Preliminary combustion designs

[niglch]

I have been working on some preliminary plans for an advanced combustion launcher using Google SketchUp 7, which I found very useful in the planning process (special thanks to Dewey-1 for providing some of the needed CAD models). It's a pretty standard over/under cannon in which I'm hoping to achieve a quadruple spark gap using an electric BBQ sparker (if the igniter is good enough for this, maybe someone here will know). The only details I haven't worked out so far is how the propane tank and fuel meter will be secured to the rest of the chamber, and where and how the trigger will be placed. I'm hoping to keep everything as neat and ergonomic as possible, but I'm also looking to stay cost effective (under $120 is the goal). I've roughly estimated the cost to be around $100, so I'm on budget for the time being.

At the moment, this is a muzzle loading design, but I have been thinking about ways to add breach loading capability. The most straight forward solution may be to add a cam-and-groove lock to the barrel and modify the barrel support so that the barrel can slide along it. Alternatively, I might replace the upper 90 degree elbow (the one attaching the chamber to the barrel) with a tee and try to find a way to load through the very rear of the gun.

I'm planning on keeping things cheap and using black steel/iron fittings for the fuel meter rather than the more expensive stuff assuming they won't corrode too easily and start leaking. Also, I'm hoping to simply clamp a 3/8" pressure rated hose onto the end of a bernzomatic torch head rather than cutting it and spending money on the tools needed to properly thread it.

Please comment and suggest to your hearts content.

I'm also trying to come up with a name for this thing haha. That's usually the hardest part.

[spudtyrrant]

nice sketch-up it is possible to get 4 sparks out of an electric one they have some specially made with more than one set of leads so that they can put out multiple sparks so in that you should be fine it looks like a solid design if you do everything right you should be able to keep it easily around $100

[Moonbogg]

Thats an outstanding layout job you did with sketchup. The cannon looks like a great design to me. As far as mounting the meter pipe and propane tank, I thought about maybe making wooden spacers like you did for the barrel support and then using hose clamps to wrap around. Another option that you may not like but I wanted to mention, is mounting tape. You can possibly make spacers or mounts and then use some really strong double sided mounting tape. It sounds lame, but they come very strong and with enough surface area it should hold pretty solid.
For the propane connection, i've seen some connectors that come pre threaded already but forgot where I saw them. I think they were really cheap to. I don't see why your idea about simply clamping it wouldn't work though.

[dewey-1]

niglch;

You truly did an excellent job in the 3D modeling of your cannon!!!
This has to be one the first sketchup drawings I have seen done with such complete detailed components.

I appreciate your usage of my components library. It makes me feel my time was well spent just to see the results that you created.

It appears that you have mastered sketchup.

You could probably easily answer somes questions asked in sketchup "sticky" posting.

If you need other models let me know. Also if you want, I could add your components to my existing library.

[starman]

Great layout work niglch!

Yep, pretty much a classic over and under and should work just fine as is. Be sure to include the 180 degree pipe in the rear as part of the chamber space calculations for fuel metering.

The spark strip looks really robust but isn't going to be effective in that configuration. The gaps are a little too tightly spaced and way too shielded from the chamber space by your upper and lower PVC screw mounts. You ideally want purely spherical detonations in the chamber. Yours as shown here will be forced out of relatively small "vents" on the side and have to rebound up and around the mounts. A purely spherical burn would have had the entire chamber ignited by the time your burns hit the side wall.

When spacing your gaps, keep them in the chamber center of course, and place them roughly equidistant apart. If you have 2 gaps, divide the chamber 4 ways and place one at the 1/4 spot and the other at the 3/4 spot. This allows the 2 detonations to meet precisely in the middle the same instant that burns complete at the front and rear of the chamber. With 3 gaps, add one in the middle and scoot the end gaps a little further out to the edges, at the 1/6 and 5/6 spots. The burns will meet at the 1/3 and 2/3 spots the same time as the front and rear boundries. As you can see, 3 gaps will clear the chamber slightly faster than 2. 4 gaps offers almost no improvement at all over 3.

Almost everyone probably knows by now that I'm a fan of outboard fueling/metering. As a suggestion, you can save cannon weight and money on a metering system if you go with something like this...



Just something to think about.

[niglch]

Thanks starman, I'll definitely be taking all that into consideration. Just out of curiosity, should I bother trying to compensate for the chamber volume in the 90 degree bends when placing the spark gaps or should I just focus on getting the main body of the chamber to ignite as quickly and uniformily as possible?

[starman]

Thanks starman, I'll definitely be taking all that into consideration. Just out of curiosity, should I bother trying to compensate for the chamber volume in the 90 degree bends when placing the spark gaps or should I just focus on getting the main body of the chamber to ignite as quickly and uniformily as possible?

For the gaps, I would just concern myself with the main chamber body...it will be wasting time to compensate down to that low level...and honestly 2 or 3 gaps will be plenty for your setup. However, be sure to take the extra space in the 90s into account for the fueling.

Having the chamber fire quickly like that will approach the same effect a burst disk has.

[niglch]

Just as an update, I have got this project nearly done. The gun is operational and has been field tested with fairly good success. I just need to finish fixing up a few kinks in the ignition system and come up with a good paint job. I'll post it over in the showcase once I've got it all pretty. I stuck mostly to my design as shown but ended up putting the threaded fittings for servicing on the opposite side of the chamber. I also added handles and used STHORNE's modded electric BBQ setup. The gun has a push-button trigger, a fan switch, and a safety switch.

Just as a general question, when I calculate the amount of propane needed per shot, should I use an additive or displacive method? I'm thinking some air will be displaced during fueling around the potato and such.

[Moonbogg]

Just as an update, I have got this project nearly done. The gun is operational and has been field tested with fairly good success. I just need to finish fixing up a few kinks in the ignition system and come up with a good paint job. I'll post it over in the showcase once I've got it all pretty. I stuck mostly to my design as shown but ended up putting the threaded fittings for servicing on the opposite side of the chamber. I also added handles and used STHORNE's modded electric BBQ setup. The gun has a push-button trigger, a fan switch, and a safety switch.

Just as a general question, when I calculate the amount of propane needed per shot, should I use an additive or displacive method? I'm thinking some air will be displaced during fueling around the potato and such.

If your ammo is not an air tight seal in the barrel, I would go with the displacive fueling method. More experienced spudders will probably chime in as well on what they do.

[jackssmirkingrevenge]

Your comparison dude is looking a bit emo, modified accordingly

[niglch]

^^Yeah, I figured I should talk to him. His girlfriend just broke up with him. Something about him spending to much time making potato cannons . . . he took it pretty harshly.

[King_TaTer]

You won't need to worry about air leaking around the potato, that is if its cut to fit the barrel. You will with a loose potato or some other loose veggie or object. I would fire it with in a decent amount of time from fueling just to be sure, but then again a misfire due to this will be quite rare.

Also, if you haven't already, take a fluid volume measurement of your chamber and meter pipe. This will make it a whole lot easier when calculating the right amount of propane. Just cover the breech with tape or you could even plug the barrel with a potato to simulate a realistic shooting scenario. Then you can fill and empty the chamber from the ball valve. Remember to close the ball valve on the meter pipe where it meets the chamber. Take the meter pipe off the chamber when measuring its volume, It's much easier.

Hope this helps, and good luck. If you have any more questions, just ask.

[chenslee]

Well played sir. Emo indeed.