SpudFiles

Metered combustion (advanced combustion) with a fan is much easier to build and get working than a piston air cannon. I like engineering challenges so I opted to build air cannons to try valve ideas instead of a combustion.

I eventually plan on building a combustion, but with my background, I will be more than a basic combustion. Initial plans are for a full automatic.

i really appreciate all of the help, fellas.

here is a good idea for you, simply make a overly huge chamber.... say a 5' stick of 3'' PRESSURE RATED pvc..... and use a crazy long (full stick [10']) barrel.... this will help to reduce the loss caused by a slow valve....

Something if this size may even reach the 400 yard range with a simple spring operated ball valve.

jhalek90 wrote:this will help to reduce the loss caused by a slow valve

Long barrel, yes. But as I have discussed before, only good valves can make use of a high C:B ratio.

It's simple logic - a large chamber is there so that the chamber pressure falls less during the shot. With a poor valve, so little air flows through the valve that the chamber pressure doesn't drop much anyway, negating the purpose of a large C:B ratio.
In short, the worse the valve, the less point there is in a big chamber.

Combustions are a bit better at using larger C:B ratios, as their higher temperatures and (usually) less restricted flow between chamber and barrel means they perform more like they have a "perfect" valve, although they will still approach an asymptotic limit simply because of the increasingly insignificant pressure drop in the chamber.

hah its looking like maybe i need to start small. how far do you think i can get a gb to travel with like a 4' barrel and a chamber maybe 2.5' long, 3" diameter, using a modded 1 or 1.5" sprinkler valve?

With those specs, a 1" sprinkler valve, and a 100 psi pressure, I'd say about 200-250 yards. That'll only give about the same velocity as you could expect from a reasonable golf drive, so the range is going to be similar.

A 1.5" valve and 120 psi might give you a bit over 300 yards.

But 400 yards is going to take something with more guts behind it.

so help me learn a little here. with the numbers i just said up there ^^, how could i increase distance? would lengthening the barrel add distance or making the chamber bigger? i'm not really understanding how the size of the chamber helps (i know this probably sounds dumb to you all). i'm just saying that a chamber with a volume of 2liters (hypothetical and random) at 100psi, versus a chamber volume of 4liters at 100psi. when it comes down to it, the air is going to be released down a barrel, pushing a golf ball out. so why does it matter how much air is used? as long as the pressure (100psi) is the same and there is enough air to get it out of the end of the barrel, shouldn't it go the same distance? this probably sounds so dumb, but i just can't get past it in my mind.

wolfie22 wrote:would lengthening the barrel add distance or making the chamber bigger?

Lengthening the barrel would help a little, but not by a huge amount. Making the chamber bigger would be pretty pointless - I explain why below.

i'm not really understanding how the size of the chamber helps

As you let air out of the chamber, the pressure falls. But with a bigger chamber, it falls less. As the muzzle velocity is defined by the forces on the projectile, a system where the pressure falls less during firing (and thus has higher force on the projectile when it's further down the barrel) will result in a higher muzzle velocity.

However, the caveat is that you can only benefit from this if you have a valve good enough to let all that air through. Sprinkler valves are not up to handling really large chambers effectively.
In this case, I wouldn't recommend a chamber larger than about 1:1. You'd still get about the same velocity and range if the chamber were half that length.

In this case, your really limiting factor is the valve - it just can't let enough air through. Even modded, sprinkler valves are pretty poor.

so its either piston valve or combustion. hmmm.

how can i determine the maximum sizes of my design? i wish there was a simulator out there where i could build my cannon and it would tell me all of my numbers.

wolfie22 wrote:how can i determine the maximum sizes of my design? i wish there was a simulator out there where i could build my cannon and it would tell me all of my numbers.

Umm...

pneumatic simulator

combustion simulator

Umm...

pneumatic simulator

combustion simulator

yikes. can you tell i'm new?

(thanks)

Ya know what wolfie, just go to the hardware store with $40 and buy some stuff for a spray n pray.

Since this is your first build, it's better to just get out and build something than wade through days of planning. A 3"x1 foot chamber and 5 foot barrel will work just fine.
Bring a golf ball with you so you can check for the correct pipe. Know how to identify pressure ratings and learn to glue properly.

(Trust me, if you haven't build a full size combustion before you'll think it's the best thing ever for at least a couple days, then you'll want to build a bigger one. The same thing happens (again) to most of us when we build our first hybrid.)