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Slowing Down Projectile?

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lauriethepigeon

Wed Jan 23 2013, 02:08PM

Registered Member #7669 Joined: Mon Oct 29 2012, 02:55PM
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Posts: 19

I am building my test rig for a 3 stage coilgun, with theoretical speeds of 5-30m/s with 1-5g armatures.

Since this is for an academic project, I have to keep it safe, and ideally I wouldn't let the armature leave the tube if I could help it. I have integrated optical speed measurement which is attached around the clear tube, but at the end of the tube I need to slow / stop the armature in a controlled manner.

Has anybody investigated methods of slowing / capturing their projectiles? I was going to just shoot into a block of expanded polystyrene or something for now, but there must be a better way of doing it? I will be doing hundreds of test shots with different parameters, so emphasis is on being able to easily recover the armature after the shot (having to dig it out of a block of oasis foam with a pair of pliers every time would soon become tedious)

I did look at pellet traps used for air gun shooting but there is too much potential for it to ricochet out.

I also considered something like a round electrical junction box, where the tube shoots straight into the junction box and the armature rattles around inside and stops, then I can open the lid and take it out.

Perhaps eddy current braking could be used somehow?

DerAlbi

Wed Jan 23 2013, 02:26PM

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

You are talking about 2.25 (lets say 3) Joules of energy here :-/ Just put hard plastic bottle over the barrel ending and catch it. With 2.25 joule there is no real danger to anyone... not even to the bottle.

Maybe you could fill the bottom of the bottle with something hard... like glue?

lauriethepigeon

Wed Jan 23 2013, 02:42PM

Registered Member #7669 Joined: Mon Oct 29 2012, 02:55PM
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Posts: 19

Yes, there will be 37J in the capacitors, assume 1-2% efficiency, that's 0.4-0.7J KE which is even less.

A bottle is the sort of thing I am going for, but I would like it a bit neater, maybe I will buy a project box with a hinged lid or something, perhaps I could put it on an incline so the armature will roll down to the bottom and drop out of a slot?

Maxwell

Wed Jan 23 2013, 04:22PM

Registered Member #8497 Joined: Tue Dec 04 2012, 06:24PM
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Posts: 74

I'm with DerAlbi. 1J of KE is like flicking a pencil with your finger. It moves a little but even if the sharp end hits your hand, it's not going to hurt.

Cut up an old t-shirt and make a soft pillow for the armature to impact with.

ZakWolf

Wed Jan 23 2013, 06:01PM

Registered Member #3114 Joined: Sat Aug 14 2010, 08:33AM
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Posts: 608

magnets on the end of the barrel to slow and stop the projectle ? my two cents

Yandersen

Thu Jan 24 2013, 04:12AM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

I used my dad's old trousers to stop 10J projectile flying at 54m/s. Looks like the only thing those trousers ever missed to stop is my dad's farts...

lauriethepigeon

Thu Jan 24 2013, 09:56AM

Registered Member #7669 Joined: Mon Oct 29 2012, 02:55PM
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Posts: 19

Maxwell wrote ...

I'm with DerAlbi. 1J of KE is like flicking a pencil with your finger. It moves a little but even if the sharp end hits your hand, it's not going to hurt.

Cut up an old t-shirt and make a soft pillow for the armature to impact with.

I shot my hand with a 0.07J KE CG (pointed end), and it did hurt. Didn't break the skin, but it left a little dent.

Maxwell

Thu Jan 24 2013, 03:48PM

Registered Member #8497 Joined: Tue Dec 04 2012, 06:24PM
Location:
Posts: 74

0.07J!?!??!

With no ill intent - I don't believe you're accurately measuring the velocity or mass of your armature.

Please re-run some simple kinematic equations.

Dropping a 10g pen from 1m would result in >0.07J KE...Unless you sharpen the end to a needle point - it does not hurt.

MrFlatox

Thu Jan 24 2013, 04:56PM

Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102

Fire a 1J airsoft gun in your hand, it hurts.

Dr. Slack

Thu Jan 24 2013, 05:53PM

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

Attach a length of oversized, not too flexible plastic pipe to the end of your barrel. Or maybe if plastic proves to be too soft, some copper tube. The internal diameter must be comfortably larger than the projectile diameter, but much smaller than its length.

The projectile will glance off the inside repeatedly, slowing down as it rattles its way along. If the launcher is on a table, then I have a feeling that a few feet of pipe would be more than enough to tame it, and just long enough to take it down to the floor where it would drop into a metal cup with a satisfying clunk. You need to have the brake tube turning downwards, as if the projectile runs out of energy half-way down, you want it to fall the rest of the way

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