Fins/ motor mount assy

[rsbhunter]

 

[bjphoenix]

I have built a number of rockets this way, I think it makes a very strong system. If you surface mount fins there is always the chance that the glue will separate from the tube and the fin will come off. With fins extending to the motor tube the fins would actually have to break to come off.

You have to cut slots in the outer tube for the fins and the design has to be such that the fins extend to or almost to the rear of the tube. I haven't had any problem with cutting the slots, after the fin can assembly is complete I use it to mark where the slots go. It's a lot more steps to do, more fillets to make, etc. The only other negative I've seen is that I can't use my fin jig to mount the fins to the motor mount tube. I think there are a couple of workarounds that would allow me to use the jig so I might try that next time.

 

[rsbhunter]

I under stand the work involved...already made the 3rd centering ring. I thought I would have witness marks for the mmt and centering rings relative to the body tube. I am attaching wood blocks to the inside top of the bottom centering and top centering rings for rail buttons. I figured i would insert the motor mount and centering rings into the bt, align with witness marks, and "tack" the fins to the motor mount tube using a fin guide. When dry, remove fin guide, motor mount tube and epoxy fin roots to tube and centering rings. Then place epoxy in body tube for top cr, insert , shoot epoxy through fin slots in body for top of fin cr, and around inside of bt bottom for aft cr. But, in theory, everything works.....time will tell. rsbhunter. P.S. This is on a Madcow Super DX-3 paper and plywood....bottom of fin slots on bt are maybe 3/8" from bottom of bt.....

 

[bjphoenix]

Another trick if you are using 3CR- don't glue on the rear centering ring yet, just slide it on and tie a string or piece of tape. Slide the whole assembly into the outer tube and epoxy the front CR from the front of the tube. Now slide out the rear ring and epoxy the middle CR with access from the rear of the tube. Then last you can slide the rear CR back in place and epoxy it from the back.

 

[Art Upton]

The build a complete fin can outside of the main body tube is decades old.
Then you cut slots down on the main outside body tube and drop it on the fin can.
Even the LOC Goblin offers in the instructions how to do it similar:
https://cdn.shopify.com/s/files/1/0568/7489/3503/files/GoblinInstructions2020.pdf?v=1623658483

I don't glue that body tube on, only secure it with sealer, it's an AeroSkin only.
Some rockets, including my L3 rocket, I have put 3 or 4 mini screws into a centering ring to hold it in place but able to remove it for repair. One of them can be your rail button screw, that will be done on this one as the fins extend above the top centering ring.

I don't fold any tabs or things over. I don't glue the fins to the outside, only to the motor mount.
I seal the slots with silicone or light yellow glue mini fillet, so I can remove a damaged tube. If you want you can put a silicone or yellow glue sealer on the end centering ring to the body tube as well. Again if body damage you could peel the sealer, glue away. It also makes repairing, replacing a broken fin easier as well when you can remove the body tube.

Hopefully I will commit to finishing this one before Christmas. The fins are being aligned in this picture. You can see the one on the right needs pushed in more to the motor mount tube.

 

[Art Upton]

The only other negative I've seen is that I can't use my fin jig to mount the fins to the motor mount tube. I think there are a couple of workarounds that would allow me to use the jig so I might try that next time.

What kind is your jig? If you put only the fins on the motor mount tube with the jig and then put the centering rings on after they are set, then you could use just about any jig?

But I like the method in my picture above with the large jig I have for centering rings done first, then fins.
I set the centering rings on the jig first so they were very tight on the fin tabs with CA spots, then use epoxy or Gorilla glue on the sides not facing the fin tabs. You can glue that side of the rings after the fins are glued in.

Also, avoid the desire to slather glue all over the fin can, adding weight to the wrong place in the rocket.

 

[rsbhunter]

I have a new printed fin jig coming from HPR tools. I have my centering rings spot ca glued to the mmt . Then i planned on epoxy the fin tabs to the mmt while in the body tube, place the fins into the guide, when set, remove the mmt/fin assy. Put fillets on fin to mmt and fins to centering rings ( smaller fillets) . Then after set, assemble the fin can into the body. Quick question, why are alot of rail buttons sets sold where the button will NOT fit over the t nut shaft? The set i recieved with my PML AGM 2.56, they fit great....others I've had to drill the button out ( LOTS OF FUN) to fit the t nut. But , other than that, the build is going great...rsbhunter

 

[Art Upton]

I don't know why on the T-nuts? I have never used them. Mine either simply go in the tube with an Epoxy rivet on the inside from just gluing them while it dries with the button facing down, or in bigger rockets the screw into a centering ring; even if the screw is larger then the ring. It is just there to 'stabilize' it simply gluing it in.

 

[bjphoenix]

What kind is your jig? If you put only the fins on the motor mount tube with the jig and then put the centering rings on after they are set, then you could use just about any jig?

I looked up photos of the past 2 fin can projects I've built. One was a mid power built around 2019, the other was a Fat Boy clone I made a couple of years ago. These days I use the fin jig I found in old posts of this forum- 2 strips of wood with 45 degree angles cut on one side, rubber bands, etc., but I didn't have that built when I did either of these rockets.

Here is the Fat Boy clone. I couldn't use a jig on this one because the motor retainer hook and the wraps to secure it made the outside of the motor mount tube irregular and it wouldn't register properly in the jig. Barely visible in the photo- I wrap a strip from old body tube around the motor mount then put the hook over it, that way there is more material holding the front part of the hook to resist it coming out of the rocket. Then I wrap another strip of old body tube around the outside to hold the hook in place. This Fat Boy was built to fly on 18mm motors but unfortunately is now hanging in a tall tree next to our launch site.

 

[bjphoenix]

This is a mid power rocket I built in 2019. I reused an old motor mount tube that already had the centering rings on it. I wasn't using a fin jig back in these days but had the idea to make one for this project. I used 2 strips of wood and glued on a couple of small blocks to each one so the centering rings would register between the small blocks and center up on the gap between the boards. It was obviously specific to this particular rocket. I think the main reason I came up with this idea was to help flatten out the plywood fins. I bought the plywood from a hobby shop and it had a bow in it that I couldn't get out no matter what I did. With the fins sandwiched between the 2 boards and glued to the motor mount tube, they stayed flat through the rest of the assembly.

 

[rsbhunter]

Finally ordered a HPR Tools fin guide...took the plunge and epoxied the fins to the mmt by marking where the fins needed to be indexed, then ran a thin bead of 15 min epoxy between the centering rings on the mmt. Inserted the mmt and centering rings assy into the bt.. aligned the 2 , inserted the fins, put on the fin guide. Placed a rubber band tightly around the flats on the fins to hold them tightly against the mmt. Once that set up...I ran fillets on the mmt to fins, fins to centering rings...I am going light on the fillets to keep weight down, yet still get a strong joint....Honestly, any rocket I can assemble this way, I will. It is so much easier....Thanks to all who helped me along this project...rsbhunter