Are internal fillets really necessary?

[dhkaiser]

Are internal fillets really necessary?

I have just finished building a Madcow 2.6" FG DX3 SD. There were no references in the instructions to internal fillets. In the next few weeks I will construct a Madcow 2.6" FG Tomach, again no internal fillet instructions. If Madcow does not feel internal fillets are necessary, why should I consider them? Extra peace of mind? Desire to over build?

Surely Madcow would include them if it were required. Seems they will add extra weight for little on the plus side.

 

[dixontj93060]

Look at the physics. It is a leverage thing. Strong bonds at the MMT with the fulcrum at the airframe edge can help guard against unintended hits on the tip, thus keeping the fin in place.

 

[kevinkal]

Great question. My opinion: I've thought about what are the largest loads on the fins and motor tube. Sure a punchy reload will hit hard, but it's not that much relative to the forces/stresses that could be generated from other flight/landing related events.
Fin flutter will give the fin-2-body tube joint a workout. The dynamics and actual alternating stresses are complex.. and thus calculating any meaningful forces would be difficult. Better to build a strong bond and be done with that concern.
Landing a bit fast under canopy on a hard surface with the motor mount/retainer or fin hitting first will put a large shock load through the fin-2-motor tube or fin-2-body tube bonds.
For those reasons alone, I think it's worth it to add the internal fillets. Considerations need to be made for what materials you're working with too.. for instance a cardboard motor tube might fail in compression near the aft end before any of the fin-2-mmt fillets fail. However a CF or FG tube might pass all the stresses right to the fin glue bonds and they'd better be capable to handle it.
It will be interesting to read other thoughts on this.

I like dixontj's picture and point.

 

[RocketFeller]

I prefer to use internal fillets on the MMT and interior of the fin-BT joint and then use relatively small fillets on the exterior fin-BT joint. I don't like the look of huge external fillets and I wonder how much strength they add after discussing the subject with someone much more knowledgeable than myself.

 

[REK]

If your pushing this past Mach I would consider doing internal fillets. I flew a four inch rocket with a three inch motor mount when I was with the school and the fin flutter destroyed the blue tube. I did not do any internal fillets or foaming.

Now this was blue tube, I dont know about fiberglass, but I would still do internal fillets to be safe.


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

 

[NateLowrie]

Look at the physics. It is a leverage thing. Strong bonds at the MMT with the fulcrum at the airframe edge can help guard against unintended hits on the tip, thus keeping the fin in place.

This is exactly right. Those internal fillets are going to help immensely to protect against hard landings and also flutter precisely because the TTW setup acts like a lever around the outer wall pivot point. You will probably do fine on the DX3 just using the epoxy on the base edge, but in the future if you do TTW it would be to your advantage to fillet the internal connection between the fin and motor mount tube. Everyone does internal fillets differently. I prefer epoxy injection, especially when the space is small. Most of the time you leave the back centering ring unglued so you can pull it off after the fins are tacked on by the base edge.

Also, consider putting 1/4" chopped FG or CF into the epoxy when you do those fillets. It helps strengthen the fillet some more. You only need about a dime size pile in a pumps worth of epoxy.

 

[samb]

I like the leverage analogy but I think you can extend that to include the aspect ratio of the fins and the length and depth of the fin tabs in determining whether they are necessary. My own ideas align with what RocketFeller posted.

 

[RocketFeller]

I slot the BT clear out the back and build the fin can prior to sliding it into the BT for most rockets.

 

[MikeyDSlagle]

I slot the BT clear out the back and build the fin can prior to sliding it into the BT for most rockets.

Yep. Best way to do it. You can easily get fillets everywhere you need em.

 

[ksaves2]

Yep. Best way to do it. You can easily get fillets everywhere you need em.

Ummmmmmm, Slotting to the end is the easiest way to do it. The other way is to use a long enough MMT to epoxy the one or two forward CR on the MMT first. Slather the epoxy and slide up the assembly with a press fit aft CR. Gotta make sure the aft end is clear of epoxy 'cause you wanna be able to pop off the aft CR after it cures so you can put on the fins. Tack epoxy them in place and then I use a syringe with tubing to lay an internal bead/fillets on the MMT two at a time. If I get really paranoid, I'll lay fillets against the fin and the internal diameter of the BT for a total of 24 fillets on a four finned rocket. Last step before doing the AeroPack retainer is epoxy the aft CR in place. Yeah, real PITA and really is only practical with larger diameter, 3" or greater tubes. That diagram used by dixontj93060 is a fantastic visual.

If not spending much time at Mach, you might get away with it without a serious internal fillet. A robust fincan leads to resilience on landings though besides resisting the effects of flutter. If not going stupid fast making the rocket strong can lead to a longer life. (Unless you lose it!) Again, look at your materials and be realistic in your desires. Going stupid fast for a stupid long time, you'll need to beef up that can to prevent disassociation. Kurt

 

[EeebeeE]

First to answer your question.

Try this experiment. Take a 1/8" piece of plywood about 6" long and 1" wide. Hold it on the edge of a table so about 4" of the long side hangs out. Put your thumb right at the point where the wood is at the edge of the table. Put your finger on the back edge of the ply. Now pull up the wood hanging over the edge slightly with your other hand and let it snap loose. Notice the vibration. This represents a fin held in place by an outer fillet and the glued fin root on the motor tube. Now...do the same thing again only this time, press your entire hand down on the wood that represents the fin root. Notice how much less the vibration is.

This is the effect of internal fillets. When your fin vibrates in one direction from the airframe outward, there is tendency, although much slighter, for the fin root between the airframe to vibrate the opposite direction because the point where the fin meets the airframe is a fulcrum. The effect of the internal fillet is to stiffen up the internal vibration, and in so doing, you stiffen up the external as well.

Al the advice that has been offered for internal fillets is good, but I personally find this approach to be my favorite.

Instead of gluing in your entire MMT/CR assembly, just glue in the forward CR where it should be, being careful not to block the fin slot. Then just slide in the MMT and the aft CR so everything is in place, but removable. Now tack your fins in place at the airframe. Once the glue has set, slide the aft CR and the MMT back out. You now have a lot more room to build up some very strong internal fillets. Here are photos of my L3 project to give you a better idea of what I mean. This is a Rocketry Warehouse Terminator, which has split fins. One set on the airframe and the other on the tailcone.

 

[DavidMcCann]

Madcow's tabs are pretty short. Would they make it stronger? sure. Do you need it that strong? I don't think so. Put on some fat externals and you should be fine. My screech has 4 trips past the sound barrier. Still got fins. I had tried leaving the back off and using a kabob stick to do internals. It got an ineffective amount of epoxy in there.

Nice and fat-

zoom zoom, still got 'em-

These rockets are going to barely break mach even on 38/1200's. The single deploy, I wouldn't even blink at no internals. The new Tomach, unless it's a 54mm mount and you plan on throwing a 2800 load at it.... I wouldn't lose sleep. Just fill the slots with epoxy, butter the fin root, and push it in.

 

[mpitfield]

I am a believer in purposely overbuilding, within reason, why? Because why not, if you think you are smart enough to build it just the right amount, without any more than required, then I think you are kidding yourself. So who is to say what amount of over-building is too much, not me. I overbuild because I am not setting any records, if I ever try to then I will make an effort to build with "just enough", but with all the rest of my builds I will add a bit more simply because I like to fly my rockets not repair them.

 

[DavidMcCann]

Overbuilding can be good...but...is it effective, or worth the effort? sometimes yes, sometimes no. I see a lot of people dumping epoxy and threaded rod where it's not needed. It may not hurt much in a non-record bird, but it's wasted cost, effort and time.

So internals, if you can easily? oh boy yes do them. I injected my DS and wild man. With the short tabs on the madcows, I don't think it adds enough to be worth the effort, so I just overload the externals.

 

[Swissyhawk]

If they were really necessary, minimum diameter rockets would not work.

 

[H_Rocket]

There is a downside. Break a fin and you got that much more crap to dig out.

 

[plugger]

If they were really necessary, minimum diameter rockets would not work.

Exactly. They are completely unnecessary. The fact that you're going through the wall to an internal motor mount pretty much guarantees you'll stay below Mach 1.5 in most cases. And while all the leverage arguments above are technically correct and come with a pretty picture they're mitigated by the fact that you'd still bond the fin to the motor mount even IF you didn't run internal fillets. So you'll still have two bonded points of contact between the fin and rocket, one on the motor mount, the other on the fillets on the external airframe. So the leverage arguments are made moot by that fact. And if there's enough leverage to separate the two bonds per fin, I'm pretty sure you're rocket is already in a world of hurt.

 

[mpitfield]

If they were really necessary, minimum diameter rockets would not work.

An extreme MD rocket typically has some additional fin reinforcement or a special purpose fincan. It is only at the lower end of the spectrum of MD rockets where it is generally thought that you can "get away" with just sticking the fins on and adding some fillets. However start bumping up the performance and you are going to start ripping them off at a higher rate, let alone popping them off in the case of falling 2 miles with an undersized drogue @ 90 f/s and landing on a hard surface. I have done just that with my 54mm Tomach, picked it up dusted it off and started prepping for the next flight.

I obviously have no way of knowing this, however I have seen lighter rockets fall less height onto a softer surface and pop a fin off. In my case I am convinced that had I not invested the few hours of time, a small amount of epoxy and CF cloth to reinforce my fins, that my fall would have resulted in a repair job.

There were other reasons why I opted to build this way, one of which was I wanted to reinforce the fins because I purposely undersized my main in an effort to get it on the ground faster, so even when my main goes it still comes in at 25-30 f/s. I also wanted the hands on experience for when I really need the know-how and skill-set. There is no sense in trying it for the first time on a rocket that counts, and I plan on building some extreme MD rockets...I may even attempt a Tripoli record.

There are many more reasons why adding fillets or additional reinforcement may be chosen, but the point being "necessary" is subjective and the comment is too general and should have more context to be fair.

Heck maybe someone's reason is just because they want to, and that works for me.

 

[Nathan]

I am a believer in purposely overbuilding, within reason, why? Because why not, if you think you are smart enough to build it just the right amount, without any more than required, then I think you are kidding yourself. So who is to say what amount of over-building is too much, not me. I overbuild because I am not setting any records, if I ever try to then I will make an effort to build with "just enough", but with all the rest of my builds I will add a bit more simply because I like to fly my rockets not repair them.

+1
I figure that too much is better than not enough so my rockets all have big fillets, inside and out. I can fly any of my rockets higher than I want them to go by just using a bigger motor so a few extra ounces of epoxy isn't going to make any difference.

 

[DavidMcCann]

He asked about 2.6" madcow kits. Just going to go ahead and quote myself here, in case you missed it.


If anyone has photos of a 2.6" madcow kit tearing itself apart because it didn't have internal fillets....I'm listening.... until then, I leave you this....again.

Those K1127's pulled 50G's guys.

(said with love people , don't freak out.)

Madcow's tabs are pretty short. Would they make it stronger? sure. Do you need it that strong? I don't think so. Put on some fat externals and you should be fine. My screech has 4 trips past the sound barrier. Still got fins. I had tried leaving the back off and using a kabob stick to do internals. It got an ineffective amount of epoxy in there.

Nice and fat-

zoom zoom, still got 'em-

These rockets are going to barely break mach even on 38/1200's. The single deploy, I wouldn't even blink at no internals. The new Tomach, unless it's a 54mm mount and you plan on throwing a 2800 load at it.... I wouldn't lose sleep. Just fill the slots with epoxy, butter the fin root, and push it in.

 

[plugger]

An extreme MD rocket typically has some additional fin reinforcement or a special purpose fincan. It is only at the lower end of the spectrum of MD rockets where it is generally thought that you can "get away" with just sticking the fins on and adding some fillets. However start bumping up the performance and you are going to start ripping them off at a higher rate, let alone popping them off in the case of falling 2 miles with an undersized drogue @ 90 f/s and landing on a hard surface. I have done just that with my 54mm Tomach, picked it up dusted it off and started prepping for the next flight.

I obviously have no way of knowing this, however I have seen lighter rockets fall less height onto a softer surface and pop a fin off. In my case I am convinced that had I not invested the few hours of time, a small amount of epoxy and CF cloth to reinforce my fins, that my fall would have resulted in a repair job.

There were other reasons why I opted to build this way, one of which was I wanted to reinforce the fins because I purposely undersized my main in an effort to get it on the ground faster, so even when my main goes it still comes in at 25-30 f/s. I also wanted the hands on experience for when I really need the know-how and skill-set. There is no sense in trying it for the first time on a rocket that counts, and I plan on building some extreme MD rockets...I may even attempt a Tripoli record.

There are many more reasons why adding fillets or additional reinforcement may be chosen, but the point being "necessary" is subjective and the comment is too general and should have more context to be fair.

Heck maybe someone's reason is just because they want to, and that works for me.

I find it funny that you speak to the subjectivity of the question and yet comment about an "extreme MD rocket" which I find is a very general term and should itself have more context. Here's an example. Nic Lottering built "A Serpentine Offering" with the main goal to Build a 3" dual deployment minimum diameter rocket that can be used with virtually any commercially available 3" APCP motor. He flew this rocket on both a M1610BL as well as a M1450. I don't have the data in front of me but I'd put money on the fact that the M1450 flight was in the Mach 2+ regime. The fins of this rocket were tacked on and then had a generous fin fillet applied. No layups/tip to tip, no fincans, just a regular old "glue then fillet the fin" approach. And it flew twice without issue.

Here's a thread on the rocket as well as a photo of the airframe with fins bonded and filleted.

https://www.ausrocketry.com/forum/viewtopic.php?f=10&t=3802

If your fins rip off during boost then most likely they're suffering from fin flutter. This is a fault of the fin design, not the bonding and filleting of the fin to the airframe.

 

[noffie79]

He asked about 2.6" madcow kits. Just going to go ahead and quote myself here, in case you missed it.


If anyone has photos of a 2.6" madcow kit tearing itself apart because it didn't have internal fillets....I'm listening.... until then, I leave you this....again.

Those K1127's pulled 50G's guys.

(said with love people , don't freak out.)

This is a tad off topic, but are you using a Möbius camera on that rocket? My shroud from Additive Aerospace looks just like yours. Any links to some videos you can share? I'd love to see them!

 

[DavidMcCann]

This is a tad off topic, but are you using a Möbius camera on that rocket? My shroud from Additive Aerospace looks just like yours. Any links to some videos you can share? I'd love to see them!

pm'd

 

[dhkaiser]

Thank you everyone for the comments. As a new L1 I will fly the DX3 SD well under mach speeds. The Tomach will be flown SD and DD as a friend has a Eggfinder TRS. With a 38mm motor mount the DD flights will probably be near mach 1 but not sure they will reach it.

Before constructing the Tomach I will fly the DX3 which has minimal internal fillets. That will have to wait until next month's Indiana Rocketry launch. After a few flight I will decide whether to do internals on the Tomach.

 

[NateLowrie]

I find it funny that you speak to the subjectivity of the question and yet comment about an "extreme MD rocket" which I find is a very general term and should itself have more context. Here's an example. Nic Lottering built "A Serpentine Offering" with the main goal to Build a 3" dual deployment minimum diameter rocket that can be used with virtually any commercially available 3" APCP motor. He flew this rocket on both a M1610BL as well as a M1450. I don't have the data in front of me but I'd put money on the fact that the M1450 flight was in the Mach 2+ regime. The fins of this rocket were tacked on and then had a generous fin fillet applied. No layups/tip to tip, no fincans, just a regular old "glue then fillet the fin" approach. And it flew twice without issue.

Here's a thread on the rocket as well as a photo of the airframe with fins bonded and filleted.

https://www.ausrocketry.com/forum/viewtopic.php?f=10&t=3802

If your fins rip off during boost then most likely they're suffering from fin flutter. This is a fault of the fin design, not the bonding and filleting of the fin to the airframe.

I agree with your findings. However, it's all about risk mitigation for me. TTW mounting with internal fillets is a best practice that drastically reduces the risk of failure from flutter or hard landing. You don't have to do TTW, but on rockets that are not MD it is a low weight option for dramatically strengthening that area.

On a MD airframe, tip to tip FG can dramatic reduce the risk of failure from flutter or hard landing. Whether or not you "need" it is a design preference of how much risk do you want to eliminate versus how much performance are you looking for. If you want to go after weight savings for performance sake that's great as long as you recognize and account for the reduced safety factor in the airframe.

At the end of the day, there are many ways to achieve strength on the fin can. This thread outlined a few of them and there are more like bolt-on fin cans that we haven't discussed. Determine the goal of your rocket and pick a design that meets your needs.

 

[mpitfield]

I agree with your findings. However, it's all about risk mitigation for me. TTW mounting with internal fillets is a best practice that drastically reduces the risk of failure from flutter or hard landing. You don't have to do TTW, but on rockets that are not MD it is a low weight option for dramatically strengthening that area.

On a MD airframe, tip to tip FG can dramatic reduce the risk of failure from flutter or hard landing. Whether or not you "need" it is a design preference of how much risk do you want to eliminate versus how much performance are you looking for. If you want to go after weight savings for performance sake that's great as long as you recognize and account for the reduced safety factor in the airframe.

At the end of the day, there are many ways to achieve strength on the fin can. This thread outlined a few of them and there are more like bolt-on fin cans that we haven't discussed. Determine the goal of your rocket and pick a design that meets your needs.

Well put Nate. That was essentially my point as well, it is a design choice and there are other benefits to consider beyond will it work without it.

 

[Nick@JET]

Thank you everyone for the comments. As a new L1 I will fly the DX3 SD well under mach speeds. The Tomach will be flown SD and DD as a friend has a Eggfinder TRS. With a 38mm motor mount the DD flights will probably be near mach 1 but not sure they will reach it.

Before constructing the Tomach I will fly the DX3 which has minimal internal fillets. That will have to wait until next month's Indiana Rocketry launch. After a few flight I will decide whether to do internals on the Tomach.

Hm Indiana rocketry launch you say? I may just have to show up for that one! Great field

 

[plugger]

I agree with your findings. However, it's all about risk mitigation for me. TTW mounting with internal fillets is a best practice that drastically reduces the risk of failure from flutter or hard landing. You don't have to do TTW, but on rockets that are not MD it is a low weight option for dramatically strengthening that area.

On a MD airframe, tip to tip FG can dramatic reduce the risk of failure from flutter or hard landing. Whether or not you "need" it is a design preference of how much risk do you want to eliminate versus how much performance are you looking for. If you want to go after weight savings for performance sake that's great as long as you recognize and account for the reduced safety factor in the airframe.

At the end of the day, there are many ways to achieve strength on the fin can. This thread outlined a few of them and there are more like bolt-on fin cans that we haven't discussed. Determine the goal of your rocket and pick a design that meets your needs.

Hard landing I can accept Nate, but using ttw mounting to mitigate flutter is the wrong way to go about resolving that issue imo. If you're suffering from fin flutter then you should revisit your fin design instead of relying on "extra strengthening" from fillets.

Basically, appropriately designed fins (as in, fins that won't be subjected to flutter within the performance envelope of the rocket) and a conservative stability margin are much more important than things like ttw mounting when it comes to mitigating the risk of losing a fin during flight. This goes for all rockets, from fat stumpy ones to MD beasts. Sure, ttw might save a fin from a hard landing. But it might not. Last flight I had on my Talon 2 (which had internal fillets btw) had a fin pop from a hard landing. So it can happen. I reckon I received bonus points as my magnetic apogee detector didn't kick off my nose cone, I was saved by the backup motor eject on the H410 VMAX motor I was flying on. I didn't think those worked... :eyeroll:

Long story short, if you rocket is well designed then internal fin fillets are unnecessary imo. A nice to have? Sure. But not necessary.

 

[watermelonman]

Are internal fillets really necessary?

I have just finished building a Madcow 2.6" FG DX3 SD. There were no references in the instructions to internal fillets. In the next few weeks I will construct a Madcow 2.6" FG Tomach, again no internal fillet instructions. If Madcow does not feel internal fillets are necessary, why should I consider them? Extra peace of mind? Desire to over build?

Surely Madcow would include them if it were required. Seems they will add extra weight for little on the plus side.

Necessary for what? There is practically no rocket and motor combination in that class that would require it for flight forces.

Then again, consider ground forces. There are hard landings, transportation mishaps, and humans knocking them off tables.

 

[CzTeacherMan]

I am a believer in purposely overbuilding, within reason, why? Because why not, if you think you are smart enough to build it just the right amount, without any more than required, then I think you are kidding yourself. So who is to say what amount of over-building is too much, not me. I overbuild because I am not setting any records, if I ever try to then I will make an effort to build with "just enough", but with all the rest of my builds I will add a bit more simply because I like to fly my rockets not repair them.

+1. I like how you said that. "Overbuilt within reason"...
I'm not dumping epoxy by the bucket load, I'm placing a fair amount in places where it makes sense. It might be overkill, but my rockets will all withstand the occasional undesirable flight profile, whatever is. I also know for a fact that my rockets will withstand the biggest, punchiest motors I can fit in them. I also know that my rockets will last as many flights I can get in over the next twenty years or more. Basically, I'll get maximum enjoyment for my dollar.
And if i ever try to set records, I'll build accordingly.