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GGDT lies

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Unread postAuthor: jackssmirkingrevenge » Tue Jan 29, 2008 12:14 am

VooDooCC wrote:I see you guys talking about velocity and muzzle energy. I noticed that When I only filled the bottles about 3/4 of the way they seemed to fly farther, but that may have been the trajectory of them as well. So for distance should I be aiming more for velocity or more for muzzle energy?


The distance you can achieve depends on many things:

velocity - the faster you throw something, the farther it will go, as simple as that

drag - the less streamlined your projectile is (in this case, a water bottle is not aerodynamically stable so it will tumble end over end, causing a lot of drag) then the quicker it will slow down and travel less distance

sectional density
- the relationship between projectile weight and surface area. For example, compare a golf ball to a table tennis ball - similar size, so similar area, but the golf ball is significantly heavier so if you had to launch them at the same speed, the golf ball will slow down at a much slower rate and travel faster. A perfect example of a good ballistic coefficient would be an arrow, because looking at it from the front it has a very small area, but it's long so it has a relatively high weight. These same features allow it to penetrate targets in a manner disproportionate to their velocity when compared to bullets.

In your case, the best idea would be to fill the 20 oz bottle with about 100mL of plaster, clay or anything similarly heavy and sticky and let it set. This way, the top of the bottle will act like a stabilising tail, preventing it from tumbling in flight. I recommend you leave the cap off though, so that on firing the pressure will enter the bottle and help prevent the heavy bit from coming loose due to the shock of acceleration.
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Last edited by jackssmirkingrevenge on Tue Jan 29, 2008 12:00 pm, edited 1 time in total.
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Unread postAuthor: VooDooCC » Tue Jan 29, 2008 12:33 am

Okay, it's official, you are my new hero Jack. You may not have the chopping block avatar anymore, but you still amaze me with every post.

note: The water bottles are not the only thing I'll be shooting, I just had a lot laying around so I figured why not?
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Unread postAuthor: jackssmirkingrevenge » Tue Jan 29, 2008 12:39 am

Always happy to help :)

If you really want maximum distance, may I suggest you take the time to build something like this:

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(since you have a 3" barrel, you might want to use something a bit bigger than a nail for the main body)

the important thing is that the end result fits neatly in the barrel, and the fins are long enough to ensure stability in the barrel. Note that the foam disc is just as sabot, not attached to the projectile. You'll need some good epoxy (I use araldite) to attach the fins, and maybe a jig to ensure you have the angles and dimensions right.

This is the benefit of the sabot round, maximum area in the barrel but low weight - for good acceleration - then when it leaves the barrel, the sabot strips off so you're left with a high speed projectile with a very good ballistic coefficient.
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Unread postAuthor: VooDooCC » Tue Jan 29, 2008 12:50 am

I also have a 2 inch and a 1 inch barrel, I was using the 3 for testing.

Since I'm broke, I have the multiple barrels that way I can shoot many things and not have to worry about having a full gun for each item 8)
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Unread postAuthor: jackssmirkingrevenge » Tue Jan 29, 2008 1:19 am

For a 1" barrel a 3-4" nail set up as above will be the ultimate penetrating projectile, worth trying if you have a bit of patience.
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Unread postAuthor: SpudBlaster15 » Tue Jan 29, 2008 3:20 am

I almost put my head through my computer screen reading 2 posts by "Likimyscrotchus5". The conception of physics that some people on this forum have is just downright annoying.

i know that you can get better performance with that barrel to a certain extent, but it can be much better if you go with a larger chamber than you barrel and increase the size of the barrel somewhat.


Of course a larger chamber will increase performance to some extent (by creating a higher average barrel pressure), but the statement "That won't have very good performance at all" is completely idiotic. The most efficient C:B ratio for a pneumatic launcher pressurized to 100PSIG is ~0.2:1, and the OP's ratio is sitting right around 0.6:1, which is making fairly efficient use of the energy available in the chamber. I'm sure VooDoo doesn't want to exert additional effort just to obtain a 10% increase in muzzle energy.

Distance you want velocity, power and force is where you want muzzle energy.

so choose a smaller light weight projectile for distance.


No, for distance you want high velocity values, high kinetic energy values, high mass values, and high density values. Drag force is proportional to the square of velocity, so "Choosing a small, lightweight projectile" will yield inferior results to a more dense, slower moving projectile.

Do you even know how "power" and "force" come into play in projectile ballistics, not to mention how they are affected by kinetic energy and velocity?

*Waits for Likimyscrotchus5 to run to Wikipedia, then return with the answers*
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Unread postAuthor: jackssmirkingrevenge » Tue Jan 29, 2008 3:33 am

We're all here to learn, it's just that folk should do their research before making categorical statements.
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Unread postAuthor: jimmy101 » Tue Jan 29, 2008 11:58 am

Jasksmirk:
I think your definition is off a bit;
ballistic coefficient - the ratio between projectile weight and surface area. For example, compare a golf ball to a table tennis ball - similar size, so similar area, but the golf ball is significantly heavier so if you had to launch them at the same speed, the golf ball will slow down at a much slower rate and travel faster. A perfect example of a good ballistic coefficient would be an arrow, because looking at it from the front it has a very small area, but it's long so it has a relatively high weight. These same features allow it to penetrate targets in a manner disproportionate to their velocity when compared to bullets.


Isnt it;
BC = SD/i
where
BC is the ballistic coefficeint
SD is the sectional density (frontal area/mass)
i is the form factor or coefficient of drag.

Or, replacing SD with frontal area/mass;
BC = (frontal area)/mass/i

What you describe is just SD (or 1/SD).
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Unread postAuthor: jackssmirkingrevenge » Tue Jan 29, 2008 11:59 am

wrong choice of words on the part of yours truly, edited.
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Unread postAuthor: jimmy101 » Tue Jan 29, 2008 12:09 pm

VooDooCC wrote:That was the other thing I was going to use GGDT to play around with, and I have heard so many different things about the right ratio. So I figured I would just 0.8 like Burnt Latke (I know there's is for combustion)

A handy trick for using GGDT is to enter a barrel length that is much longer than you think it should be. If you think the barrel should be 5 feet then tell GGDT it is 50 feet.

After you run the calculation, change the X-axis of the graph to "Projectile Position" (bottom left of the GGDT window). This will give you graph of muzzle velocity versus barrel length.

This makes it much easier to visualize the tradeoff between muzzle velocity and barrel length (and total gun length). Usually, if you don't mind dropping the muzzle velocity by just 10% below the max, you can cut the barrel length by more than a factor of 2. If you don't mind dropping the muzzle velocity by 20% below the max, you can cut the barrel length by a factor of ~4.
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Unread postAuthor: LikimysCrotchus5 » Tue Jan 29, 2008 3:22 pm

SpudBlaster15 wrote:I almost put my head through my computer screen reading 2 posts by "Likimyscrotchus5". The conception of physics that some people on this forum have is just downright annoying.


Well i am sorry that i am not as knowledgeable as you. I will try not to give inaccurate information. I dont mean to annoy you, i just dont know as much as you.

SpudBlaster15 wrote:the statement "That won't have very good performance at all" is completely idiotic


I would agree i would say that sentence was more extemporaneous and it was "idiotic" part of me talking.

SpudBlaster15 wrote:Do you even know how "power" and "force" come into play in projectile ballistics, not to mention how they are affected by kinetic energy and velocity?


No, i dont but i will learn soon enough.

SpudBlaster15 wrote:*Waits for Likimyscrotchus5 to run to Wikipedia, then return with the answers*


Im sorry but i wont be going there because i feel that it is inefficient for what i want to learn, and also it can be edited, im not the average idiot who runs to wikipedia to look for answers.

Jackssmirkingrevenge wrote:We're all here to learn, it's just that folk should do their research before making categorical statements.


Yes we definately are here to learn and i was wrong and i am sorry if i mislead anyone.

Here is a perfect example, and somewhat ironic.

Jacksmirkingrevenge wrote:wrong choice of words on the part of yours truly, edited.


As you said, we are all here to learn, like learning to make better choice of words, accident or not. :)

Sorry for my inaccurate information. I am a sophomore in highschool and have completed half a year of chemistry, so i am not knowledgable of all physics explained on this website, but it doesnt give me the right to mislead someone, even though i wasnt trying to.

Although your post made me feel somewhat like an idiot :cry: (arent we all :D ) i am not mad, i have very good patience and i dont resort to profanity to make me feel better about myself and making you feel like an idiot, we all have our strengths and weeknesses.

Now to be on topic,

that is a good idea, i should try it, now to learn how to use GGDT :roll: .

Also Voodoo, do you have pictures, so i can get a better visualization and try not to mislead you... again :)
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Unread postAuthor: MrCrowley » Tue Jan 29, 2008 3:32 pm

Liki, on the GGDT website he has a FAQ section or something that explains all the parameters that you need, what they do and usually how to get them.

GGDT is incredibly easy to calculate pneumatics, to a certain extent of one's knowledge of course. But modeling piston and sprinkler valve cannons is like child's play.

Edit: If you have MSN or anything, just add me on that and i'll help you whenever.
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Unread postAuthor: LikimysCrotchus5 » Tue Jan 29, 2008 3:40 pm

Thank you for the offerm but sadly i dont have msn, but i might just make one so i can refer to you from time to time.

I will check out that page on GGDT website, im sure ill learn to use it..eventually :)
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Unread postAuthor: BrianMigs » Tue Jan 29, 2008 4:16 pm

But the more mass you have for a projectile wont it go farther?

Or is that only when you have the same amount of energy pushing behind a heavy projectile, and a lighter projectile?
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Unread postAuthor: jackssmirkingrevenge » Tue Jan 29, 2008 11:56 pm

BrianMigs wrote:But the more mass you have for a projectile wont it go farther?

Or is that only when you have the same amount of energy pushing behind a heavy projectile, and a lighter projectile?


If you have 2 identically shaped projectiles with different weights and fire them at the same speed, the heavier one will lose velocity slower and therefore will travel farther.

If you take these two projectiles and put them in the same launcher with the same conditions, the heavier one will leave the barrel slower than the lighter one, so you can no longer make a direct comparison.
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