Epoxy bead question for fins

[ManOntheMoon]

I'm currently building a 4" Madcow Frenzy after losing my previous one over the summer due to a Cesaroni motor failure. My previous Frenzy burnt up on the launch pad on an extremely hot day over the summer (over 100 degrees C on the field). The 54 mm casing separated from the end-enclosure, I believe due to thermal expansion of the end-enclosure. The grains from the motor burnt the lower stage of the rocket. I had many previously successful flights and want to bring back the Frenzy to my fleet. On the previous build, I didn't apply an epoxy bead to the fins and outer airframe. However, I did fill the cavity between the motor mount and the airframe with expanding foam which added to the structural integrity. The fins were obviously epoxied to the motor mount. I never had any damage to the fins, despite a few hard landings.

As I rebuild the Frenzy, I question if I need an epoxy bead on the fins. I pose the question to the forum community if there is any scientific reason or actual examples of why I would need an epoxy bead on the fins attached to the external airframe if I am using expanding foam between the motor mount and the airframe?

 

[djs]

If you're flying where it's over 100C, you might want to contact the VAR (Venus association of Rocketry)

 

[ManOntheMoon]

LOL. I hadn't had enough coffee this AM. I mean degree F.

 

[BDB]

I was there for the death of the 1st Frenzy. Previous flights from this rocket were the impetus for me to get into HPR. It was a great rocket and a spectacular fire. Here are a few pics that Boris posted on his "Clusters are Fun" thread.

View attachment 307643View attachment 307642View attachment 307640View attachment 307641

As for the epoxy bead question - you mean the external fillet, right? If you are foaming the fin can, it should be sufficiently strong; you probably don't need an external fillet. Others here may have better advice, but your old Frenzy flew fine on L2 motors without them. That said, for me it is just gratifying to make a smooth transition from the fins to the body tube.

 

[Bat-mite]

I always apply my external fillets before using foam on the inside. If you don't, the foam will ooze out of the gaps between fin and slot. It is a mess to clean up.

 

[Buckeye]

The Frenzy TTW fins are very solid. After your foam, the fillets are for better aerodynamics and looks.

 

[dr wogz]

I'll agree, the outer fillets are more of looks & "smoothness" of the fin / body tube transition.. assuming of course, other structural additions are made: foam, internal fillets, etc..

 

[fyrfytr310]

If you're flying where it's over 100C, you might want to contact the VAR (Venus association of Rocketry)

Ahhhhahaha! Nice.

 

[soopirV]

I fillet all my builds because I prefer the way it looks. It's always a bit trickier on split fin rockets, but the rest of the responses are correct- on the Frenzy they probably aren't structurally necessary, absolutely not necessary if the can is foamed I bet (have yet to try to foam anything myself).

 

[ManOntheMoon]

I was there for the death of the 1st Frenzy. Previous flights from this rocket were the impetus for me to get into HPR. It was a great rocket and a spectacular fire. Here are a few pics that Boris posted on his "Clusters are Fun" thread.

View attachment 307643View attachment 307642View attachment 307640View attachment 307641

As for the epoxy bead question - you mean the external fillet, right? If you are foaming the fin can, it should be sufficiently strong; you probably don't need an external fillet. Others here may have better advice, but your old Frenzy flew fine on L2 motors without them. That said, for me it is just gratifying to make a smooth transition from the fins to the body tube.

I am glad the previous flights from the rocket were the impetus for you to get into HPR! The Frenzy has been quite a spectacle, even at it's death.

I do mean external fillet. Given all the inputs, I am going to skip applying the external fillet. I actually like the cleaner look without the fillet. The good news is no foam came out as one member had posted. I had a tight fit between the fins and the airframe slot.

 

[BDB]

I am glad the previous flights from the rocket were the impetus for you to get into HPR! The Frenzy has been quite a spectacle, even at it's death.

I do mean external fillet. Given all the inputs, I am going to skip applying the external fillet. I actually like the cleaner look without the fillet. The good news is no foam came out as one member had posted. I had a tight fit between the fins and the airframe slot.

I can attest that it felt like 100 C out there. It was so hot that the launch controllers started having heat-related issues and we had to completely stop launching HPR for the day.

I assume you are rebuilding the 4" kit. Standard tubing or are you going to try FG this time?

 

[Woody's Workshop]

I do fillets on inside, outside, big fins, little fins, TTW fins, surface mount fins...
But what do I know, I just put the biggest LPR Engine Mount in rockets as I can.
Launch and say goodbye to the Tree Gods.

 

[ManOntheMoon]

I can attest that it felt like 100 C out there. It was so hot that the launch controllers started having heat-related issues and we had to completely stop launching HPR for the day.

I assume you are rebuilding the 4" kit. Standard tubing or are you going to try FG this time?

I am rebuilding the 4" kit with standard tubing which I have coated with epoxy. I also use 2" of 9 Oz fiberglass tape on the upper airframe before the nosecone to reinforce that section and avoid zippering. I have avoided FG for rocket this big since the field size is a bit small for anything heavy. I would hate to squash a car parked on the field if there is a failure on my recovery system!

 

[AdAstraPerAspera]

All fillets are primarily structural. I doubt you could push the unglassed cardboard tubes harder than the fins can take without external fillets, but up to you. You said it flew fine without on version 1.0, so unless you were going to try significantly punchier motors, you should be fine this time around too.

 

[Woody's Workshop]

Found "THIS" on Jim Z's site, pertaining to fillets and drag.
See Page 12, under Fins.
Fillets do reduce drag according to this document.