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I have build one potato cannon before, but I have a few questions before I begin building a second. The first cannon I build was combustion, had a one inch bore, piezoelectric ignition, and shot pretty well. On this new project it will also be combustion, but I would like it to be more of a "potato rifle" than a cannon.
I am designing it to be bolt action, have a .5" bore, a 2" diameter combustion chamber, and inject fuel (propane) with the assistance of a solenoid valve. I plan for the barrel to be 36" long and the chamber 3.4-4.5 inches long depending on space restrictions, giving me a C:B ratio between 1.5/1 and 2/1.
What kind of pressures will I be dealing with?
What would be a safe material? sch 80 brass?
Will having a ninety or 180 degree bend between the chamber and the barrel
effect velocity and or pressure?
I assume I would need to inject air as well as propane into the chamber, correct?
What material would be needed to use say oxygen and propane?
Any suggestions for a smaller vessel to store the propane in other than a standard blowtorch cylinder?
Typically less than 100 psi for a combustion cannon, meaning you're not going to get much power moving to a half inch bore to be honest. At this size, I would either go hybrid or pneumatic, assuming muzzle energy is one of your requirements.
Sch 40 PVC is more than enough.
Probably not enough to notice without a chronograph.
There's a difference between air and oxygen, injecting oxygen will allow you to burn up to 4 times more fuel giving a lot more power, but is also more dangerous to handle and can wear your materials faster.
Anthing that can safely take 200 or so psi should be fine.
Not quite true. A lot more dangerous is an understatement. Air is about 20% Oxygen. When fuel and air are burned, the remaining 80 percent of the air is heated in the process so the rate of burn is limited as well as the temperature rise.
Not only does a pure Oxygen fuel mix burn faster, hotter, etc, the mixture becomes explosive resulting in a detonation. Flammable materials become explosive and many items that won't burn in air become flammable. Some fuels can auto ignite or explode when pure Oxygen is added.
In summary, Don't mess with adding pure Oxygen. The reaction does not simply scale up like a 4X hybrid.
Here is an example simply lighting a piece of cloth.
Here is an example of a type of fire that you can't even do with just air.
A note on safe Propane handling. Overfilling a container is easy with a home made container. This can result in liquid expansion when heated with no room for expansion resulting in extreme pressure much the same way freezing water in a sealed full container will do resulting in breaking the container. Having a home made propane container burst when the sun hits it or you ship it in your car on a warm day is a safety concern.
If you build a small container for propane, be sure you have a proper way to monitor the fill level inside so the top 20% remains empty for expansion and it has a proper pressure relief valve that opens at slightly more than 300 PSI.
Commercial BBQ tanks are required to have both an over fill protection device (OPD valve) and a certified pressure relief valve.
Disposable small camp stove and lantern bottles are filled by weight by the manufacture and contain the required pressure relief valve.
Thanks for all the info! I was trying to avoid pnumatic, because I don't want to have to lug around extra equipment. I also don't own a compressor.
Hybrid is a possibility... I would assume I could build a combustion gun and later adapt it to be hybrid, correct?
What velocities could I expect from a standard combustion? Power is a concern but not the largest one. Is another, less volitile, oxidizer a possibility? No2 perhaps?
Edit: I did electrolysis once, and ignited a balloon about 2 inches round of the oxygen hydrogen mix. Scary stuff, yeah oxygen probably isn't the best idea.
A hybrid can be fired as an ordinary combustion, but it needs to have a means of containing pressure. Are you looking for a single shot type launcher or a semi-auto/rapid fire repeater?
I think you need to have a good look at RamboNucke's channel he gets a lot of power with some interesting fuels and extremely basic material techniques.
I was looking for something that could be reloaded very quickly, within a few seconds. This why I was leaning towards combustion. What fps could I expect from a simple combustion? Could an oxidizer not be used to increase the power? What material would be proper for a hybrid? I have read sch40 steel.
http://www.spudfiles.com/forums/combust ... 10402.html
http://www.spudfiles.com/forums/cartrid ... 20150.html
Generally, not a lot.
Instead of guessing, you could always download HGDT and do some virtual modelling
I've used PVC
It depends what mix you want to use really.
I keep going back to that self-contained cartridge idea. It'd be so nice to not have a "conventional" attached chamber to deal with, but I am having trouble justifying the horrible lack of power without using cartridges that were about the size of a typical chamber to begin with without getting all hybrid-ish and eliminating the simplicity I wanted. It would satisfy my desire for quick rate-of-fire with a larger cannon.
JSR, when you did your testing, I noticed you never used a burst disk. Or did you, and I missed it? Perhaps that'd help some. Then again, implementing that may destroy the relative simplicity of the cartridge designs you made.
I've tried to avoid using burst disks because of the appeal of using the projectile itself as a seal in terms of simplicity. The idea with cartridges is that you would need to make them in quantity, and threading is a chore.
I defininely need to revisit the "swaged projectile seal" idea where a lead projectile is hammered into place into a smaller diameter port which makes for a leakproof and pressure resistant seal.
Cartridges seem more complex than I was aiming for, and may well be above my skill level... I think I may start with a simple combustion, and then work my way up to a hybrid if need be. Also I have tried to download HGDT but failed to get it to run. It kept saying missing file. Thanks for all the input!
You probably need to install a Visual C++ runtime pack. Been a long time since I installed it - anyone remember?
The cartridge isn't really that complex; it's just that it's not overly powerful for our uses.
Try using "Run as administrator".
yea running hgdt as admin worked on my win7
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