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 hybridhighflyer » Thu Sep 29, 2011 12:59 pm

How to fold

First fold 2 edges into the centre

Image
Then fold it in half
Image
Then fold it diaganally
Image

Then fold again to the shape desired.
Image

Here is what you get for the main edge. The back edge only has 1 fold.
Image

Here is the edge that will be going against the fillets.
Image

With the first fold ensure you have a generous layer of epoxy and fibre(glass or carbon)

Vacume seal to form again ensuring a generous layer of epoxy on the outside.

Sand lightly and buff smooth.

Try this and lets see if it works better than what was done ;)

Thanks

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

Postby hybridhighflyer » Thu Sep 29, 2011 12:59 pm

ROCKet STAR wrote:
hybridhighflyer wrote:
Image



WTF :?:


Thats where I am hence cant upload rocket pictures

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

Postby rocket_troy » Thu Sep 29, 2011 1:16 pm

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.


Yes, I use to use balsa for my fin substrates or core materials, but found too many problems with them over time. No doubt some of these issues were due to the crushing strength of the balsa. Never tried orienting the grain direction from skin to skin but I'm still not totally convinced about using balsa for "assertive" velocities that might place high stresses on the fins. I'm not sure about the scale of the shear forces the internal structure of the balsa is under during very intense bending and fluttering moments.

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

Postby OverTheTop » Thu Sep 29, 2011 2:30 pm

In fact it's basically the standard in the industry to construct composite parts with stressed skins over a lightweight core of foam


I think the stressed outer layer increases the natural frequency of the assembly (a trick that has been around for a long time), hopefully keeping it away from resonances.

The foam probably does a good job of absorbing any vibration (increases damping in the part) so destructive excursions are less likely when resonance is actually hit. Sometimes it isn't about the strength of the materials, but the cleverness of the combination.
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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby Passaretti » Thu Sep 29, 2011 2:50 pm

There is one fundamental mechanism here that I don't completely understand, please share if you do.

From the wiki page on aeroelasticity/dynamic aeroelasticity: "Flutter can occur in any object within a strong fluid flow, under the conditions that a positive feedback occurs between the structure's natural vibration and the aerodynamic forces."

I know enough about resonance, and control systems to understand the basic process here, but what I can't seem to grasp is why or how a "strong fluid flow" can induce this feedback to begin with. There has to be more to it than that... My hunch is that the "strong fluid flow" being a natural system in itself is not perfect and has its own set of dynamics that contribute to/form the excitation components necessary to start the feedback process. Would that be correct? Can anyone explain and preferably pointing to any literature in the area?

If the "strong fluid flow" was an ideal laminar flow of say air (planar to the surfaces of the wing/fin). Would the feedback process begin? Forget about rockets, planes and bridges for a second. If I put a fin in a wind tunnel under the above conditions - what would happen?

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

Postby cryoscum » Thu Sep 29, 2011 3:34 pm

We seem to be mostly discussing the resonance issue, and no doubt it is the one item known to be responsible for lost fins, but there is one observation that I can't really explain at this point and that is the fact the 2 of the fins on the carbon rocket had some torn and frayed tips, but had been literally folded flat against the BT, but the fillets were fully intact. It kind of looked like the fins were subjected to a lot of heat, softened and folded, then set in the unnatural position. Assuming this observation was actually caused by high heat, would it have been friction or motor? Motors are supposed to be able to take something like 200 deg C, OK for aluminium but not for epoxy. The whole body was heat treated to about 75 deg for several hours, but I have heard that even well cured epoxies return to near gel states if subjected to higher curing temperatures than it was cured to previously. All the discussions thus far have assumed the fact that the composite itself has remained unchanged during the flight.

What ever happened to the temperature logger someone built on the forum? (sorry, I forget now who it was). Maybe worth an experiment... Longer alu tip to the NC and alu leading edge to the fins? Would certainly help the stiffness of the fin too...

On that point, has anyone actually read about Mach 3+ carbon finned rockets or are we into aluminum country?
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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby hybridhighflyer » Thu Sep 29, 2011 3:42 pm

Hey Mike,

I do get your point and agree somewhat.

My view on this is of a more simple way. The rules of equilibrium constant. The world likes to be ballanced.

If preassure is applying on a force to such a rate the changes in pressure around the object will be such that it will try and keep the item equil. This creates vibrations resignating throught the object. The resignating frequency and the matirials tendancy to resignate will change the rate of vibrations from preassure. So in summary lead will vibrate with a higher frequency but require more energy and wood will vibrate easier but have a lower frequency max. Its all about the matirial the configuration of the matirial with the maluable and ductile properties creating a resignating frequency to release the bonds causing a fracture or shred at the weakest point. At a certain point design stops. All you can do is ensure its the best to have the strongest point at the highest stress point and reduce the load on the weak points. Also ability to control wind flow around the object through design may help however I dont think its too much of a use in a rocketry design application much to effect rockets easily reaching mach 1.

Practical applications to the above in case your confused.

1: Matirials are important. - A potato will not have the stability to go mach 1 the bonds in the item can not take the preassure.
2: Design is not everything - A potato designed for the best aerodynamics will still not go mach 1 however carbon fibre could with enough power.
3: Matirial structure is important - A straw when a potato is droped side on it bends the straw. when the top is showing it goes through the potato.
4: A design has to come together - All aspects have to flow and work together. The world doesnt like things to go mach 2 or more so you have to work for it and all conditions need to exact.

Happy thinking let me know.

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

Postby PK » Thu Sep 29, 2011 3:57 pm

Hmm, too technical for my mortal brain.

How about:
wind hits fin
Fin bends
Bent fin acts like an airfoil and additional forces are generated that help force the fin back into shape.
Fin snaps flat and bends a bit the other way under it's own inertia.
Bent fin acts like an airfoil (on the opposite direction)...
Etc. Etc
(this is the feedback bit Mike was on about).

The speed that all this happens at a speed deteined by the stiffness and mass of the fin.
PK

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

Postby Passaretti » Thu Sep 29, 2011 4:14 pm

PK wrote:Hmm, too technical for my mortal brain.

Come on PK don't tap out now!

And Wayne, you lost me at ...
hybridhighflyer wrote: If preassure is applying on a force to such a rate...


I'm now reading about something called "fluid structure interaction" ...
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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby Lunar_Rendezvous » Thu Sep 29, 2011 4:30 pm

More reading, an early study by D.Martin NACA...

http://ntrs.nasa.gov/archive/nasa/casi. ... 085030.pdf

I can imagine more heat being produced by the actual flutter, than by airflow over the leading edge of the fin, I know 5 min epoxy is not as heat resistant as 24H araldidte, you might have to phone cgcomposites for more info on the epoxy you used.

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

Postby ROCKet STAR » Thu Sep 29, 2011 4:32 pm

cryoscum wrote:
What ever happened to the temperature logger someone built on the forum? (sorry, I forget now who it was). Maybe worth an experiment... Longer alu tip to the NC and alu leading edge to the fins? Would certainly help the stiffness of the fin too...


It's flown once so far, though only to Mach 1.1. Initial flight didn't yeild very useful data as the thermocouple went open circuit briefly at the crucial moment. That said, I think I have resolved the issue so should get better data next flight. Next time I fly it I hope to push it >Mach2.

I have also been looking at these sensors:

(Edit: wrong link)

http://www.sparkfun.com/products/10264

http://www.seeedstudio.com/depot/piezo- ... th=144_146

They could be very easily installed into a composite fin as it is being made and could potentially give an indication of how much deflection or vibration the fin experiences during flight.
Last edited by ROCKet STAR on Thu Sep 29, 2011 4:49 pm, edited 2 times in total.
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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby cryoscum » Thu Sep 29, 2011 4:41 pm

ROCKet STAR wrote:That said, I think I have resolved the issue so should get better data next flight. Next time I fly it I hope to push it >Mach2.

I have also been looking at these piezo vibration sensors:

http://www.seeedstudio.com/depot/piezo- ... th=144_146

They could be very easily installed into a composite fin as it is being made and could potentially give an indication of how much deflection or vibration the fin experiences during flight.


Nice! Would love to find out what sort of heat we're dealing with. To ablate a carbon-epoxy NC in less than 3 seconds suggests some seriously high temps...
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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby rocket_troy » Thu Sep 29, 2011 4:47 pm

Nice! Would love to find out what sort of heat we're dealing with. To ablate a carbon-epoxy NC in less than 3 seconds suggests some seriously high temps...


Heat indicating tapes, crayons, adhesive labels and whatnot are popular for analyzing that sort of thing too.

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

Postby Sumo310 » Thu Sep 29, 2011 5:22 pm

Hi Nic,

Just a quick query - did you cook the fins constantly at 75 degrees, or build the temperature up over time?
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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby cryoscum » Thu Sep 29, 2011 5:42 pm

Simon, ramped, but admittedly the buildup was fast, maybe 30 minutes. Really need to come up with some sort of electronics that can do the ramping...

Troy, I couldn't keep stickers on the BT on my Mach 2.2 flight at Williams, so some tape may struggle to stay on too, but the crayon sounds like a good idea. Do you have a product name?
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Total impulse for 2016: 32,458 Ns (thus far)
Total impulse for 2015: 84,231 Ns
Total impulse for 2014: 40,757 Ns
Total impulse for 2013: 62,927 Ns


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