Basic Aerodynamic Theory- OTM vs Polymer tipped

[hardlyworking]

To any of you guys thinking of holding back on the sciency stuff... don't.

Thank you to everybody contributing thus far I don't know ANY of this info, (ask me about DNA and I'll set you straight) but even if its above my pay grade, I have little tangents to go googling and can spackle in my gaps.

Keep it up!

[milotrain]

I asked about lead free and they told me it could not be done. The lead was needed for stabilization.

The lead doesn't leak out. Certainly not during flight. Soft point bullets indeed act like polymer tipped bullets but they typically don't have the profile of polymer tipped bullets. A HPBT is simply a function of manufacturing, the hollow point has no other reason for being there.

[badhabit90]

my head just exploded.

however, it is great reading and keeps the theory going....my brain is hungry. more please.....

[jb octane]

Level5Bear
When the tip is added it adds to the overall length of the bullet so naturally it lowers gyroscopic stability that is why we have a Greenhill formula.
If you take an OTM bullet and out it in a tipping die to reduce its diameter naturally the BC goes up. Try putting a sheet of plywood on your front bumper flat and standing. The greater frontal surface area will always create more drag. In short Mack trucks are flat and Corvettes are pointy.
Your comment in number 3 is were you need to keep focusing. Center of mass xy rebated boattails and wastebands should all be brought up to your professor.

A friend of mine is working on a 6mm bullet with a 0.800 BC number right now.

I don't see how your analogy relates to bullets. Unless the plywood was tapered it would still be impacted upon by the shockwave at supersonic speeds. To be more a more accurate anology, polymer tip vs OTM with Corvettes would be a standard Corvette (polymer tip) vs a Corvette with a recessed and concave front. Even then it fails because cars incorporate downforce for stability and bullets need uniform forces for stability.

I agree with you that the rest of the bullet characteristics need to be brought up. I think that the main piece of information to gather from Level5Bear's first post is that the OTM's cavity creates an air pocket that is inherently less instable due to a buffering zone between the bullet and the shockwave. Ballistic coefficient being a factor in long range shooting comes second to stability*, but we have become so advanced in both that stability can be overcome by twist rates and ballistic coefficients are the most notable thing to improve.


*Obvious example is a wadcutter vs a musket. A sphere has a notably higher ballistic coefficient than wadcutter but the stability provided by a twisted wadcutter is far and away more accurate than an untwisted sphere.

[level5bear]

Hello folks, sorry for being so slow on responding back. Just got finished with a round of midterms.

I went in and spoke to Dr. Nossier today and asked a few more questions, as well as grabbed some shadowgrams from the internet instead of the drawings.

Come to think of it, you may have been correct. If you consider the wake behind the projectile, it would act like a barrier of high pressure. And since air moves from high pressure to low pressure, the wake would try to move towards the higher velocity flow (lower pressure) causing the flow around the projectile to turn into itself, thus creating a shockwave. I think the same thing happens with jet/rocket nozzles when you see Mach diamonds.

I looked through some shadowgrams to confirm this and I think you are right. As seen in the image linked below, there clearly is an aft shockwave, but it is not attached to the rear of the bullet as I drew above. It occurs where the flow clearly turns in on itself from the effects of the bullet's wake.

level5bear - do you have any insight into what makes "transsonic stable" bullets stable? I remember reading something about the boat tail design being an important factor...

I do not, unfortunately. I went to Dr. Nossier on this one, and he has no insight to offer, either. He says that transonic aerodynamics are "a complete different animal" and that it is too complex to be modeled accurately using any of the theoretical tools available to us. Most design work done for transonic flight is performed with empirical investigation, or by interpolating between pure supersonic and subsonic models.

To better illustrate why this regime is so complex, I have included a shadowgram of a bullet in transonic flight below. The airflow at the tip of the bullet is subsonic, but then accelerated to supersonic speeds by the curve of the bullet. When the flow reaches supersonic speeds, it creates a complex three dimensional network of normal shockwaves attached to the body of the bullet. These shockwaves are highly disruptive to the flow around the bullet and often result in the "boundary layer" separating from the sides of the bullet. Boundary layer separation creates very substantial turbulence and drag.

Level5Bear
When the tip is added it adds to the overall length of the bullet so naturally it lowers gyroscopic stability that is why we have a Greenhill formula.
If you take an OTM bullet and out it in a tipping die to reduce its diameter naturally the BC goes up. Try putting a sheet of plywood on your front bumper flat and standing. The greater frontal surface area will always create more drag. In short Mack trucks are flat and Corvettes are pointy.
Your comment in number 3 is were you need to keep focusing. Center of mass xy rebated boattails and wastebands should all be brought up to your professor.

It is more complex than the simple fact that polymer tips add length. The greenhill formula and modern alternatives for polymer tips underestimate the stability of OTM ammunition. As I said, I do have empirical evidence showing this to be the case. I pulled the following quote from the study in question as an ideal example of my point:

"It is notable that the original Miller twist rule (Miller,
2005; Miller, 2009) predicts Sg(CD) < 1.0 for the
whole range of experimental data shown in Figure 1
for the 1 in 14” twist .222 Remington. Consequently,
the empty space in the tip of the bullet significantly
enhances (increases) stability and allows the bullet to
be stable under conditions where a constant density
bullet of the same dimensions would not be stable."

If you would like to read the report in detail, it may be found here through the Cornell University Library's webpage here:

http://arxiv.org/ftp/arxiv/papers/1401/1401.4187.pdf

As far as the corvette vs truck example, yes of course a long thin object is usually more aerodynamic than a wide one. Drag is directly proportional to area.

It is worth pointing out, however, that subsonic and supersonic aerodynamics are vastly different. Scale plays an enormous role in boundary layer effects, as well. So, comparing bullets to cars is a very apples and oranges comparison.

In this case, the normal vs oblique shock at the nose is a very important consideration in terms of drag, because the normal shock that creates the subsonic area at the tip of a OTM is very drag intensive. The manner that it effects the drag coefficient goes beyond the simple change in slenderness ratio.

I think that the main piece of information to gather from Level5Bear's first post is that the OTM's cavity creates an air pocket that is inherently less instable due to a buffering zone between the bullet and the shockwave. Ballistic coefficient being a factor in long range shooting comes second to stability*, but we have become so advanced in both that stability can be overcome by twist rates and ballistic coefficients are the most notable thing to improve.


*Obvious example is a wadcutter vs a musket. A sphere has a notably higher ballistic coefficient than wadcutter but the stability provided by a twisted wadcutter is far and away more accurate than an untwisted sphere.

I'd say this is a pretty good summary of the key points above. Thanks for bringing it down to a short succinct form.

Thanks to everyone for coming in and sharing your experiences, questions, and objections. I feel like this has been a fruitful discussion, and I am glad that everyone is enjoying the thread.

[LynnJr]

The hollow cavity means the center of mass is further back so naturally this helps stability.
The polymer tip assures us of a more uniform BC number shot to shot and that is why most long range shooters trim there meplats to a unform length then close the meplat to a smaller diameter to improve the BC number.
Sorry for all the errors in my earlier post auto correct trashed me.

[milotrain]

That movement of the center of mass is also why (typically) FMJ bullets are not match accurate. Because the process is injecting the core from the back, the less "filled" part of the bullet is the open base, this moves the center of mass towards the front. This may also be why the stability calculator is over-conservative with OTM bullets and polymer tipped bullets. It may have been initially created with slugs or FMJ bullets in mind.

I still want to know what unicorn blood they use in the D46 though.

OOOH actually, level5Bear wanna go on another "Dr. Nossier run" and ask about rabbeted boat tails? I always assumed they had increased accuracy because the boat tail is less effected by any swaging effect that the bullet experiences in the bore, but I have no real idea. Corbin's page seems to have some information but he's also trying to sell you dies.

[morthrane]

And here I thought the D166's (and D46) rebated boat tail was just some kind of manufacturing byproduct. There's actually a reason for it?

[LynnJr]

The rebated boattail and now double rebated boattail is put there for a reason but most manufacturers over do it in an attempt to make super bullets.
Ask your professor if the meplat should be 10-15 % of caliber bore. Ask him/her what happens if we make the meplats as sharp as we can with regards to accuracy

[BillyGoatCrawler]

Great topic, OP.

I agree with you that the rest of the bullet characteristics need to be brought up. I think that the main piece of information to gather from Level5Bear's first post is that the OTM's cavity creates an air pocket that is inherently less instable due to a buffering zone between the bullet and the shockwave. Ballistic coefficient being a factor in long range shooting comes second to stability*, but we have become so advanced in both that stability can be overcome by twist rates and ballistic coefficients are the most notable thing to improve.


*Obvious example is a wadcutter vs a musket. A sphere has a notably higher ballistic coefficient than wadcutter but the stability provided by a twisted wadcutter is far and away more accurate than an untwisted sphere.

I was under the impression of the above. To expound on it;

I also had thought that the tipped projectile's stability would suffer some because of the longer tip acting as a more exaggerated moment arm(lever).
Combine that with a the more abrupt disruption of fluid than the OTM and it would further exaggerate the effects.

Another thing I thought might have a small affect is the distribution of mass radially due to longer length of tipped bullets. Given the same weight of projectiles but different overall length, the tipped bullet will have less radial distribution of mass and a lower moment of inertia. This will lower the gyroscopic stability....

All effects combined, lower gyroscopic stability, increased moment arm to create more leverage, and a more defined and abrupt shock wave all culminate to a less stable flight.

[ducky_0811]

I am in no way qualified to jump into this conversation with any REALLY science. But I would like to bring up something that just popped into my head. I am a Marine artilleryman and I know a little bit about aerodynamics. In the 60's artillery could not fire in heavy rain bc of the possibly of having an impact Fuze detonate in flight from hitting the rain.

To solve this problem cross drilled holes were made in the tip of the Fuze that created and air pocket around the tip of the round while in flight. This made the Fuze "rain insensitive" I'm wondering if this could have a measurable affect on a bullet's stability or ability to remain supersonic to longer distances if the same principle was applied to a bullet.

Obviously it's easier to drill a hole in something the size of your first as opposed to the size of your pinky finger but it just crossed my mind.