SpudFiles

Well im a new member, and i just built my good combustion spud gun.
Haven't fired any potatos yet but i will soon.Its very basic. Fires 1 1/2 nurf balls good and a 2'' rubber ball pretty good so we will see what it can do with potatos.

Specs
All sch 40 pvc piping
Dark gray texured and green krylon paint job
18x3 Chamber
35x1 1/2 Barrel
35x2 Barrel
Pulse ingnition from bbq, 5 or 6 sparks a second, works good

Here are some pics



1 1/2 Barrel


2" Barrel




Starter


Well i did some upgrades to the cannon, i took some advice and bulight a longer barrel with a handel, i came across some awsome couplers that i couldn't pass up. And i added a handel to the chamber and a 12v chamber fan with 8 aa battries so it runs at full speed,and a little toggle switch.

Thats a nice looking cannon! Very simple and clean; should be fun and easy to use. That is an interesting ignition; I don't know a lot about combustions, so I don't know whether it is good or bad; but if it works, I suppose its good . I only have two concerns:

1) Hose clamps and paint jobs are not very friendly with each other; you might want to consider wrapping a strip of black electrical tape around the chamber, and then placing the hose clamp on top of this strip. Not only does it prevent the hose clamp from scratching the paint, it also holds it in place.

2) Your barrels are a bit short. For a high efficiency launcher that uses propane, a chamber to barrel ratio of 0.8:1 is desired. With this in mind, you would want to have a 2"x50" barrel and a 1.5"x90" barrel. However, I see now that this launcher probably uses an aerosol spray, for I don't see a metering system. With an aerosol spray, people have reported having the most power with a chamber to barrel ratio of 1.5:1, meaning you should have a 2"x27" barrel and a 1.5"x48" barrel to maximize the power in your launcher. considering that aerosol sprays can be kind of random, the barrel lengths you have now should be fine. But if you do upgrade to propane, keep the ratio in mind.

thanks 4 the input, i used some foam weather stripping on the inside of the hoseclamp so it wont scratch. I might try a longer barrel see what happens.

A longer barrel definatly would yeild a great increase in performance.
That is a nice simple combustion although you might want to consider so upgrades like a propane meter, chamber fan, duel ignition, barrels in the correct ratio the list goes on and on.
And were did you get that ignitor and how much did it cost?
Are the sparks you get from it large?

i want to keep it simple, how do i figure out correct barrel ratio?

NECROMANCER wrote:i want to keep it simple, how do i figure out correct barrel ratio?

It's just a volume thing, chamber volume/barrel volume. Since you've got a circular chamber and barrel, use formula "pi*radius*radius*length" for each, the chamber and barrel. Assuming you measured all dimensions in inches, this will give you a measurement in cubic inches. An example of a 1.5:1 c:b ratio would be a 150 cubic inch chamber and 100 cubic inch barrel. Obviously, the smaller the diameter of the barrel the longer it needs to be to maintain a particular ratio.

Be sure you use the inside radius, not the diameter of your pipe in this calculation. pi = 3.1415

If you load your chamber down with a fan and couplers/weird shaped connectors, etc., you should compensate for these as they do take up or add a certain amount of volume, although in practice you'll not notice a significant difference on large guns unless you go way overboard. The smaller the gun the more precise all this needs to be.

Well i did some upgrades to the cannon, i took some advice and bulight a longer barrel with a handel, i came across some awsome couplers that i couldn't pass up. And i added a handel to the chamber and a 12v chamber fan with 8 aa battries so it runs at full speed,and a little toggle switch.

Nice, gun, very clean and simple, and I like the ignitor as well. I was going to try one on my first combustion, but wasnt sure if it woulda worked. now I know they do!! Also where did you find those couplings??