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High ROF Propane Gatling Cannon, 2" Pykrete Slugs

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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High ROF Propane Gatling Cannon, 2" Pykrete Slugs

Unread postAuthor: cannon freak » Wed Jan 14, 2009 4:13 pm

Well hello all, its been some time since I've posted on this site so forgive me if I'm not "up" with whats new in the spud gun world.

Alrighty well the title basically says its all. I'm planning on building a propane powered gatling cannon capable of reaching 2 shots per second. Now yes this is by no means amazingly fast I would personally find it an accomplishment to achieve such a ROF with a combustion based launcher.

The projectile is simply pykrete frozen in some 2" pipe then cut to a desired length.

The cannon will consist of a series of 6 cannons arranged in a circle, firing at 1/2 second intervals. Thus each one firing once every 3 seconds. It is my belief that 3 seconds is completely sufficient to allow for venting, fueling, loading, and creating an even distribution of fuel in the chamber.

Venting has already been figured out. I managed to locate a large 1"-1/4" one way valve which can allow air flow in for venting but then when ignition takes place it immediately shuts due to pressure. Now its not perfect however any loss in pressure is very minimal. I used it with great success on the last cannon i made. For the gatling cannon a shop vac motor would be attached to the vent allowing for easy venting. This is all set up so that once the gun has fired that chamber is vented for only a short time, approximately 1 second, with a powerful vacuum this is more than enough time. I'm not going to explain how i do this because it really needs a picture to understand it.

Loading is accomplished via a simple gravity feed from a hopper. The weight of the pykrete slugs should be enough to force them down into the firing position. At the same time these are breach loaded so the breech will be pulled open at the time of loading them snapped back in place by springs.

I'm undecided for fueling at the moment, i can either use ball valves and have them open and close for a predetermined amount of time or use an electric valve and do it all electronically. I'm personally leaning towards the mechanical method yet it may be fraught with timing issues. But I'm modeling all this on solid works and can do simulations to check everything.

Ignition would be a stun gun circuit hooked up to a relay. The relay would simply be activated when the cannon reaches its firing position.

You also have to remember that this entire assembly is rotating while the base is not. this allows for the rotating parts to interact with stationary ones such as a ball valve moving around and hitting a small piece of wood thus opening it, then a split second later hitting another shutting it closed.

As for dimensions I was thinking something like 3' long 4" chambers and 8' of 2" pipe, this C:B ratio mainly for the loud noise and effect.

Initial cost estimation is around a few hundred dollars.
This is what happens when you get bored.

I am mainly interested in what you guys think of the main concept, not for you to simply nit pic over everything I've just said.

Interested to know what you think.

Thanks
Cannon Freak
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Unread postAuthor: ALIHISGREAT » Wed Jan 14, 2009 4:25 pm

i don't think pykrete would make a good projectile.... try and use something like mini nerfs or JCS paintball 'rockets' that i do believe are both 2"
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Unread postAuthor: cannon freak » Wed Jan 14, 2009 4:40 pm

My main goal is for massive destruction and well more importantly a uniform cheap projectile so it can work well when being loaded. Pykrete keeps its form for a fair amount of time so shouldn't melt within the time frame I'm looking at. If there's any other "cheap" 2" or 1"-1/2" uniform projectiles id be happy to use them. Preferably something very easy to make.
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Unread postAuthor: psycix » Thu Jan 15, 2009 9:23 am

If there's any other "cheap" 2" or 1"-1/2" uniform projectiles id be happy to use them. Preferably something very easy to make.

Potatoes.

But they may be too inconsistent for you.
I'd do it with golfballs or something like that.
How will you feed the fuel to the chambers?
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Re: High ROF Propane Gatling Cannon, 2" Pykrete Slugs

Unread postAuthor: jimmy101 » Thu Jan 15, 2009 12:26 pm

cannon freak wrote:Ignition would be a stun gun circuit hooked up to a relay. The relay would simply be activated when the cannon reaches its firing position.

This isn't going to be easy. You can't switch the high voltage with a relay. That means you need to switch the 9V power supply to the stun gun. You probably don't need a relay for that, just a snap action type switch should be sufficient. Something like this;
Image
http://www.allelectronics.com/make-a-st ... R/-/1.html
The roller is just pushed by the a chamber when the chamber is in the firing position.

You may have trouble with the delay between when the stun gun gets 9V and when it actually creates a spark. IIRC, stun guns spark several times a second and there may be as much as a couple tenths of a second delay between power and first spark.

Another way to do it we be the same way a car distributor works. Put a spark gap between the firing chamber and the stationary part of the gun. When the chamber is in the firing position the spark jumps the gap from the frame to the chamber and then the igntion caps inside the chamber. You would probably need a default gap so the spark ahs somewhere to go when there isn't a chamber in firing position since stun guns hate to be triggered without a working gap.
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Unread postAuthor: jackssmirkingrevenge » Thu Jan 15, 2009 1:24 pm

This would be quite a demanding undertaking to achieve. Have you considered using preloaded cartridges?
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Unread postAuthor: starman » Thu Jan 15, 2009 1:58 pm

Sounds pretty ambitious...and with 6 barrels and 11+ feet long plus the inevitible mount you're going to need, you're definitely going to stand out.

Here's some thoughts:

- Why not use golf balls for ammo? They are plentiful and cheap and designed to move hastily through the air and easily hoppered.

- I'm with Jimmy on the ignition...design a spark distribution system based on the rotation position of the barrels. This will need to be fairly robust...I would consider using a standard engine coil as the heart. If you don't do that, you could configure a stun gun for each chamber (6 of them) and apply trigger voltage through rotating contacts that then fire the stunner. This contact should be a momentary type or work up some electronics that offer only say 1/4 second firing time. You don't want the ignition firing continuously in one chamber when the rotation stops.

- Metering and injecting your fuel will be a little challenging but not too bad. I suggest mounting a propane tank and regulator that rotates with your barrels the in the middle of your chamber cluster and build a common feed to 6 - 3 way valves, each connected to a fuel meter and chamber, something like this. With more rotational contacts, activate the 3 way valve as appropriate for the firing position it's in. The limit switch model Jimmy shows should work perfectly for your rotational contact work.

You should think about something on wheels, possibly a tow behind trailer mount...certainly detachable barrels. This will be one big mother so working on it's logistics is something you should put some serious effort into.
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Unread postAuthor: ramses » Thu Jan 15, 2009 3:20 pm

I haven't worked with ignition coils, but they are meant for a near constant duty cycle, while stun guns are not, so they would be the prefered ignition source.

It sounds like it will be a pain to make it work, but if/when it does, it should be amazing.
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Unread postAuthor: POLAND_SPUD » Thu Jan 15, 2009 3:32 pm

I suggest using spherical ammo... it really makes life easier when it comes to semis or full autos
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Unread postAuthor: cannon freak » Thu Jan 15, 2009 8:53 pm

I appreciate all the input.

Golf balls are certainly a consideration for ammunition, however my main reasoning behind pykrete was its ease of manufacturing on a somewhat large scale, cost, and being reasonably uniform. I thought about golf balls but i have yet to find a good source for them, and the kinetic energy difference between a gold ball and a giant slug of pykrete is very substantial. But again its still all on the table.

For ignition i like your ideas. That type of switch looks perfect and is kind of what i had in mind. A switch activating a relay thus activating the ignition circuitry. With a few of my friends we could figure out some kind of ignition circuitry that is up for the demands that would be placed on it. I had second thoughts on the stun gun ignition, I'm not sure exactly how it works and if it could stand up to the demands that would be placed on it so I may get a few of my friends who are a little more well versed in electronics to help me out in that department.

Personally i think the hardest part of the project is going to be fueling. getting the timing down will be difficult but not impossible. That valve looks perfect as well, i have been trying to find one but haven't had any luck so far.

One main reason I'm going for a large cannon is simply because the main bulk of the parts will be in all the valves, electronics, ect. . . So since I'm spending the money i say why not go large.

I have thought about detachable barrels but I'm not sure how that would work yet because everything will need to be securely held in place so it can spin. Cam locks may be a potential solution to that. If possible I would really like to have the entire barrel assembly come off so i could interchange the barrel diameters for different ammunition. this would be somewhat daunting though since it would require a versatil loading system capable of handling different sized ammo.

the rotation by the way is from a 5hp electric motor. Unless I find a reasonable alternative this cannon will most likely need to be plugged into an outlet to provide power to the motor and shop vac motor for venting.

Just in case it isn't obvious, this will not be built very soon, I'm broke after paying my tuition and if Ive learned anything from my ME education so far its that planning and simulations are so important in keeping cost down. so therefore this entire thing will be modeled in solid works before anything is built to work out all the bugs. Ive yet to learn the cad package very well but in a few weeks I should be able to do motion analysis ect on it.

P.S. Thank god for spell check I can't spell to save my life.

Cannon Freak
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Unread postAuthor: psycix » Mon Jan 19, 2009 5:18 am

Why use a switch + relay if you can also just use nothing more then a switch (like jimmy101 said)
I wonder if you will actually build this. (most projects like this die in design phase or are never finished)
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Unread postAuthor: Hotwired » Mon Jan 19, 2009 10:10 am

Not many people have the money to build to their imagination :P

Besides, something that sounds like a good plan might seem to be of dubious value after two weeks of work on planning. Better to find out after only using some computer and research time than after you started shelling out for components.
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Unread postAuthor: SilentJ20 » Mon Jan 19, 2009 10:58 am

cannon freak,

I've been planning a project like this for some time. Always in the design stages, and never in the build phase :lol: As you can tell, I don't frequent these forums often, it depends on which one of my many hobbies has my attention at the time.

Here's some things I was planning, you can use them if you wish, or take them as "advice". Since you're still designing, why not go for portability? Slightly smaller may help, but I was thinking more along the lines of electrical systems. Maybe base the chamber evacuation on 12 volt fans. A couple of high-flow computer case fans can clear a chamber in a fraction of a second (under ideal conditions).

12 volt propane solenoid valves are common in the RV and marine industry, and can be found somewhat easily. Link these to switches triggered by a rotating cam, and metering/injection can be done simply and timing can be adjusted as necessary.

I was initially planning to have ignition done by timing a stun gun, however, the circuit just isn't designed for it. I agree with others that basing it off of a vehicle ignition system with a distributor and coil is probably the best way to go (and that's 12 volt as well).

Hook the whole thing up to a 12 volt motor (MANY different types and power ratings are available) to spin everything and you can take the gun anywhere you can drag a car battery (or possibly a smaller battery pack for hand-held minigun awesomeness as seen in Predator). 5hp seems like a lot. If 5HP will take a 400 pound gokart and rider to 30 mph, it seems like a bit much for rotating something significantly smaller. It will need more torque than power, you may get what you need out of a geared 12 volt wiper motor from a car.

I'll second (or triple--whatever it is) the recommendation for golf balls. Many golf courses will sell used range balls for cheap at their shop.

For ammo delivery, breach loading and springs seems like too many moving parts. The fewer moving parts, the more reliable it is. Think about a chain/belt feed system, with the chain simply sliding between the barrel and chamber as with a revolver pistol. Getting the fit tight enough to keep blow-by minimized is the only real hurdle. Another advantage to belt-feeding is that you can use whatever ammo will fit in the belt. Pykrete, golf balls, spuds, eggs... the possibilities are endless.

Wow, I typed a lot more than I thought I would. I hope this doesn't sound like nit-picking--I'm just sharing from my experience to hopefully make this venture of yours successful. I WANT to see you do this! I wasn't going to share many details originally(think personal trade secrets :D ), but I don't know if I'll ever get to building it. Maybe someone else can use the knowledge. If you can do this, you deserve all the kudos in the internets. Hope some of my ideas can help. Good luck,
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Unread postAuthor: Ragnarok » Mon Jan 19, 2009 11:32 am

SilentJ20 wrote:Hook the whole thing up to a 12 volt motor (MANY different types and power ratings are available) to spin everything and you can take the gun anywhere you can drag a car battery (or possibly a smaller battery pack for hand-held minigun awesomeness as seen in Predator). 5hp seems like a lot. If 5HP will take a 400 pound gokart and rider to 30 mph, it seems like a bit much for rotating something significantly smaller.

A typical minigun does actually use a multiple horsepower electric motor for spinning the barrels. Also, due to the power requirements of this, the minigun in Predator was powered by a pair of truck batteries that were out of shot, so basing your expectations on that is overoptimistic.

Ah, the gatling cannon ideal. I think everyone considers it every now and again.
If I had a machine shop and a large bank account, I would return to my BB minigun design, which was designed for 1,500 rpm upwards, using a method I designed to refuel a small chamber within a few milliseconds.
Unfortunately, I can't see the design would be any use for a 2" cannon - my reckoning was that you'd struggle to be able to use it for a cannon much larger than 1/2".

Anyway, best of luck to you.
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Unread postAuthor: SilentJ20 » Mon Jan 19, 2009 12:18 pm

I agree it is kind of an ideal, but not an unreachable one.

Doesn't a typical minigun spin a heavier barrel assembly at much higher speeds(1,000-10,000 rpm for the predator gun 3.2 HP max)? If he's only wanting 20rpm from maybe 50 pounds of plastic, 5hp seems a bit overkill to me. But it really depends on the friction in the system. If the mechanical resistance is low, a smaller motor should do it, especially if other devises are electrically--not mechanically--actuated. However if it doesn't spin freely and there is a lot of resistance, a larger motor may be necessary. Imagining what is needed verses building it and finding it out are very different. I suppose I was trying to emphasize simplicity and efficiency by recommending a smaller motor--it just didn't come out right.

(BTW, googling the XM214 minigun brings up lots of neat info)

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