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Some gun info....

October 28 2002 at 5:21 PM
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D Tom  (Login DTom)

Response to Swank gun, cool designs....

Thanks for the response Latke. Your site has been very helpful as well and is very nicely done! Unfortunately, I didn’t 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 haven’t measured distance yet, but I’m getting hang times of 10-11 seconds when shot straight up. The main thing I’ve 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 it’s out in the garage and I’m 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 I’ve 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 it’s accuracy. I’ll be writing another post on how to accurately establish a chamber pressure on a combustion gun. I’m 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.

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  1. re:some gun info... - _latke on Oct 29, 2002, 12:41 AM
  3. i need help! - kaleb on Nov 6, 2002, 9:28 PM
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