Butane Combustion Cannon

[M.J.P.]

Hey all, I'm really new to combustion cannons so i would like to upload my design and specs just so it can be looked at and improved (don't want to have to rebuild it). I have done a fair bit of research and have spent a bit of time planning this out, I am merely posting this so it can be checked and i also have a few questions. I hope you guys can help out

BCC1- 1st Butane Combustion Cannon

First of all I am fueling this on butane. Simply because it is easier for me to get, cheaper for me to buy, and from what i can tell you need slightly less fuel volume than propane (3.8% compared to 4.2%). It is going to be co-axial (barrel only runs 200 mm into chamber however) and I am using a fuel meter to inject the butane. I am using a barbecue piezo to ignite the chamber as a stun gun is hard for me to get. I'm not sure if a spark strip will work with a piezo (maybe not enough voltage?) but I am planning on testing a 5-gap spark strip with 1mm gaps. The launcher is is threaded where the barrel connects to the chamber so I can breech load it and it also has a threaded cap on the end so I can turn on a computer fan that i plan on inserting into the back. The butane cannister hangs below the chamber. The butane meter is hose clamped to the side of the chamber.

On the attached diagram of the butane meter its shows a 'butane cannister valve'. This is basically the top valve that i got off a portable butane-powered blowtorch. It shows on the diagram that a hose barb will connect to this valve; i will most likely weld this on. If the pressure hose fits nicely to the butane cannister valve i will simply clamp the hose straight to the valve, but it is not an issue.

Specs
Chamber dimensions (diam. x length): 50mm x 600mm
Chamber volume (when barrel is connected): 1400cc
Barrel: 40mm x 1000mm
Barrel volume: 1250cc
Butane meter volume: 90 cc
C:B : 1.1:1 (not ideal i know!)
Fuel volume: 53.2 mL
Pressure in meter (doesn't sound right to me): 9 PSI

Here are my questions:
1) Piezo: Does a piezo supply enough voltage to create 5 1mm sparks? Or will it provide enough voltage to make more gaps?

2) Spark strip: I have tried to research them, but i am still quite clueless on spark strips. I think I read someone saying that bits of coat hangers work well? If i cut up some bits of coat hanger, glue them the wall of the chamber 1mm apart from eachother, and connect them to the circuit would it work?

2) Computer fan: What do they usually run on, 9V batteries? This may sound stupid, but i was thinking the other day, doesn't it break when the explosion inside the chamber occurs?

3) C:B : Is 1.1:1 too bad or should it really be in the 0.6-0.8:1 range?

4)Meter: My maths isn't great, but after doing some for my meter i got the result of filling the butane meter to 9 PSI. First of all this doesn't make sense to me as the atmosphere pressure is 14.7 i think, or does this mean 9PSI on top of the atmosphere pressure? Or maybe i need to reduce my meter volume to increase this pressure (i already have the parts for a 90 cc meter volume but i can change if need be)?

I have attached some of the basic sketches i did on paint (mainly for myself) but i will put them up anyway. Sorry, i think all the names for the parts are wrong but i originally just made these diagrams for myself but i hope you are able to understand them anyway!

Any input/ideas/improvements/answers would be much appreciated!

[Technician1002]

First thought is why is this coaxial? There is very little space between the OD of the barrel and the ID of the chamber. It is in a hard to mix space and would contribute negligible power.

Chamber dimensions (diam. x length): 50mm x 600mm

Barrel: 40mm x 1000mm

I'm in the US. I don't know the wall thickness of your pipe, but a 50 mm chamber and 40 mm barrel would leave a space of 5 mm on each side if the barrel has no thickness. Less when counting the pipe thickness.

Have you picked up pipe and checked the gap?

[M.J.P.]

Yes your right there isn't much space.
My only reason for going semi coaxial in the beginning is because i wanted to reduce the total length of the launcher without making the C:B ratio worse.
As i said i have little to no experience with combustion cannons and i thought 3-4 mm would be enough space for fuel to mix with a chamber fan going.
But maybe it's not enough space so i might have to reconsider. Thanks

[Biopyro]

If you're using metered propane, then you should be going for a C:B ratio around 0.7 : 1.
1.1 : 1 will give you a very noisy cannon, although co-axial combustions tend to experience a loss in power compared to inline ones.
Your best bet for making it shorter, is to either get a wider chamber (which will also increase power somewhat), or just shorten the chamber. A 90cm barrel with a 40cm (50mm) chamber gives 0.7 ratio, and about 250J for the average potato slug (pretty good). A 15cm (90mm) chamber with a 1m barrel gives a 0.75 ratio and 340J for the same piece of potato.

Also, you'll never get five 1mm sparks from a cigarette piezo. You'll be lucky to get 2. Best I've got is from a BBQ igniter which gives about 1cm. I'd recommend making each gap about 2mm, and as close to the centre of the chamber as possible, but your idea would work fine (you'd only need 2 in a 15cm chamber).

A computer fan will need anything from 5v ( little ones from routers) to 18v (big 8cm ones). Most are 12v. Because you're only using them for short amounts of time, you can use a 9v battery, which will provide ample air movement.
The "explosion" doesn't damage the fan because of several reasons - the force of the explosion acts equally on every side, so no damage there, and the hot air has a very low specific heat capacity. It is hot, but has very little heat energy to transfer to the fan or chamber (hence why nothing melts).

I'm probably wrong here, but my basic maths suggests to me that your fuel mix is a little lean, with 32ml of of fuel in the chamber (2.2%). However, I haven't worked with meters at all, so I could be wrong. I'm

[M.J.P.]

Biopyro thanks for the advice on the ratio's; I'm thinking of using a 90cm barrel with a 40cm chamber as i cannot getting any pressure rated pipe larger than 50mm at my local hardware store.

I'm not too sure what a cigarette piezo is but I'm using an igniter from a BBQ (might have been a misunderstanding). Good to hear that they will produce 1 cm so yes i will use 5 2mm gaps. Have you had any experience with coat hangers though?

I understand about the computer fans thanks for explaining that.

Also my fuel volume is 53.2 not 32 ml.

Thanks

[MountainousDew]

There is a guy on Youtube by the name of MushiYoshi, and he has an assortment of combustion cannons. Here on SpudFiles, he goes by TheBuG, and he has written a "How-to" on one of his advanced combustions, which is here.

If anything it may just help you with some minor questions during the build, or it may inspire you with a new design. Either way, I hope it helps...

[Biopyro]

Ahh, I thought your meter volume was 53.2, so I used that in my calculations. That was correct, but your new chamber volume is 785cc, so you'll have to re-do those calcculations

That is the correct volume of fuel, but I don't have enough information about your meter to check the pressure you need to get that value. (meter volume * meter pressure = volume of propane introduced to chamber).

The value you're getting (9psi) is measured in PSIG or gauge pressure. It is called this because it is read from a gauge calibrated to 0psi for atmosphere. Therefore, the pressure in your meter is 9psi above ambient pressure.

A cigarette (lighter) piezo is the type found in an electric lighter, and is great for very basic systems. A BBQ igniter will usually be more powerful, but you should measure it yourself (all are different - I have another which gives 3mm). Connect a wire to one electrode, and move it close to the other, whilst clicking occasionally. See what the maximum length of spark you can get is. Estimate, because you can't really measure it. Take a few mm off, and this is your maximum combined gap length (gap 1 +gap 2 +gap 3 etc).
I say take a few mm off, because if your total length is more than the igniter can handle, it won't spark and so won't ignite.

I haven't personally used a coathanger, but that will work fine, anything conductive is good. Technician explains why any metal is fine: just replace screw with coathanger

[jimmy101]

First of all I am fueling this on butane. Simply because it is easier for me to get, cheaper for me to buy, and from what i can tell you need slightly less fuel volume than propane (3.8% compared to 4.2%).

Butane provides virtually the exact amount of energy as does propane so if it is more convient then it is better.

1) Piezo: Does a piezo supply enough voltage to create 5 1mm sparks? Or will it provide enough voltage to make more gaps?

Like others have said, probably not. A good BBX piezo might jump a 3mm gap consistently. So you need to use fewer gaps or smaller gaps.

2) Spark strip: I have tried to research them, but i am still quite clueless on spark strips. I think I read someone saying that bits of coat hangers work well? If i cut up some bits of coat hanger, glue them the wall of the chamber 1mm apart from eachother, and connect them to the circuit would it work?

As above, several 1mm gaps might be a bit too big for a piezo. The gaps should be out near the centerline of the chamber, not up against the wall. A piece of small diamter PVC pipe sliced lengthwise makes a convienent holder for the wires that make up the gaps. Keep the PVC near the wall of the gun with the wires extending up to the centerline. Any wire will work electrically but heavier gauge wire will be less likely to get bent during firing. Sharpen the wires at the gap to a point.

3) C:B : Is 1.1:1 too bad or should it really be in the 0.6-0.8:1 range?

The performance will be better at 1.1:1 than it is at 0.6-0.8:1. It'll also be louder.

I would avoid the co-ax design. It does make the gun somewhat shorter but the effectiveness drops as well since the effective chamber shape is less than ideal. If you want to keep the total length as short as possible consider an over-under design.

Since this'll be a muzzle loader you want to double bevel the muzzle. That is, sharpen the outside of the barrel to a knife edge, then sharpen the inside a bit as well.

[Biopyro]

Better performance with 1.1:1?
Burnt latke doesn't seem to agree!

The difference probably wouldn't be noticeable, but it will definately be louder.


Do note that my BBQ igniter with a 1cm spark is the most I have ever seen from a piezo, so I wouldn't be suprised if yours was only up to 2 or max 3 gaps.

[jimmy101]

Biopro

You need to look closely at what Latke measured and I should have been a bit more careful and said a ">0.8 CB will outperform an 0.8 CB for a given barrel and ammo".

Latke's data is for a fixed chamber size and a variable barrel length. To get to the >1 CB's he shortened the barrel. So the CB comparison is a bit misleading. Bascially it just says that, for a fixed chamber volume, if you shorten the barrel you decrease the performance.

If you keep the barrel the same length then vary the size of the chamber then the performance will increase as the CB gets bigger. A CB>1 will outperform a CB 0.8. 1.5 is even better, 2.0 is even better. (Though there are diminishing returns.)

So, in this thread we are talking about a larger chamber on a fixed barrel. The larger chamber will indeed boost the performance.

Eventually you may get to the point where a bigger chamber on the same size barrel will start to decrease performance but that doesn't appear to start to happen until you get to very large CB's of at least 3 and perhaps not till the CB gets over 10. Or it might not ever happen.

Since adding volume to the chamber typically results in a fairly minor increase in the gun's length (since the chamber is larger diameter than the barrel) and cost, a larger chamber is usually an easy way to boost the performance of the gun.

[Biopyro]

Ah ok, was just going out so I only pulled that site from memory, and didn't have time to actually check it.

I thought it was a little counterintuitive, but didn't really think about it at the time.
Also, for personal enjoyment, and impressivness, loud is good!

[M.J.P.]

There is a guy on Youtube by the name of MushiYoshi, and he has an assortment of combustion cannons. Here on SpudFiles, he goes by TheBuG, and he has written a "How-to" on one of his advanced combustions, which is here.

Thanks MountainousDew i have checked the video out and picked up some really neat ideas on creating multiple sparks and making a handle.

The value you're getting (9psi) is measured in PSIG or gauge pressure. It is called this because it is read from a gauge calibrated to 0psi for atmosphere. Therefore, the pressure in your meter is 9psi above ambient pressure.

A cigarette (lighter) piezo is the type found in an electric lighter, and is great for very basic systems. A BBQ igniter will usually be more powerful, but you should measure it yourself (all are different - I have another which gives 3mm). Connect a wire to one electrode, and move it close to the other, whilst clicking occasionally. See what the maximum length of spark you can get is. Estimate, because you can't really measure it. Take a few mm off, and this is your maximum combined gap length (gap 1 +gap 2 +gap 3 etc).
I say take a few mm off, because if your total length is more than the igniter can handle, it won't spark and so won't ignite.

Yes that makes sense about the PSIG. I think i will be changing my meter volume so that it fills to about 20 PSI anyway.
I am about to do some testing on a coathanger and my BBQ igniter. Sorry everyone about the confusion of me calling it a 'piezo' as i have heard them called 'BBQ piezo's' before in Australia. I will take into consideration that taking a few mm off is a good idea so i can be sure of a spark. I will get started on testing my igniter soon.

As above, several 1mm gaps might be a bit too big for a piezo. The gaps should be out near the centerline of the chamber, not up against the wall. A piece of small diamter PVC pipe sliced lengthwise makes a convienent holder for the wires that make up the gaps. Keep the PVC near the wall of the gun with the wires extending up to the centerline. Any wire will work electrically but heavier gauge wire will be less likely to get bent during firing. Sharpen the wires at the gap to a point.

I will probably use something like TheBug's design to make the spark strip more 'centreline'.

The performance will be better at 1.1:1 than it is at 0.6-0.8:1. It'll also be louder.

I would avoid the co-ax design. It does make the gun somewhat shorter but the effectiveness drops as well since the effective chamber shape is less than ideal. If you want to keep the total length as short as possible consider an over-under design.

I am scrapping the coax design and making it a simple inline cannon. As I am not too worried about noise and am looking for more power i will probably go for a ratio of about 1.5:1. Thanks for the advice.