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Most Efficient Projectile Shape

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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Unread postAuthor: jackssmirkingrevenge » Wed Nov 12, 2008 3:56 pm

How about a frangible dart filled with powder or dye that will disintegrate on impact and give a puff of smoke (without being a pyrotechnic device) that will give an observer a good indication of where to look.
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Unread postAuthor: CYANIDEGENOCIDE » Wed Nov 12, 2008 4:21 pm

did you wiki drag coefficient? there is a pretty good page there to get you started
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Unread postAuthor: jackssmirkingrevenge » Wed Nov 12, 2008 4:56 pm

CYANIDEGENOCIDE wrote:did you wiki drag coefficient?


An easy route is to take a look at state of the art anti-tank kinetic munitions that represent the ultimate in aerodynamic efficiency - as their function is to retain as much velocity as possible downrange, they cannot afford to be made otherwise.

Recent developments however seem to favour slightly less aerodynamic (lower length to diameter ratio) form in favour of greater strength, but in general this is the sort of shape you want for maximum range.
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Unread postAuthor: CYANIDEGENOCIDE » Wed Nov 12, 2008 5:28 pm

I disagree, Jack. APFSDS is as you say designed "to retain as much velocity as possible downrange", this does not produce the most areodynamic design, fins are inherently drag prone, they do stabilize the projectile but there are other means of doing that. APFSDS also moves several times the speed of sound and it needs a point on the tip to concentrate the force. The OP has none of these requirements or parameters, I am of the opinion he would be better served with a tear drop shape with the tip composed of lead and the remainder of the body some lighter material wood perhaps. Just the same as a shuttlecock; center of mass ahead of center of drag.
I do recognize the value of letting someone with better funding do the leg and reaping the rewards, my point of contention is I don't think this analogous to KE weapons.
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Unread postAuthor: rp181 » Wed Nov 12, 2008 5:33 pm

a teardrop shape with fat part pointing forward is effecianct, alot of projectiles neglect the fact a vacume created behind a projectile with a sharp decrease.
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Unread postAuthor: jackssmirkingrevenge » Wed Nov 12, 2008 5:55 pm

CYANIDEGENOCIDE wrote:fins are inherently drag prone, they do stabilize the projectile but there are other means of doing that.


Any method of stabilisation is going to induce "drag" of some sort. The reason modern tank guns have smoothbore barrels is to avoid the friction causeb by rifling.

APFSDS also moves several times the speed of sound and it needs a point on the tip to concentrate the force.


Not necessarily, modern shells are "capped" in order to spread the force of impact over a wider area to avoid shattering the shell. Not APFSDS though, granted.

Image

In any case, consider that while flying, a projectile is essentially "penetrating the air", therefore anything which makes it good at punching holes in armour (reduced drag form, high velocity, high sectional density) will give it better resitance to losing speed in flight.

I am of the opinion he would be better served with a tear drop shape with the tip composed of lead and the remainder of the body some lighter material wood perhaps. Just the same as a shuttlecock; center of mass ahead of center of drag.


Moving the centre of gravity still means the projectile is drag stabilised as it would be if it had fins. Besides, there are other advantages to the APFSDS type projectile other than aerodynamic shape, namely light weight in the barrel that will give a much higher velocity than a full bore projectile would, while retaining the large surface area for the pressure to act on while in the barrel.

As a practical example, look at the ammunition fired by the K5 German WW2 artillery piece. This was purely a long range heavy gun made for bombardment, with no intention of penetrating armour. The Peenemünder Pfeilgeschoss developed for it is a discarding sabot finned round, and with it the K5 could fire almost 100 miles away, much more than it could with conventional full bore rounds.

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Unread postAuthor: CYANIDEGENOCIDE » Wed Nov 12, 2008 8:05 pm

I agree the sabot shod projectile will yield the highest muzzle velocity, but the teardrop shape is the most areodynamically efficient shape; that is why rain takes this form instead of being shaped like darts :D. I think finned projectiles are uniquely adept at their designed function: poking holes in things, thus the reason for their adoption by military forces, not necessarily because of their areodynamic value. Even modern naval ships use bullet shaped projectiles and they are capable of reaching out to 20 miles, if the APFSDS projectile was the best areodynamically why not use it in the naval capacity as well? It would be easy enough to implement with bearings in the sabot to negate the rifling.
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Unread postAuthor: Ragnarok » Wed Nov 12, 2008 8:27 pm

CYANIDEGENOCIDE wrote:but the teardrop shape is the most areodynamically efficient shape; that is why rain takes this form instead of being shaped like darts.

No no NO!!! Where did you learn that?!?!

Rain is not teardrop shaped. Any liquid in freefall takes a spherical shape under it's own surface tension.
Look up shot towers if you must. These were a way of making round ball for muskets. Molten lead was fed through a grate so it broke up to droplets, which took spherical shapes. These droplets then cooled over a fall of a hundred feet or so, finally falling into water at the base, setting them into their shape fully.
If they made teardrop shapes, then they would have been useless.

Teardrops are not aerodynamically efficent. They have a horrible sectional density, and travelling point last, they pose a nose with a tangent directly perpendicular to the direction of travel. This is not good, as the pressures created by the air flow then have a vector directly opposed to the projectile's travel. If what you said was in the slightest true, why would the military use darts over teardrops? Answer: They wouldn't.

Teardrops are aerodynamically superior to spheres for various reasons, but that's like saying a Baboon's arse is prettier than Margaret Thatcher. (EDIT: Now corrected)
It may be true, but they're both uglier than sin!
Spheres and teardrops are both very poor aerodynamically, especially when compared to a dart (to finish the metaphor, consider the dart equivalent to whichever supermodel you like most)

The reason that darts are not used in a naval capacity is that the munitions fired from ship guns are explosive, and typically lobbed. The actual kinetic energy or penetrative capacity of the shell itself is fairly irrelevant, as it's the explosive charge that does the job.
Darts are kinetic energy weapons (not explosive), require direct hits, and are thus best fired near flat.
The two should not be compared.

I recommend you take some time to brush up on your aerodynamics knowledge.

PS: If you are a baboon, and thus find Baboon rear ends attractive, consider the dart to be the Ape's buttocks, the teardrop to be the model and the sphere to be the infamous Iron Lady.
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Last edited by Ragnarok on Thu Nov 13, 2008 3:47 pm, edited 1 time in total.
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Unread postAuthor: CYANIDEGENOCIDE » Wed Nov 12, 2008 8:50 pm

No no NO!!! Where did you learn that?!?!


well highskrewl and
here http://wiki.answers.com/Q/Most_aerodynamic_shape
here http://cr4.globalspec.com/thread/22706/ ... amic-Shape
here http://www.imps4ever.info/tech/aero/tech_aero.htm
here http://www.f1-country.com/f1-engineer/a ... amics.html[/url]
these are off the 1st ten results when googling "most areodynamic shape"

so I guess everyone else is wrong?
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Unread postAuthor: rp181 » Wed Nov 12, 2008 8:55 pm

im not sure about the teardrop being effeciant, but i agree with him about water falling as spheres. As water falls, addhesion (or is it cohesion?) pulls it together, making each molecule touch as many molecules ( i think). Theodore gray did a section on shot towers in popular science with the molten lead =p

It seems that the best shape goes from a point, widens out, and goes back to a point, like 2 tear drops put together, one flipped. Spheres are very effeciant for low speeds, be cause it has the lowest surface area for a given volume. Naturally, the longer the projectile, and steeper the angle, the better it pushes air away, and is more effeciant.

In addidtion, Jet's are shaped like tear drops, but the point is added to prevent shock. Wings are also tear drop shape.

Also, you said its for a science fair, so shouldn't you being testing all these shapes?
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Unread postAuthor: CYANIDEGENOCIDE » Wed Nov 12, 2008 9:05 pm

Alright I was wrong about rain, shot towers, anna nicole marrying for love, global warming, staying at a holiday, electricity and water, lunchmeat and refrigeration, hamsters, antique computers increasing in value, gasoline as aftershave, sandpaper condoms, speeding through school zones, government conspiracies, and using IIT during the day.

when you figure out the most areodynamic shape send me a pm cuz im done with this thread
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Unread postAuthor: Ragnarok » Wed Nov 12, 2008 9:37 pm

CYANIDEGENOCIDE wrote:So I guess everyone else is wrong?

Everyone else hasn't written ballistic simulators and isn't doing a degree in mechanical engineering - which oddly enough, describes me.

In turn:
Wiki answers - Useless. You might as well ask someone at the bus stop, and they'd be as likely to be right. The average person's accurate science knowledge is about nil. (I for example, am not average, and pride myself on being NOT normal)

CR4 - Electroman has exactly the right answer. Cut cross-sectional area, and angle all faces away from the direction of air flow. Get all surfaces as smooth and frictionless as possible.
Del the Cat, techno and Anonymous Hero are also on the right line.

Autozine: Utterly incorrect. The drag co-efficent of a true (spherical) water drop is approximately 0.4 - that of a tear drop is closer to 0.3. Only the best of darts can hope to break 0.1, let alone 0.05 for a Cd.

And your last link is broken.

when you figure out the most areodynamic shape send me a pm cuz im done with this thread

I can't help but profess a degree of relief. I would only feel compelled to correct your errors. :roll:

I would PM you, but assuming you actually read Electroman's posts on the CR4 forum, you already know.
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Unread postAuthor: jackssmirkingrevenge » Thu Nov 13, 2008 2:11 am

CYANIDEGENOCIDE wrote:I agree the sabot shod projectile will yield the highest muzzle velocity, but the teardrop shape is the most areodynamically efficient shape; that is why rain takes this form instead of being shaped like darts :D. I think finned projectiles are uniquely adept at their designed function: poking holes in things, thus the reason for their adoption by military forces, not necessarily because of their areodynamic value.


As Ragnarok pointed out, while being very aerodynamically efficient, darts can hold very little explosive and therefore while being able to reach out further, when they actually get to the target they offer a much smaller damage radius, hence the retention of full calibre shells.

Incidently, OTO-Melara are developing a sub-calibre naval round called the Vulcano - modern guidance systems mean that direct hits are much more likely and therefore, a lot of explosive is not essential and advantage can be taken of the increased range of saboted darts.

As another example, look at the Martlet family of gun-launched space vehicles. Certainly not designed to hit anything, just reach out as high as possible, "poking holes through space" as it were - and still FSDS shape.

No doubt the teardrop is an efficient shape, ask the navy ;) - but in the world of high speed projectiles, a saboted dart has the best characteristics both inside (low weight for a large surface area) and outside (high weight for a small surface area) the barrel.

when you figure out the most areodynamic shape send me a pm cuz im done with this thread


The point of forums is discussion ;)
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Unread postAuthor: psycix » Thu Nov 13, 2008 3:34 pm

rp181 wrote:Spheres are very effeciant for low speeds, be cause it has the lowest surface area for a given volume.

But since only the frontal area counts (well, other surfaces also count on drag, but surely not as much as the frontal area), cylinders may have an even lower surface area for a given volume.

When searching for the most effective round in terms of aerodynamics, you will probably come out to a projectile with a low frontal area, yet a high mass and pointed tip. And this applies to most darts.


@CYANIDEGENOCIDE
Please note that a dart with a pointed tip approximates the shape of a teardrop: pointy, and long-ish, causing a low frontal area in respect to its lenghth and mass.


Ragnarok wrote:Rain is not teardrop shaped. Any liquid in freefall takes a spherical shape under it's own surface tension.

Spherical, but flattened due to the force of the air resistance when they are larger then 1mm.
Half spheres would describe their shape best.
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Unread postAuthor: Brian the brain » Thu Nov 13, 2008 3:39 pm

saying Margaret Thatcher is prettier than a Baboon's arse.


Maybe a bit embarassing, but you got it the wrong way around...change it before people see it...




how could you insult a fellow primate like that...

A thin long needle shape should do best, if you cab stablise it in flight.
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