Construction of a fin

Discussions on high/extreme altitude and mach busting rockets.

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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby ROCKet STAR » Wed Sep 28, 2011 10:01 am

rocket_troy wrote:It's important to note however that multiple fibre cloths or mats laid up together *in the one process* can't delaminate from themselves. The potential for delamination comes from the combination of different materials or different processes.


That is interesting. Does that suggest that the common practice of "tip to tip" laminating of fins after they have been attached to a rocket is not optimal?
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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby Scoop1261 » Wed Sep 28, 2011 10:20 am

ROCKet STAR wrote:
That is interesting. Does that suggest that the common practice of "tip to tip" laminating of fins after they have been attached to a rocket is not optimal?


Yes.....unless you pre-prepare the "substrate" in an adequate fashion to promote cross bonding.
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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby rocket_troy » Wed Sep 28, 2011 1:17 pm

Yes, exactly what Dave said. Laminating 'Tip to Tip" is fine providing you're sure the base surface is ready for bonding. If you're unsure, just scruff up or mechanically remove the top fraction of the surface to be bonded to.

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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby cryoscum » Wed Sep 28, 2011 10:02 pm

I've wanted to try random oriented strands in resin, i.e. make a thick paste of resin and lots of finely chopped carbon and compress it between 2 sheets of glass with a set of packers off to the sides to ensure an uniform and predictable thickness. From what I would guess the random orientation of the strands would at least provide uniform flexural capacity in any direction, unlike laminated layers of cloth that depend on a change in cloth orientation to increase stiffness. This method may also provide some options to the designer, such as fins that start thick at the root and taper towards the tip, something hard to achieve with layered cloth.

May need to try it soon...
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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby Lunar_Rendezvous » Thu Sep 29, 2011 7:33 am

When I was building boats, we always layered csm between sheets of woven rovings, this type of layup was compulsory when a hull was built to survey spec. we also used wire rollers to squeeze out most of the air during layup, but that was using polyester resins.

my 2 cents say 1mm fins were just too thin, 1.5 probably would have survived, the only other cause is what Mr Passaretti has already mentioned, the vibration from fin flutter reached the same frequency as the natural frequency of the rocket and....

...google Tacoma narrows bridge.

vey much doubt there was anything wrong with Nic's layup, I'd blame Blake's prep :wink:

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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby Kryten » Thu Sep 29, 2011 7:41 am

cryoscum wrote:I've wanted to try random oriented strands in resin, i.e. make a thick paste of resin and lots of finely chopped carbon and compress it between 2 sheets of glass with a set of packers off to the sides to ensure an uniform and predictable thickness. From what I would guess the random orientation of the strands would at least provide uniform flexural capacity in any direction, unlike laminated layers of cloth that depend on a change in cloth orientation to increase stiffness. This method may also provide some options to the designer, such as fins that start thick at the root and taper towards the tip, something hard to achieve with layered cloth.

You will achieve non-directional strength, but it won't be as strong as using cloth:
The fibre length will be much shorter
The resin:fibre ratio will be much higher
So you won't have as much fibre in the laminate.
Cloth enables you to orientate the fibres to achieve the strength where you need it.
Which now prompts the question - how were the fibres orientated in the fins in question? How many layers? Did at least one of the layers have the fibres at a different orientation?
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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby hybridhighflyer » Thu Sep 29, 2011 9:35 am

cryoscum wrote:Anobody able to decipher that?


Its ok thats prob why mine dont shred ;) I understand the science.

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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby RobAust » Thu Sep 29, 2011 9:36 am

Hey Nic,

I believe your construction of the fins was correct, for sure we should look at adding chamber (aerodynamic shape) to the fins airfoil, although the sizes we are dealing with this is measured in MM's. Typically the most load is on the first 1/3rd of a wing construction which remained intact and your fillets held.

I would focus on the size of the fins for high speed flight - the faster you go, the less airfoil you require. The resultant shape of the fins now is tell tale and would be a good design to follow, I would change the fin design in Rocksim to match these, verify the stability, keep your fin construction the same and give Blake a second attempt !

My synopsis was the the de-laminated area was in the 'flutter zone'; the boundary layer of air lifted, turbulence resulted which created voilent forces until the fin(s) suffered overload.

Just my thoughts.

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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby RobAust » Thu Sep 29, 2011 9:49 am

Hi Wayne,

I also reviewed your response and was not able to understand the message you were attempting to send.

As a picture is worth a 1000 words, can you please post a few images of your mach busting rockets / construction techniques you described above so we can view these in relation to your response ? This would be really helpful.

Just out of curiosty, how high did your rockets fly to ? As you know, one of the challenges of mach busting flights is slowing them down and remaining within the ranges altituide waiver.

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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby PK » Thu Sep 29, 2011 10:05 am

hybridhighflyer wrote:
cryoscum wrote:Anobody able to decipher that?


Its ok thats prob why mine dont shred ;) I understand the science.

So you wont have any problem posting pictures of them then.

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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby cryoscum » Thu Sep 29, 2011 10:45 am

Kryten wrote:Which now prompts the question - how were the fibres orientated in the fins in question? How many layers? Did at least one of the layers have the fibres at a different orientation?


I had the 2 outer layers (200GSM 2x2 twill) oriented perpendicular with the BT and the central 411GSM bias weave (stitched 3 layer 45 deg, 0 deg, 45 deg) was in there with its outer layers at 45 deg's to the twill layers. Don't really see how the orientation could be improved upon. Maybe the fins were, in the end, just too thin?! :?

I could maybe have tried Wayne's method, but I don't know what that method is. Maybe the photo's he's going to post will help.
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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby Andrew Burns » Thu Sep 29, 2011 11:06 am

Also note that it's not stifness but resonance frequency that matters for flutter. You want the resonance frequency to be higher which means you want more stiffness and less mass. Therefore it may be better to insert a lightweight sandwich material into your fins than simply make them thicker out of solid composite. In face it's basically the standard in the industry to construct composite parts with stressed skins over a lightweight core of foam, balsa or honeycomb. Of course if you do introduce a core remember that the crush strength of the core will likely dictate the overall strength of the fin (rather than the failure mode being the fin snapping it will be the skin buckling and the fin folding), which is why it's better to use end-grain balsa with the grain pointing from skin to skin for example.

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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby Kryten » Thu Sep 29, 2011 12:05 pm

cryoscum wrote:I had the 2 outer layers (200GSM 2x2 twill) oriented perpendicular with the BT and the central 411GSM bias weave (stitched 3 layer 45 deg, 0 deg, 45 deg) was in there with its outer layers at 45 deg's to the twill layers. Don't really see how the orientation could be improved upon.

Nice!
cryoscum wrote:Maybe the fins were, in the end, just too thin?! :?

Possibly
cryoscum wrote:I could maybe have tried Wayne's method, but I don't know what that method is. Maybe the photo's he's going to post will help

It may prove interesting. I got lost in his description
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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby hybridhighflyer » Thu Sep 29, 2011 12:42 pm

I will make a build thread when I get home. I will even do it to the same peramiters that this was done.

It is a tad complicated process but worth it.

Image

The key to this method is the ends of carbon fibre or fibre glass sheet are not around any of the fins edges of the finished product. Also around the edges you have a lot of folds. If you have many layers also try changing the grain of the threads. Another key is a generous layer of epoxy around the outside to ensure no bumps.

Some people lay serveral sheets flat, cut around them and lather a heap of epoxy and expect this to go mach 2. The ends will simply fray under the forces.


I will upload a few pics for you in a second.
Again this is about aerodynamics. They fold AL around a wing's edges and dont expose flat surfaces.

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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby ROCKet STAR » Thu Sep 29, 2011 12:58 pm

hybridhighflyer wrote:
Image



WTF :?:
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