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19 round Fully auto Gatling gun concept (To be Built)

Built a pneumatic cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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19 round Fully auto Gatling gun concept (To be Built)

Unread postAuthor: mrphilip246 » Mon Jan 04, 2016 11:23 pm

gatling air cannon.png
19 barrel.6.png


the base model is a potato cannon on its own. I will briefly describe how it works, but for more description, you can visit Curt Graber's videos.

Ok to start, this is a very simple design, one of the most effective BUT, SIMPLE designs.For a potato cannon. If any of you have built potato cannons before then you know that the faster the valve opens the more efficient the cannon can be. Well this model uses a varient of the diverter type of valve. the valve is very simpilar to a piston valve but without the weight of the piston that casues a loss of efficiency. This model has the efficiency that is extremly simpilar to a burst disc, wich by the way is the best it can get and its reusable.

The main valve is made up of a diapram disc that moves backwards to allow the inner chamber to release all the air. When the air hose is attached and air starts moving in, it moves around the diaphram into the large pipe around the barrel. There is a rubber drain hose that is clamped over the back of the barrel which servs as the seal keeping in the pressure also holding the diaphram in place. As soon as the trigger valve is activated, the air pressure suddenly becomes less than whats in the air chamber pushing it backwards. the moving air keeps pressing the diaphram backwards as the air escapes the barrel.

If this isn't clear enough, plese visit this video and build tutorial https://www.youtube.com/watch?v=zSDVTdThz_M&list=FL81Pkm6L_yoPh8Z2M389N2A&index=3 but this isn't about one of these cannons, but 19 of them!!
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I should say before I get into the experamental deisgn that i haven't built this yet, but it works exactly the same as what has been built and the operation of the componets seem promising. NOW saying that, im asking for those experiences fellows on their input for this design variation.
I recomend if sombody were to build this before i do that they would only add 1 or 2 cannons to the series testing the concept before adding 19 of them
Allthough this design is not your typycal $20 weekend build, it is reletivly cheap and easy to build
Let me start with the opertion of the seperate cannons in the series. the only change needed when adding cannons to fire in the series, is the addition of a exaust to every cleanout cap and the only one with a trigger valve being the initial cannon. For explanation simplicity, im going to talk about only 2 cannons. each cannon will have 2 exaust connections. each one will be used in the pressurization of the cannons. when the air hose is connected, the 2nd exaust connector will send air to the next cannon. as they pressurize every cannon will fill simultaneously acting as one air chamber. the last cannon in the sequence needs only a single exause port to fire and to pressurize.
Now lets talk about firing.
when the trigger valve is activated the first cannon will fire, BUT that 2nd exaust connector will bleed air. which is the only necessity for fire sequence. because every cannon uses the same type of valve they pressurize simultaneously, but the series of exaust connectors cause the sequence of triggers.
it takes a fraction of a second for the diaphram to react and time for the air to leave the small exaust ports. so this is what causes the delay for the machine gun effect.

The edition of multible cannons will heavily increase the importance of no leaks. if a couple of the high number of connectors leak, then the cannon will not get to full pressure.
Also the seal that presses against the diaphram will need to be lubricated reguly to all the cannons to get to full pressure. Above you can see the direction of the pressurization of chambers, but when the trigger valve is activated, the direction of air flow will reverse in sequence starting with the one connected to the initial cannon.

The Fire rate of the cannons can be adjusted by changing the exaust size or pinching each exaust hose equaly.

Now, i plan to build these 10 feet long and 1 1/2" barels. 1 1/2 inches will allow for a potato, but will also allow giant spears to be used as ammo. Another issue would be reusable/cheap ammo. spears would be reusable as long as they didnt expereince too heavy of impacts. Ice is used alot with potato cannons, but why would i use ice when i can make pykrete. They melt, so you dont have to worry about the firing range being littered.

if you can imagine this is a beast of a potato gatling cannon, not gatling gun but gatling cannon. 19 700+ ft/sec projectiles that weigh a couple ounces is some serious power.

to compare.......i built a potato cannon once that was about 1/5 the power of this and it fired a fence post 6 inches into a tree (UNSHARPED).

please visit Curt Graber's Youtube channel.

Also here's another youtuber that built the same cannon and fired 500 ft/sec potatoes.
https://www.youtube.com/watch?v=lp8HjOlskNg

If you've read this far then im sure that your intrigued, so your input will be greatly appreciateed on the design and operation of the cannon.
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Re: 19 round Fully auto Gatling gun concept (To be Built)

Unread postAuthor: Ragnarok » Wed Jan 06, 2016 6:58 pm

the diverter type of valve

In all honesty, I see nothing about the "diverter valve" which would lead me to classify it as something other than a piston valve. Pistons don't have a dictated shape, size or weight - they just can't, unlike in the similar diaphragm valve, be attached to the valve body (where the diaphragm operates by deforming, rather than a piston's moving).

If any of you have built potato cannons before then you know that the faster the valve opens the more efficient the cannon can be.

... yeeeeeeees. What many people forget to mention is that it's a matter of diminishing returns.

The important factor is really how far the projectile moves while the valve is opening. School physics teaches us, and quite accurately, that kinetic energy is force times distance (it's also half-emm-vee-squared, but that's not what's relevant here) - so we want as much force over as much of the barrel's length as possible.

So, the problem with a slow valve is that the projectile is only under full acceleration for some of the barrel (or none, if it's really slow). So, more school physics:

Distance = (acceleration * time ^ 2 / 2) + (initial velocity * time)

The important part here is the quadratic I've bolded. Every doubling of valve speed quarters the distance the projectile has moved before the valve is fully open, meaning that what might have cost you 16" of full acceleration in a 40" barrel now only costs you 4" and, doubled again, only 1".
Correspondingly this means 24", 36" and 39" of full acceleration. So the first doubling gives you a 50% increase in energy (if we assume negligible acceleration before the valve is open, which isn't the case, but the general point holds), but the next one only 8.3%.

Given the huge forces involved in the opening of a piston valve, it's easy to get even a relatively heavy piston to open in a couple of milliseconds, which doesn't give the projectile much time to move in.

Actually, the simplest estimation of this is to compare the masses of your piston and projectile. If your barrel and valve seat are the same diameter, then they have pretty much the same force on them*. Multiply the required piston travel by the piston mass, divide that by projectile mass and you'll get an estimate of how far the projectile moves during valve opening.

* A piston valve starts to open when the forces on it are in equilibrium. The uneven force that opens it is the chamber pressure acting on the now exposed valve seat area - the same area and pressure** as is acting on the projectile, ergo the same force.
** Yes, I know, fluid dynamics. It's close enough.


so this is what causes the delay for the machine gun effect.

I'm not convinced this will work as you imagine. Pilot C will be bleeding into pilot B about as fast as pilot B is bleeding into pilot A, and pilot D into pilot C the same, et cetera, et cetera.

To do it that way, you'd need to be able to precisely and independently control the bleed rates between every chamber so that the last one leaks into the second-to-last one slower than that leaks into the third-to-last... which is just likely to result in some cannons where the chamber is leaking back into the pilot just as fast (particularly without the seals of a fully O-ringed piston) and they therefore just leak without ever firing.

What would be far more effective is to link each cannon's pilot to the last cannon's chamber.
Your rate of fire will then depend to an extent on the mass of the projectiles you're using (a heavier projectile stays in the barrel longer, therefore the chamber pressure doesn't fall as quickly), and very heavily on the C:B ratios you pick...

... but my guess is you'd probably get a convincing 1000-3000 rpm out of it. (And then half an hour of reloading, but hey, you came up with the idea).

Just one warning though. Doing it that way will be a complete chain reaction. Once you pull the trigger, all nineteen barrels are going to go off in sequence, short of something failing or an act of the gods.
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Does that thing kinda look like a big cat to you?
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Re: 19 round Fully auto Gatling gun concept (To be Built)

Unread postAuthor: mrphilip246 » Wed Jan 06, 2016 9:15 pm

I greatly appreciate your input on the design. I fully agree about the piston valve concept. The diaphram piston already opens fast enough that a small barrel would be only subjectable to its importance, but here i plan on a longer barrel. So true, the acceleration difference in a traditional piston valve isnt much. But saying that, I wouldn't need to make a piston either.
....for the delay concept, I got the idea from using a modified sprinkler valve. It works similarly as a piston valve with equilibrium forces. As soon as those forces are unequal air escapes. I experimented by only allowing small amount of air out at a time to trigger the valve, and it does work. If small smounts are leaked at a time it can easily take 0.5 seconds to trigger the valve. I just believe that I would most likely need small tubing transfering the chain reaction of valves. Either way i believe the fire rate would be too fast if i used just standard 1/4 inch tubing for the transfer. I could maybe use 1/8" and if that isnt slow enough, then i could just fasion somthing to slow it down.I don't know about the neccesity of varing the air transfer between everyvalve though. As long as i could slow it enough then it would matter if the fire rate wasn't exactly consistant.
....And your definantly correct about the chain reaction....i might as well strap it to a tree just to make sure the barrel dosent start getting a mind of its own.
The greatest variable i believe in this concept, would be make sure that none of the diaphrams leaked. lubrication would be needed most likely every shot.
... thank you for your side on the design.....I would love sombody to test this concept of the chain reaction.
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Re: 19 round Fully auto Gatling gun concept (To be Built)

Unread postAuthor: mark.f » Thu Jan 07, 2016 10:06 am

Ragnarok wrote:What would be far more effective is to link each cannon's pilot to the last cannon's chamber.
Your rate of fire will then depend to an extent on the mass of the projectiles you're using (a heavier projectile stays in the barrel longer, therefore the chamber pressure doesn't fall as quickly), and very heavily on the C:B ratios you pick...


No pictures, explanation intact. :)
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