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Intro and a combustion cannon

A place to ask general spud cannon related questions.
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Unread postAuthor: Heimo » Thu Oct 29, 2009 11:56 am

qwerty wrote:Cool!, was that just with a spud and also did it go through or just shatter it?
Pretty good cannon if you ask me :)


spuds just turn to vapor...

there was a small fishing sinker in front of the spud and it went straight through I think I could go through a thicker piece that was indeed 20mm thick I measured and here is one more shot for you guys

Image

those projectiles still fly fast enough when exiting on the other side that I am unable to find one or see it when it exits.
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Unread postAuthor: psycix » Thu Oct 29, 2009 2:15 pm

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Till the day I'm dieing, I'll keep them spuddies flying, 'cause I can!

Spudfiles steam group, join!
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Unread postAuthor: Heimo » Thu Oct 29, 2009 3:09 pm

psycix wrote:Do you live in a blue world? :P
Neat damage pics!


three words : sunset no flash :)
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Unread postAuthor: Brian the brain » Thu Oct 29, 2009 4:35 pm

If this truely enhances the common combustion cannon...could the jet ignitor also upgrade a metered propane with a chamberfan perhaps?

Or better yet...a metered cartridge without any means of mixing?
I feel the idea needs to be explored further..
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Gun Freak wrote:
Oh my friggin god stop being so awesome, that thing is pure kick ass. Most innovative and creative pneumatic that the files have ever come by!

Can't ask for a better compliment!!
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Unread postAuthor: Heimo » Fri Oct 30, 2009 5:41 am

Brian the brain wrote:If this truely enhances the common combustion cannon...could the jet ignitor also upgrade a metered propane with a chamberfan perhaps?

Or better yet...a metered cartridge without any means of mixing?
I feel the idea needs to be explored further..


I won't put a chamber fan in it. it won't survive but I agree this idea needs further development and that is why I posted it here.

I won't use PVC for this type of gun but that is just me I have no idea what kind of pressure builds inside this thing and I don't know how PVC will react to the pressure spike generated by this thing ,but any one is free to try it. I just don't know how safe it is.

As to metered propane I bet this will work even better than it works at the moment cause this is still essentially a spray and pray ( not for long tho I just never let go of an idea it stays in development forever )

also some theory I have about this and I have tested to a point (not enough for a clear conclusion tho) is that for maximum effectiveness of the jet ignitor the flame coming out of the ignitor should be the same volume than the interior of the main combustion chamber and then effectively ignite all the fuel at once but this is still in the theory stage and remain unproven.
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Unread postAuthor: jimmy101 » Fri Oct 30, 2009 12:51 pm

Folks have tried "jet igniter" type systems on spud guns before. DR, aka BigDog, tried it.

I really doubt you can get it to make a spud gun perform enough better to be worth the effort. There are a couple problems and there are easier ways to do what the jet igniter is designed to do.

There is a pretty big literature on jet ignition and it has rarely (if ever?) resulted in a system that had long term utility. Several old styles of engines used the technique but they aren't used very much anymore because more effective techniques have been developed.

The jet igniter is just meant to give a faster burn. You can accomplish the about the same thing with a chamber fan.

The jet igniter requires that you adequately mix two chambers before firing.

The jet igniter has more surface area than the same sized standard chamber so there is more heat loss.

The jet port is a smallish orifice and robs a fair amount of heat from the jet.

You can get the same performance increase, for nearly zero cost, by just increasing the size of the chamber.
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Unread postAuthor: Heimo » Sat Oct 31, 2009 9:02 am

jimmy101 wrote:Folks have tried "jet igniter" type systems on spud guns before. DR, aka BigDog, tried it.

I really doubt you can get it to make a spud gun perform enough better to be worth the effort. There are a couple problems and there are easier ways to do what the jet igniter is designed to do.

There is a pretty big literature on jet ignition and it has rarely (if ever?) resulted in a system that had long term utility. Several old styles of engines used the technique but they aren't used very much anymore because more effective techniques have been developed.

The jet igniter is just meant to give a faster burn. You can accomplish the about the same thing with a chamber fan.

The jet igniter requires that you adequately mix two chambers before firing.

The jet igniter has more surface area than the same sized standard chamber so there is more heat loss.

The jet port is a smallish orifice and robs a fair amount of heat from the jet.

You can get the same performance increase, for nearly zero cost, by just increasing the size of the chamber.


How a bout a link to BigDog's results

"You can get the same performance increase, for nearly zero cost, by just increasing the size of the chamber"

Not entirely true
I have made a pretty big gun a few years back unfortunately I cut it up for a higher purpose but i can still show the combustion chamber and part of the barrel The problem with increasing the volume of the combustion chamber is that it only helps up to a point. What happens inside a really huge combustion chamber is the projectile exits the barrel before all the fuel has burned up cause the fuel don't burn fast enough. When a certain pressure has been reached the projectile starts to leave the barrel even if the fuel have not finished burning and this is where I got the idea for the jet igniter. it was created for the purpose of speeding up combustion but less talking here is some pics of my very big and unfortunately cut up gun

Image

Image
just for scale purposes that barrel had the exact ID to fit a golf ball
and the barrel used to be longer

"The jet igniter requires that you adequately mix two chambers before firing. "

True but there are ways to overcome this but that is still part of my new design the design is more or less finished but going from the design stage to the build stage is still gonna be a while.


"The jet igniter has more surface area than the same sized standard chamber so there is more heat loss. "

In my experience the heat loss is very slight, too little for me to worry about remember my cannon is made of steel and steel conducts heat much better than plastic and when I finish a shot only the first four inches of the barrel have heated up enough for me to even notice it


"The jet port is a smallish orifice and robs a fair amount of heat from the jet."

The jet port don't have to be small. mine is around 15mm but that is my current one another of of my previous guns was made from 2 freon tanks welded to each other the port between them was about half the diameter of the tank and man did I get crazy performance from that thing but once again I had scrapped that gun a long time ago and I don't have it anymore.

"The jet igniter is just meant to give a faster burn. You can accomplish the about the same thing with a chamber fan. "

maybe but remember the chamber fan also add to the restriction to the flow of gasses in the combustion camber and I doubt a chamber fan will survive in one of my guns . I also don't like the Idea of adding something in a hard to reach place that can (and probably will) break eventually

I guess I don't think in the same way as most of you guys

Heimo
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Unread postAuthor: jimmy101 » Sat Oct 31, 2009 1:36 pm

I would say several of your points are incorrect.

A jet ignition system is extremely touchy. In research papers I've seen on the system they tend to be very inconsistent. It'll work one shot then not the next.

Most of the published work is done in a closed chamber. An operating spud gun is actually quite a bit different than a closed chamber. Movement of the gases in a gun can induce burn accelerating turbulence.

The increase in burn rate is significant but not all that much.

To keep the projectile from exiting before combustion is complete you can;
1. Add more spark gaps. Two gaps make the chamber burn ~2x faster.
2. Get the sparks away from the chamber's wall.
3. Increase the static friction between the ammo and barrel. For Veggie ammos this is very easily done by beveling the inside of the muzzle (or whatever is used to cut the ammo).
4. Induce turbulence with fan(s)

Fan(s) are not necessarily in the gas flow path, or at least, they don't necessarily represent a large portion of the chamber's cross section. A 3" diameter fan will probably be sufficient for a 12" diameter chamber.

Furthermore, since the fan position isn't critical for the combustion process, the fan can be almost all the way to the back of the chamber. The fan might only be 2" in front of the back of the chamber even with a 2 foot long chamber. Very little gas has to move through the area defined by the fan's blades. (That's also why a fan will usually survive in a properly designed gun, it really isn't subjected to much pressure during firing. It's probably subjected to more force after the projectile leaves the barrel and cool air is sucked back into the chamber.)

A high speed fan positioned within a couple inches of the spark gap is probably going to give exactly the same affect as the jet igniter. The high speed gas movement will be turbulent and turbulent gases burn up to 10x faster than do stationary (or laminar flow) gases.

"You can get the same performance increase, for nearly zero cost, by just increasing the size of the chamber"

Not entirely true

For all practical designs using practical materials and getting practical results, yes it is.

It is true though that an extremely large chamber might have poor performance. If the CB ratio gets up above perhaps 3 then you might be getting into the "mine sized chamber affect" domain. I don't believe anyone has ever actually demonstrated that affect in a spud gun and, IIRC, HGDT doesn't predict a "mine sized chamber affect". HGDT predicts the performance goes asymptotic to some value.

HGDT does predict that chamber volumes bigger than CBs of perhaps 3 or so do not significantly increase the performance of the gun. Even with a burst disk (which should out perform jet ignition and is much simpler to implement and get to work) very large CB ratios don't help according to HGDT.

A perfectly tuned combustion system would be one that reaches ~120 PSIG before the projectile moves, and the chamber volume is so much bigger than the barrel volume that the entire transit is done at essentially 120 PSIG. To put it another way, 100% of combustion is done before ammo starts to move and the chamber is very big. The easiest way to realize that is to use a burst disk and large chamber. I don't see any way that a jet ignition system can get anywhere near the performance of a burst disk. Indeed, I doubt the jet ignition system will have any measurable affect on the gun's performance.

Furthermore, the orifice size and ratio of the pre- and main chambers appear to be critical to a successful jet ignition. "Successful" means the chamber actually burns faster than a single large chamber of the same total volume. Get any of the parameters wrong and there is zero benefit, and may even be a decrease in performance. Get any of the parameters outside the critical range and the main chamber won't even ignite.

A jet ignition system may well have problems with the ammo moving too soon. If the pre-burn chamber raises the pressure above the static friction then the ammo starts to move before the main chamber is ignited. If the pre-ignition chamber is big enough to actually work as a jet igniter it will probably raise the pressure enough to start the ammo moving prematurely.

A typical combustion spud gun has the ammo moving after about 10% of the chamber has been burned (CB of 0.8 or so). That means that if the pre-chamber is 10% or so (or bigger) of the main chamber then the ammo is still going to be moving before main chamber ignition.

The photo's show a gun that may well behave as a "mine sized chamber". I bet if you made the chamber smaller the gun's performance would increase.

BTW, for some nice photos of a jet ignition process see http://www.td.mw.tum.de/tum-td/de/forsc ... er/324.pdf
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Unread postAuthor: Technician1002 » Sat Oct 31, 2009 5:07 pm

A jet ignition system is extremely touchy. In research papers I've seen on the system they tend to be very inconsistent. It'll work one shot then not the next.


On the antique gas engine that I have, it runs very well. It is one of my better running engines. I presume the reliability comes from the premix of carburetor fuel delivery.
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Unread postAuthor: Heimo » Sun Nov 01, 2009 4:50 am

Technician1002 wrote:
A jet ignition system is extremely touchy. In research papers I've seen on the system they tend to be very inconsistent. It'll work one shot then not the next.


On the antique gas engine that I have, it runs very well. It is one of my better running engines. I presume the reliability comes from the premix of carburetor fuel delivery.



That is similar to the way the fuel is gonna be distributed be in my new design. The fuel is going to be premixed in a separate chamber and then be pumped into the chambers from the primary chamber into the secondary chamber thus ensuring that fuel is distributed evenly and all the chambers contain the same mix almost like a propane metering system. but not entirely the same I am considering switching to propane for next cannon cause liquid fuel like my present one will make things more complex
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