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Penatration, and what causes it.

A place to ask general spud cannon related questions.
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Unread postAuthor: joannaardway » Tue Apr 17, 2007 9:27 am

Can't blame you. It took me a while to notice.
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Novacastrian: How about use whatever the heck you can get your hands on?
frankrede: Well then I guess it won't matter when you decide to drink bleach because your out of kool-aid.
...I'm sorry, but that made my year.
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Unread postAuthor: jackssmirkingrevenge » Tue Apr 17, 2007 9:40 am

joannaardway wrote:I hate to tell you this, but area is measured in square centimetres, not cubic centimetres.


curses, foiled again!

...and to think I even went to the trouble of making it look <sup>prettier</sup>

:oops:

Which is what makes the arrow such a neat projectile: the old cultures were really onto something! Achieving aerodynamic stabilization and high sectional density all in one.


That's one thing which always struck me, after all these years of cannon-balls and shells we're now back to the arrow in design terms :roll:

Incidentally, the increased penetration of the arrow's design is the reason why it's possible to take down large game using bows that put out much less ft/lbs of energy than their firearm counterparts.
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Unread postAuthor: boilingleadbath » Tue Apr 17, 2007 3:45 pm

I got bored once, and analyzing penetration data into ballistics gelatin of various handgun rounds, found that penetration was predicted fairly well by M*V/A<sup>2</sup>, not MV<sup>2</sup>/A<sup>2</sup>.
Note that at, even at handgun velocities, tissue acts mostly like a fluid, so the retarding force on the projectile can be predicted pretty well by our typical fluidic drag formulas.

And I'd argue that it's not the high sectional density of the arrow that allows it to take out the large game, but rather the fact that it has very low drag in tissue (because it's pointy, and the blades don't add much, being... swept back and pointy) and makes an large wound cavity (comparable to that created by fire arm that is sin(60<sup>o</sup>)*1.5 inches wide, perhaps?)
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Unread postAuthor: jackssmirkingrevenge » Tue Apr 17, 2007 10:24 pm

And I'd argue that it's not the high sectional density of the arrow that allows it to take out the large game, but rather the fact that it has very low drag in tissue (because it's pointy, and the blades don't add much, being... swept back and pointy) and makes an large wound cavity (comparable to that created by fire arm that is sin(60o)*1.5 inches wide, perhaps?)


A spitzer bullet is also pointy and has even less drag than the arrow, but fired at the same velocity will tend to penetrate less because it doesn't have the sectional density to back it up and maintain its momentum.
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Unread postAuthor: LucyInTheSky » Wed Apr 18, 2007 12:04 pm

Dosnt that mean its the momentum, not the KE cousing the deeper penatration?
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Unread postAuthor: jackssmirkingrevenge » Wed Apr 18, 2007 12:24 pm

LucyInTheSky wrote:Dosnt that mean its the momentum, not the KE causing the deeper penatration?


In a word, no. I refer you again to the airgun pellet example:

A typical 0.22 pellet weighing 16 grains travels at 581 fps for 12 ft/lbs.

A typical 0.177 pellet weighing 8 grains travels at 822 fps for 12 ft/lbs.

The 0.177 pellet only has 70% of the momentum of the 0.22 pellet, but will penetrate further by virtue of its higher velocity and smaller diameter.


In this case, kinetic energy is equal and it's the projectile with the least momentum that penetrates further (It's assumed both pellets have the3 same shape and are made of the same material). For the purposes of the argument, I'm assuming that we're talking about depth of penetration here as a measure of "good" or "bad" penetrators.

There are simply too many variables in penetration to be able to categorically state that one of them is the major factor in penetration.

Can I ask why you're concerned with this particular matter, perhaps we can offer more specific advice?

Freud would have a field day interpreting all this interest in the subject :roll:
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Unread postAuthor: LucyInTheSky » Wed Apr 18, 2007 12:34 pm

Well the reson i mainly wanted to know because i was wondering one the bus about the car example, then i thought how fast would the car have to travel, then i thought obviously so the KE was equal, then i thought no, maybe its the momentum.

My bus trips to the gym are like a worlwind of mystery lol
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Unread postAuthor: jackssmirkingrevenge » Wed Apr 18, 2007 10:27 pm

In the car/bullet scenario you mentioned, the difference in velocity is tremendous, so if the target in this case were to be a steel plate of considerable thickness, I'd say the bullet would penetrate it while the car would simply deform it. Again, there are many variables to be taken into account, if the target were to be a soft medium like ballistic gel the car might well have the edge.
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Unread postAuthor: dewey-1 » Thu Apr 19, 2007 5:26 pm

--
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Last edited by dewey-1 on Sun Apr 20, 2008 4:17 pm, edited 1 time in total.
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Unread postAuthor: bboymatty » Mon Apr 23, 2007 7:09 am

i think in the case of cannons, where we are dealing with low end pressure, its really a matter of what your firing, what your firing it at, and with what pressure.

I once shot a potato right the whole way through a top dollar boogie board a few months back, and right now i fire masonry nails with my latest cannon that completely embed themselves in most materials including hardwoods (thats 3.5" penetration").

If i had to rank them:

1. pressure
2. what your firing at
3. Projectile
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