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Trying to figure out what my cannon should look like.

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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Trying to figure out what my cannon should look like.

Unread postAuthor: Igrekun » Sun Apr 17, 2011 11:20 am

Hello. I've built 3 cannons. This one has been my first.
Image
Other are not with me now. After reading through all technical aspects of building a spud gun, I realized that I must have been VERY lucky, that all of that cannons actually worked.

Now, to our business. According to calculations and theory, my gun should look like the left one. Common sense tells me to enlarge chamber at least to make it like the right one.
Image
Tech Specs:
Barrel: 1 meter length, 1.54 cm inner diameter.
Chamber:
5.82 cm inner diameter
10 cm length on the left image
20 cm length on the right image
Rates are:
1.428 and 2.858

Both of the chambers are larger than they should be, and smaller than I expected. Please, can you tell me how acceptable these designs are.
I'd like to go with larger chamber, in order to place a fan and other stuff,
but do I really need a fan, multi spark ignition, direct metered propane injection for such a small cannon?
Definitely, I would like to have direct propane injection. Is it possible to inject amount as small as 25 ml, or how much do I need for a 540 ml chamber?
I can go without a fan, and I suppose multi spark ignition, if they give small performance boost.
Thanks in advance! Any help appreciated.
PS: In my first cannons CB ratio was pretty close to 9 :D
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Last edited by Igrekun on Sun Apr 17, 2011 11:37 am, edited 3 times in total.

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Unread postAuthor: jackssmirkingrevenge » Sun Apr 17, 2011 11:27 am

That looks like a Russian neighbourhood :)

Here's some datafor a 3/4" combustion cannon from a test by burnt latke with various chamber ratios:

Image

As to fuel injection, herewith some shameless self promotion:

[youtube]http://www.youtube.com/watch?v=0nM603eOQU4[/youtube]
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Unread postAuthor: Labtecpower » Sun Apr 17, 2011 11:32 am

Dick! I was copying the link to that vid :D

So: optimal ratio: .8:1
and (0,032*540)=17.28 ml of butane.


@jsr: I found that a small piece of shrinkage tubing over the nozzle of the syringe works very well for filling it. zero leaks :)

Also, do you have problems with butane leaking out of your hybrid chamber when filling it from the syringe?
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Unread postAuthor: Igrekun » Sun Apr 17, 2011 11:37 am

You are absolutely right :)

So, direct injection with a propane meter is not an option for small cannons, right?
And multi spark ignition, or one spark is enough?
Fan? Or in a 20cm chamber it is going to be destroyed?

The best would be to keep to the 1.2 ratio? Higher ratio will only decrease performance?

I read somewhere to choose the largest avaliable diameter for the chamber. Should I?

Thanks for the replies!
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Unread postAuthor: jackssmirkingrevenge » Sun Apr 17, 2011 12:10 pm

Labtecpower wrote: Also, do you have problems with butane leaking out of your hybrid chamber when filling it from the syringe?


I just hold the chamber threaded end up, inject the butane (not too quickly) and pop the burst disk on.

So, direct injection with a propane meter is not an option for small cannons, right?


It is, you just need to be terrible precise about the size of your meter.

The best would be to keep to the 1.2 ratio? Higher ratio will only decrease performance?


As you can see from the results above, the best velocity was obtained with a 0.8:1 chamber:barrel ratio.

I read somewhere to choose the largest avaliable diameter for the chamber. Should I?

The optimal chamber is considered to be a sphere, with ignition in the middle. However this is impractical so the next best thing is to have a chamber that is as long as its diameter. In general, a short fat chamber is better than a long thin one.
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Unread postAuthor: Igrekun » Sun Apr 17, 2011 12:40 pm

Well. I'm going to stick with the left, although it looks strange.

My previous cannons suffered from use of Piezoelectric ignitors and a hairspray. I changed Piezo to Tescoma Uno electronic gas lighter. It performed better, but still left a lot to be desired.

Which ignition option is going to be the most reliable for a mini?
I will do my best to make it ergonomic too, because my second design was exactly like this one.
Image

I pumped air to get the right percentage of flamable gas. As you can see it looked pretty ghetto. Was pretty unstable, like 60% it won't shoot.
Besides it was difficult to aim, pump and push the trigger button.

One good thing. If it explodes, it won't do serious harm.
(Checked myself. Actually I built 4 cannons. One of them exploded. You can see it above)

PS: I just calculated number of shots I will be able to make with a 1L propane gas holder. I got some insane numbers like 50000 shots with just 1L of liquid propane. I know I need only 10ml to shoot, but still. Did I make a mistake in calculations?
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Unread postAuthor: jackssmirkingrevenge » Sun Apr 17, 2011 2:57 pm

Igrekun wrote:Which ignition option is going to be the most reliable for a mini?


A piezo with a 6mm or so spark gap is perfectly reliable with a good fuel mix.

I just calculated number of shots I will be able to make with a 1L propane gas holder. I got some insane numbers like 50000 shots with just 1L of liquid propane. I know I need only 10ml to shoot, but still. Did I make a mistake in calculations?


That sounds about right, you need 10mL of gas but the container contains one litre of liquid ;)
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Unread postAuthor: Igrekun » Tue Apr 19, 2011 9:04 am

Image
This is about what I want to build. My goal is to build reliable and safe cannon. Thus, I moved the chamber back, that even if it explodes, it will happen behind, not in your face. All pipes are rated to 360psi 95C.
For this reasons I decided to go with spark plugs, as they can be screwed securely. They provide a nice spark, but not in the center of the chamber. And again, as I heard screwing something into the back cap is bad.
Perfomace is on the first place. I don't want it to be THAT safe. I don't mind if it fails, not in my face though :D
Is there any way to provide a spark and a power to the fan, without drilling the chamber? Maybe a tee with a spark plug, or something in what you can be sure.

Thanks in advance.

PS: This is all I need to know to order the parts and put my gun together. I'm stuck because I'm not sure about connections and ignition circuit.
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Unread postAuthor: jackssmirkingrevenge » Tue Apr 19, 2011 10:55 am

Igrekun wrote: I moved the chamber back, that even if it explodes, it will happen behind, not in your face. All pipes are rated to 360psi 95C.


That's a safety factor of at least 3 on the working pressure, so I would not worry about it blowing up.

Perfomance is on the first place. I don't want it to be THAT safe. I don't mind if it fails, not in my face though :D


Again, the materials you are using are more than capable of safely taking the pressure.

Is there any way to provide a spark and a power to the fan, without drilling the chamber? Maybe a tee with a spark plug, or something in what you can be sure.


Ideally you would drill in the place where the cap overlaps the chamber for extra safety, but really drilling the chamber directly isn't going to cause it to fail, unless you drill a hole with a rusty pickaxe ;)

What are you going to use as an ignitor? A spark plug sounds a little overkill for a 1x combustion.
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Unread postAuthor: Igrekun » Tue Apr 19, 2011 11:07 am

I'm thinking of using an ignition coil and 9V battery. Something like 30KV igniter i saw on ultimate spudgun. If not spark plugs, then what would you advice to put into the chamber? Screws?
Should I place fan alongside the chamber, or horizontally?
Can I use a valve for ventilation? I suspect that otherwise thread on the back cap will fail earlier than I finish my propane tank. :shock:
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