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machined APFSDS projectiles

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Re: machined APFSDS projectiles

Unread postAuthor: jackssmirkingrevenge » Sun Nov 10, 2013 9:19 am

The flechette loadings played with in the SPIW development days were not known for their accuracy.

The Steyr AMR also seems to have had its own problems with hitting the target, even though when it did the claimed penetration of 40mm of steel at 1000 metres is phenomenal performance.

Image

Modern tank guns firing APFSDS rounds on the other hand are incredibly accurate, hitting their targets 3,000 metres away and beyond.

The full bore fin design dispenses with a centering sabot, it just needs a pusher plate which should make for an extremely clean separation. I think the first two shots in the last DR video bear this out, both shots hit dead straight.

Somwhere between is the Russian fullbore fin design with a discarding centering disk:

Image

Nadezhda. huh.

Image

Well, I say Russian design, it was originally used by the Germans for their Peenumünde arrow shell - Peenemünder Pfeilgeschoss or Flugstabilisierte treibringgeschoss. It was of 12cm caliber and 1911mm (75.23 in) long, with a 31cm diameter three-piece discarding ring sabot at the waist and four fins at the rear. ( some early model had six fins ) On firing, from the K5 Glatt smoothbore gun, the ring was discarded and fell about 2km ( 1.25 miles ) in front of the gun, leaving the fin-stabilized projectile to fly to the target. development began in February 1940 and the long-range trials, fired at the Rugenwalde range,gave a maximum range of 151 km ( 93.83 miles ).

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Re: machined APFSDS projectiles

Unread postAuthor: Lockednloaded » Sat Nov 30, 2013 7:06 pm

Have you ever thought about using a mild glue to hold on your sabot/stabilizers? In the videos, they seem to separate too soon and once the round leaves the barrel it seems like they'd be ripped away regardless of how well attached they are.
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Re: machined APFSDS projectiles

Unread postAuthor: jackssmirkingrevenge » Sun Dec 01, 2013 6:38 am

The idea is that the sabots separate as quickly as possible though.

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Re: machined APFSDS projectiles

Unread postAuthor: Ragnarok » Sun Dec 01, 2013 1:19 pm

I'm not sure it'd separate at all. Drag forces aren't actually that high.

A quick back of the envelope - 0.5 x Drag Coefficient x Velocity^2 x Air Density X Cross-sectional area... drag coefficient might be as high as about 1.3, velocity will be about 400 m/s, air density's going to be about 1.25 kg/m^3 and 12 gauge cross section is 0.00027 m^2...

... 35 Newtons, maximum. Even were the projectile of an arbitrarily high inertia, a glue that would hold in the barrel (forces of 20,000+ Newtons) almost certainly wouldn't separate after it.
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Re: machined APFSDS projectiles

Unread postAuthor: jackssmirkingrevenge » Mon Dec 02, 2013 8:51 am

Yup, keeping this in mind I instructed both Matt and Jeff to tape the sleeves very lightly, just enough to allow them to hold together until the round was loaded.
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Re: machined APFSDS projectiles

Unread postAuthor: cammyd32 » Mon Dec 02, 2013 12:32 pm

Jack, I know someone would probably have mentioned it before but, have you ever considered putting spring loaded deploying fins onto your rounds? Similar to the RPG-7:
Image

Would allow for a larger, heavier projectile in a larger bore?

I know it would probably cost more than it would be practical, but it could be designed such that the deploying fins came off on impact, alternately it would make a brilliant one-off projectile for a keen buyer.
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Re: machined APFSDS projectiles

Unread postAuthor: Ragnarok » Mon Dec 02, 2013 1:51 pm

I'm not really seeing the appeal myself. The RPG-7's design is intended to deliver a warhead, but as we're not firing explosives, the benefits of increasing the weight more than can be done with a Brenneke or Foster design would seem to be fairly negligible, and we're generally trying to fire sub-calibre projectiles anyway.
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Re: machined APFSDS projectiles

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Re: machined APFSDS projectiles

Unread postAuthor: cammyd32 » Mon Dec 02, 2013 2:08 pm

Well, not so much the larger bore, but the deploying fins would definitely add a greater degree of in-flight stability, now I know that a properly engineered dart would produce a good flight pattern straight out, but from the videos of jacks projectiles so far, I have seen a few to tumble as they come from the muzzle, especially detrimental to the penetration of the rounds penetration at shorter ranges when the smaller fins don't have time to right the projectile.
A greater righting force caused by the large deploying fins would seem to me to be able to align the projectile a lot faster, ergo, more penetration.
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Re: machined APFSDS projectiles

Unread postAuthor: jackssmirkingrevenge » Mon Dec 02, 2013 9:15 pm

If you're going for fullbore, no need for the complication of spring out fins. A simple nose heavy slug is enough, like the bolt-in-delrin rounds Jeff tested here: http://www.youtube.com/watch?v=yoVpxg6R ... page#t=210

Unfortunately full bore means the impact is spread over a larger area, resulting in lower penetration. The latter parameter was what I was going for. As usual :roll: :D
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Re: machined APFSDS projectiles

Unread postAuthor: ramses » Mon Dec 02, 2013 9:45 pm

What about a full-bore aluminum (or even lead) foster slug tipped with a long pointy piece of HSS? Perhaps gently glued into a hole in the nose of the slug so as to be able to break off and fully penetrate the target?
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Re: machined APFSDS projectiles

Unread postAuthor: CS » Mon Dec 02, 2013 11:29 pm

I know you are attributing the tumbling to the sabot, which is it's own problem.

But for in-flight stabilization I look at it this way:
-Weight towards the front
-Drag towards the back

To me it seems you don't have enough drag to stabilize the weight of your projectile. Something like a badminton, or a pilot chute.

http://www.skydive-safety.com/Images/Ma ... In-Tow.gif
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Re: machined APFSDS projectiles

Unread postAuthor: cammyd32 » Tue Dec 03, 2013 4:23 am

jackssmirkingrevenge wrote:If you're going for fullbore, no need for the complication of spring out fins. A simple nose heavy slug is enough, like the bolt-in-delrin rounds Jeff tested here: http://www.youtube.com/watch?v=yoVpxg6R ... page#t=210

Unfortunately full bore means the impact is spread over a larger area, resulting in lower penetration. The latter parameter was what I was going for. As usual :roll: :D


Okay then, sub caliber dart, with deploying fins.
I suppose the full bore element of it is more a thing that would appeal to the 'bigger is better' people, but I more proposed the idea to counteract the tumbling effect as your projectiles leave the muzzle, I noticed that in the slow motion shots the projectiles didn't have time to right themselves before hitting the target, resulting in little penetration.
Hence, more drag at the rear of projectile = more force that levels the projectile = projectile hitting the target closer towards 90' with the target = more penetration.
It's just a thought though, I know other factors are at play, and I'm no expert after all...
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Re: machined APFSDS projectiles

Unread postAuthor: jackssmirkingrevenge » Tue Dec 03, 2013 6:53 am

CS wrote:But for in-flight stabilization I look at it this way:
-Weight towards the front
-Drag towards the back

To me it seems you don't have enough drag to stabilize the weight of your projectile. Something like a badminton, or a pilot chute.


I don't think that is the issue. The basic premise for stability is that the center of mass is ahead of the center of pressure.

This is well understood in the field of model rocketry: http://modelrocketnews.blogspot.ca/2009 ... ility.html

I'm not about to argue with NASA on the issue either ;) http://exploration.grc.nasa.gov/educati ... tstab.html

All the projectiles I made have the CG ahead of the CP, which is why none of them (that left the barrel intact at least) tumbled end over end, but rather yawed and recovered.

It is true that in some cases, the CG was only barely ahead of the CP, meaning the yaw was quite significant.

One of the issues was that since I was concerned about the projectiles surviving the shock of firing, I made the fins much thicker than I would have liked for aerodynamic purposes, so th CG was not as far forward as would have been desirable.

Have a look at Shot 4 in Jeff's last video: http://www.youtube.com/watch?v=p32xmVXY ... page#t=180

The projectile is one of the two to the left of this group.

Note that the fins on these are considerably longer than the others, meaning the CP is further back, and it flew in a much more stable fashion.

I more proposed the idea to counteract the tumbling effect as your projectiles leave the muzzle, I noticed that in the slow motion shots the projectiles didn't have time to right themselves before hitting the target, resulting in little penetration.


I would surmise that this is due to the issues described above.

Hence, more drag at the rear of projectile = more force that levels the projectile = projectile hitting the target closer towards 90' with the target = more penetration.
It's just a thought though, I know other factors are at play, and I'm no expert after all...


Folding fins are awesome, I would love to make something like this:



The swing out fins on bore-parallel axis as on the Hydra would also be interesting to imitate:

Image

The issue is that since these rounds are destined for destruction, there's a limit to how much effort I'm willing to put into them individually. I'd rather make 10 simple rounds than one super-sophisticated one.
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Re: machined APFSDS projectiles

Unread postAuthor: CS » Wed Dec 04, 2013 9:10 am

Thanks for the links. Talk about a exponentially growing rabbit hole of mind f*ck. I'm running out of room for tabs.

The big question I haven't figured out, what negative happens if the cg and cp are to far apart?

Also a badminton like structure on the back of a projectile would move its cp back?

The basic premise for stability is that the center of mass is ahead of the center of pressure.


Sure, I may have said it in my own layman terms though. However how much flight time are willing to sacrifice for your projectile to spend yawing, before correcting itself? The amount of drag and lift will effect how fast the projectile corrects itself. Firstly amplified by the distance between the cd and cp.

Practically, gluing a shotgun wad in reverse might do the trick.

Better yet I can STFU, I'm the Sarah Palin of external ballistics. <Weird how that seems funnier to say in a face to face situation, for some reason in text it seems like I'm throwing my hands up in a pity party.
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Re: machined APFSDS projectiles

Unread postAuthor: Ragnarok » Wed Dec 04, 2013 12:59 pm

CS wrote:The big question I haven't figured out, what negative happens if the cg and cp are to far apart?

Two things really.

It may cause the projectile to face into any cross wind, much like a weather vane. This is not of dramatic significance here, in part because this isn't a model rocket (and has no thrust) and in the other because of the relative velocities of wind and projectile.

It's more a sign that the projectile has too many fins, increasing the energy lost to drag.

My estimation - and I admit it was a very rough estimation, using some brutal misuse of several innocent equations and mathematical models - was somewhere about 80% of the drag on the long range darts I designed years ago was from the fins, and probably a similar sub-percentage of that solely from skin drag. In practical terms, being reliably able to produce the darts in a three fin configuration, rather than the four fin version, would have reduced the drag on them by up to about a fifth.
(However, given the drag was already relatively low, durability and build consistency were of greater importance).

In an ideal world, it would have been nice to have explored fin designs in a wind tunnel, seeing how the leading edge could be reshaped in order to change the air flow around the fins and overall reduce the drag, but actually getting access to a wind tunnel in the region of Mach 0.8 is a little tough.

What would be ideal is a silver bullet to reduce skin friction, but there's no real candidate I can see for that.
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