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HyperSonic

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I need to bond my CF fins to my CF airframe and as always, I could use your help. The fins will be
solid CF around .125 thick made from 6K IM7 twill fabric .015 thick. There will be 8 layers of fabric
with 2 layers at 0 degrees, 2 at 45, 2 at 90, and 2 at 135. They wont be in that exact order when
layed up with Duralco 4461SS. The shape is in pic. Its a 4 inch MD CF airframe and the fin root chord
is 15 inches, span 4.5, leading edge sweep is 12.75, tip cord is 1.25. There will be 4 fins on this
rocket.


My game plan as of right now is to tack fins on to airframe. Then create L brackets on each side of
fin with thin unicarbon ( three or four layers) that only go about .3125 up fin and onto airframe and
run the lentgh of the fin. I will try to vacuum bag the L brackets section if I can. Then when I put
the fillets on each side of fin, they will cover up the L brackets completly. After the fillets comes
the 2 or 3 layers of T2T.


The things in question are: the type and thickness of CF for the L brackets and the T2T.
is the L bracket design a good or bad idea to begin with.


This will be an extreme flight pushing high mach (3+) at low altitude, thus the need to hang on to my
fins! They are highly swept back leading edges to minumize drag and maxamize speed. Its basicly a
5800 wraped in carbon fiber with a VK fiberglass nose cone. My goal here is to tame the 5800 with
composites and live to tell the story with HD video. Every second that goes by after launch this
flight will cross a new mach number. 1 sec into flight, mach 1 at 600 feet. 2 sec into flight, mach 2
at 2200 feet. 3.3 sec into flight, mach 3, at only around 6000 feet. It will take the next 35k feet
to bleed off all that energy that was generated in the first 3.3 seconds of flight!! And I built this
on my kitchen table. Truely amazing to me if I can pull this off!


Of course it may never happen without your help and guidance. As always I'm open to any advise that
anybody is willing to offer. There is only one thing that I dont want to hear, and that is SLOW Down!


All epoxy used will have a high Tg such as products from Duralco.

Wont let me upload jpg of fin.
 

tfish

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Very ambitious. I tried making my own fin stock a few years ago, I gave up. I could never get it "flat" enough or strong enough. I'd say making flat plate, strong enough and able to survive mach (3+) is about 10 times harder to do then making your own body tube for this mission.

Tony
 

Nytrunner

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There was a guy doing a build thread of his successful L3 flight which was essentially an N5800 flying case with composite Fin band at the back. I was looking forward to the fin part of the write-up, but he quit writing for some reason. (perhaps he didn't like his "Anonymous" handle linked to his name via Tripoli lol)
 

HyperSonic

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I could never get it "flat" enough or strong enough.

I have never made my own CF plate before but that is about to change. My plan is to vac bag all the layers in the correct order to a piece of granite that is 18 by 24 by 3. This granite is within .0001 of being flat. I'm hoping that as it cures it will stay flat. As far as strength goes I was shooting for a thickness of about .225. Thats after all the T2T

20180111_104527.jpg
 

FredA

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Read the article in the current issue of Rockets Magazine if you want a great idea on how to make your own CF fins.
 

AlphaHybrids

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The only plate I've seen that comes out very flat came from a shop that had an autoclave and some flat granite pieces. The epoxy required elevated temperature curing. They did the layup, got everything just perfect, then put it in the autoclave and cured it under pressure.

I have seen a couple people use two granite plates to smoosh their layup between, and then vacuum bag around that to compress. I never saw them on a rocket.

Edward
 

watheyak

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I’ve done a bit of plate. The only really structurally useable plate was all uni and vac bagged onto caul plate similar to the method you’re using. The first few came out warped until the order if the 0s, 90s and 45s correct in the stack. I’m not at home to be able to share that order, but I can later if you’d like. Woven fabric made pretty cruddy plate, your mileage may vary. And never use two plates. You can’t control pressure or thickness across the layup with a plate on top.
 
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FredA

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I plan on resubscribing to that magazine

Might be a good time.
 

HyperSonic

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I’ve done a bit of plate. The only really structurally useable plate was all uni and vac bagged onto caul plate similar to the method you’re using. The first few came out warped until the order if the 0s, 90s and 45s correct in the stack. I’m not at home to be able to share that order, but I can later if you’d like. Woven fabric made pretty cruddy plate, your mileage may vary. And never use two plates. You can’t control pressure or thickness across the layup with a plate on top.
The order I was going to use for all the layers is:
First layer at 0 degrees
Second layer at 45
Third layer at 90
Fourth layer at 135
Fifth layer at 135
Sixth layer at 90
Seventh layer at 45
Eighth layer at 0
What do you think? How did you do yours to keep it flat? The 4461SS epoxy I'm using only needs 250 F for four hours. I have a oven for that. If you split my 8 layers in half, the top half is an exact reversal from the bottom half. Do I have that right?
 

AeroAggie

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The order I was going to use for all the layers is:
First layer at 0 degrees
Second layer at 45
Third layer at 90
Fourth layer at 135
Fifth layer at 135
Sixth layer at 90
Seventh layer at 45
Eighth layer at 0
What do you think? How did you do yours to keep it flat? The 4461SS epoxy I'm using only needs 250 F for four hours. I have a oven for that. If you split my 8 layers in half, the top half is an exact reversal from the bottom half. Do I have that right?
Looks fine to me. If the laminate is balanced and symmetric, it should hold its cured shape (i.e. flat in your case). Make sure you use a release film or mylar between your granite and the laminate or you'll find out the hard way how porous granite is.
 

AeroAggie

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Maybe you got the whole thing because you’re already a subscriber? I downloaded twice and got the same partial set of pages each time. They were not sequential so I don’t think it was a result of partial download. I got the first page or two of each article.


Sent from my iPhone using Rocketry Forum
 

Steve Shannon

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Perhaps, but a subscription is free. I’m not even certain that I am a subscriber.


Steve Shannon
 

FredA

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Just hit the "New Subscriber" tab under the subscriber login and give them your email - I think that's all that is required.
 

AeroAggie

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Just hit the "New Subscriber" tab under the subscriber login and give them your email - I think that's all that is required.
That did it. Thanks! Since past issues appear to be available by purchase only, I guess I assumed the subscription was paid as well.
 

HyperSonic

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Just downloaded my copy of Rocketry Magazine. Very good article on airfoil fins and how to make them. I just don't think this technique would work on my fins. (pic in post #4) My root chord and Leading edge sweep are to long (root chord = 15 inches, LE sweep =12.75 inches) to have an airfoil. Plus after the T2T, that would cover up the airfoil and deform it. I really like this idea of all my fins being exactly the same. If your fin was not to long (root chord) and the LE sweep not a big % of the root chord, then I see this technique really shining. I need to buy a 3D printer. I might want to do something like this airfoil technique in the future. Any suggestions on which one to get?
 

Reinhard

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The order I was going to use for all the layers is:
First layer at 0 degrees
Second layer at 45
Third layer at 90
Fourth layer at 135
Fifth layer at 135
Sixth layer at 90
Seventh layer at 45
Eighth layer at 0
What do you think? How did you do yours to keep it flat? The 4461SS epoxy I'm using only needs 250 F for four hours. I have a oven for that. If you split my 8 layers in half, the top half is an exact reversal from the bottom half. Do I have that right?
If you go with this order, cut the fin in a way that the 0° layer is perpendicular to the body tube. The 45° is probably best orientated if it is swept backward (roughly parallel to the leading edge). This way, the fin will be stiffest in the direction that you need it the most. Maybe it would be slightly better, if the 135° and 90° layers would be swapped, but the inner layers don't contribute much to the fins bending stiffness, so there will not be much difference.

Reinhard
 

HyperSonic

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I made my first piece of CF plate (Hyperplate) Pics below. It turned out very flat. 8 layers, each .015 thick, after cure is .135. All the layers weighed 375 grams before lay up. After cure the plate weighs 493 grams. I wasn't thinking about all 4 fins from one plate had I made the plate a little larger. As it sits now, I can get 3 fins from this plate. These will be the core, with more layers to come. Im shooting for .21 final thickness after the T2T. I used FinSim to come up with the thickness that can handle the 5800 as for as flutter and divergence goes. I ended up using Aeropoxy 2032 with the 3665 hardener. Cotronics website told me the viscosity of Duralco 4461SS was 600 cps, only to find out later that isn't true. Its really 3600cps, so I decided to use the epoxy that was made for layups. The core doesn't need to be high temp anyway, but the outer coats will be 4461SS.

20180130_023927.jpg20180130_023658.jpg20180128_160237.jpg
 

OverTheTop

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Len B

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Thanks for connecting me back with Rockets Magazine. I grabbed a bunch of back issues as well.

I have a CF plate that Mick Kelly made for me several years ago. I don't know what I was thinking when I asked him to make it. It's a bit thick. ;-)
I must say that it is perfectly flat and if you tap it, it rings! I believe it was made in a manner very similar to your method HyperSonic.
 

HyperSonic

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I would be considering slotting the airframe to glue the fins into. You can then do the L-brackets and T-T to butress the joint some more. This is a method I use when attaching fins, as I think it gives a stronger result, since the joint relies on something other than peel-strength for robustness. Works well for me. Here are a couple of builds you might get some idea from:
https://forum.ausrocketry.com/viewtopic.php?f=6&t=4758
https://forum.ausrocketry.com/viewtopic.php?f=6&t=5019&start=45
I remember reading a thread a while back about whether or not slotting a MD airframe would be beneficial or not. Half the guys thought so, but the other half not so much. Some thought it wasn't necessary. To me personally, I think that slotting the airframe like you did in your builds will make a stronger joint than just epoxying to outside of airframe. Is it necessary, I don't know. My flight will be pushin it to Mach 3 at low altitude, and it very well might need everything I can do to hang on to my fins. I'm definitely going to consider it.

I would have to send my airframe out to someone who can do it right, as I don't have the mill (or whatever they use to do it accurately) to do it myself. Also, I like duralco 4525 instead of JB Weld because of the tensile strength difference. Beautiful builds by the way!
 

HyperSonic

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I have a CF plate that Mick Kelly made for me several years ago. I don't know what I was thinking when I asked him to make it. It's a bit thick. ;-)
I must say that it is perfectly flat and if you tap it, it rings! I believe it was made in a manner very similar to your method HyperSonic.
I was cutting my CF plate to size today, when I heard a Ping (sound) When I tap my knuckle against the plate, It rings just like you said. Mine sounds like the ping you would hear in the movie The Hunt For Red October, when they are in a submarine and getting painted with sonar from another submarine (or boat) hunting them.

I wonder if different thicknesses and type of fiber would change the frequency at which it resonates. I want to make a drum set of CF skins. I may be on to something here. The new drum beat of the future. And I owe it all to building and launching rockets!
 

HyperSonic

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I want to add a couple layers of Hexcel IM7 5.7 oz 12k UNI Directional Fabric to each side of my .125 cores. The UNI is .013 thick. Since UNI is only good for one direction, which way should I line up the fabric? With the tows perpendicular to the airframe? What direction on a fin needs the most strength on a high mach flight?
 

Reinhard

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Since UNI is only good for one direction, which way should I line up the fabric? With the tows perpendicular to the airframe? What direction on a fin needs the most strength on a high mach flight?
Perpendicular is ok. Depending on the sweep and the flutter behavior of the fin, it might be advantageous to sweep the fiber direction back a bit too, but a better answer here requires an understanding of the oscillation modes of your particular design.

Reinhard
 

Zebedee

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nice plate - can I commission some from you?
 

OverTheTop

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I want to add a couple layers of Hexcel IM7 5.7 oz 12k UNI Directional Fabric to each side of my .125 cores. The UNI is .013 thick. Since UNI is only good for one direction, which way should I line up the fabric? With the tows perpendicular to the airframe? What direction on a fin needs the most strength on a high mach flight?
If, as you say, you want to put a couple of layers of UNI on each side, why not put them orthogonal to each other and end up with mostly isotropic behaviour?
 

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