SpudFiles

hey guys i decided to build a cannon to mess around with while in maine for the week launching crap across the lake... it was a quick and free build i used a 3 in chamber and 1.5 barrel,, barrel lenght 4ft and chamber lenght 18, i used propane as fuel ran a hose to the tip of the propane into a barbed ball valve to a 90 elbow...(i didnt want to drive to lowes for another ball valve and psi gauge and what ever else to make a exact metered cannon like i said just a quick build) this thing fires balled up duct tape pretty far but the 3 times i shot potatoes it seems to have no power..i am going to try to add a fan and see it that helps but im just looking for tips should i have used a 4in chamber
or just use more or less fuel i generally use 10 sec of

Welcome to spudfiles

Too rich or lean a mix of fuel will certainly affect power, you'd better sort that out before thinking of adding a fan.

"10 seconds" is certainly not a reliable measure of fuel metering, a simple way to fuel precisely without entering the complexities of meter pipes and gauges is to simply use a large syringe to inject fuel directly into the chamber.

At 18 inches long, your 3" chamber has a volume of just over 2 litres, so a 4% concentration of propane can be achieved by injecting around 80mL, that's two squirts from a 50mL syringe.

Another point, make sure you're venting the chamber between every shot to make sure there's fresh air when you add the fuel.

sounds like your using to much fuel, if you calculate the volume of the chamber then use a syringe to mesure (i believe) 4% of that in propane, just short of that try slowly decreeing the amont you put in there until the power is at optimal, also good ventilation and a chamber fan would help

edit: damn i was beaten to it, if you pay more attention to the first post youll probly have better luck anyways

ya ur prob right ...and i do vent after each shot ... also i have the hose connected to the tip of the propane nozzle but i did not cover the holes that mixes it with air in the nozzle should i slide the hose all the way on???

When you remove the end cap to vent the chamber, quickly insert a lit match. If the junction of fresh air and the spent chamber gas can be lit, it is too rich. If the flame does not go out when inserted into the spent gas, it indicats left over oxygen. Ideally, the flame should go out as the flame transitions from fresh air to the spent combustion gas in the chamber. It should neither be flamable or support a flame by providing oxygen.

ok thanks i will try that... but what about the hose position question on the nozzle cover the vent hole or not

Cover those. They will leak out your propane. Thats why you must wait 10 seconds to achieve any kind of a mix. The majority of the propane you think is going into your cannon is exiting the system through those holes.

Welsome to Spudfiles!

I do like how you did the grips.

With the holes open, air is added with fuel. This would be for a chamber that is using the fresh charge to vent or displace the old spent gas.

Normally the holes would be covered so the chamber is vented with fresh air, then some fuel is added without adding extra air.

If you try to add air and fuel by leaving the holes open, you don't use a hose at all. The torch extends directly into the chamber as the back pressure on torch from the hose lowers the flow and causes a rich fueling condition.

The Contest winner with the Tennis ball launcher uses a torch in this manner. No hose on the torch. Torch extends into the chamber.

Here is his torch extending into the chamber.

i covered the holes and put fuel in...i keep hitting the igniter and eventually it fires and wow!!! now i think i need to make the spark bigger and just get the right mixture

critter wrote:get the right mixture

As previously advised, a simple syringe will be enough to give you the perfect mix very easily.

Your C:B ratio is 1.36:1
A larger ratio such as this will produce a loud report, but a larger report doesn't equate to a more powerful cannon. Your chamber volume is actually large enough to support a 7-1/2' long barrel of 1-1/2" pipe.

Spray-n-pray is not the ideal method for fueling a launcher, even if you're using propane. Without a well-designed meter-pipe (or, as suggested by others - a more accurate way of measuring the fuel), you'll always get inconsistent results.

If I do a psi gauge and pipe what psi would I use

critter wrote:If I do a psi gauge and pipe what psi would I use

2psi in 11-3/4" of 2" SCH40 PVC pipe.