Note: Wrote this post over some time, so it might be a tad out of date.
theepicmool wrote:The reason for the large chamber is because of spudtechs recommended 4:1 ratio.
Completely unnecessary excess. Anything over 2:1 is highly wasteful.
Spudtech's information pages haven't been updated in years, and you won't find an active veteran on this forum who would tell you 4:1 was worth it over 2:1. The 4:1 school of thought is long since dead.
keep in mind my valve will be modded
That doesn't change anything, because it's not the opening time that's the matter, it's the choked flow of the valve.. A 1" Sprinkler valve has a Cv (flow coefficent) of about 6.
Compare this to what your 2" barrel is capable of... which is of the order of 100.
This flow choke of 6% Valve:Barrel flow means that any chamber size increases over about 2:1 can't come into play, meaning you're adding all that extra air for no muzzle energy gain. Essentially, you halve your efficiency for absolutely no reason.
For an example, when I made
HEAL - a decent enough example of a flow efficent cannon, which has a barrel of just 0.8" diameter, I used a valve with a flow of ~11.2, compared to the barrel's flow of ~16 - and even with that huge 70% V:B flow ratio, I still only used 1.5:1 C:B, because more would have given me almost nothing in the way of an increase.
(Also, it would have made the cannon look funny to give it more chamber)
If i got 10ft of 3" i would be cut back to a 1.5:1 ratio.
4ft of 3" pipe would give a 1.5:1 ratio, which is actually about where the power gains from increasing the chamber size become increasingly insignificant.
Doing a quick model of your cannon in GGDT, assuming a 100 psi pressure and a 150g projectile, a 4ft 3" chamber will generate about... 237 ft-lbs of muzzle energy.
A 6ft 4" chamber gives you 247 ft-lbs of muzzle energy.
That's a pitiful 4% increase in muzzle energy, which will be utterly unnoticeable except with a shooting chronograph (and even then, spotting it within natural variation would be hard), for a 166% increase in chamber size and fill time - and a bulkier, less practical cannon.
That my friend, is what we call in the business, an utterly pointless exercise.
It also points out the critical factor of valve flow. You're generating an efficiency of about 8% there.
To skip back to HEAL - although it uses 300 psi
(but due to it's small chamber, uses a mere third of the air relative to what a 100 psi 4' long 3" chamber would), it generates 300 ft-lbs in a cannon a fraction of the size you're talking about, and efficiencies of about 40%. (Not
quite worthy of the High Efficency Air Launcher acronym, but not bad by any means)
It's worth nothing that if you can find a larger valve, you'll get a much more powerful cannon. With the 3" Dia 4' long chamber, with a 2" sprinkler valve instead (and thus 4 times the air flow), you get 700 ft-lbs, almost three times the muzzle energy from no changes but a large valve.