need estimated FPS on my potato cannon

[rastetter]

I built my own pneumatic potato cannon with solenoid valve with 9v battery launch trigger thing

tank is about 20 inches long of 3" pipe
I fill usually to 100psi with a bike pump(although i have gone to 160, yeah i know, not smart[but good exercise])
barrel is 1.5 inches, and i use the cheapest potatos i can find, usually $3 for a bag of like 30 here in nyc (yeah i know shooting potatos near 2 major highways in queens nyc is once again, also not smart)

I would guess the potato goes 1000 feet if not further, while aiming for rooftop airconditioners 5 houses up my block, uphill, about 400 feet... i aim it staight, and it usually goes straight to the airconditioners, sometimes going above and sometimes below do to air resistance (didnt do any damage, dont worry) soo 400 feet line drive easily

shot straight up it is hard to do, and it often lands down my block and has a hang time of like 8-10 seconds (as i remember it)

put a 1.5inch hole in my friends shed (1/4-1/2 inch particle board, painted)

broke a glass sliding door (haha whoops)

Anyway, just some things i have done.. I guestimate that i am shooting 250-300fps at the second it leaves the barrel, but i base this on no knowlege at all... maybe someone with a similar cannon can guess what i am getting

I am doing a speech on my cannon in english class, and would like to state a legitimate speed, rather than my guestimit

BTW
pictures

http://photobucket.com/albums/v611/rastetter/Potato/

replaced barrel since then (25"? X 1.5" inch) and added my own little altoids trigger with 9v battery and safety switch (nice)

[spudshot]

if you're hitting windows, air conditioners and sheds, you're way too close to people to be firing that thing, whats worse is you're aiming for the air conditioners, i hope you're caught and your gun is taken away.

[rastetter]

windows were on an abandon house, air conditioners too... they knocked the house down a week later, and i knew they would. Actually only hit the air conditioners once, didnt do anything, they are industrial metal boxes. undershooting hits the brick house, overshooting goes over the park and into the woods

shed was in my friends backyard, i was aiming for a little chalkboard target, i figured if i missed it would hit the shed and nothing would happen, didnt expect it to go through the wood haha whoops, but we fixed that

edit: I usually just shoot it at the local park, which has empty woods behind it, or i shoot it in my backyard at a hard wooden table on its side

I just used those examples because they best expain its power

actually add to that, i shot a friends white ford van (gave me permission, and actually convinced me to do it, after my warnings, made a nice dent in that, but we kicked it back out from the inside

I never would cause harm to people or there possessions, and i take necessary precautions in doing so... I dunno if you consider shooting at an abandoned house property damage, or a serious offense, some do, some don't... technically yes, but too me, once in a while, you gotta have a little fun 8)

[rastetter]

any estimations?

[boilingleadbath]

Download the GGDT. (google search for 'GGDT pneumatic' turns up results quickly)
Enter values.
Enjoy multiple benifits: knowing how to use the GGDT, being able to tell the class about the GGDT (and you know, how it was made by a goverment researcher and all that jaz), and knowing how fast yoru gun shoots.

[rastetter]

wow, this program looks really useful... if i knew what i was doing

looking at it, some questions

under Reservoir Data

what is outer diam(eter)
and inner diam(eter)

i would assume it is the diameter of my tank, but i dont understand how outer diameter would affect volume of air, so it must be something else

For valve type
I have a 1" solenoid sprinkler valve, what do i put for Valve type?
Flow Coef?
Seat diam?

Piston Diam
Piston Mass
Vent Diam
Pilot Vol

Dead Vol?

then for Projectile Data
friction (what is typical)

all the rest i know, just those few (many) things

just a brief explanation of each would be nice, or at least just a general number by looking at my cannon

I think theres a link up there^

thanks again for mentioning this program to me, never heard of it before this

[CS]

Yea, I feel you on this one. Ive tryed GGDT several times, but since my "guessitmation" skills suck, and its somewhat complexty, it has discouraged me from spending more time with it.

I think everyone has had the question about the inner and outer diameter question. I belive it also addresses this in the "how-to".

They need the outer diameter to caculate the thickness, then the program assumes your using PVC, and from there caculates "thermal expansion". JK (I had to)

It is for use in "coaxial" lauchers, were the barrel in on the interior of the chamber. So if your not calculating such a launcher the ID should be "0".

I would like to see some simpilifications in GGDT (for people like me), but I bet D_Hall doesnt want to waste any more time on it because it had to take fair amount of time to make such a program with complex equations.

[boilingleadbath]

Answears to most questions can be found <a href="http://www1.iwvisp.com/thehalls/GGDT/us ... l">here</a>
The sprinkler valve info, however, is <a href="http://www1.iwvisp.com/thehalls/GGDT/li ... l">here</a>

Friction isn't very important. The defult value is fine.

[rastetter]

I was unsure about some of the inputs

Piston Diameter is unclear, i dont know what to put there,

Flow Coef i put 30 (my sprinkler valve says Flow Range: 0.25-30 GPM)

Vent diameter i put .1 inch... i am assuming that isthe little metal piece on the bottom of the actual solenoid that retracts when electrified? or the screw that you can unscrew to also trigger the valve... so i put .1 inch, doesnt really effect performance either... just a little delay

Pilot Vol... i put 3 cubic inches... i dont see how this can affect performance, and playing with the number doesnt change my results

Friction i put 2 psi... sounds right

what does a 3 X 1.5 inch potato weigh? I guess that is pretty important
and I get WRN: Valve coef impossibly large

[theregoesthewindow]

[rant]Look in the linkin BLB's post! People put links in thier posts for a reason![/rant]

potato mass in calculated like this: http://www.spudfiles.com/forums/viewtop ... tato+mass&

[rastetter]

[rant]Look in the linkin BLB's post! People put links in thier posts for a reason![/rant]

I did look at his link, and it answered most of my questions... for some it just didnt explain it enough and/or they are values that i have no idea how to figure out, and thats why i posted that

[boilingleadbath]

The values provided on the sprinkler valve page are accurate. Just use them, no figuring needed. To use them, you'll have to go to the configuration pull-down menu and change the flow coefficient to percent.

[rastetter]

Ok, i think i got it now

results are interesting, says i am getting 275ft/s ( my guess was 250-300), so i think i did well..

what is interesting, when i double my barrel length from 25inches to 50 inches it goes to about 360f/s

and when i double that, to 100 inches... it says 450f/s
200" -> 500f/s
300" -> 510f/s
then at 350" it says barrel unnecessarily long

can that be right? id figure after 100 inches the air in the tank is depleted enough that it cannot accelerate the potato any more

[boilingleadbath]

At typical pressures, it takes a pretty small C:B ratio to deplete the air - your numbers sound about right. There's a graph option that allows you to see the acceleration drop off as a function of time or distance traveled down the barrel... in truth, there's a graph option of most everything of interest except projectile energy.

So, what pressure were those numbers calculated at? 120 with a 75 gram projectile, by any chance?

[rastetter]

100psi with a 90gram projectile (2 inch by 1.5 inch potato)
(3.53 in^3 X 15.4 g/in^3 = 54 grams) 54 grams seemed to light, so i upped it to 90,
but at 54 grams it says 360fps
and for 90 grams it says 280 fps
i guess a 2 inch by 1.5 inch potato would weigh ~54 grams