Laminating epoxy for fillets?

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David_Stack

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Good Evening All;

So I've managed to put myself into a state of 'paralysis through analysis', and could use a smack upside the head courtesy of the forum members...

Working on a Loc 3" kit, 1/8" plywood fins. This will fly on H & I motors (intended to be my L1 cert rocket if I don't pursue putting an H in my PSII Nike Smoke).

Fins are attached with BSI Slow-Cure 30 min epoxy, and now it's time for fillets. I have both the Slow-Cure and BSI Finish-Cure on hand, along with West Systems 407 Low-Density Fairing Filler. I like the thinner viscosity of the Finish Cure, it seems to mix better with the 407, but am I best served from a strength perspective using the Slow-Cure epoxy for fillets and adding the 407 to that? (fillets on a scratch MPR build were done with the Finish-Cure / 407 combo and turned out well, but have not been stress-tested)

I know, aliphatic glues are fine for cardboard and plywood rockets that aren't pushing the edges of the envelope, but the amount of shrinkage is too much to deal with, particularly when forming fillets of a significant radius (3/8" / 9mm in this instance). I've been through the forum courtesy of the Search function, and most all the 'hits' talk of using laminating epoxy for applying fiberglass or similar to a other materials as a layup, not as I am proposing to use it.

Likely I'm making 'much ado about nothing', but what would you do?

Thanks all
 

bjphoenix

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Fillets are one of the holy grails of rocket building.
I've seen lots of people show different ways of doing them.
My fillets on past LOC kits have been made by laying the rocket on its side and pouring on the epoxy. It self levels into a triangular fillet. I put pieces of tape on the front and back of the fin root to form dams to control the flow of the epoxy then I file/sand the ends to look more presentable.
On my latest build I used epoxy putty to form the fillets. I put it in place and then used a round rod pulled along the root to shape the putty. I used JB Weld wood putty because I had some left over.
 

David_Stack

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Thank you all (especially you Jim) for the feedback.

I feel far more comfortable using laminating epoxy as a 'base' for my fillets now. (and I really appreciate the hint to wet out the tube and fin with 'straight' laminating resin before laying the fillet).

r/
Dave
 

MidOH

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I drag the epoxy up the fin and tube from the puddle, then let it run down and form a triangle fillet like BJ said.

The bit I dragged sands easy enough, and surface tension ads a little bit of a curve at the ends. If the fillets have a dip, bubbles, or a weird spot, I use Bondo later on to pretty them up.

I'm done dragging fillets with a round tool. Fun, but looks no better to me.
 
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Good Evening All;

So I've managed to put myself into a state of 'paralysis through analysis', and could use a smack upside the head courtesy of the forum members...

Working on a Loc 3" kit, 1/8" plywood fins. This will fly on H & I motors (intended to be my L1 cert rocket if I don't pursue putting an H in my PSII Nike Smoke).

Fins are attached with BSI Slow-Cure 30 min epoxy, and now it's time for fillets. I have both the Slow-Cure and BSI Finish-Cure on hand,I'm along with West Systems 407 Low-Density Fairing Filler. I like the thinner viscosity of the Finish Cure, it seems to mix better with the 407, but am I best served from a strength perspective using the Slow-Cure epoxy for fillets and adding the 407 to that? (fillets on a scratch MPR build were done with the Finish-Cure / 407 combo and turned out well, but have not been stress-tested)

I know, aliphatic glues are fine for cardboard and plywood rockets that aren't pushing the edges of the envelope, but the amount of shrinkage is too much to deal with, particularly when forming fillets of a significant radius (3/8" / 9mm in this instance). I've been through the forum courtesy of the Search function, and most all the 'hits' talk of using laminating epoxy for applying fiberglass or similar to a other materials as a layup, not as I am proposing to use it.

Likely I'm making 'much ado about nothing', but what would you do?

Thanks all
RocketPoxy the external fin fillets, exclusively; so easy to work with. Aeropoxy for internal if I really am concerned about the the strength, otherwise, West Systems, without fillers if I am doing an epoxy dam or with colloidal silica if doing a standard epoxy bond.
 

DeeRoc29

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I'd use the BSI 30 min you've got, mixed with your filler of choice. Use masking tape for clean edges on those fillets.
 

David_Stack

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Went to work last evening, taking into consideration the responses received here...


Thanks once again to all who contributed.
 

bjphoenix

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Here are some fillet photos. The first one is the back end view of my LOC LilNuke. (I changed the fin design and made the fins TTW.) These are basically triangular fillets made with "runny" epoxy. I lay the rocket on its side with the valleys for 2 fillets pointing up, dribble on the epoxy slowly and let it flow to a smooth surfaced puddle. I put a dab of masking tape at the front of the fin root and another at the back of the fin root to create dams to contain the epoxy. I get as much epoxy on there as I think it needs and let it level itself and cure. The fillets come out triangular and fairly smooth, as good as I like for most of my rockets. If I do 2 fillets at the same time then that sets the angle of the surface. This rocket has 3 fins so laying on its side with one fin pointing down the remaining 2 fins angle up at 30 degrees above horizontal, so the angle between the surface of the fillet and the surface of the fin is that 30 degrees. If I was willing to take twice as long and do one fillet at a time I could have made them more like 45-45-90.
DSC_7237b.jpg

This is a rocket I have under construction, currently in white primer. Its fins extend to the motor tube. After sliding in the assembled fin can I put small epoxy fillets on the surface, then covered that with some leftover JB Weld epoxy wood putty that I had. It did not take much of the JB weld to do all 8 fillets on this 2.6" diameter airframe. After putting on the epoxy I used a round steel piece to drag along the epoxy and shape it (I think I used a 7/16" deep socket). This created a pretty good shape but there is some friction between the epoxy clay and the steel to keep the epoxy surface from being perfect. After the initial shaping I used my finger dipped in rubbing alcohol to smooth them out a bit more. After everything cured I put on a thin layer of Elmers wood putty and then sanded that smooth. These are the nicest fillets I've ever done. On an all white rocket it is hard to show the smooth shape, the photo has some sanding marks that might help show the contour. I've been working on this one a long time and I'm still trying to decide how I want to paint it. If I can find the right color in Rustoleum metallic I might use that, otherwise I'm thinking about Duplicolor metallic.
DSC_7234b.jpg
DSC_7235b.jpg
 

prfesser

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FWIW I just made fillets for my Super Big Bertha using laminating epoxy and "Fairing Compound" from US Composites. Not perfect but they'll sand up just fine.
 

hobie1dog

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I did a NARCON video where I talked about using laminating epoxy for fillets.
Jim: I've been reading old high power rocket magazines lately and your name is mentioned so many times with pictures of your multi-stage rockets, and now I finally get the understand what you're all about.
thanks so much for doing this
 

bjphoenix

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Going through old photos last night and I stumbled on this photo I took of a fin fillet. This is a 2.6" phenolic airframe, the fillets were made with JB weld wood putty and the surface finished with a little bit of Elmers wood filler. I believe that's a 7/16" socket which measures 0.65" outside diameter. I put on the wood putty and then drug that socket along the joint to smooth out the putty. This worked pretty well but the putty was a bit sticky and would try to drag along with the tool causing slight roughness in the surface. I then wet my finger with rubbing alcohol and went over it lightly to smooth things out. After the epoxy cured I wrapped sandpaper around the socket, maybe even the next smaller socket size and sanded the surface. Then I patched the imperfections with Elmers wood filler and sanded more.
DSC_5549c.jpg
 

waltr

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Wetting the radius tool with alcohol helps a lot to create smooth fillets.
Then wipe clean and re=wet for next fillet.
 
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