# Best Methods for Epoxying Internal Fin Fillets ?

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#### DMcCauley

##### Well-Known Member
Yeah, I know this question has been asked a million times, but I'm always looking for a new and more novel method of doing it. There just has to be an easier way folks.

As an intro, the way I've done it, with my large L2 and L3 builds, was to simply route out the fin slots all the way to the end of the tube, and build the fin / motor can first, and then insert at the very end. Of course, you still need to fillet the inside of the tube to the fins.

Any thoughts?

Thanks

#### Steve Shannon

##### Well-Known Member
TRF Supporter
I just use dowels or chopsticks reaching in with the rear centering ring removed. I do the outside fillets first so epoxy doesn't leak through, then pick up the epoxy on the stick and reach in and deposit it near the aft end of the fin tab. I let the epoxy run towards the front and push it with the stick as needed. I do one at a time and let it partially cure before moving to the next fin.
I've considered industrial epoxy applicators, the kind with thin replaceable mixer tips, but never got around to buying any.

Steve Shannon

#### dr wogz

##### Fly caster
I have:
Added a massive glob as the ease to reach end, and let gravity take it down the length of the fin root..
used a long stick, and dabbed globs onto the joint until it's covered. time lets it all settle & coalesce into a nice (sorta nice) fillet
used a long stick and applied a long bead to eh stick al dabbed the long glob onto the joint. (same idea as above but to do it once or twice, not 20 or 30 times..)
Used a piece of tube (RC Plane Fuel tubing, RC plane "Gold n Rod" outer casing) and a syringe to 'inject' glue onto the joint, kinda like applying caulking..
Same as above, but used my mouth / lung pressure to squirt the glue (the syringe I had at the time was way too small
Drilled holes in the BT, along eth fin line, and injected epoxy into the cavity between the MMT & BT (For small volumes, when the MMT & BT are close in size / when the CR are only an 1/8" thick!)

Most of the above either had a flashlight held between my teeth, or a lamp craned over & adjusted to see down the tube..

I have tasted Epoxy, and I have a few shirts that prove my attempts were .. attempts! (The flights prove they were successful.. so far!)

#### NateLowrie

##### Well-Known Member
Yeah, I know this question has been asked a million times, but I'm always looking for a new and more novel method of doing it. There just has to be an easier way folks.

As an intro, the way I've done it, with my large L2 and L3 builds, was to simply route out the fin slots all the way to the end of the tube, and build the fin / motor can first, and then insert at the very end. Of course, you still need to fillet the inside of the tube to the fins.

Any thoughts?

Thanks
I have done the following methods:
• Route the slots through the end of the tube and insert the completed fincan complete with internal fillets at the very end.
• Insert the motor mount with the back centering ring loose. Glue the root edge of the fins to the motor mount. Remove back centering ring and apply fillets through the back.
• Epoxy injection through holes drilled next to the slot. The holes then get covered by the external fillet.

So far epoxy injection has worked the best for me. However, I am trying something new on the next rocket that should be easier and less prone to mistake and runs. Basically, I am gluing thin balsa planks onto the motor mount on each side of the fins to form a dam for the epoxy. They will sit 3/8in off of each side of the fin edge. Then, the mount goes in the rocket like normal. Now, when I go to install the fin, I mix up enough epoxy for the fillets and carefully pour it into the slot so it sits in between the planks. Then, you can install the fin and it will end up in the center of the planks and your epoxy, creating your fillets the same time you glue down the root edge. If this doesn't make sense I can take some pictures of the process.

#### Wayco

##### Desert Rat Rocketeer
I have used several methods over the years, including the method you describe above. I added a ring of body tube sliced lengthwise to the aft end of the fincan after installing the motor mount tube with fins into the slots, just to strengthen the back end.
Another method used for fincans that don't have much space between the MMT and body tube is to file out holes in the fin slots and inject a thin epoxy with a syringe:

I use that method on my Wildman Darkstars with West Systems epoxy, adding a bit of chopped carbon fiber to the mix.
There is also the tried and true method of removing the rear centering ring after sticking the fins and flowing in your preferred adhesive from the aft end. You have to be careful to keep the adhesive off the area where the ring goes, or it won't fit back on.
I think I have documented most of these techniques with the many build threads I have posted up, if you want to search a bit....

#### AfterBurners

##### Well-Known Member
As long as its not card board tubing I just use the two part expanding foam epoxy. All the methods listed above work great.

#### CPUTommy

##### Thrust cures All
I use the straws to make a "well", Tape off the body tube slots, Mix epoxy, pour it in the fin slot unit its "full" insert fin and hold. You will get some weeping back up through the slot, which is good, To me its not an internal fillet, its a solid bond between fin and both tubes. And it costs zero if you use straws.

#### K'Tesh

##### OpenRocket Chuck Norris
TRF Supporter
I leave my rear centering ring off, and drip 30 min Epoxy into the gap, making sure to see it drip to the CR ahead of the fin. Protecting the inside of the motor tube is important, so I seal it off using a round balloon lightly inflated.

#### Bat-mite

##### Rocketeer in MD
Or, skip it and use two part expanding foam. Heavier, but those fins aren't going anywhere!

#### Exactimator

##### Well-Known Member
I&#8217;ve only used BS 5 minute epoxy on my cardboard fleet and Rocketpoxy on my fiberglass builds. Both epoxies are pretty viscous. If you slather enough epoxy on the fin root, it makes its own fillet when the fin seats against the MMT. I saw this when I left an aft centering ring out on a cardboard rocket. On FG builds, I add black dye to my Rocketpoxy and triple butter the fin root. The black dye allows me to see how much epoxy is at the joint through the fiberglass tube.

I&#8217;ve had one failure of a joint on a cardboard Madcow Phoenix. I added more epoxy and reinstalled the fin. On a later flight of the same rocket, I had a fin snap outside the external fillet. It was MUCH easier to repair the failed epoxy joint than it was a broken/splintered fin.

If you want an internal fillet at the fin to body tube joint, the triple butter method doesn&#8217;t allow for that. But I&#8217;m of the school that those add more weight than they&#8217;re worth.

I&#8217;ve never injected fillets nor cut fin slots all the way through the back of a body tube, so I can&#8217;t say how those would compare.

#### dshmel

##### Well-Known Member
Or, skip it and use two part expanding foam. Heavier, but those fins aren't going anywhere!
I have used foam as well. I have also seen folks use a fiberglass rope saturated with epoxy first, then placed in the joint using a dowel. I have not tried that yet though.

#### seth_cooper

##### Well-Known Member
I like to "double dip".

Place a bead of epoxy on the fin root. Seat it against the MMT. Remove. Apply another bead of epoxy. Seat and stabilize the fin until cured.

The only other fillet I use is at the fin to body tube junction on the exterior.

Worked for my L3 rocket and countless others.

#### Igotnothing

##### Well-Known Member
What is this "internal" of which you speak?

<<goes off to make another MD>>

#### DMcCauley

##### Well-Known Member
Or, skip it and use two part expanding foam. Heavier, but those fins aren't going anywhere!
Does that provide enough bond to securely hold those fins (versus epoxy fillets)?

Obviously, this is a very simple and ideal method of doing it and thus attractive to me. I have used it in the past, but I've always done it in tandem with internal fillets which were done first.

#### mccordmw

##### Well-Known Member
Does that provide enough bond to securely hold those fins (versus epoxy fillets)?

Obviously, this is a very simple and ideal method of doing it and thus attractive to me. I have used it in the past, but I've always done it in tandem with internal fillets which were done first.
2 part urethane expanding foam provides a very high degree of strength and stability to the fins and the fin can. It might not bond as tightly or be as strong as epoxy over the same surface area, but the foam is covering a much greater area, so the whole is much stronger. Go with US Composites 2 lb foam.

https://www.uscomposites.com/foam.html

It expands quite a bit, so be careful with calculating how much you need, and tape off areas where it could leak.

#### H_Rocket

##### Death by Powerpoint
I've considered industrial epoxy applicators, the kind with thin replaceable mixer tips, but never got around to buying any.
I have had a good amount of success with these. I use Scotchweld DP100 by 3M with the gun for some work and if you are working with polypro nosecones DP8005 or DP8010 acrylic just plain rocks. You can stick two nosecones together with it! The trick is in matching the tip to the cartridge. A while back I located 1:1 empty cartridges and loaded hobbypoxy into them on my own. Worked pretty good. The gun runs around $75 and the tips anywhere between$1 and \$3 each.

#### DMcCauley

##### Well-Known Member
2 part urethane expanding foam provides a very high degree of strength and stability to the fins and the fin can. It might not bond as tightly or be as strong as epoxy over the same surface area, but the foam is covering a much greater area, so the whole is much stronger. Go with US Composites 2 lb foam.

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Great.

So what would be the preferred method of application?

Would it be something like this? My question applies to whether #2 below is adequate. It is by far the easiest but provides very little strength.

1. Install motor / fin can into body tube. Epoxy top bulkhead. Leave bottom bulkhead ring loose.
2. Apply CA glue to fin edge and attach through tube to the motor / fin can for alignment purposes only.
3. Apply epoxy fillets to outer body tube fin fillets.
4. Apply expanding foam to internal body / fin can
5. Secure bottom bulkhead with epoxy.

#### CzTeacherMan

##### Well-Known Member
1. Install motor / fin can into body tube. Epoxy top bulkhead. Leave bottom bulkhead ring loose.
2. Apply CA glue to fin edge and attach through tube to the motor / fin can for alignment purposes only.
3. Apply epoxy fillets to outer body tube fin fillets.
4. Apply expanding foam to internal body / fin can
5. Secure bottom bulkhead with epoxy.
Yes, this method works fine, depending on your rocket. I've used this exact method for Sport sized rockets. On my Wildman Sport, I skipped #3 entirely and it's my most flown rocket with zero damage despite one or two mishaps. (Flown on everything from low G motors to high thrust I motors)
Nowadays, I use this method, but I use JB Weld on #2.

#### Bat-mite

##### Rocketeer in MD
Yes, use epoxy for the fin root. Other than that, you got it! I once made the mistake of foaming before I laid external fillets. I will never do that again!

#### dr wogz

##### Fly caster
.. I have also seen folks use a fiberglass rope saturated with epoxy first, then placed in the joint using a dowel. I have not tried that yet though.
I like that idea!

#### mccordmw

##### Well-Known Member
Great.

So what would be the preferred method of application?

Would it be something like this? My question applies to whether #2 below is adequate. It is by far the easiest but provides very little strength.

1. Install motor / fin can into body tube. Epoxy top bulkhead. Leave bottom bulkhead ring loose.
2. Apply CA glue to fin edge and attach through tube to the motor / fin can for alignment purposes only.
3. Apply epoxy fillets to outer body tube fin fillets.
4. Apply expanding foam to internal body / fin can
5. Secure bottom bulkhead with epoxy.
I use epoxy and not CA on the fin root.

1. Install motor / fin can into body tube. Epoxy top bulkhead. Leave bottom bulkhead ring loose.
2. Apply epoxy to fin edge and attach through tube to the motor / fin can for solid root attachment.
3. Apply epoxy fillets to outer body tube fin fillets.
4. Apply expanding foam to internal body / fin can
5. Secure bottom bulkhead with epoxy.

#### dshmel

##### Well-Known Member
I like to "double dip".

Place a bead of epoxy on the fin root. Seat it against the MMT. Remove. Apply another bead of epoxy. Seat and stabilize the fin until cured.

The only other fillet I use is at the fin to body tube junction on the exterior.

Worked for my L3 rocket and countless others.
That is my standard construction method as well. I use ProLine 4500 or RocketPoxy. As I seat the fin tab against the MMT for the 2 or 3 epoxy "transfer" placements, I wiggle the fin back and forth a bit to get a good bond on the MMT tube. I then just fillet the external fin-to-fin can joint. I used this method on a WM Demon 98 that came in ballistic. The only thing sticking out of the ground was the fin can. The fins sheared off, but the epoxy fillet that was left on the root of the fin tab took the outer layer of the FG MMT tube with it. On my 5" Terminator, I used this method and then foamed the fin can for extra strength. For smaller rockets, I don't. Cardboard rockets benefit from foaming as it stiffens the entire fin can and helps keep the bottom of the rocket from crushing or denting on landing.

#### cerving

##### Owner, Eggtimer Rocketry
TRF Supporter
I've become a big fan of glue dams (similar to CPUTommy's straws). Get some basswood strips and CA glue them edgewise on either side of the fin, spacing them about 3x the width of the fin, tall enough so that they will just reach to the inside of the body tube. Glue this assembly into the tube (after sanding the fins/slots if necessary!), then dribble epoxy into the slots until it fills up the space between the dams. Insert your fin, straighten it out, and let it cure before going onto the next one. The nice thing about doing it this way is that not only does the fin get bonded to the motor mount, it also gets bonded to the inside of the body tube. It ain't ever coming out. I had a rocket come in ballistic once and it tore the fins off at the body tube, leaving the root firmly in the tube.

#### rharshberger

##### Well-Known Member
I've become a big fan of glue dams (similar to CPUTommy's straws). Get some basswood strips and CA glue them edgewise on either side of the fin, spacing them about 3x the width of the fin, tall enough so that they will just reach to the inside of the body tube. Glue this assembly into the tube (after sanding the fins/slots if necessary!), then dribble epoxy into the slots until it fills up the space between the dams. Insert your fin, straighten it out, and let it cure before going onto the next one. The nice thing about doing it this way is that not only does the fin get bonded to the motor mount, it also gets bonded to the inside of the body tube. It ain't ever coming out. I had a rocket come in ballistic once and it tore the fins off at the body tube, leaving the root firmly in the tube.
+1 for fin pockets aka glue dams. Done the way Cerving describes it is easier and weighs about the same as internal fillets.

#### CPUTommy

##### Thrust cures All
I've found that the end product ties the fins,rings, In side and outside tube to one solid unit, the straws just about touch the outside tube and after you fill the well, tape the space where your external filets go, and rotate tube in 180 degree cycles, after 10 minutes leave it fin up, the good news... the fin can't be any more secure, the bad news, the fin is secure, a cato means it's probably not worth repairing.

I'm a huge fan of trying to rip the fins off under power, I think I'll have tube issues before fin issues

#### CzTeacherMan

##### Well-Known Member
I've become a big fan of glue dams (similar to CPUTommy's straws). Get some basswood strips and CA glue them edgewise on either side of the fin, spacing them about 3x the width of the fin, tall enough so that they will just reach to the inside of the body tube. Glue this assembly into the tube (after sanding the fins/slots if necessary!), then dribble epoxy into the slots until it fills up the space between the dams. Insert your fin, straighten it out, and let it cure before going onto the next one. The nice thing about doing it this way is that not only does the fin get bonded to the motor mount, it also gets bonded to the inside of the body tube. It ain't ever coming out. I had a rocket come in ballistic once and it tore the fins off at the body tube, leaving the root firmly in the tube.
Interesting!! I never thought to do that... I've saved all my leftover balsa from Estes kits which I could easily use for this purpose. Neato!
Now, I just need another rocket to build. LoL! Dear wife, I need another rocket so I can try this new technique. But, but, but... It's for science!!

#### OverTheTop

##### Well-Known Member
TRF Supporter
I've become a big fan of glue dams (similar to CPUTommy's straws). Get some basswood strips and CA glue them edgewise on either side of the fin, spacing them about 3x the width of the fin, tall enough so that they will just reach to the inside of the body tube. Glue this assembly into the tube (after sanding the fins/slots if necessary!), then dribble epoxy into the slots until it fills up the space between the dams. Insert your fin, straighten it out, and let it cure before going onto the next one. The nice thing about doing it this way is that not only does the fin get bonded to the motor mount, it also gets bonded to the inside of the body tube. It ain't ever coming out. I had a rocket come in ballistic once and it tore the fins off at the body tube, leaving the root firmly in the tube.
Sounds a bit like what I did for my L3 1/2 Scale Nike Smoke:

You leave just enough wriggle room to be able to align the fins when you fit them. That's a 4" motor mount tube there. Worked well. Probably overbuilt in this case. If I did this again I would add lots of microballoons to the epoxy to cut down some of the mass.
Full story here if you are keen for more details: https://forum.ausrocketry.com/viewtopic.php?f=6&t=4612&start=16

Last edited:

#### rocketsam2016

##### Well-Known Member
Sounds a bit like what I did for my L3 1/2 Scale Nike Smoke:
View attachment 323899
You leave just enough wriggle room to be able to align the fins when you fit them. That's a 4" motor mount tube there. Worked well. Probably overbuilt in this case. If I did this again I would add lots of microballoons to the epoxy to cut down some of the mass.
Full story here if you are keen for more details: https://forum.ausrocketry.com/viewtopic.php?f=6&t=4612&start=16
Yeah I tried the epoxy dams on my recent frenzy XL build and it added a ton of mass. The other problem is that much of the mass is in the middle of the space between the mmt on the outer frame and does basically nothing. If I do it again I'll try the straw dams - that shape should be much more efficient.

#### OverTheTop

##### Well-Known Member
TRF Supporter
Yeah I tried the epoxy dams on my recent frenzy XL build and it added a ton of mass. The other problem is that much of the mass is in the middle of the space between the mmt on the outer frame and does basically nothing.
That's why I thought about adding the microballoons. Takes a lot of mass out of the epoxy, and given the area would still be amply strong.