Internal fillets issue - need runnier epoxy?

[billdz]

Working on the internal fillers for my Painkiller Max. The airframe is 4.5" and the MM is 3". I mounted the MM without the rear CR and planned to make the internal fillets by sticking a spoon tied to a rod up from the bottom of the airframe. I'm using Rocket Poxy, which seems thicker than epoxy I've previously used. I can't seem to get this epoxy to go where it is supposed to go. It does not flow at all. Most of the epoxy I send up on the spoon ends up on a fin or on the MM, everywhere except to the junction of the fin and MM. The Rocket Poxy instructions say the mixture can be thinned with alcohol, but thinning does not seem to fix the problem. Is there a different type of epoxy, something that easily "runs" downhill to the junction? Or is there a better way to make the internal fillets? Someone suggested blowing the epoxy into the junction with an aquarium hose.

 

[MikeyDSlagle]

The easiest way (for me) to get Rocketpoxy where it belongs.
Hold firmly over trash can
Release

Lots of folks love it but I've had nothing but bad luck with it.

I use US Composites with some sort of filler for everything. I just mix it to the consistency I need for the particular task.

 

[tim cubbedge]

Try West system or us Composite and inject it. Way easier. Look at crazy Jim’s pinned build post on HP forum.

 

[FMarvinS]

Bill, I often use West System epoxy for internal fillets, particularly if applied via a syringe (injected fillets). It flows easily and adding a small amount of powdered fiberglass or carbon fiber or kevlar- added during epoxy mixing- yields further strength. Don't add too much fiberglass or kevlar or carbon fiber-for then the epoxy will be more "peanut butter" in consistency and won't easily flow. After mixing, the epoxy should be administered (by spoon or injection) within about 5 to 15 minutes, since I think flow significantly decreases after about 20 minutes. Simple to use West Systems packets-that are pre-measured for mixing (so that no weighing or pumps are needed), can be readily purchased at West Marine stores. The powdered fiberglass can be purchased from Mr. Martinez on this forum or from Widman Rocketry. Rocketpoxy is great for external fin fillets; however, as you found out, doesn't flow easily. I hope this helps-good luck!

Fred, L2
KG4YGP
member of ICBM, S.C.

 

[tim cubbedge]

Bill this is a great build thread for just about any fiberglass kit.
https://www.rocketryforum.com/showt...x-quot-build-starts-now!!&p=446439#post446439

 

[Andrew_ASC]

The easiest way (for me) to get Rocketpoxy where it belongs.
Hold firmly over trash can
Release

Lots of folks love it but I've had nothing but bad luck with it.

I use US Composites with some sort of filler for everything. I just mix it to the consistency I need for the particular task.

I put the rocketpoxy stuff on the fin tab itself, insert coated fin tab into rocket fin tab slot. Do external fillet. Rocket fell 5400ft at 3.5 lb loaded, survived. Yeah it's not pretty, but it worked. Cotronics 4525 and 4700 make real clean fillets and run a lot. Rocketpoxy is like peanut butter unless you hit it with heat gun/blowdryer real quick. The 4700 you have to oven bake, so do a batch of rockets if your going that route, I used it on centering rings on a Wildman Saab RB-05A Sport along with some comp rockets this year.

 

[Andrew_ASC]

I don't get the logic of an internal fillet must have entire rocket smothered with epoxy. Whatever nearly slight drag reduction you gain got canned by a mass increase of filling the entire rocket structure, especially if its large rocket. Maybe its a for looks thing or some people just want a bunch of epoxy to make it aft heavy and really tuff... Does not compute.

 

[markkoelsch]

Internal fillets do not need to add that much mass. If memory serves, when I built my Wildman Extreme the internal fillets were something like 8-10 grams per fillet. So what- 2-2.5 ounces total. Mine were mixed with Kevlar pulp amendment as were the external fillets.

I used Aeropoxy 3660 laminating resin for the fillets, and then a thin layer of Polyfiber Super Fill as a finish layer.

Internal injectable fillets need thin epoxy- Rocketpoxy does not qualify as thin.


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[djs]

I found that adding the dye to rocketpoxy makes it flow a little more. I used this on my L3 build. Note- I don’t like just letting it “run”- I’d rather control it, so I spread it out with a long dowel, and check with a flashlight.

 

[RocketFeller]

I don't get the logic of an internal fillet must have entire rocket smothered with epoxy. Whatever nearly slight drag reduction you gain got canned by a mass increase of filling the entire rocket structure, especially if its large rocket. Maybe its a for looks thing or some people just want a bunch of epoxy to make it aft heavy and really tuff... Does not compute.

An internal fillet has nothing to do with drag, nor does it have much to do with looks, as it is on the inside of the airframe.

What it does do is add strength to the fin root joint. If done correctly, the fin will break off before it breaks loose.


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[RocketFeller]

The simple solution to the problem of properly applying internal fillets is to do them outside of the airframe.

I prefer slotting my airframes all the way aft and constructing the fin can prior to gluing it in. I have done this whenever possible on my HPR and have yet to break a fin loose. I did bust off a G10 fin due to a separation issue recently, but the root was still solid on the stub sticking out if the airframe.


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[DavidMcCann]

I prefer to drill or file holes and inject aeropoxy, ES6209.

Having only done it on Black rockets...I questioned if it really worked before starting my 4" wild man. it does.

The lower side of the fin you can see the epoxy...the upper hasn't been done yet and you can see the light from the two drilled holes

 

[DavidMcCann]

I don't get the logic of an internal fillet must have entire rocket smothered with epoxy. Whatever nearly slight drag reduction you gain got canned by a mass increase of filling the entire rocket structure, especially if its large rocket. Maybe its a for looks thing or some people just want a bunch of epoxy to make it aft heavy and really tuff... Does not compute.

People aren't filling the space with epoxy. It's not much weight, and it's very strong. I inject sometimes, other times go without. Always add 3/4" external fillets. Its not about drag reduction...it's overall strength.

 

[dixontj93060]

I don't get the logic of an internal fillet must have entire rocket smothered with epoxy. Whatever nearly slight drag reduction you gain got canned by a mass increase of filling the entire rocket structure, especially if its large rocket. Maybe its a for looks thing or some people just want a bunch of epoxy to make it aft heavy and really tuff... Does not compute.

Wow, you continue to prove your ignorance. Study basic physics and cantilever design in aircraft/other: https://en.m.wikipedia.org/wiki/Cantilever.

From the Ether...

 

[Buckeye]

Agreed on the RocketPoxy. Hard to work it from a distance. Good ol' hobby shop epoxy is nice and runny. 30 min or 3 hr

 

[RocketFeller]

I am currently doing the internal fillets on my Bigger Daddy. With fillets along the root edge and along the leading and trailing edges where they meet the CRs, these fins aren't going anywhere!


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[billdz]

Thanks for the replies, I have ordered some West epoxy. @Tim, I have been using Jim's build thread as a reference. I opted against his injection method because I didn't want to drill holes and it seemed there was plenty of room to go in from the rear. @djs, I also tried a dowel and other items (straw, chopstick, smaller spoon), nothing gave me good result.

 

[cerving]

I use fin pockets (or glue dams, depending on your nomenclature...) and RocketPoxy, and I have had lawn darts break fins off at the body tube leaving the tabs intact. You only need to make them about 2x the thickness of the fin on either side... that's really not much more glue than you'd probably waste trying to glob it in with a straw or a syringe, and it's a whole lot easier. It also doesn't add much weight (the dams can be even made of balsa, they're for glue retention not strength), and you're pretty much guaranteed complete adhesion of the fin tabs to the motor mount and the inside of the body tube if you fill the dams up completely. RocketPoxy works really well for this because of its viscosity and relatively long working time.

 

[billdz]

I use fin pockets (or glue dams, depending on your nomenclature...) and RocketPoxy.

That sounds very interesting but I'm not fully following how this works, do you have any pictures? How and when do you insert the balsa and the glue?

 

[caveduck]

Another way to do internal fin-MMT bonding is just to triple-butter the roots with a fairly thick epoxy. I did this for my L3 (see docs in L3 packet thread) with the aft CR off so I could see the results, which were fantastic. Aeropoxy structural adhesive ES-6209 works great for this, RocketPoxy will also work. The prebuilt fin/MMT assembly method with extending the fin slots is also usable for designs where the TE of the fin is near the aft end of the main body.

 

[rharshberger]

That sounds very interesting but I'm not fully following how this works, do you have any pictures? How and when do you insert the balsa and the glue?

Here's a whole thread devoted to fin pockets, and like cerving I use them as well on all my HPR and most MPR builds. Post #32 has an exaggerated OR figure of what fin pockets would look like sans epoxy. Basically the pocket is only slightly wider than the fin by maybe 1/16" per side, and when filled with epoxy and the fin inserted the fin self fillets between the MMT and airframe, the balsa (or whatever material is chosen can be shaped to create a convex/concave/flat fillet. Some have said the add too much additional weight, which they can if you want to use more epoxy, I do it so I can use LESS and still have a strong fincan. The balsa/whatever dams are attached to the MMT prior to instertion then the pockets are lined up with the finslots as the MMT is installed, after the MMT cures, the fin pocket is filled enough to squeeze out just a bit of epoxy as the fin is inserted and the fin is allowed to set.


https://www.rocketryforum.com/showthread.php?130757-Anybody-use-quot-Fin-Pockets-quot

 

[billdz]

Here's a whole thread devoted to fin pockets, and like cerving I use them as well on all my HPR and most MPR builds.

Thanks for the link, now I get it, will have to try that next time.

 

[Andrew_ASC]

Wow, you continue to prove your ignorance. Study basic physics and cantilever design in aircraft/other: https://en.m.wikipedia.org/wiki/Cantilever.

From the Ether...

You can study physics 1, 2, 3, statics, and mechanics of deformable solids. And still be ignorant as stink on sh*t about putting a HPR together. I balanced my fair share of cantilevers in math problems. We never did much outside of bolts textbook wise for joints.

 

[DavidMcCann]

You can study physics 1, 2, 3, statics, and mechanics of deformable solids. And still be ignorant as stink on sh*t about putting a HPR together. I balanced my fair share of cantilevers in math problems. We never did much outside of bolts textbook wise for joints.

youre comically arrogant. Wonder if you’re really that good, or just all mouth.

 

[Binder Design]

For composites, I use JB Weld thinned slightly with isopropyl alcohol. And like RocketFeller, I build the mount outside of the airframe and just slide it in when done. Strong and light with no excess epoxy running around.