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I've been meaning to get around to a metered propane launcher, and finally I did. It's not long on innovation but it was fun to build and it works great. Basically, I wanted a 1" bore because I found a source for 1" plastic balls. So the chamber is pretty small - about 5" long, and 3" in diameter. I use a BBQ ignitor to generate a single spark exactly in the center of the chamber.
I also found a source for a very small propane cylinder - perfect for the scale of the gun. There's a 2" ball valve for exhaust - I realized later that I could have used a 1" one instead - the flow would have been the same (the breech side is only 1") but with smaller size and less weight.
I wanted to come up with a straightforward breech for easy loading. What I came up with works OK but I could probably loosen up the bushings the barrel slides through to make it even easier.
Next is to add a bipod. I also plan to experiment with different barrel lengths and suppressor designs (i.e. how to vent the barrel, whether or not to vent the suppressor, etc.
This thing packs a punch!
Here's some video with construction details and some test shots with damage.
Nice cannon, looks very well made. However, I think you should get a stun gun for ignition. I don't really like piezo sparkers.
Where did you get that small propane cylinder?
Oh BBQ sparkers work great. I have used one on a launcher with 3 spark gaps and had no problems. I actually replaced my stun gun with the sparker because I was tired of changing batteries.
Love the cannon and that monster ball valve vent is kickass.
I like to play blackjack. I'm not addicted to gambling, I'm addicted to sitting in a semi-circle.
Thanks, guys! It's been a fun project. I can see it continuing to evolve for a while - the combustion chamber can serve a number of different barrel configurations, etc.
I didn't see the need for a stun gun spark gap since the combustion chamber is so small. Also, there's no simpler way to ignite than with a BBQ thingie. Plus, even though the trigger break is fairly heavy, it is very "positive". You know when it will click.
Regarding the small propane cylinder: I was keeping a lid on this until I finished the gun. About a year ago I discovered these - they are used for bike tire inflation. They are called "Big Air" and can be found in bike shops, on Amazon, etc. They seem perfect for this application since they are small and light, and so far I've done dozens of shots over several days and the first cylinder is still good.
My secret is out!
Nice looking, compact combustion! Some innovation there too with your O-ring breach fit and small propane cylinder find! Everything is neatly packaged and well thought out. You lost me a little with the plain wooden board stock, tie wraps and flat metal bracket "breach-lock".
Also, as you found out yourself, the 2" intake valve is waaay overkill. 1" or even 3/4" would have been fine for this project. On the upside the 2" valve gives it some beefy character and a cool look. I did say intake here as hopefully you have the fan mounted on the valve side of the chamber blowing toward the barrel breach. This means all exhaust goes out the barrel, fresh air in through the valve.
Oh and...where did you get that 1" "ammo"...?
Last edited by starman on Wed Dec 09, 2009 11:38 pm, edited 1 time in total.
Hi Starman, thanks for the comments.
Yeah, the stock is way too ghetto for this thing and I'm planning a much nicer one. I just couldn't keep up the patience! I wanted to fire it! Agreed about the tie wraps, too. I've already come up with a better way for the next stock.
Yes - the fan is on the valve/fuel intake side as you say, and pulls fresh air in from the back.
US Plastics is a great source for the ball ammo along with the pipe fittings, valves, etc.
I only wish they had 15/16" balls (no one does, as far as I can find) to use with sch. 80 1" pipe. I'd rather have a stiffer barrel with the sch. 80. However, I may try to sleeve the 1" sch. 40 for stiffness.
Plenty more to try and to learn about!
This forum is great, BTW.
I'm really digging the compact design and the way the barrel seals to the chamber. Smart. On the stiffening of the barrel, I've considered wrapping a chamber (I know, it's a chamber, not a barrel) with Fiberglass and epoxy before (the stuff you get to repair showers at Lowes) but never have followed through. It just didn't seem economical at the time. The problem is you have to sand the whole thing to get the epoxy to set to the PVC. That might work. Fantastic gun! I want to know more about design of supressor, though.
There's a good tutorial here, it's for firearms but the same logic extends to combustions. Plenty of other informative threads on the subject if you use the search function, like this one.
About strengthening the chamber, I would just suggest using schedule 40 pipe (3" is rated to 158 psi but has a 840 psi burst pressure) and then pressure rated fittings. If you want to go nuts, use schedule 80 for everything. Also, avoid drilling through the chamber mid-pipe, but instead go through the fittings also if you must make holes.
Pressures found in combustion cannons range from maybe 20 psi in a spray-n-pray setup to 70 psi in a metered propane - depending on several other factors, too.
Once you get into oxy mixes, hybrid cannons, etc. then you want to look at all metal construction.
BTW - I'm still tinkering with my suppressor - it's not as quiet as I'd like it to be. Part of that is the projectile - a ball does not offer much in the way of "plugging the holes". Cored carrots work better...
I'll be uploading some new photos and hopefully new video once I've updated my stock.
I appreciate the suggestion, though I already knew about most of it. I was unsure of the pressures inside combustion cannons and realize that it's the volume of air rather than the PSI that produces "successful" shots. This makes me feel a little more comfortable in our next build. Lets just say that it has a 6" chamber of sizable length. Why are you quieting the gun in the first place, might I ask? Common courtesy? neighborhood sound ordinances?
Mostly courtesy but also to avoid attracting attention. I don't get to go out of the city as often as I'd like and I do a lot of testing in the garage...
A quick update with a full-length photo and a short damage video.
1. Cut down the stock to be lighter and more in line with how the gun looks.
2. Switched to hose clamps to attach the barrel assembly to the stock. There is also a screw at the back of the barrel to keep it from sliding.
3. Added a simple peep sight.
Tell you one thing, it does appear that the Magnus Effect has a drastic impact on accuracy with this launcher. I could see the ball curving during its flight... probably an issue of a certain amount of spin induced in the barrel, then the relatively low mass of the plastic vs. the amount of "lift". I guess I can either move to more massive projectiles, some kind of darts, or look into getting rifled 1" sch. 40 pvc...
I'm also thinking about experimenting with aluminum foil burst disks to maximize the power of a 1x mix. Should allow me to get a slight bump in pressure when adding the propane.
Short damage video:
It looks better, but I still think the wooden board seems out of place. Perhaps a coat of black paint would enhance its appearance.
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