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Cannon Consistency

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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Cannon Consistency

Unread postAuthor: crashman287 » Wed Feb 06, 2008 12:15 am

Ok, so I have been assigned the task of building a potato launcher with the specific task of being able to a target 150 feet from the launch point, with a big emphasis on accuracy and repeatability. The launcher must be combustion and use the supplies provided, along with a $20 budget. Here are the important pieces:

10" length of 4" SCH 40 PVC
10" length of 2" SCH 40 PVC
5' length of 1.5" SCH 40 PVC
5' length of 0.75" SCH 40 PVC
assume I either have or can get all the necessary couplers and bushings

I'm just trying to figure out the launcher's actual dimensions at this point. I read a lot of stuff about C:B ratios and all that. I know that the max. performance ratio is approx 0.7:1, but the graph of test results from burntlatke shows a ratio of 1.3:1 to have the most consistent performance. Would this be the best ratio given my short target distance? Let me know. TIA!
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Unread postAuthor: homedepotpro » Wed Feb 06, 2008 12:38 am

dude a $20 dollar budget won't get you very far. your going to need to do al ot of dumpster diving. i buy from a commercial supplier and 20' of 2.5'' sch 40 is $40.
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Unread postAuthor: hi » Wed Feb 06, 2008 12:48 am

yea, seriously, get some funds. you will need a 10 foot barrel to achieve accuracy at that range, and probably launch something a little more aerodynamic than an Idaho spud. (to me accuracy is placing a round exactly where you aim, i don't know what your standards are) i would say that you are going to want it to be propane powered and have a chamber fan. i also suggest you make it inline. also use bbq ignitor or a stun gun for ignition, if you use a flint it will throw off the shot, if its hand-held. if its bench mounted it wont matter to much.

for a nice cannon we are talking at LEAST $60-70. take your time and do it right, i learned this when i put cannons together with the same sort of budget and decided it looked like crap afterward, so just get some money and spend a few weeks on the project. it may not sound good now, but its worth it.
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Unread postAuthor: BigGrib » Wed Feb 06, 2008 1:19 am

what the hell do you need 10' of 4" pvc for? I've never heard of a 10' chamber. If you want a combustion that will accurately hit 150' away I would personally have a chamber that is 4" diameter about 2' long with a golf ball barrel that is about 6 and a half feet long. that is the dimensions of my GB Combustion and I can reliatbly hit 150 yds away with a top distance of over half a mile. the only thing is hat you'er gonna be looking at a lot more than 20 bucks. my ignition system alone cost 30 bucks, and the fuel system was about 20. everything is hand built except the ignition system. the rest of the gun cost about another 40 bucks including aluminum cam locks to remove the barrel easily.
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Yea, that's definitely going to get you at least a tazer.

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Unread postAuthor: jackssmirkingrevenge » Wed Feb 06, 2008 2:01 am

Does it have to be potatoes? If you stick to a 3/4" barrel and use marbles as ammunition, the accuracy potential will be much higher, assuming you usae a long enough barrel to get good velocity, as well as a consistent metering system as opposed to "spray and pray" aerosol.
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Unread postAuthor: MrCrowley » Wed Feb 06, 2008 2:02 am

BigGrib wrote:what the hell do you need 10' of 4" pvc for?

Ah it's 10" not 10' :)

I thought the same first time I read it, then i looked again/
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Unread postAuthor: chartreusesnot » Wed Feb 06, 2008 2:53 am

hope this isn't too off topic, but I have started a mini combustion, that requires a very accurate fuel metering system, and was wondering where i can get a cheap, good quality, adjustable, propane regulator. I am modeling mine almost exactly off Burntlatke's M1.
this pretty much sums up what I am trying to make, albeit more messy. There is a regulator there, where can I get one like that?
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Unread postAuthor: jackssmirkingrevenge » Wed Feb 06, 2008 3:15 am

chartreusesnot wrote:hope this isn't too off topic, but I have started a mini combustion, that requires a very accurate fuel metering system


syringe ;)
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Unread postAuthor: BigGrib » Wed Feb 06, 2008 4:32 am

MrCrowley wrote:
BigGrib wrote:what the hell do you need 10' of 4" pvc for?

Ah it's 10" not 10' :)

I thought the same first time I read it, then i looked again/


damn whipper snappers with your good eyesight and all
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Yea, that's definitely going to get you at least a tazer.

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Unread postAuthor: crashman287 » Wed Feb 06, 2008 5:48 am

This cannon is for a competition that I have entered in, so the budget is fixed and the so are the supplies. I don't think I was clear enough, but all of the supplies listed are provided in the kit I have to use, and I have $20 extra to spend on additional supplies. It has to be potatoes.
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Unread postAuthor: jackssmirkingrevenge » Wed Feb 06, 2008 5:56 am

In that case, use the 0.75" barrel - it has the highest barrel length:calibre ratio and will yield the highest velocity - this will give you a flatter trajectory and therefore will be easiest to fire accurately.

Since you're limited to potatoes, the best you can do is cut your slugs out accurately to 0.75" in length, this makes them as wide as they are long and when they start to tumble, it will cut down on drag somewhat. Ideally you'd have a rifled barrel but your budget doesn't allow for it.

A good sighting system should be easy to improvise, I had made adjustable iron sights for this launcher very cheaply.

How big is the target you're aiming for? Or is it a "closest to the bull wins" style competition?
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Unread postAuthor: crashman287 » Wed Feb 06, 2008 12:13 pm

Actually, the barrel is going to be rifled. I came up with a way to get a surprisingly nice spiral bore along the barrel. I won't reveal details yet, until the cannons have been built and submitted, but it seems like it will work well.

So, I see the thinking behind the 3/4" barrel, makes sense. I will probably try and have the slugs at least 1.5x as long as they are wide, since the rifling can take advantage of that (I think? Correct me if I'm wrong).

Now that we have that established, what would be the best chamber diameter to use, the 2" or the 4"? My thought on going with the 2" is that it will minimize choke as get as much force actually pushing on the back of the spud, as opposed to colliding with the tapered coupler and being diverted into the spud. A minimal step-down in cross sectional area would be best, and since we're going with the 3/4" barrel, the 2" chamber would be a lot better than the 4".

Also, any comments on the consistency of a 1.3:1 C:B ratio versus the standard 0.8:1?

Thanks for the responses!
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Unread postAuthor: crashman287 » Wed Feb 06, 2008 12:19 pm

Couple things I should have added in the beginning:

1. The target is lying flat on the ground 150 feet away from the launch point. These are more potato "morters" than they are "launchers". As I said, points are awarded for accuracy and consistency. The target has a 1' diameter, landing on the target scores 100 points, with points subtracted incrementally depending on how far from the target you hit.

2. Fuel is controlled by the competition. It will be some kind of hairspray (not sure which) and we must use what is provided. This makes metering much more difficult, and given the limited time and budget we have, this cannon will likely be spray-and-pray (unless metering is cheaper and easier than it seems to be, haven't looked into it much). I'm pretty sure there is max amount of fuel we can use as well.
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Unread postAuthor: jackssmirkingrevenge » Wed Feb 06, 2008 1:19 pm

have the slugs at least 1.5x as long as they are wide, since the rifling can take advantage of that (I think? Correct me if I'm wrong).


The longer your projectile, the faster it has to be spinning to stabilise it effectively - I would go for the same 0.75" in length, because the faster you spin your potato slug, the more likely it is to fly apart due to the centrifugal force on what is a very weak material.

Now that we have that established, what would be the best chamber diameter to use, the 2" or the 4"?


The ideal combustion chamber is in the shape of a sphere - extending this logic, it's better to have a short fat chamber than a long thin one.

The target is lying flat on the ground 150 feet away from the launch point. These are more potato "morters" than they are "launchers"


In that case, a heavy slow projectile lobbed at an angle will serve you better than a small fast one fired in a straight line - the latter will likely bounce off the target and it will be difficult to determine the actual point of impact, and if you're not firing from a significant height it will be hard to see and aim.

My advice would be to mount the launcher on a fixed platform that's always at the same angle (use a spirit level or plumb line to ensure consistency) then experiment with different lengths of potato slug until you get the right weight for 150 feet. You could also use a fixed weight of slug and vary the angle, but that would make construction more complex.
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Unread postAuthor: dewey-1 » Wed Feb 06, 2008 1:35 pm

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Last edited by dewey-1 on Sun Apr 20, 2008 2:05 pm, edited 1 time in total.
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