Cannon Consistency

[crashman287]

Jack: Good point on the potato spinning itself apart, I will take that into account. I have reconsidered the barrel diameter, and I think I am going to go back to the 1.5" barrel. Its easier to rifle, the slugs will have more mass so they won't be as affected by cross winds, and getting a decent C:B ratio with a 4" chamber came up with a chamber length of around 2 inches and a barrel of 60", which won't work.

Dewey: Good call with that. I probably won't get that one, but something similar would be an excellent idea.

Now that we have barrel and chamber diameters, I need the best C:B ratio for my application. Remember, this is for consistency, not max distance.



According to this graph, the ideal C:B is around 0.6:1 and 0.8:1. However, the discrepancy in muzzle velocity for the C:B of 1.3:1 is noticeably the smallest (i.e. more consistent). I don't really see a reason for this, as there is no real trend in consistencies (the more efficient C:Bs are generally better, but its fairly hit or miss for the most part), but the data for it is right there. Is this just a coincidence in the experiment, or could that be considered the most consistent C:B ratio?

Thanks as always!

[jimmy101]

Couple thoughts, for what there worth..

A short fat chamber is better than a long skinny one, even with the increased choke. The fat chamber will burn faster and hotter since there is less surface area to absorb the heat plus you get a spherical flame front for a longer period of time. And, a fat chamber will probably mix better than a long skinny one.

If your trajectory is more like a howitzer or mortar than a rifle then rifling probably isn't as important.

If you can't rifle the barrel (or the rifling doesn't work like you want), and given you are firing spuds (which presumably will be cylindrical), I wonder if a hop-up will improve reproducibility?
1. A cylindrical spud will tumble.
2. The affect of tumbling is not only increased drag but also increased curving of the trajectory. Which way the spud tumbles will change from launch to launch, hence the curvature of the trajectory and where the spud lands will change from shot to shot.
3. Drag really isn't a problem and has only a minor little affect on accuracy. It is the variable direction of the tumble that will affect the trajectory the most.
4. If the spud tumbles the same way for every shot the accuracy of the shots should be better.

Not exactly sure how to build a hop-up into a spud barrel.

Use a double beveled barrel, it is important that all the spuds are cut consistently to the ID of the barrel.

The individual spuds need to be as consistent as possible. Cut the spuds in half in a plane perpendicular to the long axis of the spud. Cut several spuds and pick a few that are the same length.

The arbeit is going to be consistent fueling. Squirt-and-screw is going to give you awful reproducibility. It doesn't matter what else you do to the gun it won't make a bit of difference in the overall reproducibility unless you can come up with a way to accurately measure your fuel. You can use a syringe with a spray can. It'll be tricky but it should still be a heck of a lot more consistent than 1-potato-2-potato-3-potato counting as you spray directly into the chamber.

I wonder if you could take the valve off a WD-40 can, along with the straw WD-40 usually comes with, and use the valve+straw to connect your fuel source to a syringe. Or, maybe just take a spray valve that works with the particular spray can and chop the top off so you have just the short length of tubing that fits the top of the can. Plumb that piece of tubing to the syringe.

As to the CB ratio. I'm not sure if I would read to much into the reduced variability that Latke saw at a CB of 1.3. There is a good chance that that was just luck. Indeed, given the number of test Latke did you would expect that there would be some CBs with lower variabilities just by random chance. Personally, I think I would probably go with an 0.8~1.0 CB.


As to the chamber length, which is really the only variable left, we need to know what kinds of ranges your are looking for. If your target is at 150 feet then the gun doesn't need to be all that big. Indeed, if you make a gun that is capable of firing much farther than 150 feet I would expect it's reproducibility to be less at 150' than a smaller gun. (This assumes it is best to arc the round in instead of trying for a high speed shot with a flat trajectory.) You might want to fiddle around with GGDTs trajectory calculator to get an estimate of the muzzle velocity needed for your chosen barrel ID and ammo. Then use EVBEC to get an estimate of the chamber and barrel dimensions required to get the needed muzzle velocity. Add a bit more chamber volume to make sure the gun will indeed fire at least 150'.

You need a chamber fan. Given your budget you might try to get a "personal fan" from your local Kmart or Target. With a bit of luck you might find one for $3 or $4. That'll save you the costs of a CPU fan, wire, switch, batteries and a battery holder.

Good luck and let us know how the competition goes.

[crashman287]

Wow, thanks for the response Jimmy! You've got some good points in there. I think you're right, proper fueling is going to be a problem. The trouble is I don't know what to design around. The fuel cans are given to use on the day of the competition (maybe sooner for testing), so its tough to know how to do it well. Rifling is mostly for design points (we get judged on that too), but I think it will help keep the spud on as straight a trajectory as possible. I must say though, inducing front spin is an interesting idea. The barrel will most definitely be beveled at the end, and we will probably cut the spuds to a predetermined length. I'm trying to keep it small, because like you said, it should perform better at the distance I need than one that has a huge range. I'm working on a chamber fan. Would a personal fan be able to withstand repeated combustion cycles? It doesn't seem like it could take the heat or pressure, although both are only for a few moments.

[jimmy101]

The personal fans seem to hold up fine. Search this forum and the archived spudtech forums. DR has always been a big user of this type of fan.

To get consitent fueling, perhaps ...

Put the spray can completely in a large plastic trash bag. Seal the bag shut. Spray the can for several seconds by gripping it through the bag. Insert the syringe needle into the bag and draw out the required amount of fuel. This method should work with any kind of aerosol fuel as long as the fuel doesn't attack the plastic of the bag.

[crashman287]

Thats not a bad idea. Better than what I had before. I'm going to try and see if I can get a hold of one of the fuel cans, but if not that would probably be the best way to go. Thanks for all of you help guys!!!