interior fuel pipe

[Pete Zaria]

so i would need to flush the chamber with enough premix. so now my question would be what is enough premix? would it be 1.5X,2X. anything larger than 2X in what i was going to use as a meter will put my meter pressure too high. i will have to build a larger meter.


i have used the torch heads before, but results were less than favorable. living at 9,275 ft. above sealevel might of had something to do with it.

You need to flush your chamber - so you need to inject at least your chamber's volume, plus a little extra to be safe. I'd say your chambers volume + 15% would be a good margin.

This method would work, but as Jimmy pointed out, it will waste fuel and it's inefficient. I say go with the teeny fan idea.

Peace,
Pete Zaria.

[jimmy101]

If I had to WAG an estimate I would say you need at least 5x the chamber volume in pre-mix. 10X would be better.

A teeny fan starts to sound better and better doesn't it?

[bluerussetboy]

so it basically comes down to trial and error. i went out and played around with my setup.

my setup:

launcher
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

ammo
potatoes: barrel dia. X 1.5"

fuel meter
all parts are 1/4" NPT
male air QD
12" pipe
shutoff valve
female air QD

filling station
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.

firing range
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

misc
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


the tests

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.

[jimmy101]

Dang, that was a lot of testing.

What was the value of FD? (I think I know what it means but I don't see a value for it anywhere in the post.)

Looks like the fan was working pretty well.

The No-fan shots not only have a wide variance but it looks like the performance degrades pretty significantly after more and more shots.

Your results are pretty good evidence that a fan is a huge help in getting reproducible shots. More reproducible shots also means the gun is more powerful.

In the "pre-mix" shots you pumped a large volume of pre-mix through the chamber before loading the ammo? Or is there a vent in the chamber that could be opened (or the breech plug removed)?

[bluerussetboy]

the value of FD would be the average distance of the first ten(nine actually, the stray was thrown out) shots when using the fan. i didn't give you the actual distance because i would really like to be able to duplicate it in different weather conditions. yesterday was a beautiful day for playing with spudguns. but it is getting to be wintertime and our colorado weather patterns go haywire this time of year, so in a couple of days i could have totally different results.

FD is what i'm shooting for without the fan.

the premix shots are:
1. load potato and close breech
2. inject premix
3. fire
4. repeat

i tried to keep the purging of spent fuel to a minimum. it really showed how fast the performance suffered not having fresh air. even though i had the right amount of air and fuel by volume the chamber still needed fresh air.

the next time we go out we're going to try a few other things.
1. 2X premix shots w/ venting the chamber. (i'm not really wanting to have a hybrid)
2. using a meter amount of air to purge the chamber and then adding a 1X premix


i was also surprised at how consistent the spuds flew when using the same exact charge over and over. i'll probably convert my large spudgun to this metering system.

[jimmy101]

bluerussetboy, you should post the value of FD. I realize that the actual value of FD is very dependent on shooting conditions, mass of the spud, temp. and probably a bunch of other things but without knowing what FD is "17' <FD" has no meaning. (17' of 3000' feet is insignificant, 17' of 50' is very significant).

For your next tests you might want to consider weighing the spuds. I suspect that a fair amount of the shot-to-shot variability can be attributed to changes in the mass of the spuds. Even your most consitent fueling procedures will show less shot-to-shot variability if you can correct for the different masses of the spud.

One way to do it is to weigh the spud before loading, then collect the shavings (you do have a muzzle knife right?). Weigh the shavings and subtract from the spud's original weight to get the net weight of the round.