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Fastest Potato Velocity?

A place to ask general spud cannon related questions.
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Fastest Potato Velocity?

Unread postAuthor: Killjoy » Sat Apr 14, 2007 11:06 pm

I apologize if this has already been discussed, but i tried searching for any information i could on the subject and couldn't find anything.

That said, my question is: does anyone know what the high velocity ever achieved with a potato (unfrozen) as a projectile is? (this of course is with a regular combustion, pneumatic, or hybrid cannon).

I've been wondering because I did a recent project in which I tried to determine the maximum velocity a potato could achieve without disentergrating, and the highest velocity I could achieve was 2753 Feet Per Second with an unfrozen potato with my new hybrid.

I would greatly appreciate any information.
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Unread postAuthor: hi » Sat Apr 14, 2007 11:07 pm

i think you set the record..... thats amazing

either that, or your chronograph is broken
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Unread postAuthor: Velocity » Sat Apr 14, 2007 11:14 pm

If you actually got such numbers on your chronograph, I am going to have to assume that the muzzle of the gun was too close to the chronograph, and therefore the speed of the escaping gases was recorded rather than the projectile. Move the gun 5 feet back from the chronograph. If you are still getting 1500+ FPS, then I guess maybe you are right.

I am certainly not an authority on chronographs; I am sure some of the other members will provide more insightful, accurate information rather than my random ramblings.
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Unread postAuthor: SpudBlaster15 » Sat Apr 14, 2007 11:30 pm

A velocity of Mach 3 is highly unlikely using a potato slug, and a hybrid of any reasonable size.
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Unread postAuthor: frankrede » Sat Apr 14, 2007 11:33 pm

Thats twice the sound barrier!
I am finding that rather unbelievable.
I would think the potato would be destroyed in the process.
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Unread postAuthor: noname » Sat Apr 14, 2007 11:40 pm

Well, considering the hybrid is 55 feet long.....
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Unread postAuthor: frankrede » Sat Apr 14, 2007 11:44 pm

But velocity doesn't change with size.
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Unread postAuthor: Killjoy » Sat Apr 14, 2007 11:44 pm

Actually rmich, thats what i first thought when i saw the velocity was that the escaping gases had some how been recorded (i had the chronograph 2 feet from the muzzle). so i moved the chronograph to 7 feet away from the muzzle, and still got 2700 fps so i don't think the gases played any part in it.
I also thought before hand that maybe the burst disk would get picked up by the chrony, so i desighened it to slide out of what was holding it but not fully release and travel down the barrel so i know that it wasn't picked up by the chrony.
And i know the chony works because It picked up by bb gun at 325 fps (what it is advertised to do), my paintball gun at 300 fps (which was calibrated at a paintball field), and my dads .270 rifle at 2800 fps which is what the round we were using was advertised at be at (which could meen nothing). I have almost no doubt that the chrony works and is accurate.
Edit:
The 2700 fps was with a 4x mix just to let you'll know.
Frank,
You underestimate the strength of a potato, 2700 fps may be unlikely, but there is really nothing that says it is impossible, and i have seen the results with my own eyes (which is not of course definative proof). Also, like an idiot, i forgot to bring a camera to the testing range to take pictures of the velocity on the chrony, but i do have a picture of the velocity on the chrony from the second day of test fireing, which was 1900 fps with a potato (i know it doesn't really prove the 2700 fps claim but its something) and with a 3x mix if you would like to see it.
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Unread postAuthor: homeflake210 » Sat Apr 14, 2007 11:52 pm

Can we get some pictures of this gun?
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Unread postAuthor: frankrede » Sat Apr 14, 2007 11:55 pm

Wow if you really are getting that speed, then that is amazing.
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Unread postAuthor: SpudBlaster15 » Sun Apr 15, 2007 12:01 am

frankrede wrote:But velocity doesn't change with size.


What would cause you to believe that? The rate of acceleration of a potato slug down a barrel does naturally decrease with time, but thats not to say that it will not continue to accelerate.

If you are referring to the recent launcher scaling debate, keep in mind that when those calculations were done, everything was scaled. In this case, everything except the projectile is scaled.

If this gun does in fact have a length of 55 feet, then I have no doubt that it could reach such a velocity with a potato. It would accelerate gradually to this velocity down the length of the (massive) barrel, and would not be subjected to shock loads sufficient to destroy it.

Some pictures of the launcher would help satisfy my curiosity.
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Unread postAuthor: Killjoy » Sun Apr 15, 2007 12:05 am

Can we get some pictures of this gun?

The time will come my friend. As in when the contest happens.
However, I will give you some specs of the cannon to slightly ease your "curiosity" (SB).
It has a 6" diam sch 40 steel chamber, 63" long. The barrel is 2" diam sch40 pvc pipe and is 50 feet long (in 10 foot sections connected by modifed couplers.) It uses propane as fuel, and is ignited by 5 spark gaps spread evenly though the combustion chamber and these are connected to a 50,000 volt igniton source.

I really hope that i didn't screw anything up that would result in the velocity being miss recorded, but i am almost a hundred percent sure i didn't because i followed the instructions on the chrony and did everything i could think of to make sure i got an accurate result.
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Last edited by Killjoy on Sun Apr 15, 2007 12:11 am, edited 1 time in total.

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Unread postAuthor: frankrede » Sun Apr 15, 2007 12:07 am

SpudBlaster15 wrote:
frankrede wrote:But velocity doesn't change with size.


What would cause you to believe that? The rate of acceleration of a potato slug down a barrel does naturally decrease with time, but thats not to say that it will not continue to accelerate.

If you are referring to the recent launcher scaling debate, keep in mind that when those calculations were done, everything was scaled. In this case, everything except the projectile is scaled.

If this gun does in fact have a length of 55 feet, then I have no doubt that it could reach such a velocity with a potato. It would accelerate gradually to this velocity down the length of the (massive) barrel, and would not be subjected to shock loads sufficient to destroy it.

Some pictures of the launcher would help satisfy my curiosity.

Oh ya, I was confused.
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Unread postAuthor: Insomniac » Sun Apr 15, 2007 1:31 am

If you want to make sure it was the potato being recorded, try fireing the gun at the chrony without a projectile.
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Unread postAuthor: boilingleadbath » Sun Apr 15, 2007 2:53 am

I am seriously doubting the 2700 number - for two reasons:

1) EVBEC Live V1.5 says your launcher has to use really light projectiles (like sub 5 grams) to do this feat.
Not that this means much, given that I haven't tested EVBEC's modeling of:
Hybrids
Barrel mass flow
Chamber mass flow
Near-sonic flow
Really high velocities
Barrels with long dwell times

2) The efficiency of the launcher should decrease as the initial chamber pressure increases<sup>a</sup>, especially in this regime, and you only increased the energy generated by 4/3<sup>b</sup>, but the muzzle energy of a tater at 2700 fps is TWICE<sup>c</sup> that of the same tater at 1900 fps.
This is simply illogical.
A) Several reasons; mainly that there is more pressure at the end of the barrel and that the ratio of projectile mass:propellant mass is lower (and of course, you have to use the energy generated by the propellant to propel the propellant gasses.
B)You got 2700 fps with a 4x mix - you filled the chamber with 4 "units" of fuel-air mixture. Likewise, you got 1900 fps with a 3x mix (3 units)... so, 4/3.
C)The mass is the same, so MV<sup>2</sup> can be reduced to V<sup>2</sup>... If (2700)<sup>2</sup> = X(1900)<sup>2</sup>, X must equal ~2.019.

On the other hand, EVBEC Live 1.5 will give you 1694 fps with a 3x mix if the projectile weighs 100 grams... which, although a bit lower than 1900 fps, is close enough that I'll believe it.
(A 63g tater, incidentally, <i>will</i> give you 1902 FPS)
I'd guess that this discrepancy is related to the longer barrel dwell time of your launcher, as compared to latke's... or a error on my part in writing EVBEC.

Assuming your 1900 number is correct, I'm rather pleased; a 20% error (by energy - 11% by velocity) isn't that bad, considering the GGDT is generally within "5-10%".
Especially given all the factors I've ignored.
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