Are internal fillets really necessary?

[sharkbait]

This thread has really inspired me. I'm going Minimum Diameter Induction Stabilized (MDIS)

No centering rings, no motor mount tube, and no fins, so there's no way to use fillets, internal, external, or otherwise, fin flutter has been totally eliminated, and impossible to damage a fin on a hard landing.

 

[BDB]

I'm about to build a Binder Design Terrordactyl for my L2, so I'll be in that "sweet spot" of draggy fins + big motor.

Mike claims that wood glue is sufficient, so I'm considering doing just that for much of the build. I'll Rocketpoxy the external fillets because I like the way it works, but I'm not sure what to do for the internals. Mike recommends the modular fin can method, which I did for my Tyrannosaur, but I'm open to other ideas.

Has anyone just used wood glue for internal filets? How about "fin pockets?"

 

[dixontj93060]

I'm about to build a Binder Design Terrordactyl for my L2, so I'll be in that "sweet spot" of draggy fins + big motor.

Mike claims that wood glue is sufficient, so I'm considering doing just that for much of the build. I'll Rocketpoxy the external fillets because I like the way it works, but I'm not sure what to do for the internals. Mike recommends the modular fin can method, which I did for my Tyrannosaur, but I'm open to other ideas.

Has anyone just used wood glue for internal filets? How about "fin pockets?"

I have a Terrordactyl where I used all wood glue for every structural joint. I did as Mike suggests and built the fin can outside of the airframe (https://www.rocketryforum.com/showt...ou-do-rocket-wise-today&p=1551129#post1551129). It certainly had a benefit because the root edge of those fins are so long. When I decide to go all wood glue on a build I use Titebond II for the joint and Titebond Molding/Trim for the fillets. In general, I'm happy with the build. For a 4" rocket with that fin span it is very light. Only issue is the Molding/Trim glue never turns out as smooth as I want it so I end up with more rounds of filling and priming to get the finish I like. On last question, I don't use fin pockets. Somewhat interesting idea, but in my analysis, it just adds weight/bulk where it is not wanted/needed.

 

[BDB]

I have a Terrordactyl where I used all wood glue for every structural joint. I did as Mike suggests and built the fin can outside of the airframe (https://www.rocketryforum.com/showt...ou-do-rocket-wise-today&p=1551129#post1551129). It certainly had a benefit because the root edge of those fins are so long. When I decide to go all wood glue on a build I use Titebond II for the joint and Titebond Molding/Trim for the fillets. In general, I'm happy with the build. For a 4" rocket with that fin span it is very light. Only issue is the Molding/Trim glue never turns out as smooth as I want it so I end up with more rounds of filling and priming to get the finish I like. On last question, I don't use fin pockets. Somewhat interesting idea, but in my analysis, it just adds weight/bulk where it is not wanted/needed.

Thanks for such a detailed answer. The zip ties are a great idea too. That's why I love TRF. I'd be years behind in this hobby it wasn't for the knowledge base here.

Have you launched it on a K yet?

 

[dixontj93060]

Have you launched it on a K yet?

No, still in primer; have my color scheme picked out though.

 

[MikeyDSlagle]

No, still in primer; have my color scheme picked out though.

I know the feeling. It has rained everyday here for... weeks. My level1 bird may fly with primer only.

 

[Peter Olivola]

Thickness isn't really a solution. The single best design feature to reduce fin flutter is contained in the Handbook of Model Rocketry without ever actually saying it: 2:1 root to tip taper. That changes the dynamic frequency of adjacent cross sections of the fin so they don't contribute to amplification. I'm also not a fan of G10 fins. They have almost no internal damping. Did you do any sanding of the external fillets?

But once again, the original "fault" in the example you mention is fin flutter. And given that the best solution for that problem is to design a fin that is actually capable of withstanding the forces of the range of motors you'll be able to fly in the rocket they're attached to. After thinking about this a little more I'm able to recall another example that flies in the face of what's posted above. Specifically, I had a PML AMRAAM 2.1 kit with phenolic that I put a carbon wrap around and then upgraded the mmt from 29mm to 38mm. My fins were bonded to the mmt through the wall and then had generous Proline 5400 epoxy fillets on the outside. I flew it on a J350 and ran into a deployment failure where the NC separated but my chute didn't deploy cleanly. The rocket came in a bit hot and landed with the aft fins and retainer taking the brunt of the hit. One of the four kit provided G10 fins completely sheared along the fillet. So the fin stayed intact and didn't buckle at all along the epoxied points of contact and yet sheared off completely past those points. Moral of the story? I should have had thicker fins given the upgrades and therefore added weight to the rocket.

I find most times it's better to fix design faults instead of over-engineering around them in an attempt to mitigate the issue they present.

 

[mpitfield]

Thickness isn't really a solution. The single best design feature to reduce fin flutter is contained in the Handbook of Model Rocketry without ever actually saying it: 2:1 root to tip taper. That changes the dynamic frequency of adjacent cross sections of the fin so they don't contribute to amplification. I'm also not a fan of G10 fins. They have almost no internal damping. Did you do any sanding of the external fillets?

Peter, I made a comment regarding two-part foaming fincans, from the perspective of being a bad idea when it comes to maintenance. However I did mention that a foamed fincan feels completely different and resonates differently than a hollow fincan. Your comments "contribute to amplification" and "Internal dampening" make me wonder if a foamed fincan, vs. a non-foamed fincan has an appreciable effect on the discussion of fin flutter?

 

[Peter Olivola]

I have no experience with foamed fin cans. Never saw the attraction. Using the centering ring sandwiching the fins technique seems to achieve more than adequate structural integrity. Assuming full internal fillets, it's unlikely anything inside the airframe either contributes to or decreases fin flutter. That's strictly a function of the aerodynamic behavior of the fin, it's cross sectional shape, planform, material and velocity.

Peter, I made a comment regarding two-part foaming fincans, from the perspective of being a bad idea when it comes to maintenance. However I did mention that a foamed fincan feels completely different and resonates differently than a hollow fincan. Your comments "contribute to amplification" and "Internal dampening" make me wonder if a foamed fincan, vs. a non-foamed fincan has an appreciable effect on the discussion of fin flutter?

 

[Bat-mite]

I have no experience with foamed fin cans. Never saw the attraction. Using the centering ring sandwiching the fins technique seems to achieve more than adequate structural integrity. Assuming full internal fillets, it's unlikely anything inside the airframe either contributes to or decreases fin flutter. That's strictly a function of the aerodynamic behavior of the fin, it's cross sectional shape, planform, material and velocity.

The attraction of foaming a fin can, as I see it, is that it makes it physically impossible for the fin to move. An internal fillet can break off, allowing the fin to wiggle; but in a foamed can, there is just no room for wiggling. It isn't very heavy and adds a ton of reinforcement. Just my opinion.

 

[DavidMcCann]

If it breaks an internal fillet, it'll just as easily push foam out of the way...

 

[REK]

The attraction of foaming a fin can, as I see it, is that it makes it physically impossible for the fin to move. An internal fillet can break off, allowing the fin to wiggle; but in a foamed can, there is just no room for wiggling. It isn't very heavy and adds a ton of reinforcement. Just my opinion.

This is what I was trying to get at eariler. That empty hollow cavity you mentioned is what can cause the destruction of the main body tube.

With fiberglass I am sure its not big of a deal and David pretty much proved it to me. However, it is not the same for Blue Tube and Kraft Phenolic.


Alexander Solis - TRA Level 1 - Mariah 54 - CTI-I100 Red Lightning Longburn - 6,345 Feet

 

[mpitfield]

I have no experience with foamed fin cans. Never saw the attraction. Using the centering ring sandwiching the fins technique seems to achieve more than adequate structural integrity. Assuming full internal fillets, it's unlikely anything inside the airframe either contributes to or decreases fin flutter. That's strictly a function of the aerodynamic behavior of the fin, it's cross sectional shape, planform, material and velocity.

What your saying makes sense, however I was speaking to your previous comment "That changes the dynamic frequency of adjacent cross sections of the fin so they don't contribute to amplification. I'm also not a fan of G10 fins. They have almost no internal damping."

Perhaps I am misinterpreting your message. I was wondering if the foam would have any appreciable effect on the amplification and internal dampening you mentioned.

Beyond feeling very solid and indestructible I have noticed an obvious change in the dampening of the fincan if you tap the booster. I do not have two of the same rockets, one with a foamed fincan and the other without to compare, however the two rockets I have with foamed fincans have the same feeling. I would describe it as if you held the top of a booster in your hand and tapped it with say a piece of wood, the un-foamed one would vibrate much longer and what I would describe as a wider range. Whereas the foamed one would be more like a low thud and the vibrations appear to disappear at a noticeably faster rate.

Obviously to anyone with knowledge on this topic this is not my field so forgive my ignorance as I am just trying the relay the information in lay terms, so feel free to jump in.

 

[Bat-mite]

If it breaks an internal fillet, it'll just as easily push foam out of the way...

Push it where?

 

[REK]

Push it where?

Im pretty sure he meant bending, but that is not the case with a foamed fin can, because the foam fills in the cavity and prevents it from bending.

A foamed fin can is like an added coupler that beefs up the section or in the case of a minimum diameter, the motor acts like a beefy coupler.


Alexander Solis - TRA Level 1 - Mariah 54 - CTI-I100 Red Lightning Longburn - 6,345 Feet

 

[DavidMcCann]

Push it where?

push it out of its way. Seen foamed cans come back with no fins before. The stuff will break I'm not saying it doesn't have uses. But any force that would break epoxy fillets, is going to wreck foams day as well.

 

[Banzai88]

I saw an M shread the fin can of an upscale Red Max. It had done Ms before. All three fins departed the airframe at about 800ft agl. Rocketpoxy fin root to motor mount tube joints, full fillets throughout, and foamed fin can. Foam didn't save it.

 

[tomsteve]

If it breaks an internal fillet, it'll just as easily push foam out of the way...

seems the 2 part foam i use also acts as a glue. put a little on 2 pieces of plywood or cardboard and clamp them together and try and pull them apart.
something else i do, which idk if it adds to the structural integeity, is drill holes through the fin where it will be inside the fin can- allowing tbe foam to expand through the fin and make,what i believe to be, an even stronger fin can. i also scuff up the mmt with 80 grit.
does all that make a difference? idk, but seeing my patroit i mentioned earlier afterwards sold me.

where you saw tbe cans come back with no fins, what type of expanding foam was used?

 

[mpitfield]

seems the 2 part foam i use also acts as a glue.

I used the 2 part PML foam, which you can add drops of water to lower the density. The first time I tried to foam my BullPup, I added too many drops, and as a result I went a bit too low on the density. It was still foamed but it was more like a filler than an adhesive and did not bond well to anything. Literally you could poke your finger right into it and rub it off.

Since that initial oops experiment and subsequent clean up of the fincan, I used full strength, and when it is in there it really does act like a solid mass and bonds quite well to the everything it touches.

It is because of how well it bonds that I am not likely to use it again in the fincan unless I can find a reason. When I repaired/modified the two rockets I used it on I did what I could to remove some by using a dremel and a drill with long bit, but about 80% of it is still in there. As far as acting like an adhesive, at full strength it bonds well, but add too many drops and you may as well not use it.

 

[jderimig]

Perhaps I am misinterpreting your message. I was wondering if the foam would have any appreciable effect on the amplification and internal dampening you mentioned.

Beyond feeling very solid and indestructible I have noticed an obvious change in the dampening of the fincan if you tap the booster. I do not have two of the same rockets, one with a foamed fincan and the other without to compare, however the two rockets I have with foamed fincans have the same feeling. I would describe it as if you held the top of a booster in your hand and tapped it with say a piece of wood, the un-foamed one would vibrate much longer and what I would describe as a wider range. Whereas the foamed one would be more like a low thud and the vibrations appear to disappear at a noticeably faster rate.

The foamed fin can would dissipate energy in a fluttering fin somewhat by transmitting some of the energy through the fin into the energy absorbing polyurethane (also popular for sound damping)

However that is a relatively inefficient way of dissipating the fin energy. If you have a fin geometry that is prone to flutter a more effective and easier way to add some damping is to laminate a layer or 2 of Kevlar cloth to the fins.

 

[ksaves2]

The foamed fin can would dissipate energy in a fluttering fin somewhat by transmitting some of the energy through the fin into the energy absorbing polyurethane (also popular for sound damping)

However that is a relatively inefficient way of dissipating the fin energy. If you have a fin geometry that is prone to flutter a more effective and easier way to add some damping is to laminate a layer or 2 of Kevlar cloth to the fins.

Yeah, I heard about the "rule of thirds" which in reality is a labor intensive thing to do. It goes like this: Layup a piece of fiberglass or whatever cloth 1/3 up the finspan. Next, lay a piece 2/3rds up the span and lastly a final layer tip to tip. For a four finned rocket that would be 12 pieces to cut! I have an ongoing 4" LOC tubed project I glassed the tubes and then did the above on the 4 fins over the 1/4" plywood fins. Laid a final layer of .6oz cloth to close up the weave to make it look nicer.
The internal and external fillets (applied before laying the cloth of course) were Duralco 4525B. It was a real PITA to do and would likely be way overkill on a glass rocket unless the glass rocket was really being pushed to hyper-speeds
or the fins were surface mount. Mine are TTW by necessity of being glass and plywood. It's appropriate for a 4" cardboard plywood rocket that's going to fly on at least a 54mm L1400.

I made a 38mm MD rocket with 4mm thick 5 ply plywood fins on a LOC cardboard tube. Four fins, 12 pieces of 2oz glass on the fincan as described above. Stupidhead here flew off a dirty rail with a Mayhem Rocketry rail guide. My fault entirely as it bound on the rail and the rocket blew threw the (early version) rail guide like it was nothing. Ran arrow straight, to 8600', 950mph as "told to" by the EggFinder TRS. Both halves of the guide were destroyed and I was totally shocked there were only a few chips out of the paint on the leading edges of two of the fins with no wood damage. Whoa, that build technique is strong. Would probably gone higher if stupidhead used a clean rail!
I'm actually glad I did that stupid mistake as it showed me first hand the integrity of that build technique. Yeah, don't get me wrong. I think that's a way overkill technique with a fiberglass/CF rocket and G10.

I foamed a fincan on a Wildman Punisher Sport (the small one for 29mm motors) with full strength PML stuff for fun. I forgot I just sealed the slots in the centering ring for the Wildman type internal bridle with masking tape and a little foam leaked through. Inside the parachute bay is o.k. but the fincan does seem more solid. I don't care about the weight as I'm running an EggFinder in the nosecone. An H250 is easy if it doesn't CATO on me again like it did in another rocket but if I can
extend a delay by adding a little bit to the long delay to get close to 15-16 seconds, an I200 is possible. Using a Jolly Logic Chute release for pseudo dual deploy. Kurt

 

[Peter Olivola]

If I understand you correctly, you're evaluating the effect of fin can foaming on the basis of taping the airframe and getting a different sound. That's a bit like bouncing on the fender of a car to evaluate the shocks. Fin flutter is better imagined as the behavior of a clarinet or air horn reed and is occurring at a much higher frequency than would be induced by tapping the airframe. In any event, a basic fin/airframe external fillet isolates the external fin flutter from the portion of the fin within the airframe (with the caveat that some energy is being imparted to the airframe, internal fin portion, motor tube and centering rings.) Root to tip taper is most useful in damping flutter of the external portion of the fin.

Planform is a tricky one. Fins are more efficient in span than chord. However, fin flutter issues increase with span. So the span/chord ratio becomes a very big compromising issue. In high power rocketry, there has been a tendency to reduce span and increase chord. It looks like it should be a better design. I keep looking at sounding rockets and note the high frequency of 2:1 clipped delta designs and I ask myself, if real rocket scientists think that's a good way to go, why reinvent the wheel? So it's back to choice of materials and cross section.

What your saying makes sense, however I was speaking to your previous comment "That changes the dynamic frequency of adjacent cross sections of the fin so they don't contribute to amplification. I'm also not a fan of G10 fins. They have almost no internal damping."

Perhaps I am misinterpreting your message. I was wondering if the foam would have any appreciable effect on the amplification and internal dampening you mentioned.

Beyond feeling very solid and indestructible I have noticed an obvious change in the dampening of the fincan if you tap the booster. I do not have two of the same rockets, one with a foamed fincan and the other without to compare, however the two rockets I have with foamed fincans have the same feeling. I would describe it as if you held the top of a booster in your hand and tapped it with say a piece of wood, the un-foamed one would vibrate much longer and what I would describe as a wider range. Whereas the foamed one would be more like a low thud and the vibrations appear to disappear at a noticeably faster rate.

Obviously to anyone with knowledge on this topic this is not my field so forgive my ignorance as I am just trying the relay the information in lay terms, so feel free to jump in.

 

[mpitfield]

If I understand you correctly, you're evaluating the effect of fin can foaming on the basis of taping the airframe and getting a different sound. That's a bit like bouncing on the fender of a car to evaluate the shocks.

Correct.

I realized it is a crude test and may not translate into anything useful, however when I read your post I wondered if the foam may have a secondary effect, even if it is minimal and inefficient at best.

My take on the feedback is that there may be an effect of using the foam, that you could potentially measure, but it would not be appreciable when it comes to fin flutter, and certainly not a substitute for good fin design.

 

[Bat-mite]

For anyone planning to foam a fin can for the first time, note this: make sure you apply your external fin fillets before foaming. Otherwise the foam will push out through the gaps in the slots and make a mess of the outside of the airframe. Tape is not sufficient to prevent the foam from coming out.

I ... um ... saw someone else do this. Yeah, that's it. :wink:

 

[NateLowrie]

For anyone planning to foam a fin can for the first time, note this: make sure you apply your external fin fillets before foaming. Otherwise the foam will push out through the gaps in the slots and make a mess of the outside of the airframe. Tape is not sufficient to prevent the foam from coming out.

I ... um ... saw someone else do this. Yeah, that's it. :wink:

was this mishap on the Formula 200 or another build?

 

[Bat-mite]

was this mishap on the Formula 200 or another build?

It was my MAC Performance 3" Scorpion. I mean, my friend's!

 

[mpitfield]

It was my MAC Performance 3" Scorpion. I mean, my friend's!

To add to your point.

You just reminded me that I foamed three rockets, my PML Bull Puppy, my 3" Dark Star and my 4" Madcow Tembo. With my Bull Pup and Tembo it also somewhat deformed the boat-tail on the Bull Pup and swelled the body tube on the Tembo.

Point being if the material is flexible, the internal pressure of the expanding foam can distort the material to a degree. The external fillets also help with this as does pouring the foam in batches and allowing it to expand without being trapped, which I had happen on the Darkstar. This was because the void is so relatively narrow so when you pour the foam it can stick to the wall and narrow it as it foams...kind of like narrowing of the arteries.