SpudFiles

Hi, I have built a few combustion Spud Guns in the past (all hairspray) and wanted to experience the power of Propane. I have been reading up, a lot, on how to build one and how the metering system works, and I had some time so i drew up a sort of schematic and wanted to run the numbers/schematic through you experts to see if there was anything I could improve, or if there was some major/minor flaw in the design.

Dimensions:
Chamber: 2"x12"
Barrel: 1.5"x25"
---This achieves a .8:1 C:B ratio which I have read is best for
power/diminished sound
Propane meter: .5"x6.5" at 19 psi to achieve the correct fuel amount.

Numbers:
Chamber Volume: 37.691 ci
Volume of propane at 4.03% : 1.58336 ci
Volume of Propane Meter pipe: 1.275 ci

Components:
2" x 12" chamber We get a .8:1 C:B ratio
1.5" x 25" barrel
Propane metered system direct feed
Chamber fan (Timed run not a priority but helpful,*Just use a push
button)
Stun gun ignition (1-4 spark gaps, or spark strip)
Interchangeable barrels

*I plan to make this system as a Remote Ignition but as of yet lack the required knowledge, still need to tinker with old rc cars

Ignition:
StunGun ignition: http://www.safetygearhq.com/stunmaster.htm , 100kV for $19.99, model: SM-100s

A few random comments:

A .8:1 C:B ratio is considered the most efficient, but not necessarily the best or most powerful. A larger chamber won't decrease performance (well, if you go WAY overboard it can....), in fact it will usually increase it, but it also increases noise, muzzle flash, and "wasted power". After awhile, you hit the point of diminishing returns, of course. My point is, you might want to go with a larger diameter chamber; larger pipe tends to be easier to fit a fan into, drill/tap for spark gaps/fuel injection, etc... Also it gives you a higher "compression ratio" to the barrel size, though this is essentially caused by added chamber volume.

I highly recommend using a spark strip. I made one out of some copper clad circuit board with 5 gaps in it for my metered propane gun. This also allowed me to put all of the "holes" in the chamber where a fitting and pipe overlap, and none on a single wall of pipe - in my opinion, this should greatly increase structural stability (no weak points in a pipe wall...).

A stun gun is not really necessary for a combustion gun, though it's the "holy grail" of ignition sources. In fact, they're kind of a pain in the ass to work with, because you have to use thick-gauge, insulated wire for everything, keep them from touching at all, etc.... Personally, for a normal combustion gun like this, I'd look for a battery-powered, repeating BBQ sparker instead. Easier to work with, and just as convenient. Just my opinion, though.

Consider making it a breech-loader. This really speeds up shot-to-shot time by eliminating the need to ram a potato down the barrel with a rod between each shot. You can use cam-lock couplings or even threaded adapters to create a breech loader.

Also consider adding a ball valve to vent out the chamber between shots, so you don't have to remove the cap each time (Even with metered propane, you do need to vent out the chamber between shots. The fan helps with this, but you still need to open the chamber somehow. Most "advanced combustions" have a 1.5" - 2" ball valve for venting).

An over/under configuration allows for a more compact launcher, and gives more propane tank mounting options, but sacrifices a (very small) amount of performance to the U bend (compensate by adding a bit of chamber volume...?). Just something to consider.

All in all, great design. My advise is, look around at some of the really nice combustion guns in our Showcase section and see what you like. Keep convenience, ease of use, and portability in mind - these are the biggest strength of metered propane guns anyway.

Peace,
Pete Zaria.

A stun gun is not really necessary for a combustion gun, though it's the "holy grail" of ignition sources. In fact, they're kind of a pain in the ass to work with, because you have to use thick-gauge, insulated wire for everything, keep them from touching at all, etc.... Personally, for a normal combustion gun like this, I'd look for a battery-powered, repeating BBQ sparker instead. Easier to work with, and just as convenient. Just my opinion, though.

Thanks for the detailed reply!

About the igniter, where would i get battery-powered, repeating BBQ?
I have looked at my local hardware stores and home Depot / Do it center
and none of them have it.

I was trying to go for a smaller gun, one that i could do at a large park (when no one is around of course). Which is why i was sticking with the 2" chamber but i guess if i go with an over/under design i can keep it relatively small with added power/ease of construction.

And how much do cams usually run for, and have any of you seen them around Home Depot or Lowes, or would i have to prob order it offline?

I agree with Pete's suggestion of increasing the diameter of the chamber to 3" instead of 2". The chamber will burn quicker and will be a lot shorter for the same volume (2*2/3/3=0.44 as long in fact).

A 1.5" barrel, instead of the more traditional 2", is a good idea since properly sized spuds are cheaper and a lot easier to find.

The CB ratio is really not all that critical, anything in the range of about 0.5 to 1.0 will behave about the same. (Take a look at Latke's results here, more details are here.)

I wouldn't use a spark strip. It is easier to get multiple sparks on a strip but the strip may interfere with (slow down) combustion. If you do use multiple sparks then the optimal CB is really unknown. I don't recall anyone ever posting a gun with multiple sparks that out performed the same gun with a single spark. If you do use a spark strip the spark gaps should be well separated, roughly the chamber diameter apart. Personally, I think two or three screw gaps are just as easy to build. If you use deck screws you get very sharp points and the small diameter of the screws will have basically zero affect on the strength of the chamber even if they are through single thickness wall.

A stun gun is really more hassle than it is worth. For about the same $$$ you can get a 9V battery operated BBQ igniter. Much safer and probably much more reliable than a stun gun. There are a fair number of reports of stun guns crapping out after relatively little use.

Double bevel the barrel, breech or muzzle depending on which end you'll be loading from.

I would also suggest boosting the size of the gun. My WAG for the muzzle velocity for a 36ci chamber is ~200 fps. Doubling the size of the chamber, and the barrel, will get you up to ~300 fps, an increase in kinetic energy of ~2x.

A timer for the chamber fan is really not necessary. A toggle or slide switch works fine. There is really no reason the fan can't be running pretty much all the time.

You can get all the performance benefits of metered propane for a lot less $$$ if you use a syringe for fueling. Not nearly the bling factor but very effective. If you like the performance of the gun you can always add a more complex meter system later. Standard meters require two valves, a regulator, pipe, fittings (including the sometime hard to find propane tank fitting), plus the propane tank. A syringe just needs a Benzomatic torch, a 2$ syringe and an inch or two of rubber hose. Jacks' image says it all (though here he used butane instead of propane);