Pre assembly of mmt and fins ???

[rsbhunter]

I have seen videos of builds that the motor mount and fins were assembled , and the body tube was cut to allow the whole assembly to slide in to the body tube and epoxied. Then the tabs that were folded up to allow the insertion of the assembly, folded down and glued where the cut was. What is everyone's opinion of this technique? It obviously makes fillets from fin roots to mm alot easier, but are there negatives to doing this? Thanks for any input...rsbhunter

 

[bad_idea]

Main con is more work finishing the body after the fins are in. Pro is that it's a lot easier to do fin fillets on the motor tube.

I build most of my TTW-fin rockets this way, but either way works.

 

[kenstarr]

My Der Ed Max was done this way. 75mm motor mount 7" (?) airframe. I used fiberglass cloth on the joints where the fin is attached to the motor mount. Has 3 centering rings as well. It would be hard to get a good joint on the fins after the motor mount is installed
I think the process lends itself well to big rockets with big fins. Smaller stuff not so much
Ken

 

[Art Upton]

Main con is more work finishing the body after the fins are in. Pro is that it's a lot easier to do fin fillets on the motor tube.

I build most of my TTW-fin rockets this way, but either way works.

I finish the body and fin can Before putting the tube over the fin can. Started doing that finishing part late 2003 on a large K project and into 2004 for my 7.5 L3. Also then did it for many 2.56 and 4.0 rockets, then 5.35 and on and on.

 

[bad_idea]

I finish the body and fin can Before putting the tube over the fin can. Started doing that finishing part late 2003 on a large K project and into 2004 for my 7.5 L3. Also then did it for many 2.56 and 4.0 rockets, then 5.35 and on and on.

I meant more the finishing of the gap that had been cut in the body rather than the rest of the finishing. I agree that overall it's a lot simpler to finish a rocket and its fins in separate pieces!

 

[Art Upton]

I meant more the finishing of the gap that had been cut in the body rather than the rest of the finishing. I agree that overall it's a lot simpler to finish a rocket and its fins in separate pieces!

If talking about the gap at the end? The fins to body sealed fine with thin silicone sealer.
I wrapped a Chrome or Black Vinyl strip around the bottom covering the 'slit gap' on a few rockets simply to smooth the 'looks'

On two Supersonic rockets I filled the area thinly with micro balloons as I did the area of the fin to body. and then shot paint over that with a cover over each fin as the paint was shot.

 

[Back_at_it]

The only Con I see in doing this is that you can't really get inside the tube to do a fillet on the inside of the tube where the fin meets the body tube. This isn't really critical but it's something I like to do.

A lot of builders don't like this method as you cannot be off on you fin location or angle or it won't fit in the slots in the tube. Also, there aren't a lot of kits that come with slotted centering rings so you're pretty much left to figure out the fin placement on your own.

Personally, I like building the fin can outside the rocket as it makes it easy to do good strong fillets to the motor tube.

 

[Art Upton]

Personally, I do not have a reason for an inside fillet on the body tube of a rocket whose fins attach to the motor mount. Many will, but I am not one of them.

Back At It Quote: "Personally, I like building the fin can outside the rocket as it makes it easy to do good strong fillets to the motor tube." Yes, I really like the ability to make it strong, and also lite at the same time, not making waste of heavy glue in the rear.

Air does not travel over inside fillets and need a smoothing of the air flow. Fins tend to break right at outside fillets, especially if the Mach Monster rips them off. Also seen when you leave the fillet off on the outside, the Mach Monster rips them off right at the body tube instead; so which is better ?

An inside fillet could/might add more 'dampening' of fiberglass fins on fiberglass rockets wanting to prevent fin flapping at transonic and beyond. However Taper in Tip to Tip fins, making a fin taper in thickness from root to tip like a Nike Fin, really prevents that in fins for supersonic flight.

In real supersonic sounding rockets, the fins mostly are bolted to the airframe/motor, mostly. No fillets, just flanges and bolts. Which is where I was going in my last high power supersonic builds.

I've now sold all those metal fins and parts however in the last 3 years, as I'm not going to be traveling anytime soon again to places I could fly them.

Here is how I determine my fin placement on a pre-slotted tube below. This was to be sure the factory slotted them correctly, as I have been hearing on here, sometimes they are not cut correctly in the last few years?

On tubes I slotted myself, I built the fin can, then put the tube over it to the leading edge of the fins, and marked the spots. Then used angle stock to mark and cut them out.

 

[bjphoenix]

The only Con I see in doing this is that you can't really get inside the tube to do a fillet on the inside of the tube where the fin meets the body tube. This isn't really critical but it's something I like to do.

For mid power I have left the rear centering ring loose. It will have T-nuts in it for motor retention so it is easy enough to pull it out. Put the assembly into the outer tube, put epoxy on the front centering ring from the front of the body tube. I cut the slots myself and I can do a reasonably good job but not perfect. I put thin fillets on the outside with epoxy or titebond, just enough to seal the gaps. Now the fin can is well attached to the body tube. I can take the rear centering ring out and add all the fillets I want on the inside. After that I put the rear centering ring back in and add fillets to it.

On tubes I slotted myself, I built the fin can, then put the tube over it to the leading edge of the fins, and marked the spots. Then used angle stock to mark and cut them out.

That's the way I've done it also.