SpudFiles Wiki - User contributions [en]

Chamber to barrel ratio

2008-06-04T12:13:59Z

Psycix: Fixed link.

The '''Chamber to Barrel ratio''', or '''C:B ratio''', is the ratio of the volume of the [[chamber]] and [[barrel]];

<table border=0><tr><td></td><td></td><td>(Volume Chamber)</td></tr><tr><td>'''C:B Ratio'''</td><td>=</td><td><hr></td></tr>
<tr><td></td><td></td><td> (Volume Barrel)</td></tr></table>

The volume of a cillindric shape can be calculated with the following formula, where "r" stands for the radius of the inner diameter of the pipe and "h" stands for height, wich is basically the same as length:
:'''V = π x r² x h'''

Make sure you use the same units for both the radius and length of both your barrel and your chamber. Note that elbows or other parts also count in for the volume. If you want to be even more precise in calculating the volume of some shape, you could [[water measuring|water measure]] it.

The ratio determines how much of the available energy is transferred to the projectile. A good C:B ratio is a major factor in [[combustion cannon]] performance. The goal of an optimal ratio is to have the barrel end at the exact point where the projectile stops accelerating.

Higher ratio (shorter barrel) cannons are louder since significant [[pressure]] is still present in the chamber when the projectile exits the barrel. The high pressure results in a significantly louder "bang" from the gun, similar to the loud pop from the opening of a pressurized bottle of Champagne. Too low a ratio will also hurt performance since a vacuum is formed behind the projectile if it travels too far or if the [[combustion process|combustion]] gases cool significantly before the projectile exits the barrel. In extreme cases of low C:B ratio the projectile may even be sucked back into the chamber. [http://www.burntlatke.com/15cb-testday.html]

[http://www.burntlatke.com/launch.html Experiments] have shown that a C:B ratio of about 0.6-0.8:1 is the most efficient for a given chamber using potatoes as projectiles. The ratio will vary somewhat depending on the weight, friction, diameter and blow by of a particular projectile, as well as the [[fuel]], [[ignition source|ignition type]] and perhaps other variables. In most cases though, the optimal C:B is expected to be in the 0.6 to 0.8 range.

== C:B Ratio Studies ==

The actual affect of changing the C:B ratio on the [[muzzle velocity]] for a typical gun is shown in the two graphs below (data from [http://www.burntlatke.com/launch.html BurntLatke].) The first graph gives the muzzle velocity as a function of barrel length, the second graph the barrel length axis has been converted to C:B ratios.

[[Image:BurntLatke_CB_barrel_length_graph_15_spuds.gif]]

[[Image:BurntLatke_CB_graph_15_spuds.gif]]

As you can see from the graphs, this particular chamber / barrel / fuel / [[ammo]] combination gives the greatest muzzle velocity (521 FPS) with an 11 foot long barrel. The C:B for this barrel is 0.60.

The two graphs also indicate another key characteristic of the affect of the C:B ratio on the performance of a combustion spudgun. '''The muzzle velocity is fairly insensitive to small changes in the C:B near the optimal C:B.''' For this gun, removing 3.6 feet from the barrel, 1/3 of the barrels total length, to a C:B=0.90 only dropped the muzzle velocity by 10 FPS. That is less than a 2% change in the muzzle velocity for a 33% shorter barrel. Similarly, increasing the barrel from 11' to 13.3' (C:B=0.5) only decreased the muzzle velocity from 521 FPS to 514 FPS. For both the shorter and longer barrels the decrease in the average muzzle velocity is less than the ~5% shot-to-shot variability of this gun and ammo.

The combination of a broad and flat velocity versus C:B (or barrel length) curve and the relatively high shot-to-shot variability for shooting spuds, suggests that the C:B ratio does not have to be all that close to the optimal to get essentially optimal performance from a gun. For the gun used to generate the graphs above, shortening the barrel by 1/3 from the optimal length had essentially no affect on the performance of the gun. The red boxes on the two graphs indicate the range of barrel lengths and C:B ratios for which the muzzle velocity is within 5% of the optimal.

If you can tolerate a slightly larger performance drop of say 10%, then the C:B can range from 0.5 to ~1.2.

Note that this affect is only true over this 0.5~1.2 C:B ratio range. Outside of this range the muzzle velocity drops off very rapidly. For this gun / ammo / fuel combination, a C:B of 0.3 drops the velocity from 521 FPS to 330 FPS. A C:B of 1.4 drops the velocity from 521 FPS to 340 FPS.

A statistical analysis of this set of velocity versus barrel length data indicates that there is no statistically significant difference for C:Bs in the range of 0.5 to 1.2. (Student's T-Test, unequal variance, two tailed, P<0.05). Note that this does not mean there is no difference over this range of C:Bs. Instead, it says '''this study was unable to measure the difference'''. It appears that the relatively high shot-to-shot variability is obscuring the fine details of the relationship between barrel length (or C:B) and muzzle velocity.

One might ask the question "Can't the optimal C:B ratio be easily calculated"? A simple minded approach would be to calculate the barrel volume at which the pressure in the chamber is equal to atmospheric pressure plus the force required to overcome dynamic friction between the spud and the barrel. If we assume atmospheric pressure is 15 PSIa (absolute) and that a typical spud in a 2" barrel has 5 PSI of friction then the spud will continue to accelerate until the chamber pressure has dropped below 15+5=20 PSIa. The combustion of [[propane]] in air generates about 135 PSIa of pressure. The ratio of the combustion pressure to the minimal pressure, 135/20=6.75, is the ratio of the chamber to the ''chamber plus the optimal barrel''. Therefore, this simple minded approach says that the optimal C:B should be 1/(6.75-1)=0.17. This C:B would give a barrel nearly five times longer than the observed optimal barrels. Indeed, according to the BurntLatke studies, a spud won't even exit the barrel at a C:B this low. Clearly, there are more complex factors affecting the optimal C:B ratio of a combustion spudgun. Suggestively, the optimal C:B ratio for pneumatic guns, as predicted by [[GGDT]] are in the vicinity of 0.2~0.3, close to the simple minded calculation outlined above.

== Theories on the C:B Ratio ==

Several theories exist explaining the optimal barrel for a particular chamber. A brief summary of three are given below.

1. A chamber volume that exceeds the optimal value for efficiency will increase performance since more energy is available within the chamber. This theory is based on the "fast combustion model" in which the fuel burns quickly relative to the movement of the projectile. The pressure rises to it's maximum before the projectile has moved a significant distance down the barrel. When designing a launcher using this theory, a larger chamber will produce better performance.

2. The most efficient ratio will usually also produce the highest performance. (Internal combustion engines fall into this category, the most efficient engine speed usually also produces the most power.) This theory is based on a "slow combustion model". A chamber that is too large takes longer to burn the fuel, the pressure rises more slowly, the projectile starts to move through the barrel at lower pressures and the exits before maximum pressure is reached. This results in lower projectile speed. This model is similar to a slow opening pneumatic valve. If the chamber is grossly oversized, combustion may not complete until after the projectile leaves the barrel, further reducing performance. When designing a launcher, the chamber and barrel should be matched to each other for optimal performance. A ratio anywhere between 0.5:1 and 1:1 will be ideal under most circumstances.

3. A hybrid "slow / fast combustion model". In the first two models the combustion process is considered to be independent of the projectile's movement through the barrel. In the "hybrid" model the movement of the projectile affects the burning process. In particular, movement of the projectile allows the gases in the chamber to move, this causes turbulence which in turn increases the rate at which the fuel burns. This model may explain the broad flat velocity versus C:B (or barrel length) graphs. A slow moving projectile has a slow fuel burn speed, a fast moving projectile gives a faster burn speed. The slow moving projectile takes longer to exit the barrel and there is more time for the fuel to burn. A fast moving projectile takes less time to exit but it's movement makes the fuel burn faster.

Interestingly, gasoline engines display this characteristic "projectile speed" versus fuel burn speed relationship. The time available for the fuel-air mixture to burn in an engine is dependent on the RPMs the engine is running at. At high RPMs there is less time for the fuel to burn compared to low RPMs. (In most simple gasoline engines the fuel injected per combustion cycle is independent of the engines RPM.) Studies of the fuel burn rate as a function of the engine RPMs indicate that the fuel burns faster at high RPMs than at low and that combustion occurs thoughout most of the power stroke.

Very little hard data is available to help differentiate between these and other possible models.

== Summary ==

C:B ratios are good for predicting what barrel will yield the highest performance given a fixed chamber volume. In addition, the C:B ratio will be indicative of the noise output of the gun. With sufficient data, the C:B ratio will also be useful for determining what chamber volume will deliver optimal performance given a fixed barrel. However, a single highly precise value should not be interpreted as the optimal value for all cannons.

== Other Aspects of C:B Ratios ==

[[Burst disk]]s can be used to increase the performance of high ratio (short barrel) cannons, since they let higher pressure build up before the projectile starts to move. They will not increase the performance significantly in near optimal cannons. [http://www.advancedspuds.com/burstdisk.htm Source]

[[Multiple spark gaps]], spark position and a [[Chamber fan]] may also affect the optimal C:B for a particular cannon. Currently there is relatively little reliable data on the affect of these parameters on the optimal C:B ratio.

The C:B ratio plays a much smaller role in the design of [[pneumatic cannon]]s since factors like chamber charge pressure and valve performance play a greater role in determining overall performance. It is advisable to use [[GGDT]] to determine the best design for pneumatic cannons.

[[category:Concepts]]

Water measuring

2008-06-04T12:12:13Z

Psycix: Created a simple page about water measuring.

'''Water measuring''' is a way of precisely measuring the volume of specific parts of your gun. This is mostly needed for calculating the [[C:B ratio]] or needed for calculating the amount of fuel needed to be injected by a [[fuel meter]]. The volume of cylindric shapes can easily be calculated, but the volume of an elbow or other fittings is ridicilously hard to calculate. Therefore you can use this method.

You fill up the part you want to measure (your [[chamber]] + elbows for example) with water. Then you take the water out bit by bit and measure the volume of it using a measuring cup. When you got all the water out, add up all volumes you measured and voila, you got the volume of your chamber.

[[PVC]] is made to carry water so this won't harm your cannon, but ofcourse you should take stuff like your [[chamber fan]] out. Some types of metal, like ungalvanised steel may rust, and thus you could consider using oil.

Chamber to barrel ratio

2008-06-04T11:59:13Z

Psycix: Added formula and a piece of text for calculating the volumes.

The '''Chamber to Barrel ratio''', or '''C:B ratio''', is the ratio of the volume of the [[chamber]] and [[barrel]];

<table border=0><tr><td></td><td></td><td>(Volume Chamber)</td></tr><tr><td>'''C:B Ratio'''</td><td>=</td><td><hr></td></tr>
<tr><td></td><td></td><td> (Volume Barrel)</td></tr></table>

The volume of a cillindric shape can be calculated with the following formula, where "r" stands for the radius of the inner diameter of the pipe and "h" stands for height, wich is basically the same as length:
:'''V = π x r² x h'''

Make sure you use the same units for both the radius and length of both your barrel and your chamber. Note that elbows or other parts also count in for the volume. If you want to be even more precise in calculating the volume of some shape, you could [[water measure]] it.

The ratio determines how much of the available energy is transferred to the projectile. A good C:B ratio is a major factor in [[combustion cannon]] performance. The goal of an optimal ratio is to have the barrel end at the exact point where the projectile stops accelerating.

Higher ratio (shorter barrel) cannons are louder since significant [[pressure]] is still present in the chamber when the projectile exits the barrel. The high pressure results in a significantly louder "bang" from the gun, similar to the loud pop from the opening of a pressurized bottle of Champagne. Too low a ratio will also hurt performance since a vacuum is formed behind the projectile if it travels too far or if the [[combustion process|combustion]] gases cool significantly before the projectile exits the barrel. In extreme cases of low C:B ratio the projectile may even be sucked back into the chamber. [http://www.burntlatke.com/15cb-testday.html]

[http://www.burntlatke.com/launch.html Experiments] have shown that a C:B ratio of about 0.6-0.8:1 is the most efficient for a given chamber using potatoes as projectiles. The ratio will vary somewhat depending on the weight, friction, diameter and blow by of a particular projectile, as well as the [[fuel]], [[ignition source|ignition type]] and perhaps other variables. In most cases though, the optimal C:B is expected to be in the 0.6 to 0.8 range.

== C:B Ratio Studies ==

The actual affect of changing the C:B ratio on the [[muzzle velocity]] for a typical gun is shown in the two graphs below (data from [http://www.burntlatke.com/launch.html BurntLatke].) The first graph gives the muzzle velocity as a function of barrel length, the second graph the barrel length axis has been converted to C:B ratios.

[[Image:BurntLatke_CB_barrel_length_graph_15_spuds.gif]]

[[Image:BurntLatke_CB_graph_15_spuds.gif]]

As you can see from the graphs, this particular chamber / barrel / fuel / [[ammo]] combination gives the greatest muzzle velocity (521 FPS) with an 11 foot long barrel. The C:B for this barrel is 0.60.

The two graphs also indicate another key characteristic of the affect of the C:B ratio on the performance of a combustion spudgun. '''The muzzle velocity is fairly insensitive to small changes in the C:B near the optimal C:B.''' For this gun, removing 3.6 feet from the barrel, 1/3 of the barrels total length, to a C:B=0.90 only dropped the muzzle velocity by 10 FPS. That is less than a 2% change in the muzzle velocity for a 33% shorter barrel. Similarly, increasing the barrel from 11' to 13.3' (C:B=0.5) only decreased the muzzle velocity from 521 FPS to 514 FPS. For both the shorter and longer barrels the decrease in the average muzzle velocity is less than the ~5% shot-to-shot variability of this gun and ammo.

The combination of a broad and flat velocity versus C:B (or barrel length) curve and the relatively high shot-to-shot variability for shooting spuds, suggests that the C:B ratio does not have to be all that close to the optimal to get essentially optimal performance from a gun. For the gun used to generate the graphs above, shortening the barrel by 1/3 from the optimal length had essentially no affect on the performance of the gun. The red boxes on the two graphs indicate the range of barrel lengths and C:B ratios for which the muzzle velocity is within 5% of the optimal.

If you can tolerate a slightly larger performance drop of say 10%, then the C:B can range from 0.5 to ~1.2.

Note that this affect is only true over this 0.5~1.2 C:B ratio range. Outside of this range the muzzle velocity drops off very rapidly. For this gun / ammo / fuel combination, a C:B of 0.3 drops the velocity from 521 FPS to 330 FPS. A C:B of 1.4 drops the velocity from 521 FPS to 340 FPS.

A statistical analysis of this set of velocity versus barrel length data indicates that there is no statistically significant difference for C:Bs in the range of 0.5 to 1.2. (Student's T-Test, unequal variance, two tailed, P<0.05). Note that this does not mean there is no difference over this range of C:Bs. Instead, it says '''this study was unable to measure the difference'''. It appears that the relatively high shot-to-shot variability is obscuring the fine details of the relationship between barrel length (or C:B) and muzzle velocity.

One might ask the question "Can't the optimal C:B ratio be easily calculated"? A simple minded approach would be to calculate the barrel volume at which the pressure in the chamber is equal to atmospheric pressure plus the force required to overcome dynamic friction between the spud and the barrel. If we assume atmospheric pressure is 15 PSIa (absolute) and that a typical spud in a 2" barrel has 5 PSI of friction then the spud will continue to accelerate until the chamber pressure has dropped below 15+5=20 PSIa. The combustion of [[propane]] in air generates about 135 PSIa of pressure. The ratio of the combustion pressure to the minimal pressure, 135/20=6.75, is the ratio of the chamber to the ''chamber plus the optimal barrel''. Therefore, this simple minded approach says that the optimal C:B should be 1/(6.75-1)=0.17. This C:B would give a barrel nearly five times longer than the observed optimal barrels. Indeed, according to the BurntLatke studies, a spud won't even exit the barrel at a C:B this low. Clearly, there are more complex factors affecting the optimal C:B ratio of a combustion spudgun. Suggestively, the optimal C:B ratio for pneumatic guns, as predicted by [[GGDT]] are in the vicinity of 0.2~0.3, close to the simple minded calculation outlined above.

== Theories on the C:B Ratio ==

Several theories exist explaining the optimal barrel for a particular chamber. A brief summary of three are given below.

1. A chamber volume that exceeds the optimal value for efficiency will increase performance since more energy is available within the chamber. This theory is based on the "fast combustion model" in which the fuel burns quickly relative to the movement of the projectile. The pressure rises to it's maximum before the projectile has moved a significant distance down the barrel. When designing a launcher using this theory, a larger chamber will produce better performance.

2. The most efficient ratio will usually also produce the highest performance. (Internal combustion engines fall into this category, the most efficient engine speed usually also produces the most power.) This theory is based on a "slow combustion model". A chamber that is too large takes longer to burn the fuel, the pressure rises more slowly, the projectile starts to move through the barrel at lower pressures and the exits before maximum pressure is reached. This results in lower projectile speed. This model is similar to a slow opening pneumatic valve. If the chamber is grossly oversized, combustion may not complete until after the projectile leaves the barrel, further reducing performance. When designing a launcher, the chamber and barrel should be matched to each other for optimal performance. A ratio anywhere between 0.5:1 and 1:1 will be ideal under most circumstances.

3. A hybrid "slow / fast combustion model". In the first two models the combustion process is considered to be independent of the projectile's movement through the barrel. In the "hybrid" model the movement of the projectile affects the burning process. In particular, movement of the projectile allows the gases in the chamber to move, this causes turbulence which in turn increases the rate at which the fuel burns. This model may explain the broad flat velocity versus C:B (or barrel length) graphs. A slow moving projectile has a slow fuel burn speed, a fast moving projectile gives a faster burn speed. The slow moving projectile takes longer to exit the barrel and there is more time for the fuel to burn. A fast moving projectile takes less time to exit but it's movement makes the fuel burn faster.

Interestingly, gasoline engines display this characteristic "projectile speed" versus fuel burn speed relationship. The time available for the fuel-air mixture to burn in an engine is dependent on the RPMs the engine is running at. At high RPMs there is less time for the fuel to burn compared to low RPMs. (In most simple gasoline engines the fuel injected per combustion cycle is independent of the engines RPM.) Studies of the fuel burn rate as a function of the engine RPMs indicate that the fuel burns faster at high RPMs than at low and that combustion occurs thoughout most of the power stroke.

Very little hard data is available to help differentiate between these and other possible models.

== Summary ==

C:B ratios are good for predicting what barrel will yield the highest performance given a fixed chamber volume. In addition, the C:B ratio will be indicative of the noise output of the gun. With sufficient data, the C:B ratio will also be useful for determining what chamber volume will deliver optimal performance given a fixed barrel. However, a single highly precise value should not be interpreted as the optimal value for all cannons.

== Other Aspects of C:B Ratios ==

[[Burst disk]]s can be used to increase the performance of high ratio (short barrel) cannons, since they let higher pressure build up before the projectile starts to move. They will not increase the performance significantly in near optimal cannons. [http://www.advancedspuds.com/burstdisk.htm Source]

[[Multiple spark gaps]], spark position and a [[Chamber fan]] may also affect the optimal C:B for a particular cannon. Currently there is relatively little reliable data on the affect of these parameters on the optimal C:B ratio.

The C:B ratio plays a much smaller role in the design of [[pneumatic cannon]]s since factors like chamber charge pressure and valve performance play a greater role in determining overall performance. It is advisable to use [[GGDT]] to determine the best design for pneumatic cannons.

[[category:Concepts]]

Spray and pray

2008-06-03T21:15:52Z

Psycix: Created a simple page about spray and pray.

[[image:Basic_combustion.JPG|right|thumb|350px|Basic "spray and pray" combustion cannon.]]

'''Spray and pray''' (or spray & pray or spray 'n pray) cannons are the most basic [[combustion cannon]]s. It is the simplest way of fueling a cannon, and can be very inconsistent. You first spray in fuel at the back of the cannon. This basic fuel can be anything that is combustible and in a spray can, aerosols like hairspray or deodorant are mostly used. The cannon has a threaded endcap wich is removed and is put back right after the fuel was sprayed in.
Because getting the right amount of fuel ([[stoichiometry]]) in is very hard with this method you often have got a poor mix. It doesn't always have the same power, and sometimes the amount of fuel is so much off that the cannon wont even fire.

Its called spray and pray because you first '''spray''' in your fuel, and then '''pray''' because you don't know if its going to work and just hope for the best.

An upgrade from this type of fueling is building a [[fuel meter]] on your cannon. With that you will always inject the right amount of fuel.

Combustion cannon

2008-06-03T20:58:02Z

Psycix: spelling, link, and one interpunction

[[image:Basic_combustion.JPG|right|thumb|350px|Basic combustion cannon]]
A '''combustion cannon''' is a cannon powered by the combustion of flammable fuel-air mixtures. A basic combustion cannon is the simplest one to build, and is therefore often the choice for first time builders, though advanced designs with more power and useability are also possible. Most of the combustion spudguns are made out of [[PVC]] or [[ABS]].

Though being the simplest cannon of all, a well-made pneumatic or hybrid is usually more powerful then a combustion. The pressures in a combustion usually do not rise above 50-60 psi on [[spray and pray]] cannons and about 70-90 psi on advanced cannons. Jimmy101, a member of spudfiles showed that in closed chamber experiments the maximum pressure possible with a propane mix is a little more then 100psi in a closed chamber.

Potato guns are often painted to increase their aesthetic value and to cover unsightly primer stains. Krylon Fusion, a type of paint specifically made for plastics, is the most common type of spray paint used on potato cannons. Some combustion cannons have chambers made from clear pvc, this allows you to see the internals like the sparks and the fan (LED fans can be beautiful this way) and most importantly: a flash of fire on ignition.

==Basic elements==

* Combustion [[Chamber]]
* [[Fuel]]
* [[Ignition source]]
* [[Barrel]]

==Operation & Theory==

In order to fire, the operator loads a projectile (usually a potato) down into the barrel, adds [[fuel]] to the combustion chamber, like spraying [[aerosol]]s in the back end and screwing the cap on and then triggers the [[ignition source]] (e.g. a piezoelectric [[BBQ ignitor]]). The ignition system creates a spark in the chamber wich ignites the fuel. The fuel quickly burns up and the hot gases want to expand, creating pressure. The pressure acts out force on the back of the projectile and the air pushes the projectile out. Muzzle velocity and distance vary greatly mostly depending on the [[stoichiometry]] of the fuel/air mixture, the size and [[chamber to barrel ratio]] (C:B ratio) of the launcher and possible performance increasing additions like a [[chamber fan]].

The most basic spudguns use aerosols like hairspray, deodorant or [[starting fluid]] sprayed in at the back ([[spray and pray]]), while more advanced cannons usually use [[propane]] injection. Besides the difference of low-power aerosols and pure fuels like propane there is not much of a difference between fuels, the power of propane and butane vary no more then a few percent. Achieving the right stoichiometry (right amount of fuel and air/oxygen) is usually more important for the performance. Spray and pray cannons often have poor stoichiometry and thus inconsistent shots with relatively low power. Fast burning fuels like [[hydrogen]], [[acetylene]] and [[oxygen enriched]] mixtures are usually avoided, even if they may be a little more powerful, their fast burn rates and high chance of [[DDT]] create shockwaves that are powerful enough to rip any PVC or ABS chamber to shreds.

==Advanced improvements==

*[[Chamber fan]]
*[[Stun gun]], [[flyback transformer]] or [[camera flash]] ignition
*Multiple [[spark gap]]s
*[[Fuel meter]]
*A good [[C:B ratio]]
*[[Breech loading]] mechanism

The above improvements will increase the power and/or useability of the gun. View those pages for more information.

==Safety==
Non [[pressure rated]] "DWV" pipe is used so now and then in combustion cannons and it is generally accepted that this is safe with basic spray and pray; however some of these cannons have blown up. If you want to be safe you should definitely use pressure rated pipe and fittings, especially on advanced combustions.
As stated above specific fuels such as acetylene, hydrogen, and any oxygen enriched mixtures are not safe to use in a gun made out of PVC or ABS.
When using a normal fuel, pressure rated pipe and fittings and have properly [[solvent welding|solvent welded]] all of the joints, you can be considered 100% safe.

Some people, like parents think it is unsafe because it is an "explosion" and thus dangerous. In that case point them on the facts that it is just a [[deflagration]] and not a [[detonation]] creating no huge shockwaves, but creating just heat and pressure. This pressure is lower then the rated pressure of pressure rated pipe and fittings and that means the gun simply can't explode.

Note that cold weather makes pvc brittle and you should not use a frozen gun. However a gun that has been frozen some time but is up to normal temperature again is safe to use.
Dropping the gun isn't recommended, especially frozen guns, since this might create tiny cracks; a starting point for completely cracking open and blowing up.

==Legality==
Laws are different in each country, state or even city, check the [[legal issues]] page for more information. In some countries combustion cannons are illegal while pneumatics (possibly being way more powerful) may be legal.

Chamber fan

2008-06-03T20:40:37Z

Psycix: Rewritten the whole page. Added ALOT of information: page is now more then 2 times as long. Also added picture.

[[image:Chamberfan.JPG|right|thumb|300px|A 92mm chamber fan at the back of a cannon.]]
A '''chamber fan''' is an improvement mostly used in [[combustion cannon]]s, though [[hybrid cannon]]s sometimes also have one. The fan will improve [[muzzle velocity]], power and useability. It is a recommended upgrade and may improve the power of your cannon with 20-50%. Sometimes more then one fan is used.
For exact information on performance gains see [http://www.burntlatke.com/jpg600/fan-data.gif this latke test].

==Purposes==
They have multiple purposes:

*Mixing
*Venting
*Inducing turbulence

Their main purpose of increasing cannon performance is '''mixing''' the air or [[oxygen]] with the [[fuel]]. This creates an even mix and significantly increases performance. Unmixed chambers may have way too much fuel on one side of the [[chamber]], while a lack of fuel exists on the other side. The chamber fan fixes this and helps having a good [[stoichiometry]] throughout the chamber.

Another purpose is '''venting'''. After each shot the burnt fuel mixture needs to get out of the cannon and fresh air needs to be put in. This is because (almost) all oxygen is used for the combustion and new oxygen is needed for a new combustion. When the back end of the chamber is opened up using either a cap or some other system like a [[ball valve|ballvalve]] or quick vent plug, the fan effectively blows out all air and sucks in fresh air. With a chamber fan venting is done much faster and much more reliable.

The last purpose counts if the fan is on during combustion: '''turbulence''' is induced because the air inside the chamber is moving. This causes the flame front to speed up, and thus the combustion goes faster. This further increases the performance of the cannon.
Theories exist tell that two smaller fans blowing in opposite directions induce more turbulence though they wont do very well on venting.

==Types of fans==

The most commonly used fans are case fans out of computers. The corners may be sanded down so it fits in the chamber, and sometimes the complete frame around the fan is cut off and even the fan blades need to be cut for fitting it into in small chambers.
Instead of PC fans you could also use any other electric motor with fan blades attached. However, you should avoid brushed motors at all times. PC fans have brushless motors and thus do not create any sparks within. A brushed motor could spark with the possibility of suddenly igniting the fuel mixture, wich can lead to accidents damaging the environment or even harming people. So now and then someone still uses a brushed engine but sealed the holes in the housing off with glue, making no fuel enter the insides of the engine. The engine wont be able to cool itself anymore, but when used in short periods, that doesn't really matter.

Since PC fans are cheap, brushless and readily available in various sizes, this is the choice of most spudders and the most recommended option.
Further characteristics of fans are noise, CFM (the amount of air it displaces) and RPM (how fast it rotates). Noise doesn't really matter for a cannon, and you probably want to have an high CFM, high RPM fan for some good mixing.

== Powering, wiring and mounting the fan ==

'''Powering the fan'''

Nearly all fans are powered by a pack of batteries. A 12V fan can be ran off a single 9V battery (though it will not run at full power) or a bunch of 8 1.5V batteries. It is also useful to have a switch to turn the fan on and off. It can also be controlled automatically using a [http://www.burntlatke.com/fan-controller.html timer circuit] which turns the fan off when it's not needed. For a goodlooking cannon without duct tape and wires all over it, you build all the fan circuitry and batteries into a fan control box. Since the same can be done with ignition systems in a ignition control box, some people merge it together into one fan/ignition control box. This gives the advantage of running a fan and a stungun from the same batteries, saving space and weight. Sometimes this box is built in the handle for an even cleaner look not showing any wires.

'''Mounting & wiring the fan'''

The easiest way of doing this is having a PC fan wich is a bit too large. Sand down the corners until it fits in tight and squeeze it in. The fan will stay in place due to friction. Glue can be used to secure the fans and sometimes bolts are used too.

Since the batteries and possibly switch are outside the chamber while the fan is inside, you will need to get those two wires to the fan someway.
A leakless way of doing so is screwing in two bolts trough the chamber wall (preferably drill trough pipe+fitting so you've got two layers of PVC/ABS.) Then attach the wires on the in and the outside to the bolts and your done.

'''Fan placement'''

It is not recommended to place the fan too much near the barrel opening. Remember that all air behind the fan wants to go past it and if that is too much you will constrict flow to the barrel and most importantly: your fan will probably break. Most fans are at the back of the chamber so they can easily be taken out for service on the chamber or the fan. Note that you cant take a fan out when the electrode bolts are in the way.

==Will my fan survive the combustion?==

If you think that your fan might get burned or melt, dont worry, it wont. Since the heat in the combustion chamber lasts no more then a few milliseconds the fan doesn't have time to heat up or melt. As stated above, your fan should not be too close to the barrel. If its more to the back, it wont get blown to pieces due to the extreme flow.

A fan may have a somewhat shortened lifespan inside a combustion chamber, but it still can survive for many years. So for combustion cannons: Yes, your fan will survive.

In hybrids fans last shorter due to the more extreme environment. High-mix hybrids can break their fans on the first shot. Luckily, those hybrids dont need fans anyway. When you pressurize the fuel mixture with air after you have put in the fuel, the blow-in of air already mixes the fuel. Also because of DDT dangers you dont want to have turbulence anyway.

File:Chamberfan.JPG

2008-06-03T20:15:25Z

Psycix: uploaded a new version of "Image:Chamberfan.JPG": This is the 92mm PC case fan at the back of my first cannon. There is some sand and dirt on the blades because I put it down while venting and loading somewhere outdoors.

This is the 92mm PC case fan at the back of my first cannon.

File:Chamberfan.JPG

2008-06-03T20:12:10Z

Psycix: This is the 92mm PC case fan at the back of my first cannon.

This is the 92mm PC case fan at the back of my first cannon.

Combustion cannon

2008-06-03T14:25:25Z

Psycix: Revamped the whole page.

[[image:Basic_combustion.JPG|right|thumb|350px|Basic combustion cannon]]
A '''combustion cannon''' is a cannon powered by the combustion of flammable fuel-air mixtures. A basic combustion cannon is the simplest one to build, and is therefore often the choice for first time builders, though advanced designs with more power and useability are also possible. Most of the combustion spudguns are made out of [[PVC]] or [[ABS]]

Though being the simplest cannon of all, a well-made pneumatic or hybrid is usually more powerful then a combustion. The pressures in a combustion usually do not rise above 50-60 psi on [[spray and pray]] cannons and about 70-90 psi on advanced cannons. Jimmy101, a member of spudfiles showed that in closed chamber experiments the maximum pressure possible with a propane mix is a little more then 100psi in a closed chamber.

Potato guns are often painted to increase their aesthetic value and to cover unsightly primer stains. Krylon Fusion, a type of paint specifically made for plastics, is the most common type of spray paint used on potato cannons. Some combustion cannons have chambers made from clear pvc, this allows you to see the internals like the sparks and the fan (LED fans can be beautiful this way) and most importantly: a flash of fire on ignition.

==Basic elements==

* Combustion [[Chamber]]
* [[Fuel]]
* [[Ignition source]]
* [[Barrel]]

==Operation & Theory==

In order to fire, the operator loads a projectile (usually a potato) down into the barrel, adds [[fuel]] to the combustion chamber, like spraying [[aerosol]]s in the back end and screwing the cap on and then triggers the [[ignition source]] (e.g. a piezoelectric [[BBQ ignitor]]). The ignition system creates a spark in the chamber wich ignites the fuel. The fuel quickly burns up and the hot gases want to expand, creating pressure. The pressure acts out force on the back of the projectile and the air pushes the projectile out. Muzzle velocity and distance vary greatly mostly depending on the [[stoichiometry]] of the fuel/air mixture, the size and [[chamber to barrel ratio]] (C:B ratio) of the launcher and possible performance increasing additions like a [[chamber fan]].

The most basic spudguns use aerosols like hairspray, deodorant or starter fluid sprayed in at the back ([[spray and pray]]), while more advanced cannons usually use [[propane]] injection. Besides the difference of low-power aerosols and pure fuels like propane there is not much of a difference between fuels, the power of propane and butane vary no more then a few percent. Achieving the right stoichiometry (right amount of fuel and air/oxygen) is usually more important for the performance. Spray and pray cannons often have poor stoichiometry and thus inconsistent shots with relatively low power. Fast burning fuels like [[hydrogen]], [[acetylene]] and [[oxygen enriched]] mixtures are usually avoided, even if they may be a little more powerful, their fast burn rates and high chance of [[DDT]] create shockwaves that are powerful enough to rip any PVC or ABS chamber to shreds.

==Advanced improvements==

*[[Chamber fan]]
*[[Stun gun]], [[flyback transformer]] or [[camera flash]] ignition
*Multiple [[spark gap]]s
*[[Fuel meter]]
*A good [[C:B ratio]]
*[[Breech loading]] mechanism

The above improvements will increase the power and/or useability of the gun. View those pages for more information.

==Safety==
Non [[pressure rated]] "DWV" pipe is used so now and then in combustion cannons and it is generally accepted that this is safe with basic spray and pray; however some of these cannons have blown up. If you want to be safe you should definitely use pressure rated pipe and fittings, especially on advanced combustions.
As stated above specific fuels such as acetylene, hydrogen, and any oxygen enriched mixtures are not safe to use in a gun made out of PVC or ABS.
When using a normal fuel, pressure rated pipe and fittings and have properly [[solvent welding|solvent welded]] all of the joints, you can be considered 100% safe.

Some people, like parents think it is unsafe because it is an "explosion" and thus dangerous. In that case point them on the facts that it is just a [[deflagration]] and not a [[detonation]] creating no huge shockwaves, but creating just heat and pressure. This pressure is lower then the rated pressure of pressure rated pipe and fittings and that means the gun simply can't explode.

Note that cold weather makes pvc brittle and you should not use a frozen gun. However a gun that has been frozen some time but is up to normal temperature again is safe to use.
Dropping the gun isn't recommended, especially frozen guns, since this might create tiny cracks; a starting point for completely cracking open and blowing up.

==Legality==
Laws are different in each country, state or even city, check the [[legal issues]] page for more information. In some countries combustion cannons are illegal while pneumatics (possibly being way more powerful) may be legal.

Legal issues

2008-04-21T20:54:40Z

Psycix:

==United States of America==
===Letter from the ATF===
The bureau of alchohol, tobacco, and firearms sent the [[SGTC]] the following letter.
Another version with pictures can be found [http://www.spudfiles.com/Other/are_they_legal.php here].

<blockquote>
Department of the Treasury

Bureau of Alcohol, Tobacco, and Firearms

Washington, DC 20226

As defined in section 921(a) (3) of Title 18, United States Code (USC) the term "firearm" means --

* (A) any weapon (including a starter gun) which will or is designed to or may readily be converted to expel a projectile by the action of an explosive;
* (B) the frame or receiver of any such weapon;
* (C) any firearm muffler or firearm silencer; or
* (D) any destructive device. Such term does not include an antique firearm.

As defined in 26 USC subsection 5845(f) (2) the term destructive device includes any type of weapon by whatever name known which will, or which may readily be converted to, expel a projectile by the action of an explosive or other propellent, the barrel or barrels of which have a bore of more than one-half inch in diameter, except a shotgun or shotgun shell which the Secretary or his delegate finds is generally recognized as particularly suitable for sporting purposes; and (3) any combination of parts either designed or intended for use in converting any device into a destructive device as defined in subparagraphs (1) and (2) and from which a destructive device may be readily assembled. The term 'destructive device' shall not include any device which is neither designed nor redesigned for use as a weapon; any device, although originally designed for use as a weapon, which is redesigned for use as a signaling, pyrotechnic, line throwing, safety, or similar device; surplus ordnance sold, loaned, or given by the Secretary of the Army pursuant to the provisions of section 4684(2), 4685, or 4686 of title 10 of the USC; or any other device which the Secretary of the Treasury or his delegate finds is not likely to be used as a weapon, or is an antique or is a rifle which the owner intends to use solely for sporting purposes.

It is unlawful for anyone to make or possess a destructive device which is not registered in accordance with the provisions of the National Firearms Act.

We have previously examined that certain muzzle loading devices known as "potato guns." These potato guns are constructed from PVC plastic tubing. They use hair spray or a similar aerosol substance for a propellant, and have some type of spark ignitor. We have determined that these devices, as described, are not firearms provided that they are used solely for launching potatoes for recreational purposes. However, any such devices which are used as weapons or used to launch other forms of projectiles may be firearms and destructive devices as defined.
</blockquote>

===Interpretation of the ATF letter===
"other forms of projectiles" is typicaly interpreted to mean flammable, and/or explosive projectiles; ie, that "potatoes" were an example of an acceptable projectile, and not the only option.

===State, County, and City laws===
These vary. It is generally illegal to shoot a spudgun inside city limits.
The owner of the SGTC provides the following advice: "<i>If it is OK to shoot a shotgun in your particular location....you will have no problem with your spudgun.</i>"

*In Glendale, Arizona and Phoenix, Arizona combustion spudguns are considered firearms.
Subject: RE: potato gun legality
The legality of potato guns are as follows. If the potato is expelled
by the use of air it is OK. If any explosion occurred from gas, powder,
etc. it is illegal and cannot be fired. The "firearm" definition is
what dictates this.

==Other countries==

=== Australia ===

Information taken from my (Julz) posts on Spudfiles Forums. This information is as accurate as possible, however, may not be entirely correct as I did not speak to anybody of authority, I gained my information only from government web sites.

Please note, nowhere did I read that spudguns were definantly legal. Assumptions that they are legal are made on the basis that in a detailed list of prohibited weapons spudguns were not mentioned.

I am not responsible for any trouble you get into as a result of this information.

For the original posts, see [http://www.spudfiles.com/forums/viewtopic.php?p=1305&lighter=&#1305 here]

Victoria----> Legal with a firearms licence. Pneumatic spud guns require a Category A firearms licence (they are classified as air rifles), whilst combustion spud guns require a Category E firearms license.

New South Wales---->Illegal - Requires a permit that cannot be gained without a legitemate reason for possessing the cannon.
Quote:
Item 2(3) Any device that is designed to propel or launch a bomb, grenade, rocket or missile by any means other than by
means of an explosive, including a device known as a PVC cannon.

http://www.police.nsw.gov.au/__data/assets/file/5232/Prohibited_Weapons_General_Permit.pdf

Australian Capital Territory---->Legal? - I could find no referance to spud guns etc in this page, detailing prohibited weapons.
http://www.legislation.act.gov.au/b/db_14541/19960829-16540/pdf/db_14541.pdf

South Australia---->? I could find no referance to spud guns etc in this page, detailing prohibited weapons.
http://www.sapolice.sa.gov.au/firearms/prohib_weapon/weapon_type.shtml

Northern Territory---->Illegal

Western Australia----> A spud gun can be constructed and possessed legally, however according to state police it is not in the publics interest for one to own a spud gun, so it will be taken and destroyed with no charges.

A spud gun cannot be registered although it does fall under the firearm classification.

Queensland----> Combustion Launchers,Illegal in all forms.Home made devices not registrable.

Pneumatic Launchers,Can be legally owned as long as used in safe manner,police have right to confiscate at anytime and without reason.

http://www.spudfiles.com/forums/australian-spudgun-laws-t7419.html

Tasmania---->Unknown I could not find any page detailing what weapons are prohibited.

update on 2006/02/04 by Mountaineer
Under Australian Law, a spud gun would be considered either a Compressed Air Gun (Class 'A') or a Muzzle Loading Firearm (Class 'B') depending on the propellant being used in the individual device.
This is something of a moot point however as any home-made firearm falls under Prescribed Firearms legislation.
Other examples of Prescribed Firearms include fully automatic weapons, modified firearms (sawn-off etc), silenced firearms, disguised firearms and firearms designed to fire explosive or incendiary devices such as bazookas and mortars.
There are a few exception to the Prescribed Firearms legislation, mainly related to weapons made and used in theatrical productions.
Theoretically, a registered gun owner with (for example) a Class 'B' license could have a gunsmith make a spud gun for them which could then be registered and legally used under purpose 2 (Target Shooting - unsupervised) of Australian law. However I find it hard to imagine anyone doing that when they could simply buy a "real" gun.
I got all this information from "Firearms & the Law in South Australia" (c) 2004 Regency Publishing ISBN: 1 86393 404 9 which I am currently studying for my own license.

=== The Netherlands ===

All combustion (and hybrid) cannons are considered firearms and are illigal.
Pneumatic launchers are considered air rifles and are legal on your own terrain.

[[category:Concepts]]

Copper

2008-04-14T17:30:37Z

Psycix:

See: [[Copper pipe]]

Copper

2008-04-14T17:29:24Z

Psycix: Removing all content from page

Copper

2008-04-14T17:28:43Z

Psycix:

Copper pipe and fittings are used alot mostly for [[pneumatic]] cannons. Copper pipe can hold way more PSI then [[PVC]] (it can hold over 300PSI easily), this is the main reason most people use it. Often copper pipe is used together with a [[QEV]].
Most of the times copper pipes and fittings are soldered together, but some people use [[epoxy]] too.
Threaded fittings and pipe are ofcourse just screwed in with the use of [[PTFE tape]] or [[pipe dope]].

The disadvantage of copper pipe is that large bore copper pipe (1" and larger) is not available everywhere and that its quite pricey too. Due to this size most copper cannons are using [[marbles]] as ammo.

Copper

2008-04-14T17:27:10Z

Psycix: New page: Copper pipe and fittings are used alot mostly for pneumatic cannons. Copper pipe can hold way more PSI then PVC (it can hold over 300PSI easily), this is the main reason most people use it...

Copper pipe and fittings are used alot mostly for pneumatic cannons. Copper pipe can hold way more PSI then PVC (it can hold over 300PSI easily), this is the main reason most people use it. Often copper pipe is used together with a QEV.
Most of the times copper pipes and fittings are soldered together, but some people use epoxy too.
Threaded fittings and pipe are ofcourse just screwed in with the use of PTFE tape or pipe dope.

The disadvantage of copper pipe is that large bore copper pipe (1" and larger) is not available everywhere and that its quite pricey too. Due to this size most copper cannons are using marbles as ammo.