SpudFiles

This forum is great. I wish I had found it sooner.

I thought I'd share my propane launcher idea since I haven't really seen anything like here or any where else. It's fairly cheap, simple to build, easy to charge and produces very reliable results. To operate, load a potato, hold the igniter button half way down for about 20 sec and squeeze the igniter button all the way to fire. No unscrewing caps or metering of propane required and it can't be over charged.

This is a very old version, but it's the only pic I had handy. I'll try and post some pics of the newer versions I have with interchangeable barrels and other improvements, but this will give you the basic idea. Comments appreciated.

Could you give a diagram?

Or is this kind of like a propane meter with a twist?

Nice cannon, BTW.

No propane meter. The propane fills the combustion chamber via the blow torch (with the proper propane/air mix) while the bad air is pushed out of a vent tube at the other end of the combustion chamber. You can see the brass vent tube in the pic above.

JohnnyBOOM wrote:No propane meter. The propane fills the combustion chamber via the blow torch (with the proper propane/air mix) while the bad air is pushed out of a vent tube at the other end of the combustion chamber. You can see the brass vent tube in the pic above.

Very interesting. How well is that working?

It works most excellently. I came up with this design years and years ago and have fired many thousands of shots through many different design iterations. Depending on the combustion chamber size and barrel length, I'm able to achieve any where from 175fps up to about 250fps. And you can add about 10% to those numbers with MAAP gas....though I don't like using it.

Your idea has been done, and if you looked around hard enough you would have found it implemented. (It took me about 2 minutes to find one) The design concept of your propane injector uses the blowtorch piece that comes with standard propane kits. It does mix air with the propane at a combustible proportion; although, the mix could be anywhere from 1.9%-9.6%. The optimal mix for spud cannon should be 4.2%. Without cutting that top stem off the blowtorch and implementing a metering system you can never hit that exact mix on the nose. Too expand on the problem, there is already atmospheric air pressure in the chamber and by adding another source of air and propellant under a higher pressure you are offsetting the mix. This is all a product of this type of design. I’m sure it works much better than a spray-n-pray, but there is a reason that many people just build a metering system. It doesn’t cost that much and it will allow you to get much more consistent shots.

Sorry for the way my post came off, but I was just letting you know that your idea has been implemented before. That type of design led to the development of the metering systems you see on all the advanced cannons.

The link: http://www.spudfiles.com/forums/viewtop ... html#11425

Xxplosive42o wrote:Too expand on the problem, there is already atmospheric air pressure in the chamber and by adding another source of air and propellant under a higher pressure you are offsetting the mix.

Hmmm. That's not actually true with this design. There is a vent at the forward end of the expansion chamber that prevents any appreciable pressure from building up. So as the torch is filling the chamber, the very small pressure provided by the torch is enough to push out the bad air.

Xxplosive42o wrote:It does mix air with the propane at a combustible proportion; although, the mix could be anywhere from 1.9%-9.6%. The optimal mix for spud cannon should be 4.2%. Without cutting that top stem off the blowtorch and implementing a metering system you can never hit that exact mix on the nose.

While it may not be a perfect mix, thousands of consistent shots lead me to believe that it pretty darn good.

No offense taken BTW. I like healthy discussion. I was however unable to find anything like this design with many hours of searching on many sites. Could you provide me with a link to a similar design?

Oh. I just noticed the link you provided above, Xxplosive42o. Please excuse my noobishness. The design in the above link seems much more complex though. It has a valve, I don't see a vent of any type to allow the bad air out and it uses a separate igniter than the torch.

My design has no valves, does not require fresh air, and uses the propane torch as it's source of ignition. I like simple.

Here's an exploded view of a newer version:

His cannon has a release valve that operates the same as yours and is pictured below. The picture that shows it was apparently removed but I dug it back up. The blowtorch piece you’re using pre-mixes air exactly like his; except yours is more advanced. The only thing his setup has that yours does not is the open/close valve. It really serves no purpose because he is not using a metering system. Besides that your cannons work identically. Oh wait, yours has it's ignition built into the starter.

Cheers!

8) [/img]

It's quite a good concept, but there's no way it's venting all the bad air. Still not bad, though I think by purging the chamber after each shot and building a propane meter you could greatly improve it's fps.

Yea, he definitely would inonickname. Another issue is his short barrel. He obviously has a C:B of about 2:1 which means he could definitely squeeze some more velocity out of a longer barrel.

I'm able to achieve any where from 175fps up to about 250fps.

If you had a metering system and a longer barrel you should be around 300fps.

Hope that helps! Cheers! 8)

C:b looks more like 3 or even 4:1 to me but I really see that as irrelevant to his reason for posting here...his fueling/ignition system.

I would put this system a step above a spray and pray for sure....sort of a brute force way of charging your chamber quasi-properly every time. The downside is it doesn't offer spark gap/multiple gaps placement in the chamber and you are stuck with the fuel/air ratio the nozzle manufacturer gives you.

However, for a consistent shooting tater gun, not bad at all! But, I wouldn't use it on a high performance model. Your fps figures are at least 1/2 what is possible with an optimized barrel/chamber the size of this one and using advanced fueling and ignition methods.

I did some closed chamber pressure testing with this setup using car cylinder pressure tester and measured a pique pressure of 32.5 psi with 10 fires. I had access to a chrono only once and measured about 155fps with the above setup. I knew the barrel was too short, but it was more a proof of concept with material I had on hand.

So the above range of fps numbers I gave were calculated using gas laws qualified by my initial test data with a fudge factor for losses. I made the bold assumption that the fudge factor would be about the same independent of chamber or barrel size. So my fps numbers could easily be flawed. I do know I was able to shoot a 2" solid ball of maple wood through a piece of .75" plywood with one of my later designs.

What sort of chamber pressures are achievable with a perfect propane/air ratio and a multi-spark system?

JohnnyBOOM wrote:What sort of chamber pressures are achievable with a perfect propane/air ratio and a multi-spark system?

Theoritical peaks well over 100 psi are possible with a 1x combustion...a little less than 100 in practice. According to HGDT, my Thunder noise cannons gather a chamber and barrel pressure in the 105 psi range when using a 65 psi burst disk. A golfball will shoot 650 fps with a 60" barrel in this configuration...actually a little scary to shoot....

According to my HGDT. My cannon has a chamber pressure of 95.6 lbs and my barrel has a pressure of 104.3 lbs when using a 65 psi burst disk. The golfball will shoot at 762 fps with muzzle energy of 903 lbs.