Foam and internal fillets

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roytyson

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So I have my Super DX3 fins glued in. I ordered foam for the fincan, but was wondering if I need to fillet the fins before the foam, or does the foam essentially replace the need for internal fillets.

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Foam it. You don't need internal fillets with foam for this rocket. Doing your external fillets before foaming, however, will ensure the foam doesn't weep out around the fin slot.
 
Foam it. You don't need internal fillets with foam for this rocket. Doing your external fillets before foaming, however, will ensure the foam doesn't weep out around the fin slot.

Absolutely agree. Make sure that last sentence sinks in. External fillets first! Otherwise you will have foam forcing its way out of every conceivable crack, and it is sticky and hard to clean up. Make sure you put something inside the MMT to keep foam out of it.
 
Whereas an epoxy joint can break, foam makes it impossible for the fin to move. There are merits to using foam; however, it is heavier than epoxy, and should not be used if stability is an issue.
 
Whereas an epoxy joint can break, foam makes it impossible for the fin to move. There are merits to using foam; however, it is heavier than epoxy, and should not be used if stability is an issue.

Nonsense! I have had two rockets with epoxy foam make hard landings under drogue and the fin(s) broke.
 
Whereas an epoxy joint can break, foam makes it impossible for the fin to move. There are merits to using foam; however, it is heavier than epoxy, and should not be used if stability is an issue.

“Impossible” is much too strong of a word. Foamed in fins get knocked out and when they do it’s a chore to repair them.
If the fin tab is a large area then foaming a fin can can add rigidity beyond what epoxy fillets alone can accomplish and could even be used instead of internal fillets, but if the fin tabs are small, such as in any rocket where the body tube and the motor mount tube are close in size, epoxy internal fillets are much stronger. Unfortunately those rockets are the ones that are most difficult to apply internal fillets unless you inject them. Also unfortunate is that the rockets which have large fin tabs will gain the most weight from foam unless fin pockets are part of the design.
Foam is pretty good in compression. It’s not so great in tension.
Personally I’ve stopped using foam anywhere other than in some nosecones if I want to add weight or make the nosecone more impact resistant.
 
I'm sure there are 100 threads on fillet v. foam but the OP seemed to have already made the decision to foam.
 
“Impossible” is much too strong of a word. Foamed in fins get knocked out and when they do it’s a chore to repair them.
If the fin tab is a large area then foaming a fin can can add rigidity beyond what epoxy fillets alone can accomplish and could even be used instead of internal fillets, but if the fin tabs are small, such as in any rocket where the body tube and the motor mount tube are close in size, epoxy internal fillets are much stronger. Unfortunately those rockets are the ones that are most difficult to apply internal fillets unless you inject them. Also unfortunate is that the rockets which have large fin tabs will gain the most weight from foam unless fin pockets are part of the design.
Foam is pretty good in compression. It’s not so great in tension.
Personally I’ve stopped using foam anywhere other than in some nosecones if I want to add weight or make the nosecone more impact resistant.

Can't argue with any of that. I guess I used "impossible" like "going to the moon is impossible" ... until somebody did. :grin:
 
Hey guys, thanks for the feedback. I had in my head foam was the way to go on bigger rockets. I'm ok with not using it and just using fillets. I suppose its like which glue is better, right?
 
Hey guys, thanks for the feedback. I had in my head foam was the way to go on bigger rockets. I'm ok with not using it and just using fillets. I suppose its like which glue is better, right?

I have only used it on small rockets where internal fillets were hard to get to but after the 2 that had hard landings, I decided to never use it again. Repair work is a real pain.
 
I do all my left hand fillets with Epoxy, and all my right hand fillets with Yellow carpenter's glue..

So far, no issues..

:D :D

Seriously, though, I've never used foam, nor plan to. Wood & glue (G-10 & epoxy) when done right can make a pretty strong structure on it's own..
 
You can do neither foam nor internal fillets and probably be just fine with this kit. Madcow instructions make no mention of internal reinforcement.

If your rocket hits the ground without main chute, then bad things usually happen to cardboard and plywood, regardless of the method.
 
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