Thanks for the response Latke. Your site has been very helpful as well and is very nicely done! Unfortunately, I didnt find it until I was making my 2nd gun (Spudzuka), but the 1/8 NPT pipe die on for the propane tube helped there. For this 1st gun, I sweated on a fitting, which I have a minor leak in. Plans to redo that.
The performance is great! Man, does that potato move! I havent measured distance yet, but Im getting hang times of 10-11 seconds when shot straight up. The main thing Ive learned is to ventilate the chamber between shots. Moving my flapper/ramrod in the barrel 4-5 times
with the purge valve open takes care of that.
As far as specifics on this gun I have a 2 sch 80 barrel with a volume of 141.7 cuin. The combustion chamber is 4 sch 80. 1.5:1 would have put this at a volume of 213 cuin for a length of 18.5. I think I shortened up a little slightly cause I ignored the elbow in the barrel volume. (I think I ended up at 15 but its out in the garage and Im here
). So anyway, the metering chamber is 3/8 pipe x 9.75 from valve-to-valve, so I have a volume there of 1.9144 cuin. The 15 chamber works out to 172 cuin of volume (probably a little more if I add the elbows in). so saying I need 4% of that as propane and solving for the regulator pressure (p1 * 1.9144 = 14.7 * 6.9) gives me around 53 psi. I actually have the regulator set at 60 psi right now. I dunno if all this is right, but it shoots great.
As far as the max. pressure gauge about all Ive seen is 25-28 psi. I do not think though this conventional gauge is quite capable of reading the instantaneous pressure spike in the chamber, so I am suspicious of its accuracy. Ill be writing another post on how to accurately establish a chamber pressure on a combustion gun. Im interested on your thoughts there.
BTW, another thing I did for the spud cutter on this is I found some thinwall aluminum tube in McMaster Carr with an ID of 1.91. A 6 length of that make a great spud cutter with the walls being so thin.