Bullet stabilization, strictly RPM?

[NYLES]

blacko,

Those are terms used by the wiz kid's in here that really know way to much about bullets and such....way to complicated terminology for us common folk. Easiest way to spot one of thier type is look for the palm pilot and wind meter they also carry some type of a gadget for everything....You wont hear sounds like umm, hmm, or whaa?...they generally talk way to fast also.

I had to ask what ULD RBBT was....Sometime I feel the same as you, kinda like handing a 2nd grader a 12th grade algebra book. You know that dumb look you get?

Read the post on Parralax V/S Focus....It got me so darn confused that I now have trouble rembering what that knob is for.

 

[devildoc]

I'd have to differ on the faster spin effecting terminal performance. Any centrifugal force of the bullet is kind of internal to that system, sure it takes a bit of frictional force to stop it in the terminal media, but that little bit of spin is insignificant to the terminal performance of the bullet when compared to the momentum.

 

[X-man]

Yes I too would like to know what sg means.

Something way back in my memory bank has me thinking it is some sort of relationship between bullet length and barrel twist??

Mikecr?

 

[LewisH]

Nyles, don't get down on Blackco, I think the rest of us would like to know about these designations Sg and ICAO; what they mean, their significance, and how they're derived.

Mikecr and Overbore won't bother to educate us if you ridicule them for their knowledge.

 

[NYLES]

No way was I gettin down on Blacko I was just admitting that I to sometimes have no idea what you guys are talking about.........way over my head! Just letting him know he wasnt the only one lost in space on the subject. We figure it out one day!

 

[Mikecr]

There is a good article about it here:
Lilja Precision Rifle Barrels - Articles: Effects of Altitude and Temperature on Rifling Twist

and here
How do bullets fly?

some here
exterior ballistics

more
Twist rate - Wikipedia, the free encyclopedia

The best book I've read on it is 'Modern Exterior Ballistics' by R.L.McCoy.
This is not easy stuff to understand, or simplify. It is afterall, part of rocket science.
There is Greenhill, barrelmaker tables, and a few other rules of thumb out there. All will fail tests quickly, and some(like an nfa spreadsheet) are nothing more than monkey math. That is, they defined extremely narrow observations with math. Then put it out there as though it would work across the board. WRONG, anything outside that narrow window -falls apart completely.
Working on it for over 10yrs(very part-time), I have not been able to put together a rule of thumb which would pass all tests. I currently use McCoy's code(slightly modified) which also encompasses much of the math within JBM's calculators.
JBM - Calculations
Even McCoy's math will not predict all. It will not pass ALL tests either. His testing was limited too, although not so much as for Greenhill. But it does take way more into account than ANY rule of thumb.
To perfect this area of science requires more testing, simple as that.

 

[devildoc]

Well I'm not familiar with all of the units that Mikecr was using but I think I got the gist of what he was saying, and agree with it. So I'll take a stab at breaking it down barney-style
Basically displacement per turn shows the relationship between how much gyroscopic stabilization there is for how much drag there is.

Since we're dealing with a projectile that has a center of drag in front of the center of mass we must have more gyroscopic stabilization than drag force, which will cause the bullet to "tumble" or destabilize.

Since how much air the projectile displaced is proportional to the drag, and how fast it's spinning is proportional to it's gyroscopic stabilization, how much air the projectile displaces in relation to one revolution tells us whether the projectile is stabilized or not.

So, what that means is, the further from the center of drag the center of mass is (longer bullet), and the faster we push the projectile (more displacement/drag), or the higher the amount of drag on the nose of the bullet (frictional coefficient), the faster the bullet must be spun to stabilize it.

I think I got it right but if someone wants to correct me, please do.
(had to edit a couple times before it looked like my thoughts came out right)

 

[Mikecr]

devildoc, I think thats as good of a way to see it as any out there.
And if you think about it, it's right there in front of us..
An 8:1 twist barrel is causing the bullet to displace 8" per turn, as released from the muzzle.
8" of dense air that a completely unbalanced bullet has to traverse, without tipping. If it were balanced better, like a yard dart, it could get by without spinning at all.
Down range the bullet has slowed, but RPMs have not really, so the bullet displaces less air per turn. It's effective twist rate increasing all the way, and so is it's gyroscopic stability factor. Except, drag is changing downrange as well. With most supersonic bullets drag goes up as the velocity slows(peaking at mach1). So even with less and less displacement per turn downrange, that air seems denser to the bullet. This counters increasing stability and with a -very good- bullet, Sg downrange climbs at a reasonable rate instead of going through the roof(described as 'over-stabilization'). There is dynamic stability that comes into play here, and some of our bullets are not -very good- in this regard(like a 155smk).
But I know of no accurate predictions for dynamic stability. It's usually found through testing(with radar).

There is additional wiggles to all this and I sure as hell don't have a firm grasp on them. But stability directly comes down to displacement(not time or speed), for that turn to overcome.
How does that single turn overcome it?
I don't remember gyroscopic theory or even application anymore. I just look at it now like that turn in 8" is enough to keep the bullet from knowing which way to tip, given it's desire to do so. In 12" it may figure it out, no matter what!

 

[X-man]

After reading this stuff suffice to say that sg is a calculated number ballistic scientists use to explain gyroscopic projectile stability.

In general terms a value of 1 is "bad" 1.3 should work and 1.5 is considered "good".
Here is a link to the formula used (http://www.nennstiel-ruprecht.de/bullfly/gyrocond.htm). Trying to sort out the equation somewhat sucks you in to a bit of a rabbit hole.....It is much more complicated and deep than it at first appears.

The JBM link Mikecr provided tells most of us all need to know.

One item of interest is that a bullet will not stabilize in outer space. Weird stuff and way out of my depth!

Thanks for the mind warp!

 

[devildoc]

"One item of interest is that a bullet will not stabilize in outer space. Weird stuff and way out of my depth!"

It's not over your head at all, And they're wrong, gyroscopic stabilization still works in outer space, as a matter of fact there is all kinds of gyroscopic controls on a space shuttle, for the reason that it works in space. However, it's inconsequential since there is no drag in outerspace.

 

[CatShooter]

One item of interest is that a bullet will not stabilize in outer space. Weird stuff and way out of my depth!

You could not be more wrong if you tried.


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[X-man]

You could not be more wrong if you tried.


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Interesting Cat Shooter.

If I am wrong I am at a complete loss. I thought that drag was one of the key functions of bullet stability.

Perhaps you can explain your statement and help me to understand?

 

[CatShooter]

Interesting Cat Shooter.

If I am wrong I am at a complete loss. I thought that drag was one of the key functions of bullet stability.

Perhaps you can explain your statement and help me to understand?

A spinning bullet is a long axis gyroscope. The physical laws of gyroscopes have nothing to do with the presence or absence of air (or any other gas).

Gyroscopes work in vacuum as well as in atmosphere... actually better.

It's not my "opinion" it's just the way it is.



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[X-man]

I re read the links and realized that drag is the root cause of bullet instability....No drag in space so the bullet should fly just fine!

Thanks for the patient explanations.