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Catching Projectiles (50J+)

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Dave Marshall

Mon Apr 20 2015, 05:52PM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

Imagine a plastic shoe box, with a hole a few inches in diameter cut in one end. Something like this, but with a tight fitting top.

Place a thin sheet of something that's easily penetrated but tends to return to it's original shape, perhaps like polyurethane foam, over the opening, then fill the box about 75% of the way with airsoft pellets, and put on the lid. Fire through the foam in to the box of pellets. The foam should prevent most of the pellets from escaping, especially when the sheet is new, and should definitely ensure that any that do escape simply drop out in to a tray below the box, rather than flying off.

A very similar method is used for stopping bullets without causing expansion or other deformation. You'll have to replace the foam every so often, but the sheet should be able to be quite cheap, and you ought to be able to get a number of shots from each sheet. Retrieving the projectiles is as simple as taking the top off and rooting around in the box.

Dr. Slack

Mon Apr 20 2015, 08:05PM

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

So I got in the bath this evening, thinking about your problem (hmmm, probably TMI!) I liked Dave's shoebox idea, perhaps full of sand rather than airsoft, perhaps a corrugated cardboard window rather than foam. But it's still messy, still gets damaged and has to have bits replaced, and needs a rummage around every shot.

So, back up, and rethink. You don't really want to catch the projectile, so much as stop it.

Think Newton's cradle. As the first ball comes in, it stops dead, giving up all its energy (less a tiny bit) to the next ball of the same mass, which carries on at much the same speed. Now you have to stop the next ball, so the problem has just shifted, and the projectile would probably get deformed, stopping dead in a fraction of a mm.

So, some other material than iron for the first target. How about a lump of tread from a dead auto tyre? Tough, cheap, lossy. As the rubber would absorb some energy, deforming and recoiling, if it had the same mass as the projectile, then the projectile would continue forward, albeit at a lower speed. Increasing the mass of the rubber target above that of the projectile would compensate, and at some mass ratio, related to the coefficient of restitution, the projectile could again drop at zero speed after hitting the rubber target. Arrive at the ratio by sums, or perhaps easier, experiment. If in doubt, err on the low side, so the projectile moves forwards rather than recoils. We now have a lump of auto rubber recoiling at some speed, but, we can repeat the trick, and back it with another, larger piece, which collision will remove more of its energy. You can continue this stack until the recoil speeds are manageable. I suggest hanging all the bits of rubber from strings, as pendulums, rather like a Newton's Cradle, with spaces. The spaces are so that each collision takes place independently.

See this diagram. Three stages of increasing weight rubber are shown. The projectile comes in from the left, stops dead after hitting the first piece of rubber, and drops into a towel (not shown).

Now, an important question. Is auto rubber hard enough to survive a reasonable number of shots, soft enough to avoid damaging the projectile, and lossy enough to absorb the energy in a reasonable number of stages? I think that could be an interesting experiment to do.

While on the subject of dead tyres, consider an alternative (no diagram this time). Take a whole tyre, and punch a small hole in the tread. Fire the projectile tangentially to pass through the hole. It will now skid, bounce, tumble round and round the inside of the tyre, after contacting the inside obliquely, gradually losing energy as it comes to rest.

DerAlbi

Mon Apr 20 2015, 09:22PM

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

The idea with the tire is interesting.. but i think the friction will make the rubber melt over time and just make the projectile dirty. the proectile has 90Â° edges.. so its prety sharp when it slides along.
The other thing you discribe (with the picture) is kind of whats going on in a laminated material too. Every layer increases the moved (buldged out) mass and compression absorbs energy. I think tire-rubber is quite hard and will tend to reflect instead of dampen the energy - Its not meant to absorb energy.. that would be bad for the car. You are right that i want to stop the projectile rather than just catching it.
You diagram shows exactly the needed setup, but the implementation is more compact if made of laminated material..
In the end my current setup is allready something like this: ive got the cups screwed on a wood that bends, the inner cup is flexible and can move and is dampened by foam and i just add more stages before that where the projectile impacts. and the frontend is what must endure the most abuse.
Thats why sand is maybe the best frontend. its easy to repair, has quite some mass and spreads energy hapily. The challenge is to package it..
Lets say the sand would be the second hanging thing in your grapics.. the first would still be clay to catch the projectile and the third one is the moving cup damped by foam... i just wouldnt go with rubber...
As you said, there is a impeact velocity where the projectile tends to reflect (lower speeds) and were it moves through, loosing energy. I need to cover every aspect...

BigBad

Mon Apr 20 2015, 09:44PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

It turns out that when it impacts the projectile stops and whatever it impacts starts moving; bit like Newton's cradle.

So (if the projectile doesn't penetrate like a needle) you want as much mass thickness of material as there is mass per unit length of the projectile.

Bored Chemist

Tue Apr 21 2015, 08:49PM

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

re Rubber: boing!
You may find this helpful.

which refers to this.

ConKbot of Doom

Thu Apr 23 2015, 07:15AM

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

what about a water brake, with water flowing though a pipe?
S63GPfH

Water is pumped in from the left, and the projectile enters from the right, enters the pipe, partially filled with water, where it gets slowed to a stop. The water flowing, is contained by the fitting on the end and drains back to your reservoir, and the initial splash is contained too. Maybe saw the reducing tee in half, only glue the bottom half on, and clamp the top on, so when the projectile washes its way back to the entrance, you take the cover off and grab it out. Or use a bigger drain line, and just pluck your projectile from the reservoir.

Dr. Slack

Thu Apr 23 2015, 08:18AM

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

Water brake, brilliant.

My concern is that water is low viscocity and high modulus. The first means that you'd be tempted to use a pipe barely larger in diameter than the projectile to get it to stop in a reasonable distance, the second means that if you did, your pipe would feel barrel-exploding type forces, and you'd need a very strong pipe.

Use close-fit pipe, and add bubbles to the water from a fish-tank pump or some other means, to make the water much less 'hard', like the way novice divers can dive into a pool of bubbles so they don't get really hurt when they mess up the triple somersault.

Or, instead of the long pipe, construct it from a series of Tees connected together, so there's a pressure relief channel every few inches.

DerAlbi

Thu Apr 23 2015, 06:00PM

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

I feel a bit like this would be overkill

But hey, its definitively something to keep in mind, if some day i need some test-steup for continous shooting! this concept is quite interesting.

What bothers me is the water flow rate. Or the water at all: you cant avoid asmetric waterflow or projectile entry. you kind of mess up your tube with that... right`? hmmmh. Solutions? Also in a thin tube the pressure must go somewhere. this would either result in a high water reservoir or in extrem stress for valves and/or pumps.
A thicker tube would avoid that rpoblems but the water-flow would be insane. or one uses something thicker like honey

Please note that any discussion about that would be (for now) pure theory. Its at the current state not practical to have such advanced setup.

Ash Small

Thu Apr 23 2015, 07:29PM

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

"UHMWPE fibers are used in armor, in particular, personal armor and on occasion as vehicle armor, cut-resistant gloves, bow strings, climbing equipment, fishing line, spear lines for spearguns, high-performance sails, suspension lines on sport parachutes and paragliders, rigging in yachting, kites, and kites lines for kites sports. Spectra is also used as a high-end wakeboard line.

For personal armor, the fibers are, in general, aligned and bonded into sheets, which are then layered at various angles to give the resulting composite material strength in all directions.[11][12] Recently developed additions to the US Military's Interceptor body armor, designed to offer arm and leg protection, are said to utilize a form of Spectra or Dyneema fabric.[13] Dyneema provides puncture resistance to protective clothing in the sport of fencing"

Some form of 'ballistic pendulum' padded with Dyneema might do the trick

BigBad

Thu Apr 23 2015, 11:35PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

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

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