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machined APFSDS projectiles

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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Re: machined APFSDS projectiles

Unread postAuthor: jackssmirkingrevenge » Fri Nov 08, 2013 3:43 pm

Talk wrote:I googled "Flight path of apfsds" and i found this, may it can help you guys :?


Nice find :)

i also found these projectiles are being discussed on some weaponeer.net site


Yeah I posted them there too on request of the administrator.

That got me thinking. Are you taking into account the fact that lighter materiel accelerates more easily?


The fin is fixed to the body, it can't accelerate more or less than the rest of the dart. What is true is that both muzzle blast and inconsistent sabot separation are causing the dart to yaw on exiting the muzzle - more or less what is detailed in the pdf Talk linked to - which is probably the reason for the erratic flight path.

As a matter of fact, the closer the target, the less time the fins have to correct the tumble.


Yep, I expect that towards the end of their trajectory yaw must have been minimal. Heaven knows what part of Arizona they landed in :roll:
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Re: machined APFSDS projectiles

Unread postAuthor: mako » Fri Nov 08, 2013 3:50 pm

The fin is fixed to the body, it can't accelerate more or less than the rest of the dart.


Quite true, but that was just intended to illustrate my point.

The theory is that it would be like firing an arrow through the air stream from a high-powered leaf-blower. The arrow shaft is heavy and strong, so there is little drag on it. But the light, high-area feathers have a LOT of drag, so the blower makes the arrow tumble.
The arrow is the projectile; the leaf blower is the muzzle blast.
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Re: machined APFSDS projectiles

Unread postAuthor: Gun Freak » Fri Nov 08, 2013 11:03 pm

His theory kind of makes sense... Although the different materials move as one unit, the different sectional densities make it easier for the front/back (depending on the patricular design) to swing off axis. It doesn't necessarily have to do with acceleration, rather staying parallel to the barrel. Think of it in terms of torque... With the muzzle blast being the force applied to a lever. If the rear has less density it has less inertia and is thus easier to veer off axis.
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Re: machined APFSDS projectiles

Unread postAuthor: jackssmirkingrevenge » Sat Nov 09, 2013 5:58 am

Gun Freak wrote:If the rear has less density it has less inertia and is thus easier to veer off axis.


Of course - the fins have a large area and large leverage because of the forward CG, this is inevitable if the dart is to stabilize in flight, so that element has to be retained. What needs to be improved is the sabot design and separation.

Frankly, I think the projectile with full bore fins with central support is the best design. The two shot by DR were definitely traveling dead straight.

LeMaudit has expressed reservations about the commercialization of this idea though, because of liability concerns, which are legitimate. If some crazy uses one to puncture the body armor of a law enforcement officer, the legal consequences would not be pretty.
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Re: machined APFSDS projectiles

Unread postAuthor: Talk » Sat Nov 09, 2013 6:47 am

I think the 7th projectile might fly straight too because they have a design similar to the APFSDS-T round MK829 Tank round and if someone could find the designing and manufacturing information about how they work and about their shape to find a little information about their flight dynamics ,the 7th projectile could be 1 of them which flew dead straight


This is link of a leopard firing the APFSDS round which maintains a straight trajectory, it also contains aluminum fins like your projectile 7

[youtube]http://www.youtube.com/watch?v=wIo37L_j9hs[/youtube]
[youtube]http://www.youtube.com/watch?v=LX2QRtqkSPE[youtube]
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Re: machined APFSDS projectiles

Unread postAuthor: Ragnarok » Sat Nov 09, 2013 7:05 am

We know that the fins can potentially be affected by the muzzle blast. After all, they're designed to be affected by air flow over the projectile, and the muzzle blast is ultimately another form of airflow over the projectile.

However, as Jack says, if we assume the projectile to be rigid, the whole thing has to accelerate as one - even the lightest possible fins are inherently going to be rather difficult to push around if they're firmly attached to two tonnes of lead.

The question is related to moments of inertia. Now, increasing the density of the fins will increase the inertia of the projectile... whether it'll increase the moment of inertia...

Well, it will also move the centre of gravity back. Now,if you've ever picked up a long rod (no. No. And Jack, definitely NO. I still remember that pink elephant.), you'll know that it's considerably easier to swing around from the middle than the end.
The moment of inertia from the end is mass x length^2/3. From the middle mass x length^2/12. That is to say, the moment is four times greater from the end than the middle.

Now, I can only do a rough approximation of the maths for that in my head (as calculating moments of inertia properly for complex shapes is quite messy maths), but I'm almost certain that the net result is still an increase in the moment of inertia.

However, I'm not sure that the already fairly considerable moment of inertia of the projectile is likely to be upset by what we can assume is a moderately even muzzle blast unless the projectile is already quite a lot on the cock as it's leaving the barrel.

Still, I did choose to make the assumption that it could have been a problem when I was designing darts several years ago and thus intended them to be used with a vented barrel attachment to diminish and redirect the muzzle blast away from the projectile. (That said, they never got proper testing, so I didn't find whether that was actually necessary).
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Re: machined APFSDS projectiles

Unread postAuthor: jackssmirkingrevenge » Sat Nov 09, 2013 9:00 am

Just returned and already you're making knob gags :D

The fact that none of the projectiles are spinning end over end means that the CG / CP position is more of less correct - the problem seems to lie with sabot separation.

I have a lot of ideas, but I doubt any of them will see the light of day for now.
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Re: machined APFSDS projectiles

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Re: machined APFSDS projectiles

Unread postAuthor: mako » Sat Nov 09, 2013 11:13 am

You guys are making some pretty good points, but what you have to remember is that the math doesn't always work out in practice.
@Gun Freak. Less inertia = easier acceleration.
@JSR. Your scenario is downright scary and all too true.
@Ragnarok. You say that ".....fins are inherently going to be rather difficult to push around if they're firmly attached to two tonnes of lead". This is partly true. However, take the case of aircraft "fins". The craft is steered by the shifting of these fins. As soon as the fins are no longer parallel with the centerline, the oncoming air pushes these fins "attached to MANY tons" around just fine.
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Re: machined APFSDS projectiles

Unread postAuthor: Ragnarok » Sat Nov 09, 2013 12:12 pm

mako wrote:the oncoming air pushes these fins "attached to MANY tons" around just fine.

... on a scale of seconds, using deliberately angled fins. The timescale you're proposing is milliseconds and the fins are not designed to produce uneven turning forces.

The important thing is that the inertia of the rest of the projectile is still important. The circumstance you're proposing comes down to a question of if that would increase the moment of inertia of the projectile sufficiently to have an inertial resistance to a turning force... and while that's affected by making the fins heavier, that's also not the be-all-and-end-all of how it works.

Thing is, a projectile going straight shouldn't put a large moment around the CG, and such a force would only last a short instant. I think there's another factor in play - the sabot tolerances or separation are far more likely issues.
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Re: machined APFSDS projectiles

Unread postAuthor: mako » Sat Nov 09, 2013 12:18 pm

Well, that force, in a short instant, manages to accelerate a chunk of lead really, really fast. I still think that this has quite a bit of bearing on the tumble.
However, the sabot does seem like a very good suspect. It seems like it would be hard to design one just right.
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Re: machined APFSDS projectiles

Unread postAuthor: Ragnarok » Sat Nov 09, 2013 1:52 pm

mako wrote:Well, that force, in a short instant, manages to accelerate a chunk of lead really, really fast.

By that logic, barrel length would be meaningless. Just let the projectile get accelerated by the muzzle blast.

The forces have to be contained for them to be in anyway efficient at accelerating things. And in shotguns, more than about 18" of barrel doesn't really add much to the velocity - the forces are very much diminished by the end of the barrel (even without them dissipating into the atmosphere).
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Re: machined APFSDS projectiles

Unread postAuthor: mako » Sat Nov 09, 2013 1:56 pm

True. I wonder if there is any way to properly test either of our theories. Seems like the high speed would affect any result we got. Anyway, my theory was based on which of his projectiles worked well and what the successful ones were made of.
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Re: machined APFSDS projectiles

Unread postAuthor: ramses » Sat Nov 09, 2013 5:54 pm

I suspect a muzzle brake/flash hider would remove any effects of muzzle blast yawing the projectile.

I also suspect that a sightly asymmetric sabot causing an uneven release would really mess things up badly. It could be beneficial to sand/lap the larger of the two halves until they have almost exactly the same mass. That would virtually guarantee the same cross sectional areas, assuming they were turned on a lathe before being cut.

A 'cup' style sabot, rather than a split sabot might also help, provided it can still keep the projectile straight in the barrel and release easily. I would For this, I would recommend aluminum fins and an aluminum cup.
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Re: machined APFSDS projectiles

Unread postAuthor: Ragnarok » Sat Nov 09, 2013 8:40 pm

mako wrote:True. I wonder if there is any way to properly test either of our theories.

Well, that's what it always comes down to - hard evidence.

Still, my gut instinct is that what is actually quite small as an "explosion" goes isn't going to have a dramatic effect.
Thing is, I'm used to a similar calibre launcher (~20mm, close to 12 bore's ~18.5mm) chucking out huge muzzle blasts (visibly 3 or 4 metres long, and very dense) that don't do much more than throw around loose leaves. These aren't movie explosions where people are easily thrown through windows...

ramses wrote:I also suspect that a sightly asymmetric sabot causing an uneven release would really mess things up badly.

I'm looking at those sabots and wondering how much sophistication there is in them. For some, it looks like the only thing ensuring a clean and even separation is hope.

And I don't know about others, but I am struck by the absence of "Prayer" as a variable on most of the ballistics models I've seen. :tongue5:
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Re: machined APFSDS projectiles

Unread postAuthor: Mr.Sandman » Sun Nov 10, 2013 12:45 am

I really enjoy taofladermaus's videos, so when he first started posting this video series and mentioned your channel, lets just say I was more than ecstatic. Haven't had time to completely browse the thread, however after studying your designs and comparing them to designs already used in firearms, I'm thinking that maybe something like this is in order.
Image

Also, the shotgun albeit easy to work with, isn't the best platform for this kind of thing. This is just speculation, but I'm guessing that the forcing cone (which is the first stage of shot column diameter reduction) is having a detrimental effect on the ability of the round to fly straight. Also from looking at a paused section of the video, they aren't cutting the shell hull perfectly straight. I think that this might be leading to an unstable sabot as it travels unsupported for a few fractions of an inch down the bore of the shotgun. Like I said before, this is just speculation And I'm probably wrong.
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