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projectile testing - any volunteers?

A place to ask general spud cannon related questions.
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Unread postAuthor: irisher » Wed Mar 30, 2011 10:29 pm

For the sake of cost I was thinking of turning the front and back pieces on a wood lathe (free for me) then adding lead to the nose cone to bring up the Center of Gravity. Thoughts?
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Unread postAuthor: jackssmirkingrevenge » Wed Mar 30, 2011 10:31 pm

irisher wrote:Thoughts?


Sounds about right, the CG must be significantly forward though for any hope of stabilisation so I would also suggest hollowing out the tail portion.
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Unread postAuthor: Lockednloaded » Wed Mar 30, 2011 10:33 pm

why don't you test these designs with your co2 capsule slugs? all you'd have to do is make the tails.
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Unread postAuthor: jackssmirkingrevenge » Wed Mar 30, 2011 10:35 pm

Lockednloaded wrote:why don't you test these designs with your co2 capsule slugs? all you'd have to do is make the tails.


As stated before I simply don't have the distance to shoot over before tumbling can be significantly observed.
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Unread postAuthor: irisher » Wed Mar 30, 2011 10:36 pm

I will have to start on the rounds this weekend because I managed to snap the belt on my lathe at my current location but I don't think they will take long to manufacture once I get to other lathe.
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Unread postAuthor: jackssmirkingrevenge » Tue Oct 11, 2011 9:27 am

Now that I have my own machining facilities I was thinking of giving this a go myself.

Anyone with CAD facilities care to determine the CG and CP of this design?
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Unread postAuthor: Labtecpower » Tue Oct 11, 2011 9:51 am

Here's a quick inventor sketch (threaded with m5x0.8 )

Image

Not as fappy as MiniBoy, but still pretty cool :D

I just received my study book for inventor, so I'll see if I can calculate something :wink:

I haven't got an idea about the reliability of this, but:

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Unread postAuthor: jackssmirkingrevenge » Tue Oct 11, 2011 10:31 am

Looks very pretty :) but I want cold hard data ;)

Not as fappy as MiniBoy, but still pretty cool


Well, if we can achieve more with less, and make 10 projectiles with the same effort, they will not be as precious and more testing can be done without turning spudfiles into a çookery show :roll:
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Unread postAuthor: Labtecpower » Tue Oct 11, 2011 11:21 am

Looks very pretty :) but I want cold hard data :wink:


I expected that :D
I'm a bit of a CAD-newb however, I supect there are far more capable members :P

Well, if we can achieve more with less, and make 10 projectiles with the same effort, they will not be as precious and more testing can be done without turning spudfiles into a fuçking çookery show :roll:


You still need the projectiles to be retrievable, otherwise you could just shoot pieces of potato. (unless you're going for damage)

I shot a lump of steel in the air with my QDV, because I tought I would be able to retrieve it, but I haven't seen it flying. Still curious where that one went :roll:
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Unread postAuthor: al-xg » Tue Oct 11, 2011 11:33 am

Mmm, yeah how are these projectiles retrieved ?

We need someone with those animal radio tags...


Any particular length/angle for the brass cones?

CG would be roughly 20.2mm from the tip. (with an estimate dual cone angle)


How are people estimating the CP ?

Centroid of section along the longitudinal axis is ~31mm from tip, seems that some rocket people use that...

All seems a bit dodgy to me...
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Last edited by al-xg on Tue Oct 11, 2011 4:43 pm, edited 6 times in total.
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Unread postAuthor: velocity3x » Tue Oct 11, 2011 11:39 am

jackssmirkingrevenge wrote:Anyone with CAD facilities care to determine the CG and CP of this design?


I'll make a stab at it, but it'll be another 6-7 hours before I can get to it. If you don't have an answer by then....I'll do it.
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Unread postAuthor: POLAND_SPUD » Tue Oct 11, 2011 11:49 am

Anyone with CAD facilities care to determine the CG and CP of this design


There is a relatively simple test that you can use on a model rocket to determine the stability. Tie a string around the body tube at the location of the center of gravity. Be sure to have the parachute and the engine installed. Then swing the rocket in a circle around you while holding the other end of the string. After a few revolutions, if the nose points in the direction of the rotation, the rocket is stable and the center of pressure is below the center of gravity. If the rocket wobbles, or the tail points in the direction of rotation, the rocket is unstable


So in other words >> Y U no test it yourself?


plus if it doesn't work you can drill out the inside of the delrin part and see if that helps
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Unread postAuthor: jackssmirkingrevenge » Tue Oct 11, 2011 12:55 pm

al-xg wrote:Mmm, yeah how are these projectiles retrieved ??


Assuming they are built in the same manner as the first post with two endcaps push fit in a tube, there's plenty of room to house a recovery mechanism. I'm envisioning a spring loaded or pyrotechnic device that pops the tail off on impact with the ground, releasing a foil/mylar streamer that will be highly visible in good sunlight even from large distances., especially with a slight breeze blowing.

What I drew is a small scale version, a "wind tunnel model" if you like.

Any particular length/angle for the brass cones?


Not really, can be a straight taper.

So in other words >> Y U no test it yourself?


Because I a) haven't built it yet and b) can't really tell what a tiny little projectile is doing when spinning it around, as opposed to a 3 or 4 foot rocket the method is suggested for.

As someone like dewey would eat this calculation for breakfast I thought it would be easier to seek professional help (something which friends and family have been suggesting for a long time :D)

Well it should be a doddle to machine, especially with the compound slide, might as well give it a go :)
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Unread postAuthor: al-xg » Tue Oct 11, 2011 1:04 pm

What I drew is a small scale version, a "wind tunnel model" if you like.


Right... you do realise that cad drawings don't have to be small to fit on the screen ? ;) :P
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Unread postAuthor: jackssmirkingrevenge » Tue Oct 11, 2011 1:10 pm

Oh?

I thought architects needed MASSIVE screens!

Wow.

:P

I drew it small scale because it's roughly the dimesions I intent to build it to, for testing from a 6mm pneumatic ;)
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