Internal Fillets for MMT Centering Rings?

[DaHabes]

Question. When you build a three centering ring MMT for TTW fins, you can't get an internal fillet on the bottom side of the top centering ring to BT interface, and it's not easy to get one on the top end of the middle centering ring (you could theoretically inject through the fin slots and then push through, but coverage may be spotty and is messy on the fin slots). Does anyone do internal fillets on these two interfaces via holes on the airframe or do you not bother? Building a Madcow Super DX3 for L1 cert, may use for L2 cert with DD, which is why I am asking (bigger motors, more thrust to manage). I'm using the 38mm MMT, not the 54mm. Thanks in advance.

 

[Banzai88]

Most don't bother. I know that I don't, generally. If you use plenty of the right epoxy, you'll form a natural fillet when you push the assembly up in the tube. I mean, you can do what you're proposing, but it makes for a mess and compromises the integrity of the tube. I've taken extra glue on a dowel and gunked it on the ring and tube between where the fin tabs, but I'm not convinced that it's necessary. Then again, I almost always use a 3rd ring to sandwich the fin tabs top and bottom, so I get a VERY rigid fin can.

Binder is famous for selling rockets where the fin/motor mount in made outside and fitted later......and they advertise that, properly built, wood glue is more than enough for the full range of L2 motors.

 

[Bat-mite]

I fillet top and bottom of my top and middle CRs before inserting into the MMT. However, I leave space for the fins to butt flat against the bottom of the middle CR.

My normal order is:

1. Top fillet top CR.
2. Bottom fillet top CR.
3. Top fillet middle CR.
4. Bottom fillet with gaps middle CR.
5. Insert MMT into booster.
6. Top fillet top CR to booster.
7. Bottom fillet middle CR to booster leaving gaps.
8. Insert fins with root epoxy.
9. Internal fillet fins.
10. Insert bottom CR and bottom fillet.
11. External fin fillets.