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long range ammo MiniBoy Mark I

Post about things you have launched or thought about launching. Also post about various materials used for building cannons. No posts about explosive projectiles!
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Unread postAuthor: MrCrowley » Sat Jul 30, 2011 6:34 pm

Would you recommend that I fibreglass the ABS threaded adapter or not bother because it is essentially twice as thick as the barrel?

In that case, should I be worried about the male threads on the ABS threaded adapter that thread in to the galv. iron bushing in the tee? I guess there's not really much I can do about it though.
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Unread postAuthor: MrCrowley » Mon Aug 01, 2011 6:50 am

This is the epoxy I intend to use for the fibreglass since we have some sitting in our garage. I assume it's ok considering it's for fibreglass applications.

I haven't really read up much on fibreglassing yet because I'm busy with assignments but I assume you just put a coat of the epoxy over the barrel area you want to fibreglass, apply a strip of fibreglass and then either wait a bit or continue applying more epoxy and more layers.
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Unread postAuthor: jackssmirkingrevenge » Mon Aug 01, 2011 8:09 am

MrCrowley wrote:This is the epoxy I intend to use for the fibreglass since we have some sitting in our garage. I assume it's ok considering it's for fibreglass applications.


Seems legit :)

I haven't really read up much on fibreglassing yet because I'm busy with assignments but I assume you just put a coat of the epoxy over the barrel area you want to fibreglass, apply a strip of fibreglass and then either wait a bit or continue applying more epoxy and more layers.


Pretty much. Coating with cling film when you're done prevents all the epoxy running off.

High postage costs but I'm sure that you can find something cheaper locally,this stuff would work better and look great with a bit of fine finishing ;)
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Unread postAuthor: Fnord » Sat Aug 06, 2011 10:51 am

Haven't been here for a while but...

Crowley,

You can fiberglass the adaptor, but I'd only use a couple layers if it were me.

As long as it's designed for glass it should be ok. Most fiberglass has a coating on it that is meant to adhere to specific types of resin (for this reason you should also avoid finger-printing the cloth up excessively).
Verify the pot life of your resin and that it sets up properly, as it may change with age. With only 15 minutes pot life, you'll have to be quick about it.

As to actually using it, you'll need to roughen the barrel and get it reasonably clean, then apply enough resin to 'wet' the cloth- you'll know when you achieve this because the cloth pattern will become completely transparent. Expect to make a complete mess. You'll have resin-soaked clothes and sticky hands covered with fibers for the next week if you don't use gloves.
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Unread postAuthor: jackssmirkingrevenge » Sat Aug 20, 2011 5:09 pm

It's a shame we'll never have footage likethis of the Miniboy. LeMaudit, you might have sent it to the wrong spudder :P :D
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Unread postAuthor: MrCrowley » Wed Sep 28, 2011 5:28 pm

So i'm about to buy the fibreglass as I may have some free time coming up. If I buy a 1m x 1m (200g per m^2) sheet and cut it in half I will be able to fibreglass the first 50cm of the barrel with about 10 layers for a total thickness of 3mm.

Sound good or do I need more thickness/fibreglass more of the barrel?

I'm a free man come November the 14th, I'll try get up to Mangawhai for the big shoot about a week after that. I'll see what I can do about getting a second HD camera.
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Unread postAuthor: LeMaudit » Wed Sep 28, 2011 5:33 pm

Hey, I have no idea about the fiberglass but... cool! 8)
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Unread postAuthor: saefroch » Wed Sep 28, 2011 8:15 pm

Wikipedia has some numbers on the tensile strength of fiberglass. Why don't you crunch some numbers and find out?

Too tired or I'd do it for you.
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Unread postAuthor: MrCrowley » Wed Sep 28, 2011 8:37 pm

saefroch wrote:Wikipedia has some numbers on the tensile strength of fiberglass. Why don't you crunch some numbers and find out?

So this is what it feels like to be a n00b :D

AS2634-1983 specifies that to comply with this standard a typical 6mm FRP laminate made by the contact moulded process must have a minimum tensile strength of 78 MPa

So roughly half that for 3mm thick fibreglass (I realise my fibreglass/application wont be the same as in the above quote) then adding the tensile strength of ABS would give me a working pressure of ~1000PSI (safety factor of 3) and a burst pressure of ~3000PSI according to some online calculator.

I realise that is an extremely crude way of figuring out how strong it will be but I assume it gives a good enough idea of the strength?
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Unread postAuthor: Fnord » Wed Sep 28, 2011 9:28 pm

Here I go again... should be getting sleep for work tomorrow but instead I'm spoonfeeding maths to some n00b kiwi who has nowhere near the velocity, drag-coefficient, or cross-sectional density to hit my house from there.
:D


I used DYI's equation with a yield strength of 250 Mpa* got a yield pressure of 1,798 psi for the fiberglass layer ONLY. (assuming your 50mm pipe has the same OD as us pipe).

I think I did it right, since this sounds reasonable as a theoretical burst pressure, given the dimensions.

You'll probably be more concerned with the length of the barrel you should wrap- play with the barrel psi/projectile position in hgdt and figure the approximate pressure drop at various distances. I'd use an overly heavy proj with high barrel friction in your simulation.


*based on everything I've seen online so far 250 Mpa seems within reason; garolite is basically the same thing and is often considerably stronger.
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Unread postAuthor: Moonbogg » Wed Sep 28, 2011 10:08 pm

When is this thing going to be fired?
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Unread postAuthor: MrCrowley » Wed Sep 28, 2011 10:55 pm

Thanks for that Fnord :D

I'll do some HGDT calculations tonight, I'm starting to feel like I should remote fire this thing.

@Moonbogg,

Probably mid to late November.
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Unread postAuthor: SpudBlaster15 » Wed Sep 28, 2011 11:38 pm

The equation DYI posted in that thread is based on the maximum shear stress theory, and will tend to give very conservative results with thick walled pressure vessels.

Further down the page, btrettel posted an equation he derived from the distortion energy theory, which should give more reasonable numbers.

P<sub>yield</sub> = YS * (d<sub>o</sub><sup>2</sup> - d<sub>i</sub><sup>2</sup>) / sqrt((d<sub>i</sub><sup>2</sup> + d<sub>o</sub><sup>2</sup>)<sup>2</sup> + (d<sub>i</sub><sup>2</sup> - d<sub>o</sub><sup>2</sup>)<sup>2</sup> + (d<sub>i</sub><sup>2</sup> + d<sub>o</sub><sup>2</sup>) * (d<sub>i</sub><sup>2</sup> - d<sub>o</sub><sup>2</sup>))

Where d<sub>o</sub> is the outside diameter of the pipe, d<sub>i</sub> is the inside diameter of the pipe, YS is the yield strength of the material, and P<sub>yield</sub> is the yield pressure of the vessel.

For this particular calculation, I would simply assume the fiberglass wrapped barrel to be a single thick walled cylinder, and use the lowest of the two materials' yield strengths in the equation. This will give a somewhat conservative yet fairly accurate result.
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Unread postAuthor: jackssmirkingrevenge » Thu Sep 29, 2011 1:36 am

Did you see if you can get CF cloth instead?

I'm sure that in a surf-mad area that my stereotypical view of NZ assures me you live in, it should be easy to come by.
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Unread postAuthor: MrCrowley » Thu Sep 29, 2011 1:42 am

$7.50 for a 1m x 1m sheet of fibreglass
$50 for a 1m x 1m sheet of carbon fibre.

Would I also need to buy different epoxy for the carbon fibre? I already have some epoxy for the fibreglass.

So, no, not looking like I'll be buying any haha.
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