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wich is the safest? PLZ ANSWER

A place to ask general spud cannon related questions.
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wich is safest?

pneumatic
3
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combustion
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Total votes : 7
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Unread postAuthor: jimmy101 » Sat Aug 16, 2008 5:20 pm

While I'm here, I'll make the point that combustions do not work on "explosions", although a lot of people seem to labour under the misconception that they do.


Well, for one, there are no explosive alkanes, they're all quite stable.

Folks seem to be confusing "explosion", "combustion" and "detonation".

Propane fueled spudguns most definitely involve an explosion. Modern rifles using modern powders also involve explosions. A running automobile engine involves explosions.

Any rapid combustion is characterized as an explosion. Any rapid depressurization is also classified as an explosion.

Explosions are subdivided into two categories deflagration (rapid burning but burn speed is less than the speed of sound) and detonation (rapid burning with burn speed greater than the speed of sound). Essentially all modern guns, from a propane fueled spudgun to an M-16 are powered by deflagration explosions.

Explosives that detonate are not generally used in guns, homemade or otherwise.

-------------

The peak pressure for propane + air (at 1 atmosphere) in a close chamber is 120 PSIG or less. Worse case scenario with a basic combustion spudgun is that the ammo jams in the barrel. The gun will experience a short pressure spike to 100 ~ 120 PSIG. The pressure does not last very long because of heat transfer to the chamber wall. Within a second or so the pressure in the jammed gun will be back near atmospheric pressure.

The amount of heat transfered to the gun is miniscule. A PVC gun fired once with a jammed barrel will only warm the PVC up by a few degrees C.

Many (most?) pneumatic guns are regularly pressurized to 100 ~ 120 PSIG, about what the peak pressure would be in a jamed combustion gun. As others have pointed out, the pressure in the combustion gun last for only an instant. In a pneumatic, the gun must withstand the pressure for a much longer time.
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Unread postAuthor: Ragnarok » Sat Aug 16, 2008 5:50 pm

jimmy101 wrote:Any rapid combustion is characterized as an explosion. Any rapid depressurization is also classified as an explosion.

In strict scientific terms, yes.

However, in the average person's mind, an explosion is more Hollywood special effects than any actual science, and rather more amped than what happens in a combustion.

If explosion is defined like that, if describing it to such a person, it would be perfectly acceptable to say that a combustion does not use an explosion.

As others have pointed out, the pressure in the combustion gun last for only an instant. In a pneumatic, the gun must withstand the pressure for a much longer time.

A static load is a lesser threat than a shock load - if both reach the same peak pressure, the combustion would likely fatigue the chamber materials to a greater degree.

In my opinion, the problem with a pneumatic being charged for longer is that it gives opportunity for the launcher to be dropped, damaged or knocked when under load, and then you've got a load of pressure that's all broken free to cause havoc.
It would be a freak occurrence for a combustion to suffer a knock when under pressure - and at other times, a knock wouldn't be unleashing pain and fury.
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Unread postAuthor: Insomniac » Sat Aug 16, 2008 7:01 pm

The nice thing about combustions is they cannot explode if you build them right.
An easy way to put it is this. You have X amount of chamber volume. To get combustion you need a very specific amount of propane, and a very specific amount of air. Too much or too little and it simply won't fire. This little fact means that it is IMPOSSIBLE to 'over fuel' or put too much fuel in a regular combustion. Unlike somthing like a 'real' cannon that uses solid fuels, combustion spudguns have a max power they can achieve, and using propane it is impossible to go over that. This means that while combustions vary slightly in power shot-to-shot, it is very easy to ensure the chamber can easily hold a 'perfect' shot. Simply build a cannon with strong pipe (go way overkill if your parents are still worried)

Another accidental safety feature of combustions is the ignition method. Unlike solid fuel guns (which are against the rules to discuss on this site btw), you cannot get a delayed shot. The spark is electrical. It is there for a tiny tiny split-second, and then the electricity stops flowing, and ALL SOURCES OF IGNITION DISSAPEAR. A combustion cannon either fires immediately, or it doesn't fire at all and is safe to open. This is unlike somthing like a firework, where a 'misfire' leaves the device liable to go off, and dangerous to approach.

Also, point out to your parents that the absolute maximum, utterly perfect combustion between propane and air is only around 100psi. This means the pipe in the chamber of a combustion has to hold 100psi for a fraction of a second. Compare this to a pneumatic launcher, where the chamber is under the strain of 100+ psi for extended periods of time.

One thing to point out is that due to the way the combustion gasses rapidly expand, then contract again, pretty much any combustion is gonna make a bloody loud bang when it goes of. This isn't a measure of the strain on the chamber, it is just the nature of the expanding gasses coming out the muzzle.
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Unread postAuthor: Hotwired » Sat Aug 16, 2008 9:38 pm

DYI wrote:
Ehhh I wouldn't put forward a "which would catastrophically fail first under normal use" comparison.


Uh... Why not? They'll all fail eventually under continuous use, whether it takes one shot or 400 years.

It's a given that anything which is properly built will not fail in its operational lifetime, so why include that fact?


Because a complete failure of a properly built cannon under normal use is only going to be seen on an abnormally long timescale. The kind of timescale I'd not put money on a failure either way when you consider combustions have a few other factors in their chambers besides lower pressure.

Whereas giving the information that building either correctly will result in equally safe cannons in the here and now is very much more relevant for getting one built to start with.
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Unread postAuthor: jimmy101 » Sun Aug 17, 2008 4:13 pm

Ragnarok wrote:
jimmy101 wrote:Any rapid combustion is characterized as an explosion. Any rapid depressurization is also classified as an explosion.

In strict scientific terms, yes.

However, in the average person's mind, an explosion is more Hollywood special effects than any actual science, and rather more amped than what happens in a combustion.

If explosion is defined like that, if describing it to such a person, it would be perfectly acceptable to say that a combustion does not use an explosion.

As others have pointed out, the pressure in the combustion gun last for only an instant. In a pneumatic, the gun must withstand the pressure for a much longer time.

A static load is a lesser threat than a shock load - if both reach the same peak pressure, the combustion would likely fatigue the chamber materials to a greater degree.

In my opinion, the problem with a pneumatic being charged for longer is that it gives opportunity for the launcher to be dropped, damaged or knocked when under load, and then you've got a load of pressure that's all broken free to cause havoc.
It would be a freak occurrence for a combustion to suffer a knock when under pressure - and at other times, a knock wouldn't be unleashing pain and fury.

When in doubt the correct term should be used. If there is a possibility that the correct term will be misunderstood then it should be elaborated on, as in "a combustion spudgun involves an explosion similar to what occurs in a gasoline engine. Though the peak pressure in the spudgun is considerably less than in a gasoline engine." Besides, whichever definition you prefer if you load a combustion gun with just a wad of paper towels and fire it I think everyone would classify what it does as an explosion.

It is true that shock loads are different than static loads. But a combustion spudgun really doesn't produce any significant shock loads. The process is much to slow to produce significant shock. In addition, the mass of the walls of the gun are so much greater than the tiny mass of the gases that the shock loads can safely be ignored in a basic combustion gun. Smacking a piece of PVC with a hammer produces much higher shock loads than does propane+air combustion at 1X. Heck, a pressurized water system produces much higher shock loads than does a combustion spudgun. "Water hammer" occurs when a valve in a pressurized water system is closed quickly. Electric solenoid valves found in clothes and dishwashers close pretty quickly. The shock loads comes from having several pounds of water (in the piping) moving at several feet per second suddenly decelerating to zero fps in a small fraction of a second when the valve closes. (There is no "water hammer" when a valve opens, no matter how fast the valve opens.) The shock load in that case is significant and the piping will make a noticable jump. The mass of the moving water is significant compared to the mass of the piping.
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Unread postAuthor: Demon » Sun Aug 17, 2008 6:33 pm

I do not understand why pneumatic canons fails.

if you greatly assemble it and use sch 40 pvc parts, it should never blow
if you dont go over 150 psi because the parts you use are maded for real plumbery and are supposed to hold for hour, maybe days of 150 and more psi.

but with the combustion ones the should be in theoric more dangerous because it get really hot in there and some chemical substance could damage the parts. And pvc or abs are not madded for making explode hairspray in it.
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Unread postAuthor: Ragnarok » Sun Aug 17, 2008 6:52 pm

jimmy101 wrote:When in doubt the correct term should be used.

Generally, I would agree, but sometimes if you're trying to explain something, it's sometimes best to bend the rules.

When the average person thinks about an explosion, their mind brings up images of energy dense solid explosives and detonations rather than limited energy gas combustion and deflagration - it's not a good thing to bring into their head.

Just like I try to bend around the word "gun" if I were explaining my launchers (you'll not find me using the term to describe any of my cannons - it's always cannon, launcher or occasionally spudgun - and it's not uncommon for me to correct a person who refers to one of my launchers as a "gun") to someone, although it would be reasonably accurate to use the word, it's easiest and best not to summon up their preprogrammed mental images of explosions or firearms - it only makes it harder than it needs to be.

I'm all for being honest and accurate, but if I can be honest and accurate in a slightly different way that's favourable for me but still keeps everyone happy by using other words that are still accurate but don't force me to fight mental prejudices, it'd be silly not to take the chance.

There's a story I once read where an evolutionary biologist was explaining his work to a person he was with on a plane - the guy was really impressed with his work, and was prepared to accept all the logical arguments and science, right up until the scientist mentioned the word "Evolution", and then the guy's mental prejudices slammed down hard.
(No, this is not an invitation for anyone to start an evolution debate, it's just a story I'm giving as an example.)

My point being that people can be perfectly rational and fair if you just avoid the hard road of fighting a battle they've already decided the result of.
OK, you might call it sly or manipulative, but being able to side-step around such things with subtle wording is a very useful skill to have, and allows you to have a discussion on fairer terms.
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Unread postAuthor: Insomniac » Mon Aug 18, 2008 1:23 am

Demon wrote:I do not understand why pneumatic canons fails.

if you greatly assemble it and use sch 40 pvc parts, it should never blow
if you dont go over 150 psi because the parts you use are maded for real plumbery and are supposed to hold for hour, maybe days of 150 and more psi.

but with the combustion ones the should be in theoric more dangerous because it get really hot in there and some chemical substance could damage the parts. And pvc or abs are not madded for making explode hairspray in it.


Combustion doesn't significantly heat the pipe, because while the temperature is high, there is not a lot of heat there (eg a piece of paper in an oven is very very hot, but contains so little heat energy the moment you touch it it cools)

Using propane pretty much removes the chance for chemicals to damage the plastic. Most combustion failures are from using incorrect parts, using incorrect fuels, or gluing the cannon wrong.


Pneumatic cannons can fail for a number of reasons. The recoil from firing can stress the pipe if there isn't sufficient bracing for the barrel, or if the cannon is dropped or otherwise strained while under pressure (dropping a charged pvc pneumatic on hard ground is very, very dangerous) Also, incorrect glueing, or the way the gun rapidly cools after firing could potentially cause failure (if the gun was fired so rapidly that it cooled too much. Pvc is more brittle when cool)

Also, while pressure rated PVC is quite safe, you should never take it too close to the rating. Pipe CAN be defective, and may have been damaged before you got to it. (too much sunlight can weaken pvc)
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Unread postAuthor: psycix » Mon Aug 18, 2008 3:21 pm

I agree to that Ragnarok.

After all, even while you could use the "correct term" the people you are explaining to do not know what this term means and just bring up the wrong images/thoughts with that term. And thats why you can explain things differently to "normal" people so that they will understand what you are saying to them.
When using the word explosion, your convincing of safety may convince them in the opposite direction as they simply do not know about correct terms and their correct meanings, they only know what they associate to it, and that is everything else besides the correct term.

If you really wanted to use the correct terms while explaining to them, your story will get 10 times as long as you will have to explain every term to them, and in the end, they'll tell you they lost track of it halfway...
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Unread postAuthor: jimmy101 » Tue Aug 19, 2008 10:40 am

I understand Rag's view, and I aggree, hence I also tend to describe my guns as launchers and not guns. But they clearly are guns, no ifs, ands, or buts about it, they are guns.

Anyone who has ever seen a well made, full sized, combustion spud gun fire is going to describe what happens as an explosion, or at least, describe it as being the same as a what a "real" gun does.

But back to the safety aspects of the OP. I think the key issue in comparing a combustion and a pneumatic is that both types of guns are usually operated at similar pressures. Usually, the combustion guns is operated at a somewhat lower pressures than a pneumatic but in the worst case scenario for the combustion gun, a jammed round in barrel, the gun will only experience a maximum pressure that is about the same as what the typical pneumatic is normally operated at. Some people think the "shock" that occurs in a combustion raises the effective pressure the gun must survive substantially. That isn't true, there is very little shock in a combustion gun, even with a jammed round. The shock is much less than what the pipe would be subjected to in a typical potable water system.

The heat affects of the burning fuel do very little to the PVC. The gas temperature in the chamber (especially with a jammed round) peaks at about 4000<sup>o</sup>F. But because of the low mass and low heat capacity of the gases the pipe's temperature changes very little. So little that you usually can't detect any temperatture change if you put your hand on the pipe after firing.
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Unread postAuthor: starman » Tue Aug 19, 2008 11:51 am

This parent/combustion thing comes up fairly often. Don't blame them, they're just uninformed on the hows and whys of what's really going on and they don't want their life's gift of a son "blowing themselves up".

Truthfully, about the only way to really hurt yourself or someone with an even half way reasonably constructed combustion gun is to accidently shoot someone...something that you can do just as easily with a pneumatic.

The high temperatures in a combustion last for only milliseconds so heat and flame concerns are really minimal. Also, I have actually fully contained a combustion in a fairly large chamber. You don't hear anything, you don't feel any shock. It actually feels like a misfire. Opening the air vent lets out a low pressure rush of warm air. The shock you hear and feel when you shoot is almost completely a sound effect. Any recoil is dependant totally on the velocity and mass of the projectile.

Several very knowledgable spudders here agree on this issue. If you would like, I would be glad to converse with your parents via email or even a telephone chat if you think it would help. Just PM me. I'm a parent and understand where they are coming from.
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Unread postAuthor: Ragnarok » Tue Aug 19, 2008 12:34 pm

jimmy101 wrote:I understand Rag's view, and I agree, hence I also tend to describe my guns as launchers and not guns. But they clearly are guns, no ifs, ands, or buts about it, they are guns.

Well, if we're still being technical, the term "gun" is really only appropriate for field and ship guns - big bore stuff mounted on some sort of carriage or cradle... :P:D

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starman wrote:I'm a parent and understand where they are coming from.

I'm not a parent (Despite one of my mate's insistences I have a daughter called Simone :roll:), but I can pick up a phone and pretend I am if you want. I've pretended to be worse. ;)

Any recoil is dependant totally on the velocity and mass of the projectile.

Not solely. The mass and velocity of the gasses involved is also significant. Given that in HEAL there are 18 grams of air leaving the muzzle at hundreds of metres a second, which is often a greater impulse than that of the projectile itself, it can't be ignored.
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Unread postAuthor: jimmy101 » Tue Aug 19, 2008 12:40 pm

starman wrote:Also, I have actually fully contained a combustion in a fairly large chamber. You don't hear anything, you don't feel any shock. It actually feels like a misfire. Opening the air vent lets out a low pressure rush of warm air.

Ya, I've done this type of closed chamber tests as well. The combustion process makes so little noise that the faint rattle sound a pressure gauge makes is louder than the sound of the combustion. The pressure gauge reading drops below 10 PSIG within 1 second after ignition.
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Unread postAuthor: DYI » Tue Aug 19, 2008 7:34 pm

DYI wrote:
Well, for one, there are no explosive alkanes, they're all quite stable.


Folks seem to be confusing "explosion", "combustion" and "detonation".


There are no deflagratable or detonateable alkanes, so the point's kind of moot, ain't it? :wink:

The word "explosion" is really too vague a term to be of any great use here. As far as its definition: a loud noise (generally as a result of a single pressure pulse being broken up by intervening objects and echoing), followed by the rapid displacement of things from where they used to be. :lol:
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Unread postAuthor: psycix » Wed Aug 20, 2008 8:16 am

jimmy101 wrote:Anyone who has ever seen a well made, full sized, combustion spud gun fire is going to describe what happens as an explosion, or at least, describe it as being the same as a what a "real" gun does.

The reason of people thinking of an explosion is simply because of that bang and the possible flash of fire out of the muzzle.
But actually, this bang is not "the"* explosion, it is the release of the pressure that was behind the projectile. Note that pneumatics also make a bang! (while it may sound a bit different due to the difference in temperature etc.)
The "explosion" itself ("the" explosion, the deflagration, doesn't make any sound at all. As you know, a closed chamber doesn't make any sound or shock.
And thats the whole point, the parents believe that the bang is one hell of an explosion which propels the projectile, but its simply the release of the pressure which first pushed the projectile forward and the burning of the fuel is not that bang.

*Yes I know, a sudden release of pressure may be defined as an explosion, but it is not the burning of the fuel, so its another explosion and not "the" explosion.


You guys want to call his parents and try to convince them? :lol:
Kinda makes me laugh, since his parents have no idea who they are talking to. For them, you are just some random guy from the internet, and thus they may not trust you and believe all information you tell them.
If parents have to be convinced, its the best to do that yourself, since you are their son, and so they may believe you faster if you show that you know that you are doing and know alot more then them.
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