Perfecting injected internal fillets

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afadeev

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I am building an MC Fire Flyer, a 2.6" OD airframe with 54mm / 2.1 ID motor mount. Details here: https://www.rocketryforum.com/showt...-54mm-MMT-Fire-Flyer-(MadCow-RW)-built-thread

When put together, the gap between the airframe ID and MMT OD is at most 3-4mm or 1/8", so the only way to do internal fin fillets is via injection. This is my first time doing injected internal fillets, and the first pass left a bit to be desired.

I would appreciate any and all feedback on how I can improve the outcome for the second pass.

Epoxy: Rocketpoxy, pigmented with airframe body color (green).
I ground access holes at two places in the airframe, and used a large 60ml plastic syringe to inject Rocketpoxy into the holes.
It was tough sledding, with Rocketpoxy puddling up near the injection slots, and slowly descending down courtesy of the gravity. It took me multiple rounds of injecting 10ml epoxy here and there, with 4+ rounds per hole, to get to the point where I got the epoxy to drip down to the lower motor mount centering ring on all three fins. Even then, the epoxy drip pattern was taking a somewhat parabolic path away from the fin root edges (see pics below).

In the end, I got a tooth X-ray looking internal fin fillet profiles (see pics). I'm not too worried about the strength, as even the imperfect internal fillets will provide plenty of reinforcement. The fins are going absolutely nowhere, but if there is a trick to getting more consistent and predictable epoxy flow routing alongside the fin roots, I would love to know how to accomplish that!

Fin fillet injection holes.jpgfin fillet 1.jpgfin fillet 4.jpg

Thanks in advance for your suggestions,

a

P.S.: The next pass will involve flipping the airframe upside down, and running internal fillets towards the forward/upper MMT centering ring.
P.P.S.: I'm entirely open to drilling more access holes in the airframe, if necessary.
 
Usually injected fillets are done with the airframe horizontal, and using a flowable epoxy modified with carbon or fg milled fiber. To me it looks as though the airframe was vertical which doesnt allow the fillets to lay out along the fin root and airframe ID of the fin slot. The fillets are also injected so that the fillets facing each other on two fins are done at the same time.
 
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Usually injected fillets are done with the airframe horizontal, and using a flowable epoxy modified with carbon or fg milled fiber.

OK, what epoxy would you consider "flowable" ?
Something other than Rocketpoxy?


To me it looks as though the airframe was vertical which doesnt allow the fillets to lay out along the fin root and airframe ID of the fin slot. The fillets are also injected so that the fillets facing each other on two fins are done at the same time.

I did go vertical after starting horizontal, to promote epoxy spreading.
Perhaps, Rocketpoxy is just too thick to flow in the little space afforded by Fire Flyer gap between airframe and MMT.
:confused2:

I can try going horizontal with additional access holes, and two fillets per opposing fin surface at a time.
It will be significantly slower, but if that's the right way to do it, then that's how it's going to be.

a
 
Yep, RocketPoxy is too thick for this application. You need a clear, low viscosity epoxy like West or US Composites. On colored, thin wall airfames I'd highly recommend using only milled FG and at most bit of 1/16" fiber strand as filler so you don't get the mix too thick. I moved away from carbon strands as they "ball up" the mix (although it is good for internal fillets when not using injection and not worried about see through on a colored airframe).

From the Ether...
 
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Yep, RocketPoxy is too thick for this application. You need a clear, low viscosity epoxy like West or US Composites.

Good to hear, thanks.
I don't have either one of those on hand, but do have both BSI and/or Great Plains 30 minute epoxy?


On colored, thin wall airfames I'd highly recommend using only milled FG and at most bit of 1/16" fiber strand as filler so you don't get the mix too thick.

I've seen folks mixing in strand of FG or CF, but what's the exact marginal benefit?
Intuitively, I can theorize increased resistance to sheering, but I always treated internal fillets (applied by stick until now) as "buffers" for holding the fin root to the MMT, and a method for increasing epoxying attachment surface.

I don't mind mixing some in, though expect it will only make my slow epoxy flow problem worse.

a
 
I read your build thread thus far.
You used technique I have developed, and promoted on the Forum several years ago. The "double dip".
[not saying I invented it, I'm sure someone, somewhere used it before me, I just publicized it]

Using it where the internal space is limited, & difficult to do internal fillets. double dipping is plenty strong and will form small internal fillets. [that's what it's for]
That being said, on airframes of this size, many have done surface mounted fins with just external fillets, flown big honk'n motors and flown just fine.
Think a 54mm minimum on L motors.
Your rocket has the advantage of fin tabs through wall to motor mount, providing even more robust strength. You already have more than enough .

Stop.
Continue with external fillets and all will be well & happy in rocket land.

Just some fillet pointers:
Rocketpoxy can be injected, BUT it is a bit tenuous ....first internal fillets should always be done with airframe horizontal. [99% of time, I'm sure there is an exception somewhere...lol One should never make blanket statements, they always come back & bite you in the butt]

Any how you need a temperature above 75, preferably 80-85 and gently warm the epoxy in hot water to make it flow. Having the parts warmed up also helps a great deal also.

Pull plunger out of syringe, put finger over tip of same & pour R-poxy into syringe,then insert plunger. It's virtually impossible to "suck it up" normally.

NEVER ADD ANYTHING {FILLERS} to R-poxy, you only weaken it! It's already formulated will all the fillers needed.
................................................................................................................................................................

I commend you on your injection attempt, just used the wrong stuff for a first try.
Wildman was the first to use ''injected" fillets in all his kits/instructions, over time we perfected the technique. Really simple when used correctly. You have the holes right.
Just should have used any clear epoxy...West-Aero-Pro-line-US Composites..etc. Even 30 minute Hobby types are plenty strong in rocket size you have.
Simple lay rocket horizontal, with fins hanging over edge of work table. From rear... Y looking.
Mix epoxy & inject the 2 inside fillet of the Y.
Epoxy is clear and will not hinder look of colored tubes. I use this type epoxy for all gluing of internal parts on colored tubes, sometimes with West 406 filler to thicken it for fillets around Cr's etc so it won't run.

So...in a nutshell don't overthink this anymore & just go straight to your external fillets, you will have a project more the strong enough for any motors you wish to fly. Just use the correct size external fillet. I would pull them with a 1in. diameter tube, pipe, dowel etc. Keep adding epoxy & end up with a flying tank....Lol

Hope this answers some of your fillet questions, any more.....ask away. :smile:

Edit:
For REALLY detailed info on injecting fillets go here and click on #34, it's a sticky up top , under High Power.

https://www.rocketryforum.com/showt...-quot-Stock-quot-3IN-DARKSTAR-CJ-Step-By-Step
 
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I've used Rocketpoxy but it needs to be fresh. I tried using a year old batch and it wouldn't flow very well. When I opened a fresh batch I could tell the difference right away.


Sent from my iPhone using Rocketry Forum
 
Stop.
Continue with external fillets and all will be well & happy in rocket land.

Jim,
You are right - I am over-doing it here.

But since I started down this path, I might as well learn something about internal fillet injection, even if only to needlessly bulk up the rocket weight.
:lol:


Just some fillet pointers:
Rocketpoxy can be injected, BUT it is a bit tenuous ....first internal fillets should always be done with airframe horizontal. [99% of time, I'm sure there is an exception somewhere...lol One should never make blanket statements, they always come back & bite you in the butt]

Any how you need a temperature above 75, preferably 80-85 and gently warm the epoxy in hot water to make it flow. Having the parts warmed up also helps a great deal also.

Good idea on increasing the temps of epoxy components.
It was ~70F in my room at the time, higher temps would have helped a bit. Going with

Pull plunger out of syringe, put finger over tip of same & pour R-poxy into syringe,then insert plunger. It's virtually impossible to "suck it up" normally.

NEVER ADD ANYTHING {FILLERS} to R-poxy, you only weaken it! It's already formulated will all the fillers needed.

That's exactly what I did (fill epoxy through the plunger hole, with tip covered up by masking tape).
There was zero chance I was going to get epoxy in by any other way. Even getting it out took some serious pressure, especially towards the end as Rocketpoxy started gelloing up. My wrist got a serious workout, and I'm feeling it this morning (yeah, yeah, I know, keep your jokes to yourself).


I commend you on your injection attempt, just used the wrong stuff for a first try.
Wildman was the first to use ''injected" fillets in all his kits/instructions, over time we perfected the technique. Really simple when used correctly. You have the holes right.
Just should have used any clear epoxy...West-Aero-Pro-line-US Composites..etc. Even 30 minute Hobby types are plenty strong in rocket size you have.

Simple lay rocket horizontal, with fins hanging over edge of work table. From rear... Y looking.
Mix epoxy & inject the 2 inside fillet of the Y.
Epoxy is clear and will not hinder look of colored tubes. I use this type epoxy for all gluing of internal parts on colored tubes, sometimes with West 406 filler to thicken it for fillets around Cr's etc so it won't run.

OK, great suggestions.
I might try next with 30 min BSI/Great Plains clear epoxy I have lying around.
Not because I really need those internal fillets, but because I want to learn how to get this right for the next time when I might actually need to rely on them.


So...in a nutshell don't overthink this anymore & just go straight to your external fillets, you will have a project more the strong enough for any motors you wish to fly. Just use the correct size external fillet. I would pull them with a 1in. diameter tube, pipe, dowel etc. Keep adding epoxy & end up with a flying tank....Lol

Incidentally, 1" dowel is exactly the plan.
Comically, I had damaged by 1" dowel stock (long story), and ordered what I though was 5" of 1-inch diameter plastic pipe from Amazon for $4.99.

Last night I received a giant box with 5-feet worth of 1-inch PVC pipe. My wife thought I had snuck in another rocket purchase, kids got excited. It was 12x of the 1-inch pipe I had expected.:eek:
If I was shipping something like that, the shipping cost alone wold have been higher than what I had paid for it, but OK, thanks Amazon.

alex
 
One other option that I use for close quarter internal fillets (like my recent Blue Iguana build) is: 1) to mark fin locations on CR edges (typically fin slots are a bit long and you can get to the CR edges), 2) glue (using 5-min epoxy or CA) two appropriately sized dowels to line up on either side of the marks spaced the width of fin (do this for all fin slots). Now it gets tricky, but moves the build along rapidly, 3) mix a generous amount of good structural epoxy like T-88 and thicken moderately with fumed silica and milled fiberglass, 4) slather the mix in between and alongside all dowels, 5) also apply above fin slots internal to booster and on aft motor mount CR, 6) install motor mount making sure to align dowels with fin slots, 7) quickly install fins allowing them to sink into the epoxy and be secured by dowels, 8) use good masking tape like Frog brand and wrap around all fins in a couple of separate wraps to both secure and align each fin, 9) now install (or hold) assembled booster horizontally and start rotisserie (motion or on actual) to rotate whole assembly slowly (one rotation every 10 seconds or so), 10) continue rotation until structural epoxy achieves a "tacky" (not sticky) state. Now your rock solid booster is ready for your favorite external fillets.

From the Ether...
 
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Being in the medical trade I have access to various syringes. What I've done is zip tied some latex tubing on a syringe and place the end of the tube forward and pull the tube out while steadily laying a bead. Is it perfect? Heck no! But it's a bit better than using gravity or what have you with the external drilling and "pouring"
epoxy in.

Now to do this you have to have at least two centering rings forward on the motor tube and leave the aft ring off so you can do this.

Like Jim says, don't add any garbage to the epoxy like chopped fiber for an internal fillet. Cripes it takes practice and the right glue to get that to work on external fillets. Too little and it's not adding anything and too much it weakens the bond. Best just to depend on a good "glue" for an internal fillet.

Sorry about the translucent body tubes. I don't know any way short of painting the rocket to get around the internal "epoxy staining". Just man up and live with
it. After spending hours on a nice paint job and seeing how dinged up they get over time, it doesn't bother me much anymore to go bare. Opaque color tubes
tackle that issue but the thin walled tubes have a nice lighter weight benefit. Put lights inside the danged thing and have a night flier! Kurt
 
I've used Rocketpoxy but it needs to be fresh. I tried using a year old batch and it wouldn't flow very well. When I opened a fresh batch I could tell the difference right away.

How do you know if it's "fresh"?
The label on mine says: "expires 7/12/2018".
What's the length of Rocketpoxy lifespan (production to expiration cycle)?


Being in the medical trade I have access to various syringes. What I've done is zip tied some latex tubing on a syringe and place the end of the tube forward and pull the tube out while steadily laying a bead. Is it perfect? Heck no! But it's a bit better than using gravity or what have you with the external drilling and "pouring" epoxy in.

Tubing is a neat idea, though makes for that much more fun pushing the epoxy out of the syringe and through the tubes.
What epoxy are you using?
I'm assuming something less viscous than Rocketpoxy?


I followed the "Wildman" method using West Systems epoxy and chopped carbon. It's simple to do and hasn't failed me yet. The method is covered in these instructions: https://www.wildmanrocketry.com/productimages/Instructions/Wildman/Instructions/wildman3_ins.pdf

Thanks for the link.
I like the bit on drilling perfectly round access hols within area of the external fin fillets. That's will provide better access than my semi-circles near the fin slots.
Wilman refers to "West, Pro Line, [or equivalent]" w.r.t. epoxy choice.
I don't like mixing in stuff into the fin fillet epoxy. Never figured out why that's necessary, yet to find an explanation for it. Skipping that part.

Sorry about the translucent body tubes. I don't know any way short of painting the rocket to get around the internal "epoxy staining". Just man up and live with it.

I am perfectly OK with epoxy staining.
No-one has ever accused me of being overly obsessed about appearances, within reason. Structural and functional aspects - absolutely. Cosmetics, no so much. Which is why I went with pre-colored FB tubes - no need to paint them!

Translucent tubes reveal what's going on inside the rocket under construction, and thus you learn from the imperfections. For example, I have no clue how good or lousy my internal fillets were in all other cardboard/QT airframes. None of them ever failed, but were they smooth and continuous - I doubt it. But I don't really know, and have no easy way to check, learn, and improve.

Which is my primary motivation for pursuing this activity with the current build.
As blackjack2564 mentioned, none of this is necessary for structural integrity on up to L motors that this rocket will ever see.
This rocket is now my guinea pig for mastering internal filet injection.

a
 
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