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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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Most Efficient Projectile Shape

Unread postAuthor: Floyd » Tue Nov 11, 2008 8:21 pm

Sooo, its that time of year where all the schools start a science fair and my question is......

What is the most efficient shape for a projectile when fired out of a smoothbore pnuematic cannon on the distance it flies.

So, could anyone put links to helpful sites they know of or do a quick search? It would be very much appreciated.

I would need websites to site them, but if anyone has anything they would like to add please do.

Edit: Fixed question.
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Last edited by Floyd on Tue Nov 11, 2008 8:43 pm, edited 1 time in total.
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Unread postAuthor: Lentamentalisk » Tue Nov 11, 2008 8:35 pm

Think golf ball.

Nobody has yet figured out why exactly all of those dimples make it fly better, but it reduces turbulence on the trailing edge, so it cuts the air better.

If you are looking for some sort of dart, that changes things. It also depends on what the hell you mean by efficient, as efficient is a relative term, that means absolutely nothing, and has no parameters of it's own.
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Unread postAuthor: Floyd » Tue Nov 11, 2008 8:39 pm

Lentamentalisk wrote:If you are looking for some sort of dart, that changes things. It also depends on what the hell you mean by efficient, as efficient is a relative term, that means absolutely nothing, and has no parameters of it's own.


Whoops, I guess I didn't catch that fast enough. I changed it to the distance it flies a minute ago. All shots will most likely be shot at 45 degrees.
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Unread postAuthor: Lentamentalisk » Tue Nov 11, 2008 8:47 pm

well what bore are you considering? Are you taking weight as a constant among projectiles? Are sabboted projectiles allowed? How about ICBMs with on board nuclear power plants and satellite TV? Give us some parameters for your project.

Like I mentioned, you could spend millions investigating where the dimples/ridges would best be placed in order to minimize drag, or you could go with pre-dimple technology, and just do something pointy, and you would not find a huge difference.
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Unread postAuthor: Floyd » Tue Nov 11, 2008 8:58 pm

Well jeez, 1/2"-1" haven't decided, probably go with the 1/2" one. Yes I am keeping everything constant. (weight, angle, pressure, barrel, valve) I will use saboted projectiles if I have to. Due to my budget and legality the ICBM's will mostly be out of the question.

Come on now, be reasonable, I was just asking for some sites to do some more research like on the known ballistic coefficient of different shapes of bullets/shells or similar. I wasn't asking for you guys to do the project for me I was just asking for more help because I searched everything and I want to make sure I know everything I can.
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Unread postAuthor: Ragnarok » Tue Nov 11, 2008 9:05 pm

In terms of sheer range and greatest ballistic coefficent, the answer is a finned dart with a long nose at least 2 or 3 dart body calibres in length, (and a total dart length of 10 or 12 calibres or more) and a boat tail shape at the rear.

Keep the fins as thin as possible to reduce their drag.

However, this is NOT a good shape for "lobbed" range shots (for the record, 45 degrees is only the optimum in a drag free environment - normally it sits at between 30 and 40 degrees depending). It is nigh on impossible to track, will have ranges most likely measurable in miles from any cannon with backbone, and is VERY dangerous when it comes back down.

For a consistent and safe projectile which can actually be tracked, I can suggest no better than golf balls - particularly if you can get backspin on them. Don't get too optimistic on the range though, the upper limit without spin (which is a little tricky to compensate for), even on a golfball doing around Mach 0.5 is unlikely to be more than about 400 yards.

An extra Mach number doesn't add much, perhaps 600 yards total. Even with the dimples, spheres are not an efficent shape for drag.

@Lentamentalisk: That's not true by any means - in this modern age, we have a full understanding of aerodynamics, so we do know why golf balls dimples improve it's drag coefficent. It does much the same thing as a boat tail on a bullet, helping close up the "hole" the projectile has punched through the air, and thus reducing the effects of any "vacuum" formed behind the projectile.
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Unread postAuthor: Lentamentalisk » Tue Nov 11, 2008 9:10 pm

Damn! I was really hoping for that ICBM :wink:
Sorry about giving you a hard time, just trying to get more info out of you.
At that bore, golf balls are out of the question, which is too bad, because they are really good. Barring the newfangled dimple technology, just try to make your ammo really streamlined, with a rounded front, and a long sweeping tail, and make sure to stabilize it with fins. The fins should be straight, but airfoiled on one side, to promote slicing through the air. Look at Ragnarock's recent works on his dart sniper designs for inspiration.

edit: damn, you beat me to it Rag


edit2:
Well we may have an understanding as to how it helps, but as far as I can tell, we are still investing millions in to where we should place them. We know what they do, but not how to optimize them, which is what our man here is trying to do.

edit3: (going to great lengths not to double post)
Just to nitpick, we don't exactly have a complete understanding of aerodynamics, if I am still up to date on this stuff, as if I can remember correctly, it is physically impossible for a fly to fly. Unless that has been explained, in which case, do tell, I am very intrigued.
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Unread postAuthor: Ragnarok » Tue Nov 11, 2008 9:49 pm

Lentamentalisk wrote:Well we may have an understanding as to how it helps, but as far as I can tell, we are still investing millions in to where we should place them.

Yes, this is very much the case. Although we know what they do, we don't know exactly how to apply the science for best results.
For an example - we know exactly what prime numbers are, but we don't know how to find them efficiently.

This is similar to the P=NP problem, a very major concept in science and maths, which states that (paraphrasing) "if the answers to a problem can be checked quickly, can they also be found quickly?"

We can check whether a dimple design on a golf ball is good, but we can't find a dimple design that is automatically better from the start, we have to find many and check them. That is to say, there is no way to skip to the answer. We need to find possible solutions, then check if they really are an answer.

Most experts therefore consider P≠NP - that is to say that finding a correct solution takes more effort than checking the solution.
However, although hypothesised, there is no proof that P≠NP (there may for example, be some simple pattern to the prime numbers we have not yet seen, making their calculation easy - unlikely and disastrous if true, but it is possible), and finding a proof either way is worth a lot of money.

That's not even the barest bones of the problem, but I won't bore you further. Wikipedia has a full article if you really need help sleeping. ;)

Just to nitpick, we don't exactly have a complete understanding of aerodynamics, if I am still up to date on this stuff, as if I can remember correctly, it is physically impossible for a fly to fly. Unless that has been explained, in which case, do tell, I am very intrigued.

You're a little out of date, and quoting an urban myth.

The fly/bumblebee matter assumes fixed wings - which is clearly not the case. With flapping wings, this motion creates vortexes in the air which provide the required extra lift.

I suspect the "Bumblebees can't fly" myth was spawned by someone having a joke (an engineer in a bar perhaps, to amuse his drunk buddies) which then got misinterpreted as a fact, missing the important (but possibly not originally mentioned) fact that it requires a fixed wing assumption, and passed on as true.
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Last edited by Ragnarok on Tue Nov 11, 2008 9:54 pm, edited 1 time in total.
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Unread postAuthor: Floyd » Tue Nov 11, 2008 9:51 pm

Lentamentalisk wrote:Damn! I was really hoping for that ICBM :wink:
Sorry about giving you a hard time, just trying to get more info out of you.

No problem, I should've included it anyways :)

Thanks for all your help.

If anyone else has anything to add, please do.
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Unread postAuthor: sandman » Tue Nov 11, 2008 10:45 pm

when doing golf balls, you will have to get a spotter to get exactly where it lands and use that as the distance, because it will bounce and roll quite a distance and that would result in a large error, but darts will just stick in the ground so u should be good there
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Unread postAuthor: Lentamentalisk » Tue Nov 11, 2008 11:21 pm

your other option is to shell out the big bucks for glow-in-the-dark golf balls, charge them under some lights, and then shoot them at the beach (or in the sandy desert, or onto some soft soil) in the dark. The problem with finding a normal golf ball in sand is that they are similar colored, and going at that speed they will sink in quite a ways.
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Unread postAuthor: sandman » Tue Nov 11, 2008 11:25 pm

um, no not really, golf balls aren't hard to find in sand traps, ive never encountered one deeper than the top of the ball just below the surface, and they usually only dig that much if they have lots of spin on them.
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Unread postAuthor: Ragnarok » Wed Nov 12, 2008 8:40 am

sandman wrote:but darts will just stick in the ground so u should be good there

No they won't - they'll bury straight into the ground and you'll never see them again, part of the reason they're useless for range shots.
Their ballistic efficiency means that they'll come back down at close to the speed they went up.

Golf balls and sand however - not so much of a problem.
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Unread postAuthor: Carlman » Wed Nov 12, 2008 11:29 am

pvc darts were used in a similar senerio science fair wise not too long ago, i assume you did not search for ideas first.

the instructions can be found in the howto section and another popular spudding site.
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Unread postAuthor: Floyd » Wed Nov 12, 2008 3:51 pm

Yeah, I searched, I saw them darts on Burntlatke.

So no one knows of any websites with good info?
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