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Ballistics Pendulum

A place to ask general spud cannon related questions.
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Unread postAuthor: D_Hall » Sun Dec 20, 2009 12:56 am

jackssmirkingrevenge wrote:The only decent way to do this is using a high speed camera against a metered screen. Without such equipment, I would use a series of even spaced thick card/thin ply screens, that would not only give me a measure of fragment velocity but also distribution.

That assumes a pretty considerable budget. We've got a few such cameras at the office. IIRC, the heads are something like $20k each. Further, getting *GOOD* data based on photography isn't as easy as it sounds. Your data processing/reduction really is a beotch.
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Unread postAuthor: jackssmirkingrevenge » Sun Dec 20, 2009 1:06 am

D_Hall wrote:That assumes a pretty considerable budget. We've got a few such cameras at the office. IIRC, the heads are something like $20k each. Further, getting *GOOD* data based on photography isn't as easy as it sounds. Your data processing/reduction really is a beotch.


That was the point I was trying to make, doing it properly requires resources not available to your average spudder. In the absence of hefty finances, there are several ways to approximate the energy of the projectile and any fragments after impact and in this case I think witness plates are a better idea than a pendulum.

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Unread postAuthor: Ragnarok » Sun Dec 20, 2009 10:50 am

D_Hall wrote:OK, fair enough. You're interested in the fragments and such too.

I'm trying to get a good idea of where the overall energy in the impacts is ending up, be that remaining kinetic (be it in fragments of the target or in the projectile) or energy used in piercing the target.

In the end, it gives a reliable answer, and I see no reason to complicate things any further.

yolkssmokingrevenue wrote:in this case I think witness plates are a better idea than a pendulum.

Actually, I've already got something of that order planned for some of the tests.

One of the projectiles is designed to quickly lose stability after penetrating the first target, and I'll need things of the order of witness plates to test that kind of thing.

I'm not doing just one test - I'm doing several.
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Unread postAuthor: rp181 » Sun Dec 20, 2009 12:04 pm

Im going to throw an idea I had out there, I think it is usable, though DYI thinks a pendulum would be better (Im after compact and portable).
Note: this is primarily for the railgun, where optical is useless, and break wire is a lot harder than it sounds for a soundcard.

A standard piezo disc (used in buzzers: http://www.instructables.com/files/deri ... MEDIUM.jpg on the left) is glued on the back of a material, in my case a 3" square piece of bullet resistant glass (its actually plastic :wink: ).
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the calipers are set to the pane's thickness.

Piezo's give out a voltage based on the impact, a hard flick on the pane gave me 10 mv. When shot, the disc will give out a voltage proportional to the speed (or is it the kinetic energy? need clarification on this). After calibrating with dry pneumatic shots, a model can be found, and the railgun's projectile speed (or energy...) can be measured.

I JB-Welded a piezo to the back of a piece of glass, I can test today. Anyone have an idea if the output is proportional to the speed or energy?
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Unread postAuthor: jackssmirkingrevenge » Sun Dec 20, 2009 12:22 pm

Ragnarok wrote:I'm not doing just one test - I'm doing several.


Looking forward to your results :)

I JB-Welded a piezo to the back of a piece of glass, I can test today. Anyone have an idea if the output is proportional to the speed or energy?


Test it with a pneumatic and vary the pressure incrementally? Half the pressure should be equaly to roughly half the energy.

this is primarily for the railgun, where optical is useless, and break wire is a lot harder than it sounds for a soundcard.


As part of this school project, I had to figure out how calculate velocity in the absence of a chrony, so with the help of my physics mentor came up with the system illustrated below (we happened to be covering capacitors at the time). The time across the two foil strips is calculated as follows:

time = resistance x capacitance x ln(voltage across resistors/initial battery voltage)


I never compared this system to a "proper" chrony but for the launcher being tested I got an average velocity of 30 metres per second which sounds about right.
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Unread postAuthor: rp181 » Sun Dec 20, 2009 2:23 pm

I ws planning a similar system, but I find that the values are very variable. I guess Ile give it another try, Im about to test the piezo, I need an attenuator though, just hitting it with a pair of scissors gives me 20 volts.

EDIT: I was looking at your picture, i was thinking of something else. Its so simple :shock:
gonna try it
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Unread postAuthor: rp181 » Sun Dec 20, 2009 3:51 pm

The sound card is too slow, the highest peak is composed of 3 data samples....
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Unread postAuthor: Ragnarok » Sun Dec 20, 2009 4:18 pm

jamessmeltingrecipe wrote:Looking forward to your results

Like a lot of my stuff, I think a long way ahead. No known ETA on this one yet.

I never compared this system to a "proper" chrony but for the launcher being tested I got an average velocity of 30 metres per second which sounds about right.

I've used something similar, although it was set up to discharge rather than charge the capacitor. (Which was also kinder on the batteries running it than a constant short circuit).
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Unread postAuthor: D_Hall » Sun Dec 20, 2009 4:23 pm

jackssmirkingrevenge wrote:time = resistance x capacitance x ln(voltage across resistors/initial battery voltage)[/b]

I never compared this system to a "proper" chrony but for the launcher being tested I got an average velocity of 30 metres per second which sounds about right.

Cute system. I like it!

I'm curious though... What was the accuracy of your components? At the office we find that such systems often require high precision components to provide meaningful results. I mean, if you plan for a 10 ohm resister and it's really a 12 ohm resister... In most applications it doesn't really matter. But for the app you show, I'm inclined to think it would in fact matter.

Not meant as a jab, just honestly curious. Cool concept.
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Unread postAuthor: Ragnarok » Sun Dec 20, 2009 4:30 pm

D_Hall wrote:I mean, if you plan for a 10 ohm resister and it's really a 12 ohm resister...

My concern would more be the capacitor.

Resistors may claim 5% tolerance, but mostly, they're 1% or less. Capacitors typically claim more like 20% tolerance, and although I don't test all my capacitors, I tend to find that they're far less precise than the resistors.

Of course, with a decent multimeter, you can check the actual values of both and work with much more precise figures.
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Unread postAuthor: jackssmirkingrevenge » Sun Dec 20, 2009 4:47 pm

D_Hall wrote:I'm curious though... What was the accuracy of your components?


No idea, this was done in the equivalent of a high school physics lab so I'm guessing they weren't top of the range components, I don't remember if we had verified the data with a multimeter though.
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Unread postAuthor: rp181 » Sun Dec 20, 2009 5:23 pm

don't forget leakage in the capacitor!
Thats why I like staying away from those kinds of circuits. When measuring the capacitance of the capacitors, they ranged from 300 uf below to 50 uf above, for 3900uf capacitors (they were surplus though).
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Unread postAuthor: ramses » Sun Dec 20, 2009 5:42 pm

Another application of this would be a low energy gun that shoots ridiculously fast (km/sec range). Like an LGG, or an ET/ETC gun. at 1 feet between the optical sensors, the time between pulses is 6.096x10^-5 seconds. That's pretty fast for a sound card/optical bridge. You can't make the distance any greater, because then the drag on the hypersonic, lightweight projectile will slow it immensely.

So you would put the pendulum right in front of the muzzle, and calibrate it against a dry-fire muzzle blast.
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Unread postAuthor: D_Hall » Sun Dec 20, 2009 6:05 pm

ramses wrote:Another application of this would be a low energy gun that shoots ridiculously fast (km/sec range). Like an LGG, or an ET/ETC gun. at 1 feet between the optical sensors, the time between pulses is 6.096x10^-5 seconds. That's pretty fast for a sound card/optical bridge. You can't make the distance any greater, because then the drag on the hypersonic, lightweight projectile will slow it immensely.


The same argument could be made for having the sensors 1 foot apart... You should have had them 6" apart!


That said....

Transparent PVC pipe + vacuum pump = no problem for optical sensors. :D


Or then there's doppler radar-based chronys. They're cute in that they can give you all sorts of crazy data. Multiple velocities for the same shot (wanna know how that drag is affecting things? No problem!). Measurements for multiple shots. All sorts of good stuff! Point being that they're another way to skin this cat.
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Unread postAuthor: ramses » Mon Dec 21, 2009 11:05 am

D_Hall wrote:Or then there's doppler radar-based chronys...


remember, not everyone has government funding... As for clear PVC, great idea, although there may be some problems with dispersion or a fiber optic type thing happening between the light and the sensor, through the pipe.
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