Dear jordaa-ga;
Thank you for allowing me to answer your interesting question. First,
and foremost of course, a young person should be told that while a
spud gun appears to be a lot of fun, it can in fact kill when someone
who misuses it or is not familiar with it. These devices are capable
of exploding, misfiring, or catching fire and the projectile can be
even more devastating and deadly than a bullet from a traditional gun.
It is, generally speaking, a directional (and often unpredictable)
bomb.
Enough of the advice?.now for the technical information you asked
about. The theory behind the various types of spud guns is a bit
complex so I'll over-simplify it here so you can explain it to the
kids. I will, however, provide additional (and more technical) links
at the bottom for you to read if you need more advanced information.
Spud guns (sometimes called potato guns, potato cannons or spudzookas)
are essentially pneumatic or combustion launchers. In a pneumatic
launcher, air is compressed (usually pumped in pressure) into a
chamber (tube, barrel, pipe, etc) into which the ?spud? is stuffed.
When the compressed air exceeds the pressure exerted by the potato the
spud is released. In short, the air simply presses against the potato
causing a thrust effect that basically pushes the potato out with such
force that it becomes a flying projectile much like the way a
champagne cork is pressed out of a bottle. This same concept is the
method in which acrobats are ?shot? from cannons? at the circus
(though acrobats are generally ?standing? on a platform that is
rapidly pushed upward giving the appearance that they were shot from
the cannon).
Combustion launchers are considerably more dangerous. In this scenario
a similar tube is used as a barrel except that one end is completely
sealed. A hole is drilled in the sealed end of the tube and the potato
is stuffed into the open end. Now what you have is a closed cylinder ?
the potato provides the seal in the open end. An accelerant (commonly
an aerosol spray) is injected into the hole drilled in the end. This
fills the chamber between the sealed end and the potato. Once the
appropriate amount of flammable material is injected into the cylinder
and igniter (match, lighter, fuse, flame, etc) is applied to the hole
to ignite the gasses. The result is an internal explosion. This
explosion within the cylinder cause the pressure inside to suddenly
expand many times above normal and the result is a propulsion against
the weakest point ? ideally the potato ? causing it to fire out the
open end of the tube.
Now the problem here is many-fold. If the spud is stuffed in too tight
or is too big for the opening, or for some reason becomes lodged, the
weakest point in the tube could very well be anywhere other than the
opening. The end result here is a devastating explosion that can kill
or maim everyone standing within several feet of the cannon.
I hope you find that my answer exceeds your expectations. If you have
any questions about my research please post a clarification request
prior to rating the answer. Otherwise I welcome your rating and your
final comments and I look forward to working with you again in the
near future. Thank you for bringing your question to us.
Best regards;
Tutuzdad-ga ? Google Answers Researcher
INFORMATION SOURCES
HOW STUFF WORKS
http://science.howstuffworks.com/spud-gun.htm
REFERENCE.COM
http://www.reference.com/browse/wiki/Spud_gun
THE SPUD GUN TECHNOLOGY CENTER
http://www.spudtech.com/content.asp?id=5
SEARCH STRATEGY
SEARCH ENGINE USED:
Google ://www.google.com
SEARCH TERMS USED:
Spud gun
Potato gun
Spudzooka
Potato cannon
How it works
Combustion
Pneumatic
Pressure |
Clarification of Answer by
tutuzdad-ga
on
16 Nov 2005 09:11 PST
Sure. Wow, you must really have some smart 12 and 15 year-olds on your hands.
As you will see by viewing the link below the combustive reaction
varies depending on the type of propellant used.
PROPELLANTS
http://ffden-2.phys.uaf.edu/211.fall2000.web.projects/J.%20Jones%20and%20T.%20Kenny/Propellants.htm
?Combustion or burning is an exothermic reaction between a substance
(the fuel) and a gas (the oxidizer), usually O2, to release heat. In a
complete combustion reaction, a compound reacts with an oxidizing
element, and the products are compounds of each element in the fuel
with the oxidizing element.?
WIKIPEDIA
http://en.wikipedia.org/wiki/Combustion
The simplified chemical equation of combustion is:
Reactants --> Product + Energy
The simple word equation for the combustion of a hydrocarbon is:
Fuel + Oxygen ? Heat + Water + Carbon dioxide.
?Combustion: A combustion reaction is when oxygen combines with
another compound to form water and carbon dioxide. These reactions are
exothermic, meaning they produce heat.?
C10H8 + 12 O2 ---> 10 CO2 + 4 H2O
THE SIX TYPES OF CHEMICAL REACTION
http://misterguch.brinkster.net/6typesofchemicalrxn.html
The type of combustion that is typically seen in a ?spud gun? is
called a fuel-air explosive. In this scenario a fuel source (usually
an aerosol or propane) is ignited. When the fuel fires this causes the
air in the chamber to heat and, as you know, heat greatly expands thus
creating the pressure within the tube. The pressure greatly exceeds
the maximum capacity of the enclosed chamber thus the blocking
projectile (the spud) is forced toward the point of least resistance
(in this case, ideally, the barrel opening) and is thrust upward at a
high velocity.
?The potato cannon uses the basic principle behind any reciprocating
internal combustion engine: If you put a tiny amount of high-energy
fuel (like gasoline) in a small, enclosed space and ignite it, an
incredible amount of energy is released in the form of expanding gas.
You can use that energy to propel a potato 500 feet. In this case, the
energy is translated into potato?
HOW STUFF WORKS
http://auto.howstuffworks.com/engine3.htm
?The spark ignites the fuel, causing an explosion. Pressure builds
rapidly behind the potato, which is wedged tightly in the barrel. When
the pressure in the barrel overcomes the resistance from the potato,
the expanding gas forces the potato down the length of the barrel and
into the air.?
HOW STUFF WORKS
http://science.howstuffworks.com/spud-gun2.htm
Why does an internal explosion create such pressure? Well, when a
propellant is introduced to oxygen (the air in the chamber) there
becomes a chemical bond between the two that holds them together. When
the igniter lights the propellant the oxygen and nitrogen atoms that
were once bonded tightly together suddenly disassociate. This
disassociation is evident in the form of sudden ?expansion? and in
simple terms, fills up the chamber until it can hold no more and the
force is exerted against the point of least resistance (the spud).
Since the spud is not immovably lodged in the barrel it is pushed out
like a cork in a bottle and the result is a fired projectile. An
inverted process is the same process by which rockets are designed.
For example, if you take a spud gun and place the opening against the
earth?s surface and fire it, you will have basically recreated the
same principle upon which a rocket is based. This of course is a
separate issue but I thought you might be able to use this example in
your explanation to help clarify the principle of thrust.
Tutuzdad-ga
INFORMATIVE SOURCES:
COMBUSTION INSTITUTE
http://www.combustioninstitute.org/
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