Best Methods for Epoxying Internal Fin Fillets ?

[NateLowrie]

That's why I thought about adding the microballoons. Takes a lot of mass out of the epoxy, and given the area would still be amply strong.

Why take it all the way up? I am only gluing 1/4in strips on mine and just dripping the epoxy down into the slot between the strips. Drop your fin in and you have the root edge and what amounts to normal sized internal fillets. As long as your careful about the epoxy drip your good.

 

[dshmel]

That's why I thought about adding the microballoons. Takes a lot of mass out of the epoxy, and given the area would still be amply strong.

How about adding microballoons and milled FG? Will the milled FG offset the loss of strength from the MB?

 

[CPUTommy]

I contemplated cutting the end of the body which would allow me to remove the whole assembly and make it cleaner, maybe next time. That seems much easier looking back at the time it took me to prep the internal dams. Just extending the fin slot to the end of the tube seems easier.

 

[cerving]

Sounds a bit like what I did for my L3 1/2 Scale Nike Smoke:
View attachment 323899
You leave just enough wriggle room to be able to align the fins when you fit them. That's a 4" motor mount tube there. Worked well. Probably overbuilt in this case. If I did this again I would add lots of microballoons to the epoxy to cut down some of the mass.
Full story here if you are keen for more details: https://forum.ausrocketry.com/viewtopic.php?f=6&t=4612&start=16

Nice picture. The FG dams are nice, but you don't really need them, just about any material will do. The idea is to contain the epoxy anchoring the fins, not to add more structure to the fin can.

 

[FredT]

I've been shying away from massive internal fillets. You want a fillet strong enough so the fins don't fall off in flight (doesn't take much) and to survive to withstand a hard landing, but not so strong that a fin breaks before the fillet gives. It makes for an easier repair.

 

[Banzai88]

I've become a big fan of the upper and lower centering ring fitting the fins perfectly, and injecting West Systems epoxy fillets. Seals well, flows evenly and easily.

 

[CPUTommy]

what happens when you over build your fin can. This is one of my accomplishments..

 

[OverTheTop]

Why take it all the way up?

Seemed like a good idea at the time. Probably still is. This rocket takes all the thrust on the motor mount at the front of the motor mount tube. Those sidewalls participate greatly in that task of transferring the thrust out to the airframe. If you look at the rear "thrust plate" it is very thin (relatively light) and I have designed it so the motor is not pushing on it during boost. So I have IIRC basically three centering rings taking the thrust to the airframe.

How about adding microballoons and milled FG? Will the milled FG offset the loss of strength from the MB?

There is alredy milled F/G at the joints of the dams IIRC. Just have to get the forces to the dams. Given that there was about 0.5mm clearance from the fin tab to the walls, microballoons alone would do the job in this case.

 

[Binder Design]

route out the fin slots all the way to the end of the tube, and build the fin / motor can first, and then insert at the very end. Of course, you still need to fillet the inside of the tube to the fins.

^This^ No need to fillet the inside of the airframe to fins with this method. External is plenty.