A question to the fin fillet Gurus

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AfterBurners

AfterBurners

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Fin fillets for me have not been so much a challenge, but certainly I’m finding there are many ways to “skin a cat” In my time building rockets I’ve used a combination of TBWG and TWMT for making fillets on LPR and MPR. Hobby line epoxy and epoxy clay. I guess depending on the build each application has a place.

For me I feel the challenge lies in creating the fillet as it wraps around the leading edge where it meets the airframe without having it drip. Using epoxy clay I don’t have this issues, but after it dries it requires some shaping and sanding. I know the process of taping off the fillets and using tape on each end to “dam” up the epoxy, but that does nothing to complete the connection between both sides of the fin.

How do you get it to “wrap around” without it dripping or running down the side and still make it look smooth? I know some will use Rocket Epoxy which is more like a paste and maybe that’s the only way to get the desired results?

Any of fillets gurus want to provide some insight?

Thanks!!

AB
 
Rocketpoxy is more like a slow flowing liquid. I'll be using tape dams to try and keep things corralled.
 
I never used Rocket Epoxy. I was told to mix it on a flat surface like a sheet of card board until there are no bubbles before applying it? Can you add anything. Does proper size dowel or tongue depressor soaked denatured alcohol help smoothing them out??
 
The flat surface is good advice. It doesn't allow as much heat to remain and accelerate the reaction. I'd think the bottom of a milk jug would be good because it's contained, and has corners to pour from onto your workpiece.

I don't know how good using wood on it is for smoothing. I've been using the cutoff tip of a large nosecone because it works perfectly for my current project. I'm sure you've read how people use pvc pipe scraps. The thing there is that it's a smooth surface that won't grab the epoxy.

Good luck!
 
No tape, no tools, just a finger and some isopropyl alcohol to smooth out the epoxy. And some sanding afterwards to shape it . Then a couple of coats of sanded filler primer hides any minor imperfections. When you get it right the fins just flow seamlessly into the airframe.
42293567194_daf61d9cdc_o.jpg
 
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I can understand smoothing out the epoxy but then how do you keep it in place while it hardens? I've noticed that even when it gets relatively stiff, gravity will still cause it to run. Back in cooler weather I had put some epoxy on a centering ring. The epoxy was pretty stiff because it was cool but even 15 minute epoxy was taking a long time to set enough to not run. I thought about putting in some heat so it would set faster but found that this works against you because when you warm it up a bit it gets runnier. What I've done for fin fillets is to put masking tape dams at the ends and pour on the epoxy. The epoxy self levels and you get a triangular fillet instead of a curved fillet. Then I do a little sanding at the ends but I don't obsess over it.
 
I don't do perfect fillets by no means, but using an epoxy with a filler is the best way, for me, to get the fillets to do what I want. Mix it right and it will hold its form. I use US Composites 150 medium and have laid and pulled all my fins at once with it, though the last one was a bit tacky. Using either fairing compound, Q-cells, or Cabosil as a filler depending on requirements. I tried Rocketpoxy a few times and it just doesn't work for me. I use pieces of PVC, spoons, craft sticks, or my finger. Always trying a new method. Best luck has been start with a PVC scrap and finish with my finger, pulling the fillet around the leading edge.
I don't see much benefit, other than for looks, of fillets made from unfilled epoxy. Sure it probably adds some strength but seems to me it needs a filler to act as a binder of sorts, like adding gravel and sand in cement to make it concrete. I am no expert though.
 
This will help with most everything you want to know about fillets....internal & external...from how to tape to how to pull smooth.

https://www.rocketryforum.com/threads/how-to-build-a-stock-3in-darkstar-cj-step-by-step.55019/
This is a sticky at top of high power, index # 13-34-35-36-37-38 cover everything needed to get perfect fillets.

Any epoxy can be used for this technique with proper fillers or just waiting for epoxy to thicken on it's own...Rocketpoxy.

DSCN2876.jpg
NEVER add ANYTHING to Rocketpoxy all needed fillers are already in there....adding ANYTHING else, just weakens it. Just use some cardboard to mix on, spread out thin to let air bubbles rise to surface. When you can take stick and pull ups bit of glue, into a point and it holds it shape without "melting" back into a puddle, it's ready for fillets. This can take waiting 15 -50 minutes depending on ambient temperature....at 85 usually around 15-20.
65-70 you may wait 45-hour for it to thicken. Do same for Hysol type glues also

Always start adding LESS than you think it will take in the V-groove & add to it, keeps from making a mess.
Use West 406 or cabosil to thicken laminating epoxies .

100_4214.jpg

100_7135.jpg

100_7920.jpg
DSCN0862.jpg

DSCN3205.jpg

Read those posts & you will be doing this too! Even the fillets were curved on above, it was a bit$h bending the tape to the lines!
 
Yes Rocketpoxy already has fillers. I wasn't saying add fillers to it. I've had my issues with the stuff and won't likely use it again. YMMV
 
If you plan to paint the rocket, it's not too bad. Just use Rocketpoxy as usual and then use several rounds of Bondo and sanding to get the shape that you want.

Without painting you don't have that option. I'm trying to figure this out right now for two MD kits that I am ready to build. I'm hoping to make parabolic masks out of tape or vinyl that I can use at the leading and trailing edge of the fins (and use straight pieces of tape between them) to control the shape of the fillet.
 
While I'm no expert either, I do have my own technique.

For me the key is filler. West system 405 sandable filler. mix in enough (Into 30 min BSI Epoxy for paper & wood) to be stiff, like peanut butter..
Dribble into the fillet area (I do two at a time, then do 2 more the next night, repeat..)
I then 'pull' the epoxy into the fillet shape with an old hotel room key. You can get a technique whereas you're pulling it at a low angle to 'press' it into the groove / void. Then add a bit morea, dn final pul at a steeper angel to get the desired fillet radius. (Wipe the key card after each pull, that's the secret to not getting it everywhere!) doing bathroom caulking is god practice too!! :D
I then use the edge of the key card to scrape up any left over globs and other 'runs'.

No tape, nothing else. I find the tape leaves a ridge, and there is a lot of needless prep when laying down tape..

For the leading edge, I carefully pull the card around the LE / BT joint. about 2-4 passes seems to round it out fairly nicely. slowly & surely.. they seem to come out OK..

I then come back and sand. I use a very small piece of sandpaper (about 1.5" x 3") rolled & folded over to get into the radius and fare it out onto the BT. (The paper is also somewhat wrapped around my middle finger) I then carefully push & round the LE to get a fairly decent transition. Go slow, round as you push & pull. I might add a bit of Bondo to fill in any divots..

Paint (primer, sand primer, sand primer, light sand, 2-4 coats colour.. )
 
When it comes to rocketpoxy you need to be patient. You give it at least 45 minutes to become thick like peanut butter. Maybe up to an hour, just to be assured. Once it gets thick apply and shape and it should be all good. You will still have to do some finishing touches, but it wont be as much of a hassle.
 
I've had excellent luck with epoxy putty. Lay it down, and a gloved finger dipped in alcohol smooths it right out. Works with JB weld and PC-7 epoxy as well.
 
dr wogz said:
While I'm no expert either, I do have my own technique.

For me the key is filler. West system 405 sandable filler. mix in enough (Into 30 min BSI Epoxy for paper & wood) to be stiff, like peanut butter..
Dribble into the fillet area (I do two at a time, then do 2 more the next night, repeat..)
I then 'pull' the epoxy into the fillet shape with an old hotel room key. You can get a technique whereas you're pulling it at a low angle to 'press' it into the groove / void. Then add a bit morea, dn final pul at a steeper angel to get the desired fillet radius. (Wipe the key card after each pull, that's the secret to not getting it everywhere!) doing bathroom caulking is god practice too!! :D
I then use the edge of the key card to scrape up any left over globs and other 'runs'.

No tape, nothing else. I find the tape leaves a ridge, and there is a lot of needless prep when laying down tape..

For the leading edge, I carefully pull the card around the LE / BT joint. about 2-4 passes seems to round it out fairly nicely. slowly & surely.. they seem to come out OK..

I then come back and sand. I use a very small piece of sandpaper (about 1.5" x 3") rolled & folded over to get into the radius and fare it out onto the BT. (The paper is also somewhat wrapped around my middle finger) I then carefully push & round the LE to get a fairly decent transition. Go slow, round as you push & pull. I might add a bit of Bondo to fill in any divots..

Paint (primer, sand primer, sand primer, light sand, 2-4 coats colour.. )
Click to expand...

Another thing that works well are sanding grips.

https://www.rockler.com/rockler-contour-sanding-grips
 
All this is excellent advice. I happen to have some Rocket Epoxy I bought awhile back that I haven't opened yet and I also have some WS epoxy as well. I think the 105 and 205 along with 406 silica. Also some carbon fiber shavings. Just really haven't had time to mess with it, but I appreciate all the great looking fillets and the advice. Definitely going find some scrap body tubes and glue up them for some practice.
 
Nathan said:
No tape, no tools, just a finger and some isopropyl alcohol to smooth out the epoxy. And some sanding afterwards to shape it . Then a couple of coats of sanded filler primer hides any minor imperfections. When you get it right the fins just flow seamlessly into the airframe.
42293567194_daf61d9cdc_o.jpg
Click to expand...
Nathan always admired your work. What type of epoxy were you using? Rocket Epoxy or something else??
 
AfterBurners said:
Nathan always admired your work. What type of epoxy were you using? Rocket Epoxy or something else??
Click to expand...

The fillets on the 29mm rocket in that photo was Bob Smith 15 min epoxy because that's all I used back then. These days I use Aeropoxy, or sometimes Aeropoxy with a thin layer of Bob Smith 30 min epoxy on top for appearance. That's because the Bob Smith 30 min epoxy starts out pretty thin and levels out better than the Aeropoxy.

I spend a lot of time shaping fillets by sanding. I bought some Top Flight Microballoon filler last year but have still not yet gotten around to trying it. I expect it would make the fillets a lot easier to sand.
 
I still mask off the area and use a price of PVC pipe like one of the experts on the thread taught me. It just helps me have less clean up.
 
I've used (and had) those sanding grips. I found them to be very limiting.. Sure, I'll glue a piece of sandpaper to a stick when needed.. but I found the stiffness of new paper or the suppleness of old wrinkly paper to be best..

to each his own, right?! :D
 
Several people have mentioned it, but fumed silica (aka Cabosil, West 406) is the stuff to use for fillets. It forms a non-newtonian fluid, which means that you can shear (smooth) it when you're forming the fillet, but then when you stop moving it, it will stay in place and not sag or run. It is possible to work with quite thick mixtures of the epoxy because of this property. One thing that I do on fillets is to first paint the surface with neat epoxy and then wipe it off before applying the thicker mixture. This fills the scratches, etc., with epoxy, which may not happen with the thick mixture.

Jim
 
I have a method involving paint brushes TBQ&T and water and no fingers! (I'm planning a video)

First I paint a thick bead of glue at the tube/fin joint. (For big ones I just squeeze strait out of the tube.)

Then I paint a stripe of just water, right along the edge of the bead, just close enough to break the surface tension of the glue. One stripe on the fin side one stripe on the tube side.

the glue melts into the water just a bit and self-levels into a beautiful fillet. A quick blast from a hair dryer gets rid of excess water, and let dry.

I do two or three coats of that, then light sanding.
 
i am surprised nobody has mentioned west systems Six10

https://www.jamestowndistributors.c...me=WEST+System+Six10+Thickened+Epoxy+Adhesive

this stuff is amazing. its fully thixotropic so it wont run or sag.. you dispense it with a special mixing tip and a standard caulking gun.. you can do all the filets at once, just pull them to desired shape with dowel cleaned with acetone after each pass... best of all no mixing, no mess, exact ratio every time, strength / proven west systems epoxy inside, no 'how much filer did i put last time?', etc.... its like $20 for a tube which will do a lot of filets...

i use this on pretty much all my fiberglass projects... its probably overkill for a LOC cardboard style kit...
 
Nathan said:
No tape, no tools, just a finger and some isopropyl alcohol to smooth out the epoxy. And some sanding afterwards to shape it . Then a couple of coats of sanded filler primer hides any minor imperfections. When you get it right the fins just flow seamlessly into the airframe.
42293567194_daf61d9cdc_o.jpg
Click to expand...

Dotto! I generally use Devcon 30minute epoxy for my fin fillets, mixing it about 5 minutes to insure good overall compounding. the apply with a popcicle stick or dowel and use a Alcohol dipped gloved finger to smooth out the fillet around the sides, leading and tailing edges. As the epoxy begins to set I'll give it one more wiping with alcohol.
 
What is a good radius for fillets for a 54mm minimum diameter project?
I am planning on using Aeropoxy ES6209 and West 406 filler. This one is not pushing any limits, but I would like it be strong.
 
Cool. Thanks Chris.

So, for a 7.5" root length, I am getting .45"-.6" which is slightly tighter than I imagined, but I will have to see what it looks like. I have a scrap of 5/8" torlon that will work well for that.
 
AfterBurners said:
For me I feel the challenge lies in creating the fillet as it wraps around the leading edge where it meets the airframe without having it drip. Using epoxy clay I don’t have this issues, but after it dries it requires some shaping and sanding.AB
Click to expand...

+1 on epoxy clay and alcohol to smooth it out. Use the forming tool of your choice ( gloved finger, popsicle stick, pvc pipe) and a smaller diameter to taper it around the ends. Need to smooth it out while it is still malleable. Milliput epoxy works for me. Simple, easy, no waiting, no dams,no dripping or running, minimal sanding and shaping, plenty of working time before it sets.
 
Epoxy clay tends to crack if used on cardboard/plywood rockets. After three different rockets developed cracks across two batches of epoxy clay....never again. I went straight to 4500 and have never looked back. Mark, mix, apply, pull to form, pull tape, allow to cure. Never any sanding, never any problems.
 

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