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projectile trajectory calculator

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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projectile trajectory calculator

Unread postAuthor: jagerbond » Mon Sep 28, 2009 3:47 pm

Has anyone come across a calculator that can predict trajectory/distance with inputs such as; velocity, angle, drag coef., size, weight, etc...

Thanks in advance,
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Unread postAuthor: POLAND_SPUD » Mon Sep 28, 2009 4:04 pm

chairgun (for projectiles lighter than 5g)
external ballistic calculator from GGDT
and there was one speedsheet created by Rag or joan but I can't find it...
EDIT
I found it click
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Unread postAuthor: btrettel » Mon Sep 28, 2009 4:13 pm

GGDT's exterior ballistics simulation is pretty good for the basics.

joannaardway's simulation is here: http://bb.1asphost.com/sigmarhaven/SSpi ... orV2-2.xls (edit: Nope, the link's broken.)

I wrote this coming up on a year ago: http://trettel.org/bags/code/proj_0.py

This simulation very adequate if the projectile can be treated as a point particle and the range is rather limited. I intend to write a more fully featured simulation taking into account far more, but I haven't started writing it yet.

I'd like to note that these calculators are rather basic, and even if they weren't, you'd still have to take their results with a grain of salt. If you are unsure about any input, run a range of them to see how much they affect the range. That is basically what any exterior ballistics software that produces a spread where the projectile is likely to land does.
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Last edited by btrettel on Mon Sep 28, 2009 5:59 pm, edited 2 times in total.
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Unread postAuthor: jackssmirkingrevenge » Mon Sep 28, 2009 4:15 pm

Planning on going beyond your parking lot range? :D
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Unread postAuthor: jagerbond » Mon Sep 28, 2009 5:13 pm

jackssmirkingrevenge wrote:Planning on going beyond your parking lot range? :D


It is an interesting project, I can't share many details yet... but we are looking to be out 1/4 mile.
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Unread postAuthor: jagerbond » Mon Sep 28, 2009 5:17 pm

Thanks for links! :D
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Re: projectile trajectory calculator

Unread postAuthor: Ragnarok » Mon Sep 28, 2009 5:28 pm

I was reasonably sure the link was dead on Jo's calculator... if it's not, I'm a little surprised.

Anyway, I am still working on a reasonably advanced calculator. The problem is that I have too many things I'm trying to do that eat time.

Eventually, I should have it done.
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Re: projectile trajectory calculator

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Unread postAuthor: cannon monkey » Mon Sep 28, 2009 6:22 pm

when u find one let me know please :)
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Unread postAuthor: D_Hall » Mon Sep 28, 2009 6:22 pm

Once upon a time I used to do ballistic prediction for a living. In other words, it's a topic I'm very well versed in.

That said....

A word of warning to those who endeavor to create advanced/complex models: Garbage in. Garbage out.

Once upon a time I saw wind tunnel data that was the best money could buy (ie, military/NASA installations, blah blah blah). Wanna know what asterisk most of that data came with?

"+/- 10%"

Seriously.

It was the best data available to the most advanced military in the world and it was only good to 10%.

My point?

You can bust your chops building a simulation that takes all sorts of crazy things into account(*), but you're still going to have to accept that there's a huge uncertainty in your data simply because you aren't going to get drag data that's anywhere near as accurate as your simulation.

Garbage in. Garbage out.

And THAT is why I have never busted my chops on GGDT's ballistics model (although I will concede that it would be nice if I included allowances for saboted projectiles and such).



(*) I've included the WGS-84 model (non-spherical) of the Earth as well as the Earth's rotation (NOT 360 deg/day!) in some of mine from the past. I've included gravity models that varied not only with altitude but with latitude. I've included atmosphere models that went out to 800 km and allowed for different wind speeds/directions at different altitudes. None of that changes what I wrote: Those sims results were still to be taken with a grain of salt because I still didn't know much about REAL WORLD drag!
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Unread postAuthor: Ragnarok » Mon Sep 28, 2009 7:42 pm

D_Hall wrote:I've included the WGS-84 model (non-spherical) of the Earth as well as the Earth's rotation (NOT 360 deg/day!) in some of mine from the past. I've included gravity models that varied not only with altitude but with latitude. I've included atmosphere models that went out to 800 km and allowed for different wind speeds/directions at different altitudes.

Sounds like a similar set of criteria to what I'm going for, although I'm not going for 800 km altitudes.
I know that what I'm doing is pretty moot given the accuracy of drag coefficients, but I'm now doing it out of curiosity of how the numbers fit together, and seeing how different things have their own effects on a trajectory.

Or sometimes, deciding to see how much the trajectory of a .50 rifle would be affected by being on Venus instead. It's a toss-up.
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Unread postAuthor: VH_man » Mon Sep 28, 2009 8:34 pm

Im studying this kind of thing in physics right now, and am taking an independent study in C++, so I could probably make some sort of console program that does it (or a GUI if I manage to understand it by the end of the term)
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Unread postAuthor: D_Hall » Mon Sep 28, 2009 8:35 pm

Ragnarok wrote:but I'm now doing it out of curiosity of how the numbers fit together, and seeing how different things have their own effects on a trajectory.

Ah... Sensitivity studies. What sims ARE very good at doing.

One word of caution: There are two conventions in the aerospace world regarding the calculation of lift and drag (and if you're account for wind, you WILL have to account for both lift and drag!). One is projectile-centric. The other is velocity-vector-centric.

The first tends to be used by the missile and gun community. The second tends to be used by the aircraft community.

Both are totally valid and they will yield the same numbers if you use them correctly as they are mathematically equivelent. However, if you think you're dealing with one dataset and you're really dealing with the other? Garbage in. Garbage out.

So decide early on which convention your sim will use and be sure that all the data you feed it conforms to that convention.
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Unread postAuthor: btrettel » Tue Sep 29, 2009 9:53 pm

+- 10% sounds pretty damn good. The one test I did was accurate to about +- 20% for comparison (or at least seemed to be). For what it's worth, the variables weren't controlled very well, but I figure I can do that later.

One the subject of sensitivity, this is a pretty interesting read: http://en.wikipedia.org/wiki/Sensitivity_analysis
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Unread postAuthor: Ragnarok » Wed Sep 30, 2009 9:00 am

btrettel wrote:+- 10% sounds pretty damn good.

It's not bad, but take this simulation I did a while ago of three golfballs, launched at 200 m/s and a 30 degree angle.

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- Blue is just a normal golf ball. Cd of 0.3, modelled with no lift. ~365 metre range.
- Red is a golfball with Cd 0.3, and "200% lift" (in other words, the lift generated at the muzzle velocity is twice the golf ball's weight). ~370 metres.
- Green is a ball with no lift, but Cd 0.27 (10% reduction). ~395 metres.

The points of interest here are the blue and green lines, because of their 10% Cd difference. Not too far different, but that's still not to be ignored. A 7 or 8% error in artillery fire could be catastrophic.

~~~~~

Also, for fun, a 200 m/s golfball fired horizontally 1 metre above the ground with "3000% lift":

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Unread postAuthor: psycix » Wed Sep 30, 2009 1:31 pm

Also, for fun, a 200 m/s golfball fired horizontally 1 metre above the ground with "3000% lift":

Haha.
Can you also simulate a lift to the side, so you can shoot around corners? :P
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