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Penetration contest

A place to ask general spud cannon related questions.
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Number of sheets of 3/4" plywood

0-2
1
6%
3-8
4
25%
12-15
1
6%
15-20
0
No votes
20-30
3
19%
30+
7
44%
 
Total votes : 16
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Unread postAuthor: Hotwired » Wed Aug 05, 2009 1:14 pm

There's no maybe about it, a saboted dart from the same cannon as a full bore slug will go through more sheets of board.

Same mass same speed, the dart has to go through less board than the full bore slug so with less resistance as it passes through each board it can go further.

Since penetration rather than massive surface destruction is the goal a slug will lose out to a dart if the cannon is the same for each.
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Unread postAuthor: Gippeto » Wed Aug 05, 2009 1:28 pm

If the contest goal were to penetrate a single thick layer, I would agree. But that's not the case here.

To win this contest, the projectile will have to be "unable" to tumble.

I could be wrong of course, but I don't think so. Time will tell. :)
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Unread postAuthor: starman » Wed Aug 05, 2009 2:11 pm

jackssmirkingrevenge wrote:
CpTn_lAw wrote: If such a study were to be made, we could probably find the ideal mass/size projectile.


For pure depth of penetration the ideal is simple, maximum mass and minimum size. The higher the sectional density, the easier its going to be for the projectile to penetrate and the harder it will be for the projectile to lose velocity.

This is why the best penetrators in the world have a tiny cross sectional area but are relatively long and made of the densest materials available ;)

[youtube]http://www.youtube.com/watch?v=MPgdFV19XpQ&feature=related[/youtube]


Hey Jack...........SSssssshhhhhhhhhhhhhhhhhh.... 8)
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Unread postAuthor: jackssmirkingrevenge » Wed Aug 05, 2009 2:40 pm

starman wrote:Hey Jack...........SSssssshhhhhhhhhhhhhhhhhh.... 8)


*nods gently as a gesture of tacit understanding*

:D
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Unread postAuthor: Hotwired » Wed Aug 05, 2009 3:07 pm

Gippeto wrote:If the contest goal were to penetrate a single thick layer, I would agree. But that's not the case here.

To win this contest, the projectile will have to be "unable" to tumble.


I'm failing to see the reason why this puts a rod penetrator at a disadvantage when it in fact has the advantage.

If you do a bad job of it and the penetrator is unbalanced or quite frankly totally unstabilised (i.e. a sawn off section of steel rod) it will tumble very easily.

But who am I to say how it should be done. I'm not entering. I can't get 3/4 plywood round here :wink:
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Unread postAuthor: Hubb » Wed Aug 05, 2009 3:08 pm

Personally, I think the ammo should be the same for the entire contest. With that, it would surely show the potential of a builder instead of a wonderful launcher loosing to a mediocre one.
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Unread postAuthor: DinerKid » Wed Aug 05, 2009 3:30 pm

Hotwired wrote:
Gippeto wrote:If the contest goal were to penetrate a single thick layer, I would agree. But that's not the case here.

To win this contest, the projectile will have to be "unable" to tumble.


I'm failing to see the reason why this puts a rod penetrator at a disadvantage when it in fact has the advantage.

If you do a bad job of it and the penetrator is unbalanced or quite frankly totally unstabilised (i.e. a sawn off section of steel rod) it will tumble very easily.

But who am I to say how it should be done. I'm not entering. I can't get 3/4 plywood round here :wink:


that is a good point i believe the pieces of plywood have to be 6" apart so if part of the rod was still in the first piece of plywood while poking into the second then it would remain very stable and wouldn't wobble around and tumble.
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Unread postAuthor: spudamine » Wed Aug 05, 2009 3:47 pm

I'm not entering either, I've already got one huge PVC monstrosity taking up space, I don't need another one. If I was to enter I'd be going for a saboted dart fired from a large calibre, best to play to PVC's strengths (cheap, readily available in huge sizes) rather than mess about too much trying to increase it's poor pressure holding capability.
As for standardising the ammo, wouldn't that take the fun out of it? All the cannons would have to have the same barrel size so we would never answer this argument and just get a bunch of clone cannons. Not to mention that designing and building decent ammo could be as much as a challenge as the cannon itself.
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Unread postAuthor: Technician1002 » Wed Aug 05, 2009 4:28 pm

jackssmirkingrevenge wrote:
Gippeto wrote:Jack, do you REALLY think a dart is going to win this ??

I predict that the winning projectile will be more...old fashioned.


I'm actually rather taken aback that someone of your background doesn't agree. The fact that all the world's major armies rely on saboted darts as projectiles when penetration is the requirement should be proof enough of the benefits of high velocity resulting from low projectile weight as well as high sectional density.

It may be counter-intuitive, one would think that at the same velocity a 1" ball bearing will penetrate more than a 1/4" dart becaust it's heavier, but the fact is that per unit of projectile contact area, the dart has a lot more mass pushing it through the target medium.

Don't you remember the coin challenge? You don't need a lot of ft/lbs to push a dart-like projectile through a seemingly hard target, it's the reason why bowhunters can take down such large prey with relatively low muzzle energy behind their arrowheads.


Using a larger diameter saboted round is one way to get high energy into a round from lower pressure. The larger diameter round is why in the t shirt cannon, the larger diameter barrel won the testing of various barrel diameters on the 2 inch valve. More speed can be had when combined with a larger valve.

Part of the reason I'm building the 3 inch valve is to test golf ball speeds in a 2 inch, 2.5 inch, and 3 inch barrel. Not all my tests will be just venturi into a smaller barrel.

The time to travel the last foot of the barrel is marked in the photo below if you wish to calculate the FPS of this test. I'll leave the calculation as an exercise for the viewer. The answer is below 300 FPS. It is slower than the test results on the 3 inch barrel. The 3 inch and 2.5 inch did much better than the 2 inch.
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Attachments
4 inch barrel t shirt test.jpg
Acceleration data of a t shirt in the 4 inch barrel. T shirt cannon at 70 PSI with the 2 inch valve.
60 PSI T Shirt 2.5in.jpg
60 PSI 2.5 inch test shot. The measured foot is not the last foot of travel. Travel time is 2.6ms.The speed is considerably higher than the 4 inch barrel at less pressure. The 3 inch did even better.
Last edited by Technician1002 on Wed Aug 05, 2009 4:44 pm, edited 2 times in total.
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Unread postAuthor: Gippeto » Wed Aug 05, 2009 4:29 pm

Seems I have a camp all to myself. :lol:

Not the first time,...likely not the last either. :)

Something to think about;

http://www.brotherswar.com/Perspective-4Pic.htm

This would have been at some distance.

Round ball is "one of those things" which has performance seemingly beyond the laws of physics. Ask someone that shoots muzzle loaders.
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Unread postAuthor: c11man » Fri Aug 07, 2009 12:09 pm

well my cannon just broke catistroficly so im pretty sure i out of the compititon most likely. remember always to check if the bushing is dwv :(
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Unread postAuthor: velocity3x » Fri Aug 07, 2009 10:17 pm

Gippeto wrote:Anyone know just how much energy it takes to punch a 2" hole in a sheet of 3/4" plywood?

This past weekend I built a Box O' Truth with some extremely healthy and extremely fresh 3/4", 5 ply. I shot a golf ball from 10' using 300 psi carbon dioxide. I don't have any numbers but, from that experience I can say that it takes a tremendous amount of energy! . While people are still speculating in the pole, I won't say how many sheets of plywood the GB passed through. I will say that a great many contestants may revise their estimates after they test.
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Unread postAuthor: Moonbogg » Sat Aug 08, 2009 1:01 am

I shot a golf ball at about 1.25" solid wood (don't know what kind) out of my Venom at 2X (cheaply saboted) and it friggin bounced off!
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Unread postAuthor: jackssmirkingrevenge » Sat Aug 08, 2009 1:54 am

velocity3x wrote:I don't have any numbers but, from that experience I can say that it takes a tremendous amount of energy! .


That's the point of the above discussion, a golf ball is actually a very poor projectile when it comes to penetration.

Imagine a golf ball weighs 46 grams with a diameter of 4.3cm, this means that there's around 3.2 grams of mass for every cm<sup>2</sup> of frontal area when it comes to penetrate. The same weight of steel ball will have a diameter of around 2.2cm, meaning it's backed up by 11.8 grams of mass per cm<sup>2</sup> of frontal area, 4 times greater than the golf ball a lot more momentum than the golf ball can offer.

Also, in terms of simple area, that of a golf ball is almost 4 times as great as that of the steel ball of the same weight, meaning a correspondingly large effort is needed to force it through a given material. It's the reason why you can easily hammer a nail through a piece of wood and not a marble. This means that if you use the same launcher and a lightweight sabot, the steel ball will be travelling at approximately the same speed as the golf ball and have the same energy, but will expend a lot less energy travelling through both the air and the target medium and therefore will penetrate much further.

While the launcher is an important consideration in that it must hurl a significant mass at high velocity in order to be competitive in this case, the projectile is just as if not more important if you want to squeeze the maximum performance in terms of inches penetated out of your launcher.
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