A better way for fin fillets?

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internal fillets keep the fins from pulling thru the wall
I have had epoxy only external fillets crack, so I started reinforcing
I have seen many rockets lose surface mount fins and the rocket self destructed
Yes it adds some strength, but not near as much as adding reinforcement fibers or cloth

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Its a little more complicated I think; Fillets add huge strength if done properly...

Internal fillets add strength, keep fins from pulling out and add damping to twisting moments.
Fillets can crack if too hard of epoxy is used for example. Too soft is not good either.
Surface mount fins are the hardest to keep on, that be true!

I don't like to T2T reinforce because it destroys the rocket diameter... Poop slow and low.

Question #97 - Although not a laminating expert, it seems that most of the strength gained by adding a T2T reinforcement cloth material would be along the direction of the fabric. For example Kevlar is only strong in one direction... How does this keep the fins from bending and flexing in all three axis modes?
 
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I did 3 identical big daddy rockets, one with 1/8" Baltic birch fins with epoxy fillets, one with 4 layers of 6 oz glass with epoxy chopped glass fillets, one with 2 layers of 6 oz carbon over 2 layers of 6 oz glass with epoxy chopped carbon fillets. All launched on j600 motors. Birch fins ripped off/rocket destroyed, glass fins buzzed on the way up, rocket was not fly able a second time , carbon fin rocket has flown 6 times. So I am a big believer in carbon.
 
I did 3 identical big daddy rockets, one with 1/8" Baltic birch fins with epoxy fillets, one with 4 layers of 6 oz glass with epoxy chopped glass fillets, one with 2 layers of 6 oz carbon over 2 layers of 6 oz glass with epoxy chopped carbon fillets. All launched on j600 motors. Birch fins ripped off/rocket destroyed, glass fins buzzed on the way up, rocket was not fly able a second time , carbon fin rocket has flown 6 times. So I am a big believer in carbon.

J600 motor with 1/8 inch thick plywood fins in a Big Daddy is an example of severe underbuilding. That’s certainly not evidence that fin fillets “don’t add much”.
 
All the fins were the same thickness, plywood broke the fillet before breaking, fiberglass broke above the fillet. It was a fair test.
 
All the fins were the same thickness, plywood broke the fillet before breaking, fiberglass broke above the fillet. It was a fair test.

What medium are you flying your rockets in??? I fly 1/8" plywood fins coated with epoxy and epoxy glass fillets up to 500-600MPH range quite often. 1/8" G10 fins with epoxy glass fillets to Mach 1.5 (J570). And even 0.092" G10 fins on minimum diameter rockets with fillets alone and have never lost a fin except on failed landings...

Are you talking about glass/carbon cloth over plywood for all three tests?
 
I mold my fillets using dowel rods, small PVC pipe etc, depending on the size needed. I use AeroPoxy lite as the fillet material. Since I started using this method, I've never used anything else. Start by determining what you are going to use as a form for the fillets. 5/16" or 3/8" dowel works well on large hobby or small high power rockets. Mark the area next to the fin where Aeropoxy will be applied. This is done by laying carbon paper (office supply store) down at the BT-fin corner joint and rubbing the form piece on the carbon paper to mark the tube and fin where the form touches them. Do this on both sides of all fins, then darken in the marks to make them easier to see. This is where one gets a little artful on the LE and TE. I taper the fillet off on both sides. The LE taper should be about twice as long as the TE.

Once these are marked, tape around the outside of the marks with masking tape, this marks the edge of the fillet. You will need at least two of the proper diameter forms (dowels), they need to be a little longer than the fin chord. COVER YOUR FORMS WITH WAXED PAPER! Other wise you will glue them to the fillet.
Mix an adequate amount of Aeropoxy and spoon it into the fillet area so as to fill the area between the marks and tape. Push your form down into the AeroPoxy so as to mold the fillet. Keep the excess on the tape for easy removal. I usually mold two fillets at a time. The clamping required for the forms gets a little artful. It is most important that the forms fully seat against the fin and airframe, and stay there. I generally utilize clamping against both the airframe and fin. Once the forms are in and clamped, you need to let the AeroPoxy Lite cure.
With the epoxy cured, the forms are popped out of the epoxy and the next set of fillets are molded. At this point I sand only enough to allow the next set of fillets to be molded.
Once all of the fillets are molded, one sands them out by wrapping sandpaper around the same forms you molded the fillets with. I use an adhesive backed sandpaper here. I usually start with 80 and finish with 180 grit. As you sand, you will scrub through the masking tape. That's when you stop sanding or you will cut into the airframe and/or fin.
Peel the masking tape and you'll have some pretty good looking fillets, ready for final sand and finish. Leading edges and trailing edges take a little practice, but go slow and you'll be fine.

I kinda like the idea of the molding the fillet section...but that's not the area I have a "problem" with...
and it's not a big "problem"...I was just looking for some tips to blend the leading edge and trailing edge of the fin root fillets to a nice transition.
Tips to make it EASIER... and more professional looking....
Thanks...
 
I kinda like the idea of the molding the fillet section...but that's not the area I have a "problem" with...
and it's not a big "problem"...I was just looking for some tips to blend the leading edge and trailing edge of the fin root fillets to a nice transition.
Tips to make it EASIER... and more professional looking....
Thanks...

Yay, We should get back to this... Hopefully everyone has a method that "keeps the fins on" and that works for them which is all anyone can ask.

One good way to make epoxy (with or without filler) real nice is to heat it with a hot air type heat gun. Its a little tricky as too much heat just makes it real runny, but with a little practice, once you get the sides they way you like you can add a small amount of heat to the ends and get good flow out. It's all about the meniscus properties of whatever epoxy you are using...

Also, your can scratch coat the fillets to be able to clearly see defects... IE lightly prime gray, lightly sand, lightly prime white, etc. No need for mass amounts of primer, just enough to see the high and low spots. Color contrast makes it all clear.
 
Perhaps I was unclear when I posted this:

Errrgggg
Fillets are my bane!
I don't have a problem (so much) with the contour between the fin and the body, its the transition at the leading and trailing fin root to body tube.
It KILLs me!!!
Do I extend it past the fin? Done that, not bad but not great.
Do I use tape? Tried that...didn't work for me, wasn't happy with tape to fin alignment...too much work for too little reward.
Ended up doing it without.

I just did my first 4" BT and I used a 7/16" Kobalt socket that measured 0.650" (of course I cleaned it with lacquer thinner to make sure that it didn't cause fisheyes) and that worked very well. It has a rounded top and bottom and is easy to clean the epoxy off of.
I marked it with magic marker, then did the first two fillets without dye added (used RocketPoxy) and thought "that's hard to see on a white bodytube" so on the next two fillets I added a drop of dye but I didn't think I needed the marker lines since I could see the poxy better....
Mistake!, couldn't make sure that I hade epoxy up to the "line" that blends to the fin/body tube.
One of those came out with a small low spot....hello primer and sanding!

So, I marked it with the marker trick, and the next two sets of fins came out better but....still struggling with the front and back of fin transition.....HELP!!!
LOL

My question WAS NOT "how do I DO Fillets....the question was ..are there any tips to making TRANSION from leading fin root and trailing fin root to the main body...EASIER????
Just for perspective here are some of my fillets....


and thanks for your patience.....
IMG_6517.JPG IMG_6522.JPG IMG_6523.JPG IMG_6524.JPG

Those are not the areas that I'm asking about

THIS ISIMG_6525.JPG

The leading edges and trailing edges....

Sorry about the confusion.....
but ANYONE HAVE ANY TIPS?????
 
Perhaps I was unclear when I posted this:



My question WAS NOT "how do I DO Fillets....the question was ..are there any tips to making TRANSION from leading fin root and trailing fin root to the main body...EASIER????
Just for perspective here are some of my fillets....


and thanks for your patience.....
View attachment 375919 View attachment 375920 View attachment 375921 View attachment 375922

Those are not the areas that I'm asking about

THIS ISView attachment 375923

The leading edges and trailing edges....

Sorry about the confusion.....
but ANYONE HAVE ANY TIPS?????

You just need to make sure you get more filler out past the fin leading edge... Use whatever tool you are using to make the nice side fillets and "pull" the epoxy around 1/2" or so towards the front (or back) of the airframe. Make sure to do both sides of the fin the same. You want to pull out the filler about the same length as the fillet radius. Then maybe add a small amount of heat.

Not sure you have the time but I usually do all six fillets at once for a three fin rocket. Then I will sit on my chair and rotate the airframe by hand for a couple of hours until the filler has setup enough to not run!!! I spin it CW and then CCW, nose slightly up, nose slightly down and watch the way the filler is moving... You can get gorgeous fillets this way. Any small defects can be filled with Squadron Green putty and easily sanded out. If you have big defects, your fillets are probably not that strong.

I usually save making fillets for when the wife wants to talk and then I say I have to do fillets... Not really.
 
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PS. How did you make the white ones??? These are not too bad. Do that again. Notice how the extend beyond the fin leading edge. Perfect.
 
Hooked on Rockets:
Your transitions look good.
I usually add West Systems 404 to make the epoxy very viscous, about the same as peanut butter. Then I make a fillet and smooth out the transitions with a rubber gloves finger dipped in denatured alcohol.

Great tip Steve!!! Hooked on Rockets, are you sure your leading and trailing fin edges are "sealed" to the body tube all the way around with no holes or voids??? It almost looks like in the brown pictures that the filler is migrating back inside the rocket through a void between the body tube and fin leaving a blemish...
 
Hooked on Rockets:
Your transitions look good.
I usually add West Systems 404 to make the epoxy very viscous, about the same as peanut butter. Then I make a fillet and smooth out the transitions with a rubber gloves finger dipped in denatured alcohol.

Thank You Sir!
Yes, I do the dip in alcohol ...(don't tell anyone...but I don't use gloves...)....dohh

I'm looking for easy....some of the pics are done with "fix-it"...WAY too stiff for detailing out EASILY...

I have worked out one side of the fin...then cut the excess off...down he centerline of the fin...then filleted the other side to match...the cut.... that's what is in the pics that I posted tonight concerning fillets.....I can do it the hard way....
LOOKING FOR SOME TIPS FOR BLENDING ROOTS TO BODY making it EASIER!!!

Thanks all for your input!

SS
 
Great tip Steve!!! Hooked on Rockets, are you sure your leading and trailing fin edges are "sealed" to the body tube all the way around with no holes or voids??? It almost looks like in the brown pictures that the filler is migrating back inside the rocket through a void between the body tube and fin leaving a blemish...

Dohhh...
YES SIR....there are no voids....scratching my head asking WHY cant these ROCKET PEOPLE figure out that ALL IM ASKING is for TIPS how to make the transition EASIER TO MAKE????
 
Dohhh...
YES SIR....there are no voids....scratching my head asking WHY cant these ROCKET PEOPLE figure out that ALL IM ASKING is for TIPS how to make the transition EASIER TO MAKE????

Sorry my tips aren't helpful... You said:

Errrgggg
Fillets are my bane!
I don't have a problem (so much) with the contour between the fin and the body, its the transition at the leading and trailing fin root to body tube.
It KILLs me!!!
Do I extend it past the fin? Done that, not bad but not great.
Do I use tape? Tried that...didn't work for me, wasn't happy with tape to fin alignment...too much work for too little reward.
Ended up doing it without.

Maybe you are looking more for a WORD - trowelable

Remember thermosets are trowelable and create with it!

Good day.
 
Thank You Sir!
Yes, I do the dip in alcohol ...(don't tell anyone...but I don't use gloves...)....dohh

I'm looking for easy....some of the pics are done with "fix-it"...WAY too stiff for detailing out EASILY...

I have worked out one side of the fin...then cut the excess off...down he centerline of the fin...then filleted the other side to match...the cut.... that's what is in the pics that I posted tonight concerning fillets.....I can do it the hard way....
LOOKING FOR SOME TIPS FOR BLENDING ROOTS TO BODY making it EASIER!!!

Thanks all for your input!

SS

We have no idea what you consider easy. And without knowing what method you use, we cannot judge what’s easier. But dipping an unprotected finger in a solvent that’s absorbable by the human body and then epoxy is much more likely to sensitize you to epoxy. It’s a terrible practice.
 
The leading edges and trailing edges....

Your fillets look pretty good. The transition that you posted doesn't look too bad but if you want to improve it I would think you could add some kind of putty over the epoxy to form the smoother transition, that little part doesn't have to be structural.
 
WOW,
I don't know what happened to me the other night but I pretty much started turning into a JERK about these dang fillets....
I apologize about that!!!

I guess I just assumed that EVERYONE who builds rockets has some kind of "magic trick" that they make "two passes" with a dowel
and "bada-bing" the fillets are ready for paint...

You guys stepped up and offered many good suggestions and I turned into an A$$....

Sorry about that.....!!!!

I just like to aim for perfection and get frustrated.....didn't mean to take that frustration out on Y'all....
THANKS AGAIN for the tips...AND the compliments!!!!

I'm still amazed at some of the work I see here!!!

Thanks Again!!!

SS
 
I did 3 identical big daddy rockets, one with 1/8" Baltic birch fins with epoxy fillets, one with 4 layers of 6 oz glass with epoxy chopped glass fillets, one with 2 layers of 6 oz carbon over 2 layers of 6 oz glass with epoxy chopped carbon fillets. All launched on j600 motors. Birch fins ripped off/rocket destroyed, glass fins buzzed on the way up, rocket was not fly able a second time , carbon fin rocket has flown 6 times. So I am a big believer in carbon.

It seems the fin's flexing may have made the fillets fail. I am very interested in your experiment and results but am not sure about construction... Fins are all plywood? Then uncoated, glass coated and finally glass/carbon coated? Is this correct?

How fast do you think this rocket should have gone? I ask because I appreciate your efforts and don't want to make the same thing happen on any future rockets I build...

You are correct in that carbon will be the strongest, but I am surprised the glass only one's failed if flight was below Mach.. A big daddy has kinda larger fins fin's, doesn't it?

It seems rocket science is not an exact science...
 
PS. How did you make the white ones??? These are not too bad. Do that again. Notice how the extend beyond the fin leading edge. Perfect.

Hey THANKS....!!!
Those were made with "Fix-It"...back when I first started the rocket building....
Got that tip from Apogee.....
Fix-It is a little thicker than I like, that's why I ended up extending the epoxy past the leading edge of the fin root.
I was just learning....
I do like that extension too, but it was a PAIN trying to keep everything symmetrical....which it AINT...LOL
and I guess that's what I'm looking for.....something that aint so much of a PAIN...LOL
 
Hooked on Rockets:
Your transitions look good.
I usually add West Systems 404 to make the epoxy very viscous, about the same as peanut butter. Then I make a fillet and smooth out the transitions with a rubber gloves finger dipped in denatured alcohol.
Thanks so much!!!!
I'm very interested in the West Systems epoxy.... looks solid...
It's way above my piddly little start on rocket building at the moment....I have checked it out....
ME Likey...Me wallet.....not so much...LOL
I just wanna get a rocket into the air!!!

Thanks again!!!
 
Its a little more complicated I think; Fillets add huge strength if done properly...
Epoxy is stronger in compression than tension so internal fillets add more strength than the external ones. Generally people only do tip-to-tip reinforcement for minimum diameter rockets where you can't do internal fillets.
 
WOW,

I guess I just assumed that EVERYONE who builds rockets has some kind of "magic trick" that they make "two passes" with a dowel
and "bada-bing" the fillets are ready for paint...

On one rocket, using Rocketpoxy and a 1/2" PVC pipe dipped in denatured alcohol, I was able to do a pull-swoop-flick that made the perfect leading and trailing edge transition with no re-work or sanding needed.

I have NOT been able to do it again. I decided it was some perfect combo of the amount of epoxy, temp, humidity, what I ate for breakfast that day...

Rockets I built after, pull-swoop-flick turns into pull-swoop-flick-smear-swear, try to fix with alcohol-dipped fingers/clean up with paper towels and more swearing. Especially on the trailing edges of the fins. It's the fillet equivalent of my father playing golf.

I eventually decide it's close enough (seems to coincide with the epoxy getting stiff) and it'll look good from the pad.
 
On one rocket, using Rocketpoxy and a 1/2" PVC pipe dipped in denatured alcohol, I was able to do a pull-swoop-flick that made the perfect leading and trailing edge transition with no re-work or sanding needed.

I have NOT been able to do it again. I decided it was some perfect combo of the amount of epoxy, temp, humidity, what I ate for breakfast that day...

Rockets I built after, pull-swoop-flick turns into pull-swoop-flick-smear-swear, try to fix with alcohol-dipped fingers/clean up with paper towels and more swearing. Especially on the trailing edges of the fins. It's the fillet equivalent of my father playing golf.

I eventually decide it's close enough (seems to coincide with the epoxy getting stiff) and it'll look good from the pad.

Switch to 4500 and you'll never "...smear-swear" again. Nor will you ever have to worry about sanding again.
 
Switch to 4500 and you'll never "...smear-swear" again. Nor will you ever have to worry about sanding again.
Or...work with one product enough and you learn its properties and handling characteristcs enough to have it work as well as similar products.
Speed and beauty are byproducts of practice.
 
Or...work with one product enough and you learn its properties and handling characteristcs enough to have it work as well as similar products.
Speed and beauty are byproducts of practice.

+1. I see it as a skill/practice issue, not a material issue.

I prefer to neaten up filllets before they cure. Much easier to wipe with some alcohol rather than sand hard epoxy.
 
Perhaps I was unclear when I posted this....... My question WAS NOT "how do I DO Fillets....the question was ..are there any tips to making TRANSION from leading fin root and trailing fin root to the main body...EASIER????
Just for perspective here are some of my fillets.... and thanks for your patience.

One thing I have done is to clip the tip of the leading edge where it meets the body tube. This give the adhesive more surface area.

Minimizing stress risers is also another advantage of fillets, the effects of which can be dramatic.

Clipped Leading Edge.jpg
 
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I don't add any filler... for MPR I use BSI 30 minute epoxy. I just mix it up, lay on a fairly heavy bead of glue from the front to around 2/3 - 3/4 of the length of the fin root using the pointy end of a bamboo kabob skewer, making sure that (although uneven) the bead is continuous down the length of the fin root where applied. I then use a paper towel wrapped around my index finger at least four layers deep to keep it off of my finger. I then draw my finger (which is almost parallel to the body) down the length of the fin. The paper towel seems to absorb just enough epoxy to prevent the fillet from getting too wide at the bottom. I then let it sit horizontally for at least 15 to 20 minutes by which time it's set up enough to do the next set fillets. I find I don't even need to put tape at the ends of the fins to keep it in if properly supported horizontally. Here is a photo of the fillets on my Estes Sahara:

View attachment 375542

Worst case is I occasionally have sand down little bumps that might form on the body tube-glue transition zone.

This is how I've done it as well, rockets and for model planes. With practice, it looks like a nice smooth transition.
 
My first HPR I used epoxy and created not so pretty external fillets.
After it was well cured I used wood filler and with a caulking tool made nice rounded fillets. I then wrapped sandpaper around a motor and sanded it until it was smooth. I was afraid it would crack but after a few launches including one minor crash they still look great.
IMG_6146.jpg
Am I wrong for doing this?
 
My first HPR I used epoxy and created not so pretty external fillets.
After it was well cured I used wood filler and with a caulking tool made nice rounded fillets. I then wrapped sandpaper around a motor and sanded it until it was smooth. I was afraid it would crack but after a few launches including one minor crash they still look great.
View attachment 376730
Am I wrong for doing this?
Did it work for you? Did you like the results? If the answer is yes then you did it right for you, others may not choose that method, but you have one that works for you. [emoji1]
 
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