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navyrigger46 · Joined: 10 Oct 2006 Posts: 3579 Location: Home sweet home.

It's been a while Slip, when do we get part II? This is a great writeup, I would love for it to be a series of great write-ups. Wink

Rigger
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Ravenblack · Posted: Wed May 14, 2008 9:09 pm Post subject:

slip · Joined: 15 Feb 2006 Posts: 1084 Location: Mars

I'm still for making good article about internal and terminal ballistics. I have will but I'm short with time. All I can say at this moment is that it's under construction and that it will be done. Hard question is : "when?".

Slip
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frzburn · Joined: 17 Apr 2005 Posts: 286 Location: Canada

Pen&Sword · Joined: 13 May 2009 Posts: 1077 Location: San Diego, CA

This seems pretty :-) cool - and handy!

http://ballistic.zdziarski.com/#fte

Cheers
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Pen&Sword · Joined: 13 May 2009 Posts: 1077 Location: San Diego, CA

VTaero · Joined: 01 Nov 2009 Posts: 3

As to a bullet going unstable when it drops through the speed of sound...

I am not certain, but I believe the problem is that the center of pressure moves ahead of the center of mass.

When a bullet (or anything for that matter), travels at a supersonic speed the aerodynamic forces act at the center (lengthwise). At subsonic speeds, the aero forces act 1/4th of the length back from the leading edge.

It is an elementary principle in rocket science (haha, if thats possible), that the center of pressure must be behind the center of mass for stability.

Additionally by this point the spin has degraded to the point that it cannot keep the bullet inline any longer.

VTaero · Joined: 01 Nov 2009 Posts: 3

Just sitting here thinking about it more and wow is this an incredibly complex problem. Good thing you don't have to understand gyroscopic dynamics it to hit a target or gravity to walk...

Pen&Sword · Joined: 13 May 2009 Posts: 1077 Location: San Diego, CA

slip · Joined: 15 Feb 2006 Posts: 1084 Location: Mars

Sup3rF1uid · Joined: 08 Sep 2009 Posts: 71 Location: Sydney, Australia

Woah... now this is a good article. I'm studying physics at University at the moment, did a bit of basic projectile motion (still 1st year) and what not. Ballistics is actually quite interesting Very Happy

Ok, I know this is kinda jacking your thread, but I figured I'd ask it here seeing as we're dealing with ballistics. Is Plaster's The Ultimate Sniper worth buying? I here there are mistakes regarding mil dots and the likes, but I'm quite familiar with the maths in working with mils.

Is it worth buying for all the other bits and pieces in there?
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slip · Joined: 15 Feb 2006 Posts: 1084 Location: Mars

Yes,
ballistics is interesting, but it really depends on how much time and brain you have. This is not a new science. If you go trough papers 80+ years ago you will see that most of the ballistics we know today were explained back then. It is amazing how much resources military actually spent trying to make a better and more efficient weapon for defence system.
As to Plaster book. I believe that that book is really good, but some parts just do not hold water. Wind reading is really something you wanna skip. That being first. If you really want to read the wind, you will need a spotting scope and you will have to know what to look for when observing trough it. Reading wind is not an easy task and most of the times you will have to combine multiple methods in order to properly read the wind. Even then, you will have to apply something you call experience. My instructor use to say that its not important how many rounds you fire but how you fire them.
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Sup3rF1uid · Joined: 08 Sep 2009 Posts: 71 Location: Sydney, Australia

Slip,

Thanks for the reply. I'll consider it a little more and then we'll see what happens, but thanks for the input, really appreciate it.
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Heisenberg's Uncertainty Principle... so what? The great paradox of Quantum Mechanics, we can be certain of uncertainty.

Buzzard09 · Joined: 17 Feb 2010 Posts: 36

lukasoft · Joined: 19 Apr 2010 Posts: 8

I would love to see some free body diagrams, as I have never studied rocket engineering specifically. I am an engineering student, and would enjoy seeing them for things like COP vs COM.

And for those who English is not their first language, I'm assuming you use the same symbols in physics, so I gave some English equivalents for what the symbols (and equations) represent:
P=Mv - Linear Momemtum (P)
I=MR^2 - Rotational Inertia (I)
τ=Fd - Torque (τ)
ω=dθ/dt=v/r - angular velocity (ω)

I just figured I would try to put up some terms that people seem to be taking about. For example Slip talks about rotational forces, although, I may consider the force on the side of the bullet to be friction against the wind (which is a force-but when I hear rotational forces I think torque). This friction with air is what causes the sliding effect called the "yaw of repose". The equation "6 x (M/200)(M/200) / 10" seems like a very simplified version of the yaw of repose equation and may have been created from experimental calculations as opposed to the actual physics (as the yaw is based on many factors, not just distance). I don't know how it was derived though. Also, I think this would be a negligible amount of drift unless you are at extreme distances, like beyond 1000m (but I may be wrong).

Hope my post has been helpful, and seeing any physics diagrams would be great.