A better way for fin fillets?

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Klatuso

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I'm still trying to find my way building these new bigger MPR rockets. As a teenager I had Elmer's Glue and spray paint. Now, for the fin fillets I apply wood glue, or in the case of my NCR SA-14 Archer epoxy (what a mess). After the fillet I applied slightly diluted minwax wood filler. Perhaps I applied too much, I used a Q tip and then sanded it all down first with varying Dremel tips (Minwax wood filler is damned hard) and 220 sand paper then 400. It took all morning for four fins. Holy Moly! Then I applied basic Spackle and sanded again.

Am I overworking and overthinking this process? I think I could've just applied some spackle to the first fillet and be done with it. I want to build a super clean rocket but I don't want to spend all morning sanding just to apply Spackle for the final smoothing in order to apply sander/primer to sand all over again. Welcome to MPR I guess?

What shortcuts do you find helpful in the preparation of your rockets?
 
Posted this on your other thread, but I'll copy it over here in case that one gets deleted:

The good thing with straight epoxy is you can mix in additions to work it's properties to suit, so chopped glass or carbon fibre to make strong fillets or microspheres to make it easier to sand. You can also often get a good result by a well timed wipe over with a gloved finger dipped in acetone. When starting out it can help to have some scrap material, do a mock fillet in that and practise to get the timing right. You need it firm enough not to pull from the groove but still soft so it becomes smooth.

Have a look at this thread for some excellent tips on getting nice fillets, CJ provides a lot of photos and breaks it down really nicely
 
Thank you. I wonder if we are all overbuilding our rockets?
 
Thought I posted it on the Techniques thread. However the question remains the same. Are we overbuilding?
 
If using wood glue try the Titebond Quick & Thick (formerly No Run No Drip, also formerly known as Trim & Moulding Glue), its a thick type of wood glue that makes nice fillets easy on paper and wood rockets. For fin fillets with epoxy some type of filler has to be added, usually cabosil/fumed silica (same product different names), phenolic or glass microballons (phenolic microballons will turn the epoxy mixture rust red but it sands easier than other fillers), Talcum powder (yep baby powder just make sure its not the cornstarch type, iirc Wal-marts Equate brand is talcum or at least some of it is, talcum is harder to sand than the other fillers). Epoxy needs to be about the consistency of peanut butter or mayonaise for fillets, basically non-sagging.

Plastic picnic spoons make great fillet pulling devices for most smaller MPR and LPR builds up to 4-5" diameter rockets ( I picked up that tip from Crazy Jim aka CJ ).
 
Ever since I built a kit where the manufacturer included a micro syringe for laying fillets, I've been hooked. Now I use those for all my mid/high power builds. With practice, you can lay down an even bead that self levels into a nice fillet. Much cleaner, very little mess. It's an alternative to the masking tape dams/gloved finger/denatured alcohol approach. The kit had a couple of 3 cc syringes with Luer tips, I find 10 cc works better for me. Very affordable from Amazon. I'm willing to pay a few cents to avoid a big mess.
 
A friend & I share our West systems epoxies & faring compounds. A can of West Systems 405 can go a looong way!

I've used balsa dust too, as an epoxy filler.

My technique uses a hotel room key (which I have a healthy supply of). and Epoxy don't stick to them! it takes a bit of practice, but once you get it, it works pretty well. Just have some paper towel handy to wipe up drips & excess off the card. Any 'overflow' beads can be pushed back into the fillet (or just wiped off) with the clean edge of the card.

dx3_0682.JPG

dx3_0678.JPG

dx3_0711.JPG
 
"Am I overworking?" Depends on what you want out of your rocket. Forum member Nathan puts many hours into sanding, painting and polishing his rockets, and they look like works of art. I slap epoxy on mine and don't care too much what it looks like.

The first point of a fillet is to provide fin strength. Second point is to add aerodynamics. Third point is to look good. Spackle is not very strong. Okay for filling bad spots in epoxy fillets for show, but not for fillets themselves. Don't use anything that shrinks or gets brittle.
 
I have gotten to the point I don't tape my fillets anymore. Any excess I have to wipe away but it saves the PITA of running all that tape and having tape edge dams. I've tried to pull the tape early enough so it will self level the edges but that never works. If it is thick enough to hold the shape of the fillet, it is thick enough to hold the edge dam as well. I've used card edges, wood dowels, popsicle/craft sticks, PVC fittings and so on to pull fillets and it still sucks. I hate that step more than anything I think. I have yet to find the technique I like best.

I use filler in all my fillets. Be it wood glue or epoxy. I can't see how wood glue without a filler does much good but I'm no expert in that department for sure. You will just have to find your technique and get in the groove. Then decide if that small ripple or pin hole or crease is really anything to sweat over.

Head over to US Composites and pick up some of their 150 Thick Epoxy.
Then head to the Fillers page and grab a quart of Cabosil, a quart of SM Fairing Compound, and a 1 lb tub of 1/4" Chopped Strand fiberglass. That will be enough to last you a long time and ready to build (almost) any rocket.


  • Use the chopped fiber on internals to get good strength if you can get in there too them. I build my MMT/fin assembly outside the rocket so I can get good fillets on the MMT/fin joint.

  • Use the Cabosil to lay a root fillet on the externals and smooth it with your preferred smoothing tool. Doesn't need to be perfect.

  • Use the fairing compound to finish the fillet with a slightly larger smoothing tool. It is much easier to sand than the Cabosil. The root gives you strength so the faring compound is just for looks. You can use hobby store 5 minute here if you are quick enough. No need for fancy smancy high strength epoxies to make it aesthetically pleasing.

  • If you are one of the folks who inject fillets, the Cabosil will work there as well.

They also have Fasco 110 Epoxy Glue that works well too, both for gluing and making fillets. You can also pick up mixing sticks, containers, cups, squeeze bottles and even empty caulk tubes. Nearly a one stop shop for what we do. And they have pigments to color your epoxy.

And yes, if you had a good root fillet laid, then you could've just used spackle over it to fill the holes and rough spots. But that "get you high" in a tube spot and glazing putty works better (auto section at Wal-Mart); or the fairing compound I mentioned mixed with epoxy to make a paste. I use the spot putty on occasion on my fillets but usually for dings and dents and small jobs. I don't really care for it.

It should be fairly easy to use a syringe (trim the end if need be) to squeeze out a small Cabosil fillet without too much mess, then cover with the lighter, easier to sand fairing compound to make it look good if need be. Only way to get good is to practice.

Good luck sir.
 
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
 
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
Its all about patience and practice young padawan...[emoji3]
 
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:

upload_2019-2-24_10-34-37.png

Worst case is I occasionally have sand down little bumps that might form on the body tube-glue transition zone.
 
Then there is the old fashioned way to nicely finish anything that I always seem to have to use.

Sand and sand until you think your done; And then sand 10 more times...
 
I'm working on a new 2.6" model and my plan for fillets is to use some epoxy putty. It appears that I can shape it as necessary before it cures. In the past I've used normal epoxy which flows enough to form a smooth triangular fillet but the ends are hard to form. With putty I want to form a curved fillet which I couldn't do with just epoxy. I'll get back to you with a report on whether my plan works or not. So far the biggest problem I see with epoxy putty is that I'll need 2 tubes of it and that makes it a bit expensive.
 
I'm working on a new 2.6" model and my plan for fillets is to use some epoxy putty. It appears that I can shape it as necessary before it cures. In the past I've used normal epoxy which flows enough to form a smooth triangular fillet but the ends are hard to form. With putty I want to form a curved fillet which I couldn't do with just epoxy. I'll get back to you with a report on whether my plan works or not. So far the biggest problem I see with epoxy putty is that I'll need 2 tubes of it and that makes it a bit expensive.

Remember in HPR, fin fillets kinda need to be strong as heckish more so than pretty... Make sure putty sticks good.
 
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
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.
 
Remember in HPR, fin fillets kinda need to be strong as heckish more so than pretty... Make sure putty sticks good.

The stuff seems to stick pretty well, from what little bit of it I've used. This rocket will be for a max. of G motors, but I also built it the way I typically do with fins extended to the motor tube. I put 2 centering rings on the bottom end of the motor tube so that one lines up with the rear of the fin and one near the front. I cut tabs on the fins to fit between the rings and glue the fins to the motor mount tube. Then I cut slots in the airframe to line up with the fins and slide the whole assembly into the airframe. I put small ugly epoxy fillets on the fins and put epoxy fillets on the centering rings. Last I'll put on the epoxy putty fillets. Since the fins extend to the motor mount I think the whole assembly will be strong enough.
 
Some of us have used cake fondant tools to pull fillets, the wife bought me a set off Amazon for Christmas and I have not been able to use them yet but the largest ball end is about 3/4" diameter, for putty (which fondant is similar too) they might work very well. For the most part I make my fillets of US Composites 635 with Medium Hardener and add enough cabosil, and phenolic microballons (often with a bit of 1/32" milled fiberglass fibers) to get a peanut butter thickness of epoxy. A trick for me has been to pull the epoxy the length of the fin from each direction with the final pull being toward the leading edge and I allow the tool to pull around the leading edge and off, then wipe off the little trail coming off the leading edge.
 
No matter which goopha I use, It all seems to start with the base foundation. IE step 1: Make sure the body tube to fin is sealed and glued all the way around! Otherwise, any goo fillet will get a "little runny" leaving detents and imperfections...

I think without "leaks", one can even make great fillets from white and yellow glues!
 
I primarily use 4500 for fillets, and occasionally I'l use Rocketpoxy. I got a set of those fondant balls, too. Used 'em a few times, and went back to my old standby pieces of copper and PVC pipe. The balls just pushed the extra epoxy out of the way where the pipe collects and redistributes it to make a nice, even, smooth fillet. Wait a few minutes, pull tape, done forever.
 
you know fillets don't add much strength
they have to be reinforced
OQ0XJYeSrX62pi_PToymwfYH-hXuQrQp4skoVI3v4rjQ03iKGnBzXdy8Mre960znta6PgqgdSqe2yvvALZOz4CU6hhgkYfMelgsY7mS5_wdmW-b3PJPTASnk3k7t-xp1GPPDytJq0JRDs04YFZejPu0BjHEuFsglzaFZSArytXqQ-cbsDr6xzzeqPZxwHsl2gVtqHzfddExBa7FfzP9lOs_514uLy-cTCMhxlJpF-9JlhUhT3bhfxnPw4FMOE1ty6gEcSqfiNNDtTJyU8A_5z2GrBbp26Z0r-R41sEx2lZUjqXLwCDKDvHpV2Gdg5b0TwAfzlcOrfBjLQmZmILElqFWGZJD7D7PL_4JoNe34sn7El0qz9zAAdsbnaHjjq96XvdOfsrZP-O-5dIYw6vThs9_Vo2IjSZ1CzRoIhlIXAaEvIMguhmcdNdaTo9FrgA3uJ-9DmPrVy2razt_UNaxEkzk72PLmy3_dn7YB1ZhltnVZOt7uDPQFA-HuUsIqIHat_6YjkaBpMe83QBfiyEFwFsk81Z-BND__QrUAlsI9RT51JWOjZXL64q6JmLBHCaUc-ecgNvEck7v-x-cu1kIRi2gdhzd3shK_hgv1qoJd9UlK6yLRSpMX7XM9xIc2DY5vExHHY561RfVqxXrKI7fSFMTwvcV9t0k=w3264-h1836-no
 
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

--------------------------------------------
The physical properties of composites are fiber dominant. This means that when the resin and fiber are combined, their performance remains most like the individual fiber properties. Test data shows that the fibrous reinforcement is the component carrying the majority of the load. For this reason, fabric selection is critical when designing composite structures.

https://www.fibreglast.com/product/about-reinforcements
 
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

--------------------------------------------
The physical properties of composites are fiber dominant. This means that when the resin and fiber are combined, their performance remains most like the individual fiber properties. Test data shows that the fibrous reinforcement is the component carrying the majority of the load. For this reason, fabric selection is critical when designing composite structures.

https://www.fibreglast.com/product/about-reinforcements

I’ve seen some epoxies crack along a fillet also, but not high quality structural epoxies (or even something like Aeropoxy laminating epoxy with silica or pulp added). Surface mounted fins are the most extreme use and a matrix layer absolutely greatly enhances the strength.
I simply disagree that fillets don’t add much strength. I would say “Fillets add a lot of strength, but sometimes you need added reinforcement.”
 
As far as MP and HP rockets go, plenty of evidence that using the proper fin material, properly shaped for the aerodynamic forces, properly adhered and filleted, routinely go transonic without (t2t) reinforcement......just being held on by epoxy surface mount and epoxy fillets. In the case of cardboard or blue tube, the underlying failure seems to be the flex of the tube, not the flex/failure of the epoxy fillet.
 
I’ve seen some epoxies crack along a fillet also, but not high quality structural epoxies (or even something like Aeropoxy laminating epoxy with silica or pulp added). Surface mounted fins are the most extreme use and a matrix layer absolutely greatly enhances the strength.
I simply disagree that fillets don’t add much strength. I would say “Fillets add a lot of strength, but sometimes you need added reinforcement.”

I will agree that plain epoxy fillet is better than no epoxy at all
but you seem to be agreeing with me "silica or pulp added"
these are reinforcements

dr Wogz said...I've used balsa dust too, as an epoxy filler.

the other pictures in this post are plain clear epoxy


https://en.wikipedia.org/wiki/Fiberglass

Tensile strength MPa (ksi) Compressive strength MPa (ksi)
Polyester resin (Not reinforced)[16] 55 (7.98) 140 (20.3)
Polyester and Chopped Strand Mat Laminate 30% E-glass[16] 100 (14.5) 150 (21.8)
Polyester and Woven Rovings Laminate 45% E-glass[16] 250 (36.3) 150 (21.8)
Polyester and Satin Weave Cloth Laminate 55% E-glass[16] 300 (43.5) 250 (36.3)
Polyester and Continuous Rovings Laminate 70% E-glass[16] 800 (116) 350 (50.8)
E-Glass Epoxy composite[17] 1,770 (257)
S-Glass Epoxy composite[17] 2,358 (342)


https://www.substech.com/dokuwiki/doku.php?id=epoxy_matrix_composite_reinforced_by_50_carbon_fibers
https://www.substech.com/dokuwiki/doku.php?id=epoxy_matrix_composite_reinforced_by_70_carbon_fibers


Steve, if you still want to argue, please cite your references
 
I’m unsure why you included material properties for polyester which is even less strong than epoxy, but your initial statement was simply that fillets don’t add much.
I’m not arguing that reinforcement isn’t much stronger than epoxy alone. It is. I just disagree with your statement that fillets don’t add much. They do. And the brand of epoxy makes a significant difference as well.
Aeropoxy pr2032 has a tensile strength of 9,828 psi. So, and this is an oversimplification as well, a quarter inch fillet applied to one side of a four inch fin root, provided one square inch of area. Fillets are applied to both sides of the fin. One side will be in tension while the other side is in compression when sideways forces are applied to the fin tip. Those fillet act as struts, greatly adding to the ability of the fin to resist side forces.
https://www.ptm-w.com/technical-lib...OXY PR2032 Bulletin w-4 Hardeners 24Jun08.pdf

With all that said, I would always add amendments to increase viscosity, improve the workability, and yes, increase the strength. But to say that fillets don’t add much, is just wrong.
 
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