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A little help with ballistics, please.

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A little help with ballistics, please.

Unread postAuthor: judgment_arms » Wed Mar 14, 2007 7:52 pm

I’ve been trying to figure out ballistics (internal, external, and terminal), and I can’t seem to find out how you figure muzzle velocity of a pneumatic gun, like the GGDT does taking in to account the valve and such, (preferably a little more accurately but I know that ain’t happening.).

I’d also like a little help with external ballistics. I’ve spent the last two days trying to find the formulas and figure them out, I’ve yet to find one for figuring out the muzzle velocity, and every thing I’ve found about external ballistics didn’t work, I used Excel to do the actual math on them and none of them mach up with what the GGDT or Joanna’s Launcher Range Calculator has.

If anybody can help I’d like a formula that could give me a rough estimate of how a projectile will perform too.

Any help will be very much appreciated thanks in advance.
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Unread postAuthor: rna_duelers » Thu Mar 15, 2007 3:56 am

Doesn't your GGDT show the velocity?And as how the projectile will perform once it has left the barrel would be very hard to estimate as there are so many variables such as wind speed,direction,elevation,aerodynamics just to name a few.

Why not just shoot and record the outcomes so you have useable data on the projectiles flight performance.
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Unread postAuthor: joannaardway » Thu Mar 15, 2007 4:40 am

I'm rewriting the LRC for more accuracy. I hoped to get it out ages ago, but I got bogged down with exams.

Which way does the problem occur? Is it an over estimate or an under estimate?
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Unread postAuthor: boilingleadbath » Thu Mar 15, 2007 10:41 am

There are two basic ways to calculate the tratractory of a projectile (that I know of):
1) calculus (and I'm pretty sure computers don't like calculus)
2) algorithms

Either way, you need to know the following to calculate it:
1) the Cd of the projectile
2) The mass of the projectile
3) Varius stuff like angle, velocity, air density

The Cd can be estimated by comparing your projectile's shape to shapes with a known Cd and making a geuss, or by using (compressable) CFD.
CFD programs generaly licenced out at at many thousands of dollars per year, so you won't be buying one.

Everything else is easy.

(basicaly, for the algorithms, you adjust the velocity of the projectile according to the force of the drag and it's vector... and drag formulas are pretty simple.)
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Unread postAuthor: judgment_arms » Thu Mar 15, 2007 12:50 pm

Rna_duelers, yes my copy of the GGDT does show velocities, I would like to know how it estimates velocity.
Your idea of recording my projectiles flights is a good one, but I don’t have any equipment.

Joannaardway, there’s nothing wrong with the LRC; it’s my lack of knowledge in ballistics. Well actually I find it to be a little off, it seems to over estimate the range, but it’s more accurate then the GGDT by at least ten fold.

Boilingleadbath, I know how it’s done, I just don’t know how to do it… you’ve basically told me what everybody else had, except your explained it a little better. Please excuse my ignorance.
How exactly do you adjust the velocity accurately?

All the formulas I’ve found don’t work right; one of them has a fixed angle, 45 degrees, and doesn’t even look at the projectile. The other one is missing some of the information, I found it on another forum and the guy who posted it made a text only copy of a webpage, there were links for more information/better descriptions that were not links just “click here”.

More than external ballistics I’d like to know how the GGDT figures out the velocity of a gun, I understand that it’s most likely a lot of math, but I’d really like to know.

Thanks!
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Unread postAuthor: boilingleadbath » Fri Mar 16, 2007 3:17 pm

The drag formula:
F=Cd*0.5*p*A*V<sup>2</sup>

F is force in newtons
Cd is the drag coeficient
p is the density of the fluid (kg/m<sup>3</sup>)
A is the frontal area (m<sup>2</sup>)
V is velocity (m/s)
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Unread postAuthor: judgment_arms » Fri Mar 16, 2007 5:17 pm

Thanks, Boilingleadbath. Unfortunately I’m still quite confused… man, I’m sorry about this y’all, I know what it’s like dealing with idiots, but I still don’t know what to do. I can under stand formulas, if not at first I can generally figure them out.

I understand the drag formula, thanks again, but I still don’t know what to do next. If at all possible could you show me the formula? That would help a lot.

Can anybody tell me how the GGDT estimates velocity, or another way to do so that takes into account the valve? That’s what I’d really like to know, I spent three or more hours the other day trying to figure that out.

Thanks all!

Edit:
Never mind about the trajectory stuff, I figured that out, I think…
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Unread postAuthor: boilingleadbath » Sat Mar 17, 2007 7:43 pm

GGDT uses an algorithm based method...
it's a good guess it goes something like this:

"The CALM modelling software works by computing the amount of air that passes through the valve every 100 microseconds, subtracting the from the pressure chamber and adding it to the bore. The force on the (tennis ball) Projectile is computed, the net acceleration, velocity, and new position. New pressures are computed for the bore and chamber. This is repeated until the ball leaves the bore. This allows us to see what is happening in some detail."
-<i>AKBiocca</i>
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Unread postAuthor: judgment_arms » Sat Mar 17, 2007 8:12 pm

Ahh, that sounds a lot simpler than I thought thanks! I’m going to see if I can figure this out on my own.

Oh and if anybody would like to know how I figured out the trajectory stuff, I found a web site called HyperPhysics Concepts there’s a page that has a lot of information on trajectories.
I believe that between HyperPhisics and y’all, I’ve got the trajectory stuff down.

Thanks again!
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