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600 fps -new results

A place to ask general spud cannon related questions.
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Unread postAuthor: sjog » Tue Apr 24, 2007 10:17 pm

I've heard this discussion before, the muzzle is to be well back from chrony.
My muzzle was 10' + away. Maybe hi's buddys gun is fast.
And ShowNoMercy lives up to his name!!
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Unread postAuthor: SpudBlaster15 » Tue Apr 24, 2007 11:51 pm

hi wrote:look spudblaster, i know what i saw and im not lying, if you dont believe me that fine, but i know what i saw. And why are you making such a big deal, it is only about 70 fps faster, which isnt too much. all im saying is that he has the mix close, but not perfect. end of discussion


Why am I making such a big deal of it? You are stating something that is implausible, and contradicts the chronograph results and experiences of others.

A propane powered launcher with a 5 foot barrel will not deliver a muzzle velocity of 650fps. EVBEC estimates 454fps.

You most likely had the muzzle positioned too close to the chronograph sensors. End of discussion.
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Unread postAuthor: hi » Wed Apr 25, 2007 12:11 am

you do know that the difference between 450 and 650 fps is about the speed that a good pitcher can throw baseball (which weighs more than a tater) with his arm dont you? it really isnt a whole lot if you think about it. and also think about it, if it is 30 degrees out side, then the proformance will be different than if it is 120 degreees out side. it was about 110-115 degrees out there that day.

if its cold the propane contracts, so you need more of it to get 4%, but if it is hot it expands, so it ignites faster. thats my therory, i dont know if its right.

and you know what, maybe you are right, and im wrong, maybe we were too close, but i really think that you just have to get the mix just right.
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"physics, gravity, and law enforcement are the only things that prevent me from operating at my full potential" - not sure, but i like the quote

you know you are not an engineer if you have to remind yourself "left loosy righty tighty"
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Unread postAuthor: SpudBlaster15 » Wed Apr 25, 2007 12:38 am

200fps is a huge (relative) difference. In this case, the higher value is 45% greater than the lower.

If that is not enough to convince you that such a difference is rather large, consider this. The formula used to calculate muzzle energy is E = 1/2M*V^2, with M being mass in kilos, and V being velocity in m/s.

E = 0.5*0.055*(137^2)
E = 516J

E = 0.5*0.055*(198^2)
E = 1078J

Such a velocity increase means that the muzzle energy will double.

Temperature will have a slight effect on the velocity of the projectile. That is, it will shoot faster in colder weather due to the atmospheric (and fuel) gas density being higher. It will not have an effect on the fuel mixture, because all gases occupy the same volume at a given temperature.
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Unread postAuthor: hi » Wed Apr 25, 2007 1:03 am

maybe the damn thing was broken i dont know, i just know thats what it said, so i beleived it.
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"physics, gravity, and law enforcement are the only things that prevent me from operating at my full potential" - not sure, but i like the quote

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Unread postAuthor: shud_b_rite » Wed Apr 25, 2007 1:21 am

Hey I don't know if many people know about this but google has an awesome converter that is soo simple to use. It will convert anything, currency, weight, length, speed, pressure.

Say you want to convert 578 fps in mph you would type into the search bar:
"578 feet per second in miles per hour"
It will spit the answer right back at you, very accurate too

Another example, converting pressure from bar to psi
"18 bar in psi"

This is an awesome feature of google and I don't think many people know about it. So now you do
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Unread postAuthor: jimmy101 » Wed Apr 25, 2007 10:30 am

hi wrote:you do know that the difference between 450 and 650 fps is about the speed that a good pitcher can throw baseball (which weighs more than a tater) with his arm dont you? it really isnt a whole lot if you think about it. and also think about it, if it is 30 degrees out side, then the proformance will be different than if it is 120 degreees out side. it was about 110-115 degrees out there that day.

if its cold the propane contracts, so you need more of it to get 4%, but if it is hot it expands, so it ignites faster. thats my therory, i dont know if its right.


Actualy, the difference between 450 and 650 fps corresponds to a difference of about 136 MPH. The best major league pitcher can't throw a ball anywhere near that fast. If a pitcher actually wants to hit the catcher the maximum speed is about 100 MPH.

And like Spudblaster said, the energy of the projectile scales as the square of the velocity. So the comparing 450 and 650 fps the energy scales as 650<sup>2</sup>/450<sup>2</sup>=2.08. The higher velocity has more than twice the energy of the lower one.

Your theory on the affect of temperature on the propane fuel ratio is incorrect. The stoichiometry does not change with atmospheric temperature or pressure. On a cold day the propane is <u>slight</u> denser but so is the air in the chamber, the volume ratio ends up being the same. So, doesn't mater if it is 20F or 120F, you still want 4% propane.

Besides, termperature doesn't scale like you think. You can't compare temperatures in fahrenheit (or celsius) units. You need to convert to an absolute temperature scale like Kelven or Rankine. The difference in the volume of a gas on going from 30F to 120F is not 120/30=4, it is 322/272=1.18. The volume of the gases changes by a little less than 20%, not by a 400%.
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Unread postAuthor: boilingleadbath » Wed Apr 25, 2007 2:57 pm

I'm not overly hesitant to believe the "650 fps" number, because EVBEC requires a 25g projectile in a 4" chambered/1.5" barreled launcher to achieve it.

Which implies that he probably shot a 3/4" thick wafer of potato (or something else fairly light weight) instead of an entire potato.
Which, when trying to make impressive-sounding claims, is pretty logical.
(I doubt it's an error with EVBEC; a 10% velocity margin of error only allows me to make the projectile 30 grams or so... which still isn't an entire potato)

That said, the topic launcher is pretty nice looking, but we are discussing it's velocity here, so it'd be wrong of me to not check EVBEC with your numbers...

Using a barrel length of 74" and a 4" chamber with a volume of 326 cubic inches, firing golf balls, I'm getting 566 fps.
An error of 2% or so is pretty small, so I'm happy.
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Unread postAuthor: sjog » Wed Apr 25, 2007 4:46 pm

<img src="http://i116.photobucket.com/albums/o12/sjogun/th_IMG_0276.jpg" border="0"><img src="http://i116.photobucket.com/albums/o12/sjogun/th_IMG_0278.jpg" border="0"><img src="http://i116.photobucket.com/albums/o12/sjogun/th_IMG_0281.jpg" border="0"><img src="http://i116.photobucket.com/albums/o12/sjogun/IMG_0282.jpg" border="0">
The temp. was 53 today, yesterday was closer to 63. I ran the fan longer before and after shooting. These are the only changes from yesterday.
Had faster results. 483 fps was with one dose extra on the gas. 596 fps was the first shot of the day, 602 was the last.<img src="http://i116.photobucket.com/albums/o12/sjogun/th_IMG_0280.jpg" border="0">
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Unread postAuthor: hi » Fri Apr 27, 2007 5:08 pm

i told you..... it only a 47 fps difference........
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"physics, gravity, and law enforcement are the only things that prevent me from operating at my full potential" - not sure, but i like the quote

you know you are not an engineer if you have to remind yourself "left loosy righty tighty"
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Unread postAuthor: sjog » Fri Apr 27, 2007 5:16 pm

Yup hi I did start to think when using chrony. Try to pick up some speed.
I did run the fan much longer to mix and vent. Every molecule helps!!!
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Unread postAuthor: Specialist » Fri Apr 27, 2007 5:19 pm

that gun looks like a beast. im suprised you are not getting a higher chrony rating. thats still pretty good results though.
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Unread postAuthor: hi » Fri Apr 27, 2007 5:23 pm

ya, and another thing is that maybe at 10 feet away you loose 10-20 fps because of drag. it may not be as much as 20 fps, but maybe 5-10. i have no proof of you loose that much, but its possible.

edit- so the muzzle volocity is not 602, the speed at 10 feet is 602 fps.
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"physics, gravity, and law enforcement are the only things that prevent me from operating at my full potential" - not sure, but i like the quote

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Unread postAuthor: Specialist » Fri Apr 27, 2007 5:26 pm

i always thought you were supposed to have the gun 2-4 feet away from the chrony.
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Unread postAuthor: sjog » Fri Apr 27, 2007 7:09 pm

I was told by the owner 10' back.
Of course i have not read the directions
I will try at 5'. Owner said as soon as the round leaves the barrel it starts to slow. Which makes sense.
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