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Indoor backstop

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Indoor backstop

Unread postAuthor: PeteS » Fri Sep 13, 2013 6:48 pm

I don't have a working laptop other than a chromebook so I need to do my chrono stuff indoors. So far I have been using light projectiles (wadded up plastic bag wrapped with electrical tape. I put a small piece or rare earth magnet (salvaged out of old hard drives) in each one and fire them through two coils to read the speed.

Thus far I have been firing into a 5 gallon bucket stuffed with an old blanket. I think that I am starting to exceed that method as I have been attaining higher and higher projectile speeds. So far I have hit about 653 fps at 80 psi, but am nervous about going much faster indoors with the bucket and blanket. FWIW that was with a 1.5" ID barrel, but the guns I am testing range from 0.5" to 1.5" ID barrels.

Any suggestions?
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Unread postAuthor: Technician1002 » Fri Sep 13, 2013 6:58 pm

I understand your concern. I punched out the bottom of a 5 gallon plastic bucket with a 4 inch foam ball.

What are you using to measure the time from your coils? I used a mixer and o-scope.

I used more than 2 pickup coils. Here is a data shot with a 4 inch barrel, 2 inch valve, and a foam ball with a small magnet in it.

Magnet when it passed the center of a coil is indicated by the rise in voltage from negative to positive. At center, the voltage is zero. The last two crossings are marked with the two scope cursers and the time between them is shown on the right. The coils are a foot apart, so delta time = FPS. Coils are on the barrel so this is in barrel acceleration data.

Image

One suggestion to move outdoors is to capture your data from the coils outdoors with an MP3 player with a record function such as a voice recorder and then replay the data indoors to take measurements. Use one with an external mic input. Some cell phones have a sound record function and can use an external mic. Another option is to use a bluetooth to extend the mic input wirelessly using a car hands free unit with the coils wired into the mic and using a bluetooth dongle on the pc to complete the input link.
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Unread postAuthor: dart guy » Sat Sep 14, 2013 8:31 am

another option is to use angled steel sheet 1/4" would be the safest and that angled sheet when in place in a stand can reflect your shot down ward another great idea is to put an old blanket over the enterance of your sheet metal stand so realy slow your projectile as long as it isnt pointed
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Unread postAuthor: PeteS » Sat Sep 14, 2013 12:33 pm

Technician1002 wrote:What are you using to measure the time from your coils? I used a mixer and o-scope.

I used more than 2 pickup coils. Here is a data shot with a 4 inch barrel, 2 inch valve, and a foam ball with a small magnet in it.

Magnet when it passed the center of a coil is indicated by the rise in voltage from negative to positive. At center, the voltage is zero. The last two crossings are marked with the two scope cursers and the time between them is shown on the right. The coils are a foot apart, so delta time = FPS. Coils are on the barrel so this is in barrel acceleration data.

I am using SoftChrono to evaluate. Using an mp3 recorder is a possible answer. If I go outdoors I might switch from coils to light sensitive transistors and eliminat the magnets.

I could possibly just run some wires out through the wall.
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Unread postAuthor: PeteS » Sat Sep 14, 2013 12:36 pm

dart guy wrote:another option is to use angled steel sheet 1/4" would be the safest and that angled sheet when in place in a stand can reflect your shot down ward another great idea is to put an old blanket over the enterance of your sheet metal stand so realy slow your projectile as long as it isnt pointed


I will try that.
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Unread postAuthor: Technician1002 » Sat Sep 14, 2013 5:26 pm

At high speed fabric is stressed to failure. I punched a hole in a scrap of wall to wall carpet with a rolled up T shirt.
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Unread postAuthor: dart guy » Sat Sep 14, 2013 6:48 pm

that is why i said to drape it loosely across the angled sheet metal to decrease the force to the metal
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Unread postAuthor: Technician1002 » Sun Sep 15, 2013 5:47 am

It was draped over the sawhorse. This is the end result. Hole in carpet and sawhorse knocked over. FYI, it put two holes in the carpet becasue it was draped over the sawhorse. The first sawhorse was braced against the second one behind it which collapsed.
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Unread postAuthor: PeteS » Sun Sep 15, 2013 8:13 am

After thinking about it I realized that my projectiles would likely be short lived if I use a 45 degree plate. If I go to light sensitive transistors instead of coils I guess destroying the projectiles wouldn't matter much since I could use wadded paper, foam balls, spuds, or whatever and no magnets.

I am thinking of using more bundled up blanket as well as suspending the bucket so it can swing back absorbing more energy. Either more weight or some bungees could add a bit more resistance than just the light bucket and blanket.
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Unread postAuthor: DYI » Sun Sep 29, 2013 9:36 pm

You'll want a backstop filling with low/no mechanical strength, and sufficient density to stop your projectile in the available distance without a peak acceleration high enough to damage it. You can use water hammer pressure calculation formulas to get an approximate idea of the stress on the projectile without doing any detailed analysis.

I've used all of aluminum foil balls, plasticine (low speed only, the density is quite high), water (liquid), sawdust, soil, and low density foam at some point or other for catching high speed projectiles. Seeing as how this is being done indoors, water or other liquids would likely be messy. Sawdust is probably your easiest choice here, and as a bonus you can alter its density by changing packing force and water content.

As to constructing the backstop itself - the body can be a simple wood box, with a replaceable cardboard front or similar. Alternatively, keep using the bucket and lay it on its side, with a cover on the top (now the front).
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Unread postAuthor: jackssmirkingrevenge » Mon Sep 30, 2013 7:00 am

DYI wrote:I've used all of aluminum foil balls, plasticine (low speed only, the density is quite high), water (liquid), sawdust, soil, low density foam and my garage at some point or other for catching high speed projectiles.


:D :D :D
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