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Calculate projectile data?

Post about things you have launched or thought about launching. Also post about various materials used for building cannons. No posts about explosive projectiles!
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Calculate projectile data?

Unread postAuthor: sharpshooter » Mon Mar 28, 2016 12:21 pm

GGDT/HGDT have made the calculations up to the point of leaving the barrel a dream. But what about after? High school physics was a loooonggg time ago. I found this write up http://www.dummies.com/how-to/content/calculate-the-range-of-a-projectile-fired-at-an-an.html, but it appears to be rather simplified - namely that I see no mention of air resistance.

I'm pretty sure I can get SolidWorks to spit out the Coefficient of Friction easily enough. What else might I need?

In the same vein, how would I go about calculating stability? I remember from back in my model rocket days that I wanted the center of pressure a bit behind the center of gravity. Does this hold true for unpowered projectiles? Center of gravity is easy. Is there a good way to find the Center of Pressure? I could probably figure it out using a flow simulation and trial and error if it comes down to it.

My aim here is to design my cannon/ammo to have the highest possible impact energy at 100 yds.
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Re: Calculate projectile data?

Unread postAuthor: jackssmirkingrevenge » Tue Mar 29, 2016 9:33 pm

You do know GGDT has an external ballistics calculator, right?

http://www.thehalls-in-bfe.com/GGDT/usa ... istic.html

Not extremely sophisticated, but it's something.

You can also guestimate drag coeffcient based on known shapes compared to your projectile:

Image

I remember from back in my model rocket days that I wanted the center of pressure a bit behind the center of gravity. Does this hold true for unpowered projectiles?


Yes

Is there a good way to find the Center of Pressure?


Let's ask NASA!

For a model rocket, there is a simple mechanical way to determine the center of pressure for each component or for the entire rocket. Make a two dimensional tracing of the shape of the component, or rocket, on a piece of cardboard and cut out the shape. Hang the cut out shape by a string, and determine the point at which it balances. This is just like balancing a pencil with a string! The point at which the component, or rocket, is balanced is the center of pressure. You obviously could not use this procedure for a very large rocket like the Space Shuttle. But it works quite well for a model.
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Re: Calculate projectile data?

Unread postAuthor: sharpshooter » Tue Mar 29, 2016 10:42 pm

Well I'll be. I had no idea. But it doesn't have any energy output - just distance/height. Although... I could probably work out the final velocity from distance/time. While I was hoping for something much more automated, that may have to do.

I did see that NASA page before, and frankly I am too spoiled by 3D modeling and simulation software to deal with a bunch of tweaks by hand. But, just a few minutes ago I found this gem: Open Rocket. You can build up the rocket, and adjust sizes on the fly, giving me CG and CP. Appears to be some simulation as well, but I think its stuck aiming straight up. I wish I could bring in my own model, but software like that probably isn't free.
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Re: Calculate projectile data?

Unread postAuthor: jackssmirkingrevenge » Tue Mar 29, 2016 11:24 pm

But it doesn't have any energy output - just distance/height.


See which variable gives you the longest range for a given angle - this will guarantee the projectile to "have the highest possible impact energy at 100 yds."
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Re: Calculate projectile data?

Unread postAuthor: sharpshooter » Tue Mar 29, 2016 11:28 pm

:oops: Oh duh
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Re: Calculate projectile data?

Unread postAuthor: jimmy101 » Sun Apr 03, 2016 10:25 am

Jack covered it pretty well with the NASA link. In a nutshell any simple regular shape with constant density will be unstable. A sphere, cylinder, brick, bullet, ... are all aerodynamically unstable and will tumble. To move the center of drag (center of thrust, center of pressure, ... it has several names) backwards you typically add fins. To move the center of mass forward you typically remove mass from the inside of the back end. You can do one or both.

Or you can spin the projectile. Bullets are intrinsically unstable and are spun to stabilize their flight.

The NASA approach of hanging a 2D model from a string can be done fairly well (I would think) by most solid modeling software. First, have the software calculate the center of mass of the 3D model (if the thing is constant density then the center of mass is just the center of volume). Then take a 2D slice through the model and calculate its center of mass. That second center of mass is pretty close to the center of thrust. This works well for "solids of revolution" but fins are trickier and complicate things.
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