How do fins fail?

[WizardOfBoz]

All,

I'm building my first rocket in about 20 years, and those rockets were with my son. So my first rockets were back in ... well, I can't remember.

The modern building techniques and materials (I used Elmer's glue back in the day!) are amazing. Putting in a fin can in properly seems like playing Tetris. Or maybe Jengo. There's a LOT of epoxy used, and it has to be sequenced in deviously clever ways, and supplement with syringes and clothes pins.

As an engineer, a question one must always ask is "Can we do better?". Also, "Is 'better' worth it?" To that end, are there any statistics about how fins fail? It would be useful to know
1) What fraction of fin failures occur during flight, and what fraction occur on landing?
2) What failure modes are observed? For example
a) A fin rips off during powered flight - fully detached
b) A fin joint fails during powered flight but remains attached
c) A fin fractures during powered flight - part fully detached
d) A fin cracks during powered flight - no parts fully detached
I guess e-h) would be the same for landing-induced mishaps
One could subdivide the powered flight questions into a "how?" part: did the failure occur through fin flutter? G-force and drag forces alone? Misalignment of the fin?

If all rocket failures had an after-action report with pics that would be pretty interesting, in the sense of perhaps suggesting better construction modes and materials.

Don't get me wrong: I'm sticking (sorry for the pun) with high-strength epoxy and materials. Just checking to see if Elmer's might still work....

 

[modeltrains]

I only have my personal statistics to offer: in 16 years of flying low power, LPR, balsa, cardboard, plastic, rockets I've had precisely 1 fin failure.
That was on a rocket where I'd sanded an airfoil on one side of each of the fins so as to create a one-sided lift which would impart spin to the rocket.
Upon landing from I think its third flight one fin had partway separated from the body tube in a way that left it essentially 'hinged' along one side of its root.
Fins were glued and filleted with an Elmers type yellow carpenters glue.
So, if I had not deliberately created that spin stress on the fin attachment, there would have been no failure.

For attaching low power balsa fins to cardboard tubes I've used Elemers and Titebond yellow carpenters glue and a white craft glue named Aleen's Tacky.
Low power rockets are the only type I have personal experience with.

 

[lakeroadster]

For wooden fins (balsa, Basswood or plywood) and paper body tubes you can't beat Gorilla Wood Glue or Tightbond II.

Then there are surface mounted, partial through wall, and through wall attachment methods for fins, also using Gorilla Wood Glue or Tightbond II. These methods work for LPR, MPR and low end HPR.

No need for epoxy as long as you are joining two wood based components.

The biggest issue that leads to broken fins is poor surface prep and / or insufficient glue fillet size.

​

 

[Back_at_it]

Most of my flights are low to mid-power with maybe 15 high powder flights mixed in. Over the years, I've had a number of fin failures but by far the largest number of failures happen when landing. Generally the fin cracks or breaks off a piece. I'd say that 95% of all of my failures happen during landing.

I have pushed the envelope a little bit and had a couple of fin failures during powdered flight. Turns out large fins don't like the thrust of F and G motors in small rockets

These days I'm building more and more with epoxy. It's easy to use and I like the fact that It strengthens the materials that it is applied to as it penetrates and soaks into the those materials. Yes, wood glue is stronger than balsa and cardboard but that balsa and cardboard can be made stronger with a little epoxy.

As for wood glues. If you prep the tube properly, it will work just fine for 99% of the people out there. I used Elmers glue throughout my entire childhood building rockets as I didn't know anything else and it worked fine. These days I'm more about flying them than doing repairs so I tend to over build.

 

[mh9162013]

I'm exclusively an LPR guy and almost all of my fin failures occur upon landing. Usually, it starts with a crack forming at the joint between the fin and the main body tube that I don't see after a flight. Then after subsequent flights, the fin partially detaches such that it's being held onto the main body tube with one of the fillets.

I glue my fins with wood glue or white glue. Then use Titebond Q&T for the fillets. I understand I could make attach them better to the main body tube, but I'd rather have a fin break at the joint in a clean way than stay attached too well and get dented or dinged (not easy to fix if the fin is papered balsa).

My other fin failure occured when my Estes Patriot came in ballistic and hit some tree branches on the way down. The fins suffered from dings, as you might expect.

 

[waltr]

The OP's question is very Open Ended since there are huge differences on ways to attach fins.

1- LP wood fins on outside of cardboard BT. The failure mode is typically the cardboard, the outer layer of cardboard rips away. The Glue does nor fail even if using Elmers School glue. Stronger glue will NOT be stronger.

2- MP with wood fins Through the wall (TTW) of cardboard BT. Again any glue is stronger than the Wood and Cardboard. I do like 30minute epoxy for Internal Fillets just because it is easier to work with internally. I use as little as I can so not to put too much weight in the rear of the rocket.

3- MP/HP Fiberglass Fins and BT. Way different than wood/cardboard and must use epoxy.

I have seen only a few fins fail in flight. Most were Rockets kids built with Hot Glue or simply not fitted and bonded properly. On larger, faster rockets I have seen posts here on TRF of failures when pushing well past MACH speeds.

Other wise mostly upon landing fins break not during flight.

 

[Rschub]

I saw a rocket with built up plywood fins with monokote covering. The builder used a K motor.

 

[lakeroadster]

I saw a rocket with built up plywood fins with monokote covering. The builder used a K motor.

We mock, what we don't understand. ​

 

[tsmith1315]

Many a K flight done on plywood fins!

 

[Rschub]

Built up is the key term here, as in 1/2 in sticks glued together making an open framework, then covered in Monokote. You could see light through the larger pieces of debris.

 

[tsmith1315]

Built up is the key term here, as in 1/2 in sticks glued together making an open framework, then covered in Monokote. You could see light through the larger pieces of debris.

 

[drewnickel]

1) What fraction of fin failures occur during flight, and what fraction occur on landing?

This depends on whether you are at a low/mid power or high power launch. For high power flights I'd venture to guess the percentage of shreds vs dislodging or snapping a fin on landing is significantly higher than for low/mid power. In general, the faster you fly, the more likely you are to break fins in-flight vs on the ground (stronger fins are necessary for high speed flight, so they will inherently withstand harder landings). In either case, you are statistically more likely to break a fin on landing than in flight.

2) What failure modes are observed? For example
a) A fin rips off during powered flight - fully detached

For in-flight: If the fin is intact, this is likely due to how the fin was bonded to the airframe. For paper tubes, you may see de-lamination of the outer-most layer of the tube (if the tube is glossy, it likely has a glassine coating which can make bonding epoxy or wood glue less effective). For composite parts, the airframe and fins are stronger than the adhesive they are bonded with. You usually need to scuff the "shiny" coating off of fiberglass/CF parts, the increased surface area from doing this can significantly strengthen the bond. Additionally, you need to de-grease parts before bonding; epoxy will not stick to grease, so you can use isopropyl alcohol or a very light scrub with acetone to accomplish this (acetone will soften epoxy over time if you leave it on the surface).
For a hard landing: this is common with surface mount fins. Depending on the materials being used, the bond could easily be weak enough to fail without either the airframe or fin breaking in the process.

b) A fin joint fails during powered flight but remains attached

I can't recall a time I've seen a fin joint fail and the fin stay attached while in-flight. On a hard landing you may separate the internal and external fillets and have the fin sort of locket into the slot for through-the-wall fins.

c) A fin fractures during powered flight - part fully detached

Fins fracturing during flight can be due to either the static load on the fin (think lift generated by the fin exceeds the strength of the fin), or through aeroelastic flutter/divergence (the fin flexes as aerodynamic forces are applied, and if you get into a resonance where the airspeed and the natural frequency of the fin are coupled, the forces can be significantly amplified). Flutter tends to be the "up and down" or span-wise bending of the wing/fin, while divergence is the twisting of the wing/fin.

d) A fin cracks during powered flight - no parts fully detached

Like the point I made for b, if you have a failure of the fin the odds are high it's going to flex and aerodynamic stresses will do the rest of the work ripping it off. It's fairly common with balsa and other weaker fin materials to crack the fins on landing with a heavy rocket or landing on gravel or hard dirt.

 

[bjphoenix]

Most of my experience is with LPR. I've had several rockets that lost fins on landing, the outer layer of cardboard tears off. I have always used glue fillets, these days I use fairly large glue fillets so even more of the cardboard has to tear off, but I still had one fail recently. I've had some fins split on landing, breaking along a line parallel to the grain. Everything I build these days I paper the fins to make them stronger against that type of failure. I don't doubt that they can fail, it will just be harder.

I have a few mid power rockets all built with plywood fins. All of these use LOC cardboard tubes. A couple have fins surface mounted with large fillets, a couple have fin cans. None of these have had any failures.

I have one high power, a 4" rocket with LOC cardboard tubes and plywood fins constructed with a fin can. This one has flown with H and I motors with no problems.

Fin cans aren't that difficult- put 2 centering rings on the motor mount tube, glue the fins to the motor mount tube between the rings, then take your time adding fillets here. When this is finished slide it up against the rear of the main tube and put pencil marks on where the fins align. Cut slots in the tube at these marks, slide the fin can in. Use small fillets of wood glue to seal the gaps along the slots then add big epoxy fillets. Then add epoxy fillets from centering rings to inside of the tube.

I have seen fins come off of low power rockets in flight, not my rockets, probably they weren't attached well enough relative to their size. Certainly with rockets capable of high velocity fins can be overstressed by flutter.

I'm a structural engineer. I try to build rockets to maximize the strength of the materials. Nobody knows what the real loads are, we only know what materials are in common use and what we normally get away with. With my rockets they normally work well but occasionally a rocket lands just wrong and something breaks so we get to try again.

 

[flyingeagle]

I haven’t experimented yet with an idea I had seen on you tube but going to try it. If it works then no more broken fins, dents or separation from the body tube. The fins will be literally as strong as steel and the same with the fillets. I will just have to see how much weight will be added to the fins first before moving forward. If it works I will post it in here.

 

[Art Upton]

Fin cans aren't that difficult- put 2 centering rings on the motor mount tube, glue the fins to the motor mount tube between the rings, then take your time adding fillets here. When this is finished slide it up against the rear of the main tube and put pencil marks on where the fins align. Cut slots in the tube at these marks, slide the fin can in. Use small fillets of wood glue to seal the gaps along the slots then add big epoxy fillets. Then add epoxy fillets from centering rings to inside of the tube.

^^^ This . It's what's for Dinner.

However I have only put the glue on the inside and only seal the fins to the slots, no outside fillets.
I 205s on Estes 2.5" Cardboard tubes to plywood inside fin can, no outside fillets for example.

 

[Tractionengines]

Your All forgetting one other cause, which I think is the biggest loss of fins for my family. Hanger / Road Rash. Getting them into, and out if storage, loading/unloading the car, etc.

[ Otherwise, landing shock is #1 from your list. ]

 

[Art Upton]

While most others have seen this already, I am posting this for the OP to see some fins removed in real time from on-board "transmitted to ground" video. These are G10 "thick" fins that are mounted to the 4" motor tube with Fiberglass cloth Fillets on the inside Fin Can inside the Body Tube Skin Covering.

You can jump to the 55 sec mark in the video to skip the pre-flight "Banter"



Some CFD guys took the flight accelerometer data and video and proclaimed the flat fins reached a resonance and caused them to do what you see in the video.

 

[sr205347d]

In the category of landings, the failure mode can depend on the surface it lands on and the angle of impact. For example, this landed on a hard surface (gravel parking lot) straight onto the tip of the fin with no torque on the fin-tube joint. The failure wasn't at the body tube joint, but at the tip. If it had come down into soft dirt, or grass, there may not have been any damage at all.



Preventing this kind of damage might involve a sturdier fin material, like 1/4 inch ply instead of 1/8 inch. A fin design that doesn't extend below the body tube might help too. Fortunately, it was a bolt-on fin, and repair did not involve any rocket surgery.



If the rocket comes down flat, it can put torque on the fin-tube joint. Preventing damage might involve choosing a stout material, TTW mounting, and epoxy or carpenters glue, as you mention.

Or, use sacrificial nylon bolts:

 

[lr64]

While most others have seen this already, I am posting this for the OP to see some fins removed in real time from on-board "transmitted to ground" video. These are G10 "thick" fins that are mounted to the 4" motor tube with Fiberglass cloth Fillets on the inside Fin Can inside the Body Tube Skin Covering.

You can jump to the 55 sec mark in the video to skip the pre-flight "Banter"



Some CFD guys took the flight accelerometer data and video and proclaimed the flat fins reached a resonance and caused them to do what you see in the video.

Most of the material in solid fins of dense material is just there to promote flutter and is not doing very much structural work. Hollow fins, with a thicker wall near the root and thinner near the tip, might not have fluttered. They would probably need a good shear web at the thickest part or else some kind of lighter core like foam or end grain balsa. Weight out toward the tip of the fins, especially at the trailing edge, promotes flutter.

I'd have further suggestions, but I'm not up on how flutter at Mach 1 is different from subsonic flutter.

 

[Art Upton]

Most of the material in solid fins of dense material is just there to promote flutter and is not doing very much structural work. Hollow fins, with a thicker wall near the root and thinner near the tip, might not have fluttered. They would probably need a good shear web at the thickest part or else some kind of lighter core like foam or end grain balsa. Weight out toward the tip of the fins, especially at the trailing edge, promotes flutter.

I'd have further suggestions, but I'm not up on how flutter at Mach 1 is different from subsonic flutter.

Yes , a Nike like tapered fin would not have fluttered based on the CFD work a couple of guys did back before the TRF crash. They posted their Analysis here before that with nice animated CFDs images.

 

[lr64]

How thick can you make fins that are going supersonic without significantly increasing drag?

 

[WizardOfBoz]

How thick can you make fins that are going supersonic without significantly increasing drag?

I think that there's a chicken/egg thing here. If you have high drag fins, you are less likely to go supersonic. That said, I remember that the X-15 had a huge, wide vertical tail fin. Hard to figure that.

The original question was prompted by a thought about vector forces on the fins during flight. Those forces should induce the forward fin areas to pull away from the body tube, and the rear portions of the fin to compress the body tube.

 

[ksaves2]

I didn't go through the whole thread but if I recall, if one does through the wall plywood fins, they're better off to withstand flutter. In the case of using plywood fins, if they do the 1/3rd's, 2/3d's and full span lamination with lite fiberglass cloth and epoxy lamination, it will withstand whatever a hobbyist can throw at it.

Ahhhh, it was a PITA to do so I switched to full glassed rockets with stupendously epoxied fins that can still fly today.
Putting three layers of light fiberglass on a 4 plywood finned rocket is a PITA. And..........rotating the glass matte 90 degrees before laying down another layer was another PITA!!!! I have one rocket that can still fly on an I-200 and I can get a bigger research 29mm motors in there to make it go higher as it has glassed fins on a cardboard tube TTW. It flies straight and true.

Glassing plywood fins is appropriate with wood stuff on a TTW (through the wall) and is a PITA which I don't recommend. Switch to full fiberglass fins and tubes if the launchsite allows it. It is the way to go. If one is sticking to "E" and below motors for hobby flying at a local R/C field, they don't have to worry about it.

Going into HPR is a "whole n'other" thing. Cardboard tubes can withstand an "I" motor if designed right. Going beyond that is another thing.

I saw a fellow who flew his own dual deploy rocket on motors below a "K" impulse multiple times successfully. It was 13 times. He put a "K" motor in the mainly cardboard rocket and it disintegrated into confetti on launch! Yes it was a failure but it was soooooo......... funny! We were still laughing when the main chute, many minutes later was coming down with a disconnected rocket ebay and landed onsite. At least he GOT HIS PRICEY ELECTRONICS BACK!!

The rest of the rocket I swear came down as confetti as all the club members went out there to try to pick up all the pieces to put in the garbage bags. Man we were laughing all the way to get the "broken" rocket stuff off the cornfield to be respectful for the landowner who let us fly there.

The flier told me he tested out his electronics later and they were still nominal after the mishap. I'd expect that since the main chute came out and the electronics had a soft landing. There was no rocket weight on the harness and the main chute with ebay came down very many minutes later. Man I wished I had a video camera out there but it was a long time ago. At least it's burned into my brain.

Kurt