so it basically comes down to trial and error. i went out and played around with my setup.
barrel: 60" X 1" sch40 PVC
chamber: 42.87 cu.in 2" sch40 PVC
C:B ratio: 0.85:1
ignition source: 2 spark gaps power by 80KV stungun circuit
barrel is also breechloaded
potatoes: barrel dia. X 1.5"
all parts are 1/4" NPT
male air QD
female air QD
5-gallon propane tank
5-gallon air tank @200psi
both tanks are regulated
both tanks connect to a homemade mixing valve. each has a separate s.o.v. both feed a single pressure gauge.
local ski are parking lot
elevation 9,275' above sea level
parking lot has a 1.5 degree slope
from the top of the parking lot we marked every 50' up to 600'.
60 degrees F
all shots were fired at 37 degrees. this was measured with a clinometer.
(why 37, because that's what our sandbags added up to.)
when filling the meters we first filled the gas w/meter s.o.v open to purge meter. we then closed the valve and filled with gas to the desired pressure. once gas pressure was met we added air.
the fuel tool from burnt-latke gave us for our premix:
propane: 15~ psi for a 0.042% by volume mix
atmospheric pressure for 9,275' is 10.5 psia
our first test was the launcher with the smallest fan we could find. the fan ran the entire time and the endcap was removed after each shot. chamber was purged for 15 seconds each time.
10 shots were fired at 1X premix.
9 out 10 landed within 5' of each other. The stray(4th shot) hooked right about 70' out of the barrel and went into some trees.
so we took the average and called it Fan distanceor FD.
test 2 was the launcher minus the fan. endcap was not removed and the chamber was never purged.
10 shots @ 1X premix.
first two shots were 2' <FD.
shots 3,4,5 lost 9',13', and 17' <FD.
shot 6 and 7 both 48'<FD.
8,9,10 these guys failed miserably, but i still wouldn't want to get hit by them.
we took the endcap off and purged the chamber then fired a control round to make sure of the distance. 3' <FD
test 3 was the launcher minus the fan but with the interior pipe. endcap was not removed and the chamber wasn't to be purged.
10 shots @ 1X premix.
first five shots were all 12' <FD
shot 6 was another hooker
shot 7 the breech was not closed completely
shot 8 failed to launch
we had to purge the chamber for shots 9 and 10.
we gave up for the day on this and just drank beer and played with the larger spud guns.
i've got a few more configurations to try before i give up on a fanless launcher. i really don't care about velocity or distance, i would rather have consistency.