Fully Automatic potato cannon project.

[Conrad2468]

My buddy and i are attempting this, ive been strongly urged to post this idea on the forums (not by my buddy). We plan to use propane and solenoid valves (which i just ordered) for metering. My buddy who happens to be an excellant mechanical engineer (for a 14 year old) designed (and built) an auto golf ball dispenser which will run off of some siphoned pressure. Everything is PVC our golf ball dispenser is capped so its air tight (mostly). We plan to fire this until its destroyed and make MKII out of steel with all of our design flaws fixed. If anyone has any advice so we dont kill ourselves let us know. P.S. well be firing from behind cover.

[mobile chernobyl]

sounds like you guys have it under control!

Post up some pictures of your design, or rough sketches if you want some help critiquing your design for safety hazards.

[jackssmirkingrevenge]

I only remember seeing one succesful auto combustion, this remarkable gatling design, so you have your work cut out but it can be done.

There are several feed systems you can use for ammunition, such as the blow-forward bolt, indeed if you're using hard spherical projectiles youmight not even need a bolt..

The critical issue for combusion is metering the correct amount of propane into the chamber, which given two alternating solenoids on both ends of a meter pipe should not be hard to achieve, as well as venting the chamber after each shot, so you might want to consider injecting a burst of compressed air as well, with a outlet valve on the chamber that opens in sync with the air inlet valve.

The sequence could be as follows:

burst of air -> propane injection -> ignition -> burst of air etc.

[chenslee]

Full auto combustion is the holy grail. I have a graveyard of 3D models that have failed muster.

With the right components it's very do-able, but the support equipment is going to make it unportable.

Check this out: http://www.propanehammer.com/products.htm

[Conrad2468]

I can let you all know that this auto loading system works.....it doesnt use a bolt. Does anyone know how i can get fresh air into the chamber??? I want to use a fan but i think it will melt, i cant use compressed air because it isnt that portable. any other suggestions? maybe something to the effect of a "pulse jet" that sucks the air in? "http://en.wikipedia.org/wiki/Pulse_jet_engine" how maybe a 1 way valve? Just throwin these out there.

[Technician1002]

Pulse jets require a huge pressure wave as well as a very fast reed valve to work. Simply adding a projectile takes energy away that is needed to draw in a full new charge.

Low pressure air metered with metered Propane in the proper proportion would work but a relatively high pressure blower would be needed for fast cycling.

Most cannons have too much back pressure for a simple fuel driven venturi to properly vent the chamber at any reasonable speed. See the contest winning tennis ball launcher for an example. A torch head is used, so the fuel and replacement air delivery rate is somewhat limited. Back pressure on a more aggressive volume will cause difficulty in keeping the mix correct.

What cycle rate are you looking for and how much restriction is down stream of the chamber during vent/fill cycle?

[Conrad2468]

its a 2 inch barrel that holds a golf ball...theres about 1/8 inch space in the barrel. I need to "tap" into the chamber to siphon some pressure off. This pressure will operate our autoloader (we shouldnt need more than probably 20-30 lbs in a 1/2 copper line this is however, a guess). How much pressure is generated when properly mixed? Im not looking for a high fire rate, i figure it should be about a second between shots. im going to use 2 solenoids that are programed via a BASIC stamp for metering. Im trying to avoid compressed air at all costs due to its low usability. How would i hook a blower up to my cannon?

[inonickname]

Take some design cues from a two stroke motor. Very tried and proven. Hook a barrel up to the exhaust as a proof of concept. However, it will need a re design for better power. Figure out how far the piston has to travel to complete the cycle and move the exhaust ports accordingly.

Forget the pulse jet idea. As soon as that projectile is blocking the exhaust port, the engine will stop.

Also have a look at Chenslee's tennis ball cannons. He used a venturi to meter, mix and deliver the fuel air mix.

[Conrad2468]

Ill look at that...what if i added an air pump from an aquarium? id have to fire pretty fast or it would lean my mixture...just for an extra challenge...i really dont want to use an engine. hahahah links to the cannon anyone?

[spudtyrrant]

hahahah links to the cannon anyone?

here it is, it's on the home page you know

[Conrad2468]

Ah thanks :/ im new can ya tell i looked at the siphon design for the fuel air....I cant find any way to integrate that into the system without adding a servo...which, i guess i may be able to do. in MKII which i will consider seriously.....any word on an aquarium air pump for an air source?

[Conrad2468]

Did a test today....didnt work at all!!! so we used the sort of concept that was shown in the pump action tennis ball launcher...it kind of worked. So we added pure O2 from my dads oxy acetylene cutter. and it only rolled the ball out of the barrel. has anyone had any success from a 2" barrel firing golf balls?

[Technician1002]

Hint on the aquarium pump.. Invert a 2 liter pop bottle in a tank of water with the bottle full. Use the air from the aquarium pump to fill the inverted bottle with air.

Unless you want a really slow reload time, you may wish to consider higher volume.

[jackssmirkingrevenge]

Careful playing with pure O₂, compressed air is a safer bet.

[Conrad2468]

We'll be careful! We need a way to "mix" the gasses since our spark gap is in the back of the chamber. A fan wont work. Would adding the propane and oxygen at the same time, and then firing right after i closed the solenoids work?