Are internal fillets really necessary?

This assumes their sole purpose is to reinforce the fin attachment. I've found that slotting body tubes weakens the airframe considerably and tying them to the fins at each point of contact replaces airframe integrity. Fin flutter can cause airframe failure, not just fin failure.

plugger said:

Exactly. They are completely unnecessary. The fact that you're going through the wall to an internal motor mount pretty much guarantees you'll stay below Mach 1.5 in most cases. And while all the leverage arguments above are technically correct and come with a pretty picture they're mitigated by the fact that you'd still bond the fin to the motor mount even IF you didn't run internal fillets. So you'll still have two bonded points of contact between the fin and rocket, one on the motor mount, the other on the fillets on the external airframe. So the leverage arguments are made moot by that fact. And if there's enough leverage to separate the two bonds per fin, I'm pretty sure you're rocket is already in a world of hurt.

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Peter Olivola said:

This assumes their sole purpose is to reinforce the fin attachment. I've found that slotting body tubes weakens the airframe considerably and tying them to the fins at each point of contact replaces airframe integrity. Fin flutter can cause airframe failure, not just fin failure.

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But once again, the original "fault" in the example you mention is fin flutter. And given that the best solution for that problem is to design a fin that is actually capable of withstanding the forces of the range of motors you'll be able to fly in the rocket they're attached to. After thinking about this a little more I'm able to recall another example that flies in the face of what's posted above. Specifically, I had a PML AMRAAM 2.1 kit with phenolic that I put a carbon wrap around and then upgraded the mmt from 29mm to 38mm. My fins were bonded to the mmt through the wall and then had generous Proline 5400 epoxy fillets on the outside. I flew it on a J350 and ran into a deployment failure where the NC separated but my chute didn't deploy cleanly. The rocket came in a bit hot and landed with the aft fins and retainer taking the brunt of the hit. One of the four kit provided G10 fins completely sheared along the fillet. So the fin stayed intact and didn't buckle at all along the epoxied points of contact and yet sheared off completely past those points. Moral of the story? I should have had thicker fins given the upgrades and therefore added weight to the rocket.

I find most times it's better to fix design faults instead of over-engineering around them in an attempt to mitigate the issue they present.

RocketFeller said:

I slot the BT clear out the back and build the fin can prior to sliding it into the BT for most rockets.

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I wish I would have done this on my 4" Madcow Honest John. I don't like messy fillets on the inside of the body tube even though they will be hidden and trying to do them through the rear of the rocket is a pain. Great advice RocketFeller

RocketFeller said:

I slot the BT clear out the back and build the fin can prior to sliding it into the BT for most rockets.

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Binder does this, well at least the only Binder kit I have, the Devastator, uses this method.

mpitfield said:

Binder does this, well at least the only Binder kit I have, the Devastator, uses this method.

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Yeah that's what I've read, have yet to build a Binder. Calls it a modular fin can, I think. I did it on my most recent build. A short piece of coupler can be used aft of the aft ring to help add rigidity to the tube if need be, or if one thinks there is need. I couldn't get interior fillets on the fin to BT connection, but really good connections at the fin/MMT and fin/centering rings connections.

plugger said:

Hard landing I can accept Nate, but using ttw mounting to mitigate flutter is the wrong way to go about resolving that issue imo. If you're suffering from fin flutter then you should revisit your fin design instead of relying on "extra strengthening" from fillets.

Basically, appropriately designed fins (as in, fins that won't be subjected to flutter within the performance envelope of the rocket) and a conservative stability margin are much more important than things like ttw mounting when it comes to mitigating the risk of losing a fin during flight. This goes for all rockets, from fat stumpy ones to MD beasts. Sure, ttw might save a fin from a hard landing. But it might not. Last flight I had on my Talon 2 (which had internal fillets btw) had a fin pop from a hard landing. So it can happen. I reckon I received bonus points as my magnetic apogee detector didn't kick off my nose cone, I was saved by the backup motor eject on the H410 VMAX motor I was flying on. I didn't think those worked... :eyeroll:

Long story short, if you rocket is well designed then internal fin fillets are unnecessary imo. A nice to have? Sure. But not necessary.

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Your talking about a kit here. Re-designing the fins will ruin the look of the rocket from the vendor. Due to this, some kits have huge surface area fins that are obviously more suspectable to flutter even at lower mach speeds. This is why it is necessary to do internal fillets. I am talking from experience here, because this happened to me.

Now about your talon having internal fillets. Did it go past mach? If it did no doubt the internal fillets saved the day. Otherwise your talon would have had a destroyed body tube. Now does a hard landing cause fins to pop off? Absolutely, its like being wacked by a hammer. I'd rather have a fin popped off fin versus a destroyed body tube, however.

The kit can obviously be re-engineered if the person wishes to do so, which I am sure most wont do.


Alexander Solis - TRA Level 1 - Mariah 54 - CTI-I100 Red Lightning Longburn - 6,345 Feet

Why not? The weight that is added is minimal compared to the strength that is added. Gram for gram it's a pretty good swap.

I can't get my mind past the through the wall application. If setting a fin through the wall wouldn't one apply an internal fillet for that reason? Why place it through the wall? To sandwich the tabs in the centering ring? Well shouldn't we then consider making two small tabs with two small corresponding through the wall slots forward and aft near centering rings? Wouldn't this save weight and become far less overkill?
I must say I am all for internal fillets. Three total fillets per fin. I did my L1 last weekend and saw how others created "fillets" though. Reminded me of second grade arts n crafts. Not to be rude but they looked horrible. I'm talking about consistent gauged fillets. Not heaping sloppy roads along the way with more than likely voids on contact surfaces....

plugger said:

Hard landing I can accept Nate, but using ttw mounting to mitigate flutter is the wrong way to go about resolving that issue imo. If you're suffering from fin flutter then you should revisit your fin design instead of relying on "extra strengthening" from fillets.

Basically, appropriately designed fins (as in, fins that won't be subjected to flutter within the performance envelope of the rocket) and a conservative stability margin are much more important than things like ttw mounting when it comes to mitigating the risk of losing a fin during flight. This goes for all rockets, from fat stumpy ones to MD beasts. Sure, ttw might save a fin from a hard landing. But it might not. Last flight I had on my Talon 2 (which had internal fillets btw) had a fin pop from a hard landing. So it can happen. I reckon I received bonus points as my magnetic apogee detector didn't kick off my nose cone, I was saved by the backup motor eject on the H410 VMAX motor I was flying on. I didn't think those worked... :eyeroll:

Long story short, if you rocket is well designed then internal fin fillets are unnecessary imo. A nice to have? Sure. But not necessary.

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I can agree with you in principle, but consider the following scenarios:

1. The fins may be designed a particular way to be showy or unusual. They may not be the best design in terms of being subject to flutter but that is not the goal of the design. The super batray comes to mind here as does forward swept fins.
2. If your epoxy application is perfect, that is one thing. However, I see way too many instances where the epoxy application is far less than ideal. If you don't quite get the ratios right because you eyeball the pumps instead of weighing or you don't prepare the material to be bonded right it compromises the strength of the fillet. Internal fillets are extra insurance against inferior application at a strength to time/cost ratio that is acceptable to many people.

Again, there are many ways to design a rocket and many ways to build it appropriately. Your way, being perfectly valid, is just one of several options.

Count me as one who avoids internal fillets (not say that is the way you should do it). All you who use internal fillets I raise my beer glass to you.

I use a good fillet on the fin to mmt, and one good fillet fin to airframe external. The roughest stress my fins see in order are: 1 landing, 2 transport and storage, 3 flight.

Fin flutter: An internal fillet will do nothing to make your fin more robust against fin flutter. The maximum stress on the fin during flutter is right at the fin where the external fillet blends into the fin plane. The internal fillet is much less stressed and provides no additional strength to the fin area where it is going to fracture.

All the internal fillet does is make the repair much more a PITA when I drop the rocket without a chute or when I step on it at the tent.

jderimig said:

I use a good fillet on the fin to mmt...

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Uh, John, isn't this an internal fillet?

dixontj93060 said:

Uh, John, isn't this an internal fillet?

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Is it? In my limited mind I took internal fillet as "internal" to a tube. I was thinking the discussion was about fillets on the internal airframe surface.

Of course if you use TTW you bond to the mmt tube, is there really controversy on this?

SinfulDarkLord said:

some kits have huge surface area fins that are obviously more suspectable to flutter even at lower mach speeds. This is why it is necessary to do internal fillets.

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I've seen this argument several times and do not see it as entirely valid. I do not see how an internal fillet will do anything to obviate the effects of fin flutter. The only way I know of to do that is to vary the harmonics of the fin from root to tip so that it can not set up a standing wave. The fillet inside the fin can serves no purpose to that end (that I can see). Yes, the inside fillet will keep the fin from flexing at the MMT if the external fillet fails, but that's about it.

Note to Tim: I think some of us are considering an "internal fillet" to be the one from the fin to the inner wall of the air frame - not the MMT to fin junction.

jderimig said:

All the internal fillet does is make the repair much more a PITA when I drop the rocket without a chute or when I step on it at the tent.

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Said that earlier. Same goes for foaming the fin can.

jderimig said:

Is it? In my limited mind I took internal fillet as "internal" to a tube. I was thinking the discussion was about fillets on the internal airframe surface.

Of course if you use TTW you bond to the mmt tube, is there really controversy on this?

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Yeah, not on the inside, i.e., inside outer circumference of the body tube; instead people are arguing using internal fillets fin-to-motor-mount. The issue is how you bond... Do you simply put epoxy on the TTW-to-MMT edge and stick it in the fin slot, or do you keep access open to that area so you can add fillets to that joint (that is the discussion point, to fillet or not to fillet?).

H_Rocket said:

Same goes for foaming the fin can.

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Yeah I have come full circle on this one.

I have foamed two fincans and one thing that was immediately obvious was the fincan just felt very robust, even when you tapped it, it resonated very differently. What actual effect this had on robustness, fin-flutter, etc. I have not idea. However I have also had to perform repairs and modifications to both rockets since, and if I could go back in time I would not have foamed the fincan.

Seems to be just part of the learning curve, even though some said at the time not to do it, and others said do it, in the end I had to figure it out on my own.

Now I know where my kid gets it from!

jderimig said:

Is it? In my limited mind I took internal fillet as "internal" to a tube. I was thinking the discussion was about fillets on the internal airframe surface.

Of course if you use TTW you bond to the mmt tube, is there really controversy on this?

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That is hilarious. Seriously. TTW without bonding to the MMT? Really? Is there really controversy on this?

Are internal fillets really necessary?
no, not necessary. neither is foaming the fin can.
BUT
after having a 1/4 scale PML patriot come in ballisitic into frozen ithica,mi farm soil, having tbe BT shatter into a hundred pieces UP TO the fin can, which had internal fillets both on tbe mmt and bt plus all foamed, and not one bit of damage on the fin can( the fins only got paint scraped off and the nose cone even got destroyed)
im hooked
when the build makes it possible.

My process lately has been to inject epoxy into/over the fin slot, filling the slot. butter the edge of the fin, and press it in. Then 3/4" external fillets.

Gives good fin to MMT Joint, and good hold with the beefy externals. Injecting or leaving the rear ring off seems overkill, and I'm not sure it'd add to the strength.


For the OP, there isn't a 38mm motor that'll NEED more epoxy in there than the directions call for.


Also, I built my MAC Villain in the above style. no fin damage, and it took a half mile drag plowing the field, throwing onions and all.

dford said:

That is hilarious. Seriously. TTW without bonding to the MMT? Really? Is there really controversy on this?

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Strictly speaking, you don't NEED an actual MMT. or you don't NEED CR's. Lot of redundant stuff in there.

DavidMcCann said:

Strictly speaking, you don't NEED an actual MMT. or you don't NEED CR's. Lot of redundant stuff in there.

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So let's take away through the wall and the possibility for an internal fillet.

dford said:

So let's take away through the wall and the possibility for an internal fillet.

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This is no different than a MD rocket.

dford said:

So let's take away through the wall and the possibility for an internal fillet.

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been done......

H_Rocket said:

I've seen this argument several times and do not see it as entirely valid. I do not see how an internal fillet will do anything to obviate the effects of fin flutter. The only way I know of to do that is to vary the harmonics of the fin from root to tip so that it can not set up a standing wave. The fillet inside the fin can serves no purpose to that end (that I can see). Yes, the inside fillet will keep the fin from flexing at the MMT if the external fillet fails, but that's about it.

Note to Tim: I think some of us are considering an "internal fillet" to be the one from the fin to the inner wall of the air frame - not the MMT to fin junction.



Said that earlier. Same goes for foaming the fin can.

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The internal fillets keep the fin flutter effects from destroying the main body tube. This has obsolutely nothing to do with the motor mount or nothing to do with stopping fin flutter from occuring.

Otherwise you can kiss your rocket goodbye like it happened to mine. I wont argue with fiberglass as I have never flown one before. Fiberglass body tubes are obviously way superior than blue tube.

If your flying a rocket with big fins slow, then internal fillets are not needed. If you decide one day to go past the barrier, well you better pray fiberglass will hold on to dear life.

SinfulDarkLord said:

The internal fillets keep the fin flutter effects from destroying the main body tube. This has obsolutely nothing to do with the motor mount or nothing to do with stopping fin flutter from occuring.

Otherwise you can kiss your rocket goodbye like it happened to mine. I wont argue with fiberglass as I have never flown one before. Fiberglass body tubes are obviously way superior than blue tube.

If your flying a rocket with big fins slow, then internal fillets are not needed. If you decide one day to go past the barrier, well you better pray fiberglass will hold on to dear life.

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Whut?

DavidMcCann said:

been done......

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My point exactly to the stated above comment which has zero relevance to the topic of internal fillets without a through the wall possibility.

So it is pretty hilarious to think that if one were to put a fin through the wall (which a MD probably doesn't have) they wouldn't fillet it to the motor mount tube.

DavidMcCann said:

Whut?

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......

DavidMcCann said:

Whut?

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Okay the short short version (if some of you dont know where that is from I'll be really disappointed lol).

Big fins, too fast, no internal fillets, good bye rocket lol. Again not sure about fiberglass.


Alexander Solis - TRA Level 1 - Mariah 54 - CTI-I100 Red Lightning Longburn - 6,345 Feet

SinfulDarkLord said:

Big fins, too fast, no internal fillets, good bye rocket lol. Again not sure about fiberglass.

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right, I got that part.... What I'm missing is any evidence or supporting theory that internals would save a rocket that otherwise blew up.

SinfulDarkLord said:

Big fins, too fast, no internal fillets, good bye rocket lol. Again not sure about fiberglass.

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A better, easier and cheaper solution are fins properly designed for the speed, internal fillets will not compensate.

To summarize because this thread is driving me bonkers-

The fin to MMT glue joint is a glue joint. it's not a fillet.

Internal fillets are glue added after the fin is joined to the mmt, either on the MMT to fin tab surface, or fin tab surface to internal BT.


Now, to address the OP, he was asking if the aditional glue is needed internally. not if he needed a root to MMT joint. madcow calls for a joint, not internal fillets.


on these two referenced 2.6" rockets, a root to mmt (along with the root to BT joint in the stepped madcow fin tabs) along with good external fillets, is totally sufficient to fly any 38mm motor.




As for all the other referenced "needs" of internal fillets.... sure, they may help if you land on concrete ballistically. In flight? I'm not sold. I've done them on several rockets, many actually. But I've been getting away from it. If you're trying to fly rockets with huge fins on huge motors...well you shouldn't and I doubt internals will save you. You'd be better off with better fins, as mentioned above, or using thicker material for the fins.

With solid externals, and a good root to MMT joint.... anything tearing that fin out... the fin is already blown itself apart and keeping the tab in is a useless endeavor.




plowing a freaking field couldn't tear those off.


This screech has done Mach+ 4 times now.




This is my second Screech. This time in 54mm. Hasn't flown yet. Shows how I inject the slots, then butter and press in the fin. The 3/4" fillets of 4500 outside. I think it'll hold quite well. We shall see.