SpudFiles

MrCrowley wrote: It's never modelled my piston gun decently, it's still over 110m off with it's distance calculator.
And since finding the distance heavily involves it's fps, it's muzzle velocity must be wrong too.

Not really. It's distance calculator is very basic and wouldn't do a great job on something like a golfball that probably has a variable Cd over the range of speeds it encounters and also has spin to provide lift.

On any gun that I haven't tried something goofy on like a blow forward bolt GGDT is very accurate against my chrony results


Oh, and nice job on the homemade chrony, it should give pretty accurate results. If you haven't seen jimmy's page on this type of chrony you should check it out, although it doesn't seem to be working for me right now so I can't get the link. I think he even goes into some error analysis about how accurate they can be, although I can't remember for sure.

thanks clide, yeah jimmy's page doesnt work for me either. =(

yeah i have access to a real chrony but i have to travel to get to one. i'd rather just bring a laptop outside to my back yard and get a result there even if its a bit off.

clide wrote:

MrCrowley wrote: It's never modelled my piston gun decently, it's still over 110m off with it's distance calculator.
And since finding the distance heavily involves it's fps, it's muzzle velocity must be wrong too.

Not really. It's distance calculator is very basic and wouldn't do a great job on something like a golfball that probably has a variable Cd over the range of speeds it encounters and also has spin to provide lift.

On any gun that I haven't tried something goofy on like a blow forward bolt GGDT is very accurate against my chrony results

So it's calculator doesn't take in the projectiles Cd, altitude, temperature, pressure, friction, init position, valve flow, projectile weight, firing angle etc?

It asks for all of those, which all effect distance, another basic thing it doesn't ask for is wind resistance. Like you said it also doesn't include the speed the golf ball is travelling or the spin, hell you could probably include the earths rotation speed.

But if it's calculators did include all the variables it already asks for, I would still expect a decent calculation, not one that is 110m off.

Sure there are a few things that could change it 10m give or take, but 110m is just under 1/3 of the total distance.

MrCrowley wrote: So it's calculator doesn't take in the projectiles Cd, altitude, temperature, pressure, friction, init position, valve flow, projectile weight, firing angle etc?

Well it will take into account anything it asks for, but it is still very basic because external ballistics is a very complicated matter. Heck I think D_Hall wrote a rocket ballistics program that was so complicated it sold for $200 or more.

Also what Cd are you using? That has a huge influence on range and it's probably not very easy to find an accurate Cd for the speeds we are shooting at. I have to use .2 for a golf ball to get near the actual range for my gun. If your still getting a range that is way low with a Cd that low then I would say something is probably wrong with your model.

.2

Damn I heard it was around .3

There's my problem

I only tried between .25 and .3, as I read somewhere it was .3-.35

clide wrote:Heck I think D_Hall wrote a rocket ballistics program that was so complicated it sold for $200 or more.

$200 isn't really a helluva lot for a professional navy weapons expert.

My Dad who owns a hardware store said Mitre10 (like a Home Depot) paid $20,000 for a program that singles out obsolete stock items and changes two things on that item, and it changes the same two things for each item. Nothing too complicated.

thats interesting cause i have a rocket modeling program and it cost me $100
hmm...

okay so i finally got to try my chrono on my gun. well it seems like that i am putting the muzzle too close to the first sensor and well the vapor plume that comes out is skewing my results in that i cant even see where the projectile ented and exited. i need to make a muzzle attachment with an open break to allow the vapor to get away.

i need to make one of those and i'm confused, how do yu use a chrony with photocells and leds with an audio programm?

Nice job cwazy1.

Here is how I did mine, just a pair of phototransistors mounted on a piece of sliced up PVC. It clips onto the barrel of the gun being measured. The light source is just ambient light.

For Alihisgreat; all you need is a pair of light sensors seperated by a known distance. You record the changing light level caused by the projectile's shadow as it passes over the detectors by plugging the sensors into the MIC input of a PC or laptop. You use audio recording software to record and analyze the peaks.

For other folks, yep, my web site is down. It is being moved to a new server.

Edit: The images are in the wrong order, second image should be first, third image should be second, first image should be last.

If anyone is interested, my pages are now at http://www.inpharmix.com/jps/

The chrono page is http://www.inpharmix.com/jps/Jims_chrono.html

ahh, nice design, but i do see one problem that could occur: if you are to use a very high pressure source that creates a vapor plume after the shot, that could easily skew your results.

so after adding a "break" to my design to allow vapor to escape, i clocked my marbles going 564.97fps. this is within 15fps of ggdt!! (110psi)(ggdt says 578fps)

okay so i compared my result to an actual chrony. for my DIY i had what is stated above ^^ then for a real "chrony f-1" i got 558fps. well i think thats close enough.

okay so when i chronied a bb, i got a result of 1101fps. :wav:
and also rightt after i shot the bb throuth the chrony, my whole chrony went dead...i have no idea why.. sonic boom? i did hear a small crack instead of the usual pop. could it be what im hopeing it is??

Well if you mean something like mach 1, well sorry to say you are close, but no bannana.

mach 1 = 1116.44 feet approx

So your very close, off by 15 fps

EDIT: Ahh i see that you are off by 15 in the other one. How ironic that it is the same in this case.

Very nice, I think you just compelled me to go and make my own . Where did you get the photocells, Radioshack?

crotchus: haha oh wow, i just noticed that as well. heheh well since im not just there yet, time to up my pressure from 130 to 150+

c19o: yeah i got the photocells from radioshack. they should have an assorded pack with 5 in it, its like 2.99. photocells=photoresistors. (cadmium sulfide)...but you can also use phototransistors in the same way.