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spudgun range, are we falling short?

A place to ask general spud cannon related questions.
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Unread postAuthor: ramses » Sat Apr 02, 2011 2:16 pm

Tungsten Carbide is also commonly used for penetrators. Remember Larda's ETG? Someone who works in a machine shop might be able to liberate some carbide tooling.

The issue of metal being too heavy could be solved with a sabot.
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Unread postAuthor: jackssmirkingrevenge » Sat Apr 02, 2011 2:23 pm

Some examples of sabot rounds in a carbide cannon:

Image

Image

... though apparently not strong enough to resist the shock of firing:

Dart hit the ground about 25 yards out and the fins were all bent up. I think the sabot disintegrated during firing and the dart was never accelerated to full velocity
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Unread postAuthor: Ragnarok » Sat Apr 02, 2011 2:26 pm

jackssmirkingrevenge wrote:I disagree, aerodynamics is certainly a factor.

It's a factor, but it's rather trivial in comparison to the square/cube law's effects on the matter.

The darts I designed should've clocked at about 300 m/s terminal velocity.
While the Tallboy had a terminal velocity of about 1100 m/s - if you make it 70 times smaller, matching the size of the darts, its terminal velocity is suddenly more like 130 m/s.

It's a square root (ignoring Mach number effects). Make it four times bigger, its terminal velocity will double. Hence, when you're scaling the design down by the kind of factors we're talking here, it's a massive factor on terminal velocity.

Drag coefficient is (usually) preserved when you scale something, Terminal velocity is not.
And as far as drag coefficient, the Tallboy had a Cd of around 0.1 - which isn't bad, but it's possible to do far better.
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Unread postAuthor: jackssmirkingrevenge » Sat Apr 02, 2011 2:36 pm

Ragnarok wrote:It's a square root (ignoring Mach number effects). Make it four times bigger, its terminal velocity will double. Hence, when you're scaling the design down by the kind of factors we're talking here, it's a massive factor on terminal velocity.


Ah, but isn't terminal velocity inversely proportional to the cross-sectional area, which is reduced when you scale it down?

Small air dropped projectiles that relied on gravity alone have been used effectively since the dawn of powered flight and the concept is still alive today in the shape of the CBU-107 passive attack weapon.

Image

The future is even more daunting.
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Unread postAuthor: Ragnarok » Sat Apr 02, 2011 3:57 pm

jackssmirkingrevenge wrote:Ah, but isn't terminal velocity inversely proportional to the cross-sectional area, which is reduced when you scale it down?

Mass is proportional to the cube, cross section to the square. So if you halve the size, then mass reduces by a factor of 8, and the cross section by 4. 1/8th mass divided by 1/4th area = halved Sectional density.

Terminal velocity is when the drag equals weight. Drag has quartered because of the halved dimensions, but the weight has eighthed.
Hence, lower terminal velocity.

Because drag is a quadratic function, terminal velocity is proportional to the square root (because we're going backwards through the quadratic) of the dimensions, assuming the two objects are "similar" (i.e. same shape and density, if not the same size).

It's fairly simple. If you make a dart longer, its SD has obviously improved, yes? (If you make it wider, then its increase in mass is exactly proportional to its increase in area. But if you make it both wider and longer, its additional length will give it a higher SD)
So it's got a better ballistic coefficient. And things with better ballistic coefficients are less affected by drag, so they should also have a higher terminal velocity.

It's the reason why a rain drop an eighth of an inch across has a terminal velocity of a few metres per second, but a human, despite being more or less the same density (and a less aerodynamic shape than a more or less spherical raindrop), has a terminal velocity of more like 50-60 m/s.
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Unread postAuthor: MrCrowley » Sat Apr 02, 2011 4:00 pm

MrCrowley, you said your barrel diameter was 45mm right? Or is it in inches, 1 3/4” ? Please give me a very precise value

I believe it's 42.3mm internal diameter. I will double check that though.

There's always the sabot solution. To keep the dart small (metal?) with a significant density, Surely the best choice in term of efficiency. But there's the issue of finding it after the launch Confused Unless it's long enough, like a small arrow.

I think this is the best choice. As JSR was saying before, small diameter and high density is probably best. I imagine a good diameter for the dart body would be no more than 15mm and no less than 8mm. The problem with this is if you weld fins to the back of it, the CG is way at the back where the fins are. I'm not sure what can be done to keep the CG at the front.

I don't think plastics would be a good choice, not sure if they could stand up to the forces and temperatures, aside from the fact they're not very dense.

I share Ragnarok's concern about safety. This beach of yours MrCrowley, can you be 100% sure there will be no one there when you fire the dart? Do you have a clear view all the way?

Yes, the beach is usually quite clear (visually) and there is virtually zero foot traffic. With binoculars we can see quite clearly along the beach. I think the most difficult problem would be that we wouldn't be sure how far it would travel, but with 2 miles of beach, we should be fine.
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Unread postAuthor: LeMaudit » Sat Apr 02, 2011 4:30 pm

MrCrowley wrote:
There's always the sabot solution.

I think this is the best choice.


Cool for me. I don't particularly like working with plastic. I like much more metal. No problem with the GC at the tip, my first choice would be a lead core inside a hollow pointed brass tip, and tail + fins made out of aluminum. I have all the material at hand for that in those sizes. I even have low melting point alloy, never used it but I remember it was quite heavy.

And the shape would be more like the Tall Boy JSR was talking about, so the weight will be concentrated to the tip while still having a rigid but light tail. for rigidity and alignment I think it would be better that the fins would not be welded but machined from a single block of aluminum. Besides I don't know how to weld properly :lol:

Now for designing the sabot, I'll have to research a bit, but maybe you (or others) already have at hand some links and ideas?
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Unread postAuthor: MrCrowley » Sat Apr 02, 2011 4:53 pm

the shape would be more like the Tall Boy JSR was talking about

That sounds like a good design to use, the body is similar to (A) in the designs below:
Image
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Unread postAuthor: LeMaudit » Sat Apr 02, 2011 5:02 pm

I found also that, not bad drag coeff. and the shape is a bit simpler to make:

http://www.ordtech-industries.com/2products/Bomb_Practice/BDU33.html

With some luck the tail could stick out the sand :lol:


[edit][ found some interesting reference for sabot here:

http://medlibrary.org/medwiki/Sabot
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Unread postAuthor: Hotwired » Sat Apr 02, 2011 6:43 pm

Ragnarok wrote:
jackssmirkingrevenge wrote:The Grand Slam (1) would approach the speed of sound by gravity alone, so it's a good design to emulate :)

It achieved that by virtue of its mass and sectional density, not any particular aerodynamic mastery.


Ragnarok wrote:
jackssmirkingrevenge wrote:I disagree, aerodynamics is certainly a factor.

It's a factor, but it's rather trivial in comparison to the square/cube law's effects on the matter.

It's a square root (ignoring Mach number effects). Make it four times bigger, its terminal velocity will double. Hence, when you're scaling the design down by the kind of factors we're talking here, it's a massive factor on terminal velocity.


Discussing the merits of shape vs mass is secondary to being able to put enough energy behind it for TV to matter.

A massive object may well have a much better TV but your energy source will have to be proportionally larger.

The earthquake bombs had gravity x many miles and that's an energy source to envy. But being slick wasn't their only purpose. They had to meet practical dimensions, carry a large payload and be hardened enough to deliver the payload.

So for what it is, it's a good shape.

But for a dart going for range... long skinny thing with fins.
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Unread postAuthor: LeMaudit » Sat Apr 02, 2011 6:58 pm

Hotwired wrote:But for a dart going for range... long skinny thing with fins.


I was also thinking that while looking at high performance sabot. The purpose of a bomb is to carry some (nasty) payload. For pure bullet speed, I guess the problem is different.

Still, there should be a good compromise between :

real life sabot bullet / potato gun ammo
enormous speed / reasonable speed without magic powder
very heavy metal / what we found at the hardware store
very small and high penetration / want to be able to find it with luck

The goal is to find a good compromise just to stay nicely in the air without too much turbulence.
And there's the fun effect too, a deadly killing dart have little appeal to me. A miniature Tallboy, that's something :-D
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Unread postAuthor: Technician1002 » Sat Apr 02, 2011 7:22 pm

In regards to soft sabots and damage when fired, i have had a pool noodle pass a golf ball in my 2.5 inch barrel. I quickly learned that a foam sabot collapses upon application of a couple bar pressure. This was solved by using a hard lightweight shell for a sabot. The foam inside the shell is simply used to orient the round and center it, not push it.

The AA battery with the sabot was obliterated on impact.

Golf ball sabot
Image

AA battery sabot The foam is only for orientation.
Image

AA battery destruction thread
http://www.spudfiles.com/forums/flat-aa-battery-i-challenge-some1-to-replicate-this-video-t21827,start,15.html
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Unread postAuthor: MrCrowley » Sat Apr 02, 2011 9:50 pm

Looking back on my discussions with Ragnarok there was talk of hollowing the tail of the projectile, filling it with smoke powder and stuffing the end with steel wool and hoping it would ignite.

The only other reasonable way to judge the distance would be to actually hit a target, which would be pretty cool. I'd still like to retrieve the projectile though, so that doesn't really help :?

I think I might do some more reconnaissance and see if there is a 'natural target' that could be hit from about 2000m away, I'd need a clear line of sight range as well which makes it tricky. If I kept the firing angle to around 15 degrees, that should minimize the drift of the projectile due to the wind yet still provide enough 'lob' to get the projectile out to 2000m.

I think any further out than 2000m and there is zero chance of ever finding the projectile or knowing how far it travelled.
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Unread postAuthor: Hotwired » Sat Apr 02, 2011 10:24 pm

MrCrowley wrote: stuffing the end with steel wool and hoping it would ignite.


Yes it would.

There's been videos of oversized handhelds used to fire cannisters of liquid plus steel wool to create fireballs.

MrCrowley wrote:The only other reasonable way to judge the distance would be to actually hit a target, which would be pretty cool. I'd still like to retrieve the projectile though, so that doesn't really help :?

I think I might do some more reconnaissance and see if there is a 'natural target' that could be hit from about 2000m away, I'd need a clear line of sight range as well which makes it tricky. If I kept the firing angle to around 15 degrees, that should minimize the drift of the projectile due to the wind yet still provide enough 'lob' to get the projectile out to 2000m.

I think any further out than 2000m and there is zero chance of ever finding the projectile or knowing how far it travelled.


Tracking beacons.

For only £705 you can get a tiny radio transmitter and a receiver to track it down with.

There's also GPS beacons you can buy, attach to something then pay for a service to locate it with.

Or you could do it the cheap way and rig the projectile to be rather visible/noisy.

However you do it, still a long way to walk to pick it up again.
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Last edited by Hotwired on Sat Apr 02, 2011 10:34 pm, edited 1 time in total.
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Unread postAuthor: LeMaudit » Sat Apr 02, 2011 10:31 pm

more like $100 for a tracking beacon!

http://www.gizmag.com/go/6499/
http://www.loc8tor.co.uk/Store/catalog/Products,110.aspx

That's a fun gizmo!
Maybe they are cheap as dirt in NZ? :lol:

[edit] thinking of it... if you have a iPhone you can use it as a projectile then track it on the web :lol:
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