Construction of a fin

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Passaretti
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Construction of a fin

Postby Passaretti » Sat Sep 24, 2011 12:33 pm

Been toying around with an idea to improve fin rigidity & stiffness using non-metals... hear me out:

The idea is based around the relatively high flexural strength of G10/FR4. Normally when we make fins or other structural components out of G10/FR4 we simply cut the part out of a sheet. Instead what I would like to do is cut the sheet into strips, turn the strips on end and bond or otherwise fix them back together creating a new, restructured sheet. G10 has a very high flexural strength when applying a load normal to the plane of the weave. It makes sense to me (and I could be wrong) that it is even higher when applying the same load transverse to the plane of the weave ie "on edge." Think of it like making a wood cutting board (see image) out of a single sheet of material.

Each bond that is recreated (function of number of strips) introduces a possible failure. However, G10/FR4 is readily available in sheets up to thicknesses of 2.5". Using thicker strips would mean less bonds ie failure points.

The goal is to build a core that has a high level of rigidity & stiffness but minimal cross sectional area.

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

Postby ogivemeahome » Mon Sep 26, 2011 9:24 pm

Passaretti wrote:Using thicker strips would mean less bonds ie failure points. The goal is to build a core that has a high level of rigidity & stiffness but minimal cross sectional area.


Sounds great in principle. With some layers to 'sandwich' it all?
Would there be any benefit in using a bi-convex section? - Thin leading and trailing edges, thicker at mid-chord and a 'per-centage thickness' (thicker at the root, thinner at the tip). Some increase in cross-sectional area but the 'least of all evils' aerodynamically? Think of a flat sheet of paper, now the same sheet of paper with some curves added.

What about a 'Von Karman' profile for the fin (leading edge and trailing edge mirrored)?

Easy to suggest, harder to make!
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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby Kryten » Tue Sep 27, 2011 7:52 am

Passaretti wrote:Each bond that is recreated (function of number of strips) introduces a possible failure.

Yep.
I don't think you'd gain by taking this approach.
I believe sandwich constuction is the way to go
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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby Passaretti » Tue Sep 27, 2011 12:39 pm

Thanks for your comments guys.

Yes - when used in a fin there would be at least one "conventional" layer of glass or carbon over this restructured G10 core. Haven't given much thought to aerodynamic profiling yet, more so trying to determine what the gain in stiffness/rigidity could be, as well as the compromises with this approach. Need to do a little more digging, but if I can't come across any research in the area, I just might have to test this myself.

To test it I would prepare equal (dimensional) samples of: the restructured G10, regular G10, carbon fiber sheet, plywood, aluminum, etc. Clamp and cantilever the samples with a accel & mass on the end, excite the system manually, measure the natural frequency and back out the stiffness (k [N/m]) from: freq_nat = (1/2pi)(k/m)^.5

Mike

PS - I think this conv. has just about completely veered off the original topic of the thread - sorry Blake!
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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby rocket_troy » Tue Sep 27, 2011 2:24 pm

Mike, I'm not too optimistic there would be lots to be gained from that either, but it's not an area I specialize in and I'll be more than happy to be surprised.

The way I see composites in terms of rigidity for fins is:

Composites generally get their rigidity from the combination of (a) tension on the fibres on the outer edge of the plane being bent, (b) the rigidity of the structure underneath those outer fibres and (c) the thickness of the structure underneath those fibres. The rigidity of the structure underneath the fibres provides a good base for the fibres to anchor to which forces the fibres to *stretch* if the structure is bent outwards. The thickness of the structure underneath the fibres determines how much the fibres will need to *stretch* per angle of bend on the structure ie. thicker structure(base) means the fibres will need to stretch more per degree of bend as the outside radius of the bent structure will be larger.
As glass and carbon fibres aren’t very elastic, composites can utilise this attribute to increase the stiffness of base structures such as fins as well as adding strength to the composite. So I tend to consider composite fibres (strictly in terms of rigidity) as rigidity multipliers to base structures.
Let’s assume our sheet of G10 is a composite of epoxy resin with multiple layers of fibreglass cloth. Using the above reasoning, the very inside layer of fibreglass cloth (in relation to the bend) provides very little to the rigidity of the sheet, whereas the very outer layer providing the most rigidity in relation to the direction of the bend. This is not to be confused with the strength the fibres provide to the composite.
There could...maybe... be potential rigidity gains from pre-tensioning the fibres as you lay them up over fin bases, but I'm not sure how much and it would probably depend on the base material.

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

Postby air.command » Tue Sep 27, 2011 2:40 pm

How about adding a small thin rib half way up the fin and running parallel to the rocket body. You can have one either side of each fin. The rib doesn't have to be very tall, say only 5mm. It should make the fin more rigid in the direction of airflow, and shouldn't generate too much extra drag.

What about something really crazy like an ablative fin that would predictably break bits off cleanly when they hit their resonant frequency. Breaking the bits of would change the resonance. .... yeah I'll wake up now... :D
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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby Passaretti » Tue Sep 27, 2011 4:46 pm

rocket_troy wrote:Mike, I'm not too optimistic there would be lots to be gained from that either, but it's not an area I specialize in and I'll be more than happy to be surprised.
TP

Ditto - and I agree with your understanding of composites and their effects on rigidity.

Stepping backwards for a second and back to Blake's flight - I'm going to ask everyone what the requirements for the fins are on a flight like this?

I ask because because I want to learn, this is an area of rocketry I don't have much experience with. Growing up and flying in the Northeast US, we don't get the luxury of flight ceilings much greater than 10kft. Not to mention the recovery challenges associated with the abundance of trees, neighbors, etc. I would however like to push the material/construction limits and start ripping the fins of rockets someday. Kind of like how Blake so elegantly demonstrates ... :)

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

Postby Passaretti » Tue Sep 27, 2011 4:47 pm

air.command wrote:How about adding a small thin rib half way up the fin and running parallel to the rocket body. You can have one either side of each fin. The rib doesn't have to be very tall, say only 5mm. It should make the fin more rigid in the direction of airflow, and shouldn't generate too much extra drag.

What about something really crazy like an ablative fin that would predictably break bits off cleanly when they hit their resonant frequency. Breaking the bits of would change the resonance. .... yeah I'll wake up now... :D


First part is doable, and I think already demonstrated in some fin geometries.

Second part .... good luck explaining that one to the RSO! :D
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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby ROCKet STAR » Tue Sep 27, 2011 5:54 pm

I have previously read (I cannot remember where) that one of the common failure modes of composite fins such as these is delamination of the layers of composite material. Looking at the pictures of Nic & Blakes rocket, I wonder whether that could have happened here?

I guess the question is whether delamination is a result of the physical effect of the aerodynamic forces at play on the fin, or the result of the laminates becoming weakned by the heat generated at those speeds? If the latter were the case I guess you would expect to see the delamination propagating from the leading edge, which in this case that doesn't seem to fit. If it is the result of flutter, I would imagine that delamination would propagate from the fin tips where the effect would be greatest.

I am wondering whether as well as trying to make fins stiffer, attempts should be made to protect the edges of the fins in such a way as to to prevent delamination from starting?
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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby strud » Tue Sep 27, 2011 6:29 pm

RS, if that is the case, then maybe some carbon-phenolic fins are in order.

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

Postby ROCKet STAR » Tue Sep 27, 2011 6:57 pm

strud wrote:RS, if that is the case, then maybe some carbon-phenolic fins are in order.



My thinking was that given the orientation of the fibers in the layers that make up the fin, there is lots of strength preventing the fin from bending, but little strength preventing thoes layers being pulled apart. A have no idea how it could be achieved, but if a fin could be layed up in such away that it had fibers through it's thickness, it would prevent the layers delaminating.
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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby rocket_troy » Tue Sep 27, 2011 11:29 pm

I've experienced a fair bit of delaminating on fins via various causes. 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. Some examples include: laying some composite over a fin and then as an afterthought after the resin has cured, laying another layer over the existing one without proper preparation of the surface for proper bonding, especially if you utilized some form of easy release film. Same can be said for ready made prelaid composite sheeting such as PCB board or G10. The selection of the base material can require careful attention for very "assertive" flight profiles to ensure there is the potential for a good bond and that itself doesn't delaminate from the internal stresses.

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

Postby hybridhighflyer » Wed Sep 28, 2011 8:48 am

I will let you know a secret of how I do it and I dont have this issue.

Think about it as a bunch of straws and a grain. The edge of a fin is the edge of the grain. This is the weakest grain structural point in this situation. (Dont debate this) The singe grains may be strong however when side on the grain the vibratory forces weaken the strength and contribute to seperation.

You can calculate the forces and mass of structure required to prevent seperation. When you start getting up to Mach 2 and above this really comes into concideration when looking into weight reduction.

There are 2 forces acting on a matirial when it starts to happen and 2 forces need to be concidered with bonds and strength.

First A ring is round and has no end... well how can this help..... when making the fins the grain of the sheet and how the sheet is folded is critical. Fold the sheet like making a cylander but to the length of the dimensions with both ends if you know how in the middle of the cylander. Fill the middle coat full of black epoxy (if carbon fibre) or a high hardener mix of resin (if fibreglass). Make the fins the dimension and shape you want it minus 8% thickness. DONT CUT THE SHEET. Then soak the sheet in resin or epoxy depending letting the final thickness complete. Let this set. Lightly sand and polish. I might add you can make as many layers needed to make the desired thickness. The key here is 1 sheet for all layers if possible.

Second, The matirials are critical. High quallity and well woven matirials. Thick strains so if fibreglass at least marine grade. Carbon fibre thick woven threads made of high grade strains. Epoxy and resin also needs to be high temp and have no bubbles and high quallity (not always high strength).

You can use fibreglass or carbon fibre depending on the radar absorbtion quallities required ;) If you disagree with the above raise your complaints to the real world applications using this method. I dont own this method I was just taught it.

Thanks

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

Postby cryoscum » Wed Sep 28, 2011 9:00 am

Anobody able to decipher that?
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Re: Discovery Science Fastest to 10,000' Race (LDRS30)

Postby PK » Wed Sep 28, 2011 9:14 am

cryoscum wrote:Anobody able to decipher that?

Not really, at first I though it was an attempt to apply pressure vessel concepts (hoop stress ref the endless circle) to fin design, but reading it again I'm not so sure.
Very odd....
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