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Catching Projectiles (50J+)

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Ash Small

Fri Apr 24 2015, 02:15AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

BigBad wrote ...

Just wrap some sandbags in multiple plastic bags and forget about it.

Yes, BB, I agree, but Dyneema isn't a lot more expensive than polythene, it won't need replacing as often which, I think, is Der Albi's point. Sand is also excellent to put behind whatever takes the impact. The 'ballistic pendulum' bit just seems to make sense to me, as it allows more energy to be absorbed without deforming the projectile, but maybe it's not required.

I did read up earlier on elastic and non-elastic collisions, after Bored Chemist's post (which I did enjoy reading). I'm not sure if Dr Slack's 'Newton's balls' thing works with rubber, as, against intuition, it won't be an 'elastic collision'. I did say I'm not sure about this point

ConKbot of Doom

Fri Apr 24 2015, 03:32AM

Registered Member #509 Joined: Sat Feb 10 2007, 07:02AM
Location:
Posts: 329

Oh yes, definitely not close fitting pipe, at least if you want to make it out of plastic. Perhaps injecting the water tangentially, at a steep angle, so that you get swirling going on inside the pipe, which will mix air in, as desired, and if the pipe isnt too long, you could even get the center to stay filled with air. Ideally that would try to self-center the projectile, but with a blunt end, I'd expect it to 'dig' in, and yaw the round more. Maybe add some shallow tapers to try and guide the slug where it needs to go, and if you can make it from PVC and survive the impacts, just replacing the pipe on occasion isnt the end of the world.

Or if a specific weight of oil can get frothed up and hold that for a bit as you pump that, you could use that instead of water.

Check out different firmness of clay too, they vary from soft, hand formable stuff, to really firm stuff thats meant to be sculpted with tools. Maybe a firmer clay would deform less, meaning more energy could transfer into a foam or sand damper? If the container you mount it in is stout enough, and the projectile doesnt fully embed itself, you could pull the projectile out, and touch up the surface of the clay with a hammer pretty quickly.

Bored Chemist

Sat Apr 25 2015, 11:58AM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

We may have finally found a use for porridge.

Dr. Slack

Sun Apr 26 2015, 09:07AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

This damned water brake has been nagging at my mind. I think I have the answer.

You have a long trough of water, with a magic window that consists of a 'water-diode' at the end, that lets objects in, but doesn't let water out. Fire the projectile through the window, reach in and fish it out of the trough where it's stopped.

I have a simple design for this water diode, but I'll hold back on the details for the moment. Hint, it's not a passive device.

Ash Small

Sun Apr 26 2015, 09:23AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Dr. Slack wrote ...

.Hint, it's not a passive device.

Surely, if it's not a 'passive device', a better analogy would be 'water transistor'?

DerAlbi

Sun Apr 26 2015, 01:41PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Ive got news: a total FAILURE

I went ahead and tried to stop the projectile..
The setup is made of essentially 5 buffer zones..
1) clay to avoid relfection
2) hard foam covered with 6 layers of kevlar laminated with hotglue*
3) the hard foarm is pushed against soft, but allready compressed foam
4) 3. presses against a heavy cup thich has wax at the bottom with all sorts of metals i found in it. (just to add weight). The cup is then suspended again with foam.
5) the whole thing is of course screwed to wood which also flexes..

*The Kevlar laminated with hotglue made initially a really good impression. The glue penetrated the fibers (after heatreating and massaging each layer in) and on the contrary to clay it avoids the fibers from getting pushed away. The main failure of pure clay was that the projectile slipped through holes in the kevlar.. which now seemed to be avoided. Six layers were allready stiff and hard as hell and this made me put it directly on hard foam since it didnt be spuposed to bend much anyway.
I was completely wrong in my self gut-feeling. Projectile kinetics is really not intuitive anymore.

Here the shot:

After the shot the projectile stopped in the backside-protection-clay. This was initially placed to give the push to the foam an increasingly growing compression force.

After removing the projectile one could get a good impression on the impact site:

1430055229 2906 FT170679 02 Underthehood

Where in closeup you can see how the projectile just went through the kevlar like... well like a projectile that goes through way insufficient material

The laminat looked like this initially

..and of course there is a exit wound

hen918

Sun Apr 26 2015, 02:06PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

Well done! your work on improving efficiency is certainly getting you useful projectile energies. Kevlar penetrating, next it'll go through tank armour!

DerAlbi

Sun Apr 26 2015, 02:26PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

video discribtion states its an unoptimized shot

that was just 20% for the sake of power.
What i now just really realized is that the fibers actually were not cut but couldnt sustain the stress.
I try another spot with 1.0 .. 1.5cm of a hotglue-blobb covered with 2 layers of kevlar.
What is really annoying right now is that the hotglue actually kind of welds onto the projectile which is only to be cleaned up by acetone. I hope solid hotglue can absorb a bit of energy an distribute it without breaking to bad. I know its a long-shot but.. i just want to play with the failes concept before discarding it... i just want to learn more about this...

Edit: i tried the blobb of hotglue: its actually quite effective to stop the projectile. it penetrates around 1cm deep. (the blob was covered with 1 layer of kevlar and there was 1 layer in betweeen - just for fun.) The projectile fused again to the hotglue

All this penetration.. its quite destructive to the target. I kind of want to avoid this..
it needs to stop way slower without sinking to deep into anything. hmmmmmmmmmmmmmmmh

Daaaamned. Really the watter-thingy? puuh. so much work.
i would need a 2cm(or 1inch) tube make sure the i can actually aim at it. that would need quite a flow rate and thus it self will throw the water quite far. Turbulent flow would reduce that issue.

Andy

Sun Apr 26 2015, 08:34PM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
Location:
Posts: 874

What about different layers of thin cermic and kelver, kelver by it self is type of like carbon firbre in F1 cars, its designed to disingrate, burning up the energy, unlike body armour with cermic, to spread the energy, but keep the structure inteak.

Kizmo

Sun Apr 26 2015, 09:19PM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

Stack of newspaper backed off with steel plate worked fine with my high power ETG experiment.

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