both chemical makeup and structure. It possesses similar characteristics as well. The density of butane is approximately 2.59 grams per liter or roughly 2 times more dense than air. The ideal butane mix is 31 ml per liter of air. This means you need less fuel for the same pressure to be achieved Butane also belongs to the group of hydrocarbons known as alkanes. Its chemical formula is CH3CH2CH2CH3. The structure appears like this |
. H H H H |
When Butane is burned it forms water and carbon dioxide. This relationship is represented by the formula. |
Like propane the products are non toxic and leave no gummy residue. But because the required amount of fuel is less with Butane more air and butane can be use in the same sized firing chamber and a larger pressure can be achieved. This fuel has potential to be more dangerous than propane.
Ethanol:
Ethanol belongs to the group of hydrocarbons known as alcohols. The density of ethanol is approximately 790 grams per liter this means it is a liquid. The chemical formula for ethanol is CH3CH2OH. Its structure appears like this
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| | H-C-C-O-H | | H H |
When combusted, ethanol forms water and carbon dioxide as follows: |
A liquid fuel can be desirable as an alternative to pressurized gas fuels because of the lower risk or accidental detonation. The high heat resulting from burning alcohol’s has been know to set fire to the spud gun’s firing chamber.
Hydrogen:
This is the Elemental form H2. The density of hydrogen is approximately 0.09 grams per liter or 14.5 times lighter than air. Its chemical structure is the following.H-H When burned, hydrogen reacts with the oxygen in the air to form water
H2 + 0.502 -> H20 Hydrogen is extremely dangerous because of its explosive properties, and hard to contain. This fuel is the cleanest burning with only water being the product. There have been reports of hydrogen reacting with the plastics used in Spud guns. Hydrogen fuel is also not recommended.
Acetylene:
Acetylene is lighter than air with a density being approximately 1.25 grams per liter. The ideal acetylene mix is 65 ml per liter of air. Acetylene belongs to the group of hydrocarbons known as alkenes. The chemical formula for acetylene is C2H4. Its chemical structure is
H H
\ /
C=C
/ \
H HC2H4 + 3O2 -> 2H2O +2CO2 This fuel is extremely dangerous and should not be used because it has a reputation to spontaneously combust with the oxygen in the air. This fuel however would be the best fuel for the explosive properties it possesses if it could be handle safely. With that said don’t use acetylene.
