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Honeycomb Nomex, End Grain Balsa, or other Fin Core Material

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CORZERO

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Regarding fin core materials: I will preface by saying cost is not an issue, so I am not looking for bargain solutions. This is a construction exercise. I am working on this project:

View attachment MARK13.ork

Without opening the file, fin root chord= 16.5", tip chord = 3", span = 5", core material = .250". This will be a mach vehicle.

My objective is light weight and strength. I want to construct laminated fins with a core material. Does anyone have any experience in this arena?

Construction method will employ vacuum bagging with no heat (room temp cure). Skin material will be 3k carbon fiber cloth and Aeropoxy. Have not decided number of layers and I am not interested in prepreg at this time.

Core material:

Thus far I have read that Nomex honeycomb provides the strongest and lightest per square rating. End grain balsa on the other hand, is the most widely used core material in an array of applications and is comparable in all measurements of strength.

My concern with Nomex is beveling, or fin edges. If I had a solution for edges I would use Nomex. I am leaning towards balsa for this reason, as from what I have read I will not lose any significant strength and I can bevel and cap fin edges with carbon fiber.

I have considered aluminum honeycomb, but I am not sure I have the facilities to ensure a proper bond and I will not use prepreg.

Are there any other core materials others have used with success?

Any ideas on fin edges for Nomex?

Anyone experience any failures with core fin laminates?
 

RocketFeller

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I would suggest end-grain balsa. When we were building the fins (elliptical with an 11" root chord and 27" semi-span and .5" core) for the upscale Dragonfly I did quite a bit of online research and a couple phone conversations with a composites technician from Fiberlay.

The advantages of end-grain balsa as I see them:

Good (if not the best) strength for weight ratio
Better adhesion than either foam or honeycomb core
Better thermal stability than foam core
Easier to edge than honeycomb core
Easy to cut and work with in general
 

OverTheTop

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I was looking at this a while ago. I ended up going with a G10 fiberglass core cut in an isogrid pattern as an experiment:
https://en.wikipedia.org/wiki/Isogrid

My Apache fins are 3.2mm G10 isogrid core with 0.5mm CF sheets for skins. Further details here:
http://www.ausrocketry.com/forum/viewtopic.php?f=6&t=5019&hilit=apache&start=31

fincad.jpg
AllDryFit.JPG

Tried using an ABS core originally but had troubles with bonding. Went back to the G10 and it looks rock-solid. Just waiting for the weather to dry up a little for the first flight (Mach 1.2), hopefully next weekend. Beer is for scale :)
 

Random Flying Object

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Nomex honeycomb between two pieces of CF works great. I bagged a large sheet of CF/honeycomb/CF. Cut out the fin shapes. Beveled the edges and back filled with Aeropoxy. Glued the fins in place. Did fillets with filled west systems and then did tip to tip with CF.
 

dhbarr

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I was looking at this a while ago. I ended up going with a G10 fiberglass core cut in an isogrid pattern as an experiment:
https://en.wikipedia.org/wiki/Isogrid

My Apache fins are 3.2mm G10 isogrid core with 0.5mm CF sheets for skins. Further details here:
http://www.ausrocketry.com/forum/viewtopic.php?f=6&t=5019&hilit=apache&start=31

View attachment 303649
View attachment 303650

Tried using an ABS core originally but had troubles with bonding. Went back to the G10 and it looks rock-solid. Just waiting for the weather to dry up a little for the first flight (Mach 1.2), hopefully next weekend. Beer is for scale :)
That is outrageously awesome. Seems you could get similarish results with notched & epoxied extruded CF rectangular rods? Maybe?
 

Rob702Martinez

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I have also been giving this serious thought lately. Just ordered bagging supplies so I am looking forward to this thread.

A thought would be nomex HC core, capped with g10 strips width-wise on trailing, root etc. And use g10 strips length-wise on the leading edge, about 3/8 wide. Vacuum bag the whole thing as a wet layup, degassed. You can now sand or bevel the leading edge and you are cutting into g10 as you normally would. Also tip to tip after that is my initial thought.

My second thought would be, to replace the use of g10 with divinycell.
 
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Random Flying Object

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I have also been giving this serious thought lately. Just ordered bagging supplies so I am looking forward to this thread.

A thought would be nomex HC core, capped with g10 strips width-wise on trailing, root etc. And use g10 strips width-wise on the leading edge, about 3/8 wide. Vacuum bag the whole thing as a wet layup, degassed. You can now sand or bevel the leading edge and you are cutting into g10 as you normally would. Also tip to tip after that is my initial thought.

My second thought would be, to replace the use of g10 with divinycell.
Sounds like a good idea. Sanding and back filling the HC on the edges is a real pain.
 

dhbarr

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I've seen a couple with phenolic edging for sustained high mach, hardwood for lower regimes. Lots easier to sand and shape.
 

Rob702Martinez

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G10 tri stock would be good to use for end caps. You could bevel the entire fin.
 

Rob702Martinez

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This might help. Looks like foam type core is better suited and already in use. There should be some technical publications out there...

I found this a few weeks ago...

"Alcan Airex also offers Airex R82, a polyetherimide (PEI) thermoplastic foam product with a very high processing temperature — "well above" that of crosslinked PVC or SAN, says George Dohn, director of industrial products for Alcan Baltek Corp. Because of its dielectric properties and thermoformability, PEI foam is often specified for high-performance applications like radomes for military and commercial aircraft. It also can be used as an edge treatment for aluminum and aramid honeycomb cored panels in aerospace applications, says Dohn. PEI's excellent fire/smoke/toxicity properties exceed most worldwide standards for road and rail transportation systems and commercial aircraft interiors. This led to its selection for the floor panels, front end and skirts in the maglev high-speed train that serves the Shanghai, China airport"
 

CORZERO

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Nomex honeycomb between two pieces of CF works great. I bagged a large sheet of CF/honeycomb/CF. Cut out the fin shapes. Beveled the edges and back filled with Aeropoxy. Glued the fins in place. Did fillets with filled west systems and then did tip to tip with CF.
Thank you for your response! Do you have pictures of the bagging process? What do you mean by back-filled? I was also curious how flat the skin would be after vacuum. What keeps the air from being pulled from the individual cells and causing the CF cloth to dimple?
 

CORZERO

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I've seen a couple with phenolic edging for sustained high mach, hardwood for lower regimes. Lots easier to sand and shape.
Thanks for the tip! I really like that idea as I could machine the phenolic edges and incorporate them as a core component. Do you know of any sources for phenolic? I found this:

http://uscrpl.blogspot.com/2011/10/post-balls-xx-report.html

They used this method. The project had not flown at the time of this article and I have not followed up to find out if it ever did. I also like your wood edge approach. Sanded and shaped, perhaps a coat or three of thinned laminating epoxy to protect the edges? With a waiver of 13k, surely thermal issues are of no concern at any speed one could possibly achieve without violating the ceiling as the vehicle would spend only very short periods of time at that speed.

Probably another factor worthy of noting: My waiver is 13k, however I would like to build the components to tolerate high mach, high altitude environments.
 

CORZERO

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I was looking at this a while ago. I ended up going with a G10 fiberglass core cut in an isogrid pattern as an experiment:
https://en.wikipedia.org/wiki/Isogrid

My Apache fins are 3.2mm G10 isogrid core with 0.5mm CF sheets for skins. Further details here:
http://www.ausrocketry.com/forum/viewtopic.php?f=6&t=5019&hilit=apache&start=31

View attachment 303649
View attachment 303650

Tried using an ABS core originally but had troubles with bonding. Went back to the G10 and it looks rock-solid. Just waiting for the weather to dry up a little for the first flight (Mach 1.2), hopefully next weekend. Beer is for scale :)
Thanks! I browsed your build threads. You do great work! I thought about G10 initially, as I just finished laminating a set of G10 core fins for a Madcow Black Brant II. The G10 was 3/16ths solid sheet, and 4 layers of 3k CF twill per side. I found that for their size, the G10 fins had a significant amount of mass to them. I was concerned that using this material as a core material for this larger project at a thickness of .250 even machined to a skeleton would result in a heavier component than with balsa or nomex. I have not, however, done any math to confirm this. This was a gut feeling. From what I have read, strength from this type of structure is derived from the skin and skin bond to the core and not the core material itself.
 

dhbarr

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Thanks for the tip! I really like that idea as I could machine the phenolic edges and incorporate them as a core component. Do you know of any sources for phenolic? I found this:

http://uscrpl.blogspot.com/2011/10/post-balls-xx-report.html

They used this method. The project had not flown at the time of this article and I have not followed up to find out if it ever did. I also like your wood edge approach. Sanded and shaped, perhaps a coat or three of thinned laminating epoxy to protect the edges? With a waiver of 13k, surely thermal issues are of no concern at any speed one could possibly achieve without violating the ceiling as the vehicle would spend only very short periods of time at that speed.

Probably another factor worthy of noting: My waiver is 13k, however I would like to build the components to tolerate high mach, high altitude environments.
McMaster Carr has phenolic plate. Only really needed if one plans to spend much time above mach 2.5 AFAIK.
 

RocketFeller

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G10 has a density that is about 15x that of balsa, so you would have to remove ~93% of the material to get a comparable core weight. As you mentioned, composite "sandwich" layups get their strength from the compressive/tensile strength of the core. Air has no strength to speak of, so it is hard to imagine that a skeletonized core would get you a better strength to weight ratio than a conventional core made of balsa/foam/honeycomb.
 

Random Flying Object

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Thank you for your response! Do you have pictures of the bagging process? What do you mean by back-filled? I was also curious how flat the skin would be after vacuum. What keeps the air from being pulled from the individual cells and causing the CF cloth to dimple?
I bagged the original plate on a flat stiff surface. The top layer didn't dimple.

ImageUploadedByRocketry Forum1476971085.147116.jpg
ImageUploadedByRocketry Forum1476971157.931727.jpg
 

OverTheTop

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Air has no strength to speak of, so it is hard to imagine that a skeletonized core would get you a better strength to weight ratio than a conventional core made of balsa/foam/honeycomb
The isogrid structure is remarkably similar in concept to the honeycomb core. Basically an extension of the old I-beam concept.
https://en.wikipedia.org/wiki/Isogrid

It has its place in manufacturing due partly to its strength to weight ratio.

My fin core is probably far from optimised. No FEA was performed to get the dimensions on this part of mine. I have gauged it by eye, which probably makes it overkill anyway.

IIRC my fin is about 30% of the mass of a similar solid G10 fin. It feels markedly stiffer too, but that's what you get typically with composites!
 

RocketFeller

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I can see some advantages to the isogrid structure, but it still seems awfully heavy.

A balsa core would be less than a fourth of the weight. Divinycell 200 PSI would weigh weigh less than an eighth as much. 1.8 pound/foot Nomex honeycomb would weigh less than 5% the weight.

The isogrid structure is remarkably similar in concept to the honeycomb core. Basically an extension of the old I-beam concept.
https://en.wikipedia.org/wiki/Isogrid

It has its place in manufacturing due partly to its strength to weight ratio.

My fin core is probably far from optimised. No FEA was performed to get the dimensions on this part of mine. I have gauged it by eye, which probably makes it overkill anyway.

IIRC my fin is about 30% of the mass of a similar solid G10 fin. It feels markedly stiffer too, but that's what you get typically with composites!
 

markkoelsch

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I recall Mick Kelly, composite master, using divinyl foam sheet as a core material.
 

CORZERO

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Unless someone knows of a better deal I will pull the trigger on this:

http://www.avtcomposites.com/cgi-bin/commerce.cgi?preadd=action&key=.250HCOM2X4

I have also ordered this to play with:

https://www.amazon.com/gp/product/B00YV6T028/?tag=skimlinks_replacement-20

I'll make a panel with the balsa and cut some fins for another project before building and cutting the panel with the nomex for the MARK 13. I also ordered:

http://www.ebay.com/itm/Carbon-Fibe...092294?hash=item1c73375c86:g:EhcAAOSwA3dYCTIW

and

http://www.ebay.com/itm/131493696367?_trksid=p2055119.m1438.l2649&ssPageName=STRK:MEBIDX:IT

the former for the MARK 13.

It will be interesting to see/feel the difference in materials with the different cores. I think 2-3 layers of the 12.5oz green cloth per side for the nomex panel and 4 layers per side for the balsa panel for skins. These panels will be finished to a glossy finish with 2 layers of sanded laminating epoxy and clear coat.
 
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rharshberger

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Unless someone knows of a better deal I will pull the trigger on this:

http://www.avtcomposites.com/cgi-bin/commerce.cgi?preadd=action&key=.250HCOM2X4

I have also ordered this to play with:

https://www.amazon.com/gp/product/B00YV6T028/?tag=skimlinks_replacement-20

I'll make a panel with the balsa and cut some fins for another project before building and cutting the panel with the nomex for the MARK 13. I also ordered:

http://www.ebay.com/itm/Carbon-Fibe...092294?hash=item1c73375c86:g:EhcAAOSwA3dYCTIW

and

http://www.ebay.com/itm/131493696367?_trksid=p2055119.m1438.l2649&ssPageName=STRK:MEBIDX:IT

the former for the MARK 13.

It will be interesting to see/feel the difference in materials with the different cores. I think 2-3 layers of the 12.5oz green cloth per side for the nomex panel and 4 layers per side for the balsa panel for skins. These panels will be finished to a glossy finish with 2 layers of sanded laminating epoxy and clear coat.
For making panels or fins with balsa and FG/CF/Kevlar the best strength and stiffness will come from using end grain balsa panels. The sheet balsa available at most hobby stores isn't ideal or recommended for panels, the peel resistance of the plys is fairly low since the grain runs with the plys, endgrain balsa on the other hand has the balsa fibers in tension and makes for a much stronger and stiffer panel.
Fiberglass Supply has an excellent endgrain balsa panel, they also sell, other core products and core foams.

http://www.fiberglasssupply.com/Product_Catalog/Core_Materials/core_materials.html


ACP Sales has the same type nomex honeycomb for $64 per .250"x24"x48" sheet, and I have had good service from them on the two occasions I have made purchases from them.

http://www.acpsales.com/OnlineStore.php?cat=256



 
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CORZERO

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RocketFeller

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Balsasud donated the .5" end-grain balsa that we used. It is great stuff. If you want to get fancy, the 3/8" thickness would allow you to do some root-to-tip taper for flutter resistance.

Thanks for the quick catch! I put in a request to cancel the amazon order with the balsa. I'll get on the order from the sellers you listed.

Thanks again for the save!

Edit: I found this for a slightly better deal on two panels:

https://www.amazon.com/dp/B01GNU0DDE/?tag=skimlinks_replacement-20

Apparently from these guys:

https://www.corelitecomposites.com/balsasud-core.html

It's 3/8 inch but I think I may like the extra thickness. What do you think?
 
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