Rarely is a broken fin a result of a glue failure. The two most common failures are the whole fin, with fillet and a layer of paper ripping off, and the fin breaking the actual wood just above the fillet.

IMHO the former is easier to fix. And it's also easier to reglue the fin than to deal with a dorked body tube.

The only good way to prevent BOTH failures is to properly size the fin material in the first place so it won't break from landing impact (thicker wood, or ply instead of balsa, or a bigger chute which increases drift) AND to use TTW/TTM fins so there is no glue joint to pop loose.

The only reason I prefer the epoxies, aside from their strength, is their heat tolerance. Lay a rocket down on its side a few weeks in a warmish shop, and the fins will sag over when using Elmers and certain other aliphatic glues. Titebond II is better here, though. A hot car while travelling/waiting to launch also can wreak havoc if the rockets endure it for a long period.
Fire a Skyhook off with fins attached with Elmers, then check the fins after recovery. The fins are still attached, of course, but where the joints at the BT are pliable, and you can wiggle the fins some because of the heat. Once it cools, the fins will be fine; just don't lay it down on its side while cooling!
My using epoxy has eliminated this problem, for me anyway.
The 'extra weight' claims against epoxy are overblown in sport use, I think. Do your own experiment here: Use those ml measuring cups and an accurate gram scale and compare weights of equal volumes of glues, both wet (uncured) and dry (cured). I think you will see what I mean when you're done.
Properly mixed epoxy failure rates are nil. It only takes a few seconds to measure (good eyeballing) and mix. Thin with rubbing alcohol of at least 70%, if necessary (might be if using Microballons or balsa dust filler).
Seems to work best for me.
BTW, GH laid it out nicely here ,as did Micro., also. They are right on: Do two fillets at a time with the rocket level (horizontal) with 30minute epoxy. Let tack up real good so it has ceased to flow, then roll it over, do the next two (or one, if you prefer). Most of the time, no sanding of the fillets is ever necessary. You'll love the results!
Sorry so long-winded!
Allen

ROTFLMAO!

Epoxy (at least anything you can buy over the counter) heat tolerance is an oxymoron. Even industrial high temperature epoxies I've used that are rated for high heat applications are rated for about 150 degrees. Any one know the casing temperature of a just fired A3-4T? It's higher than 150.

I used to build A SD competition models with 5 minute epoxy fillets. I remember flying one at a Ft Wayne meet, picking the model up by the fin after its flight, and the motor heat had do softened the epoxy that it sagged and bent. The glue seemed more like used chewing gum, or maybe peanut butter. That's when I stopped using epoxy for fillets on minimum diameter models, and stopped using 5 minute epoxy completely.

For heat resistance I'll take a good yellow glue over any over the counter epoxy any day.

Well, all I can say is make two Sky Hooks, one with your yellow glue, and one with 30 minute epoxy mixed properly, and put both in a hot shop, say 95 degrees F. Lay them down sideways and leave them there for a month. Then tell me which one sagged. In my experiments (or errors!!), the yellow glue always failed this test. 90 degree fins ended up about 100 degrees or more.
Also, I don't recall ever seeing too many R/C aircraft built with yellow glue, unless they are quite small. But I will say this: I built all my rubber-powered stuff with yellow glue!
Still, though, to each his (or her) own!
Over the years, epoxy will remain long after the yellow glue has 'brittled out'.
I've been building more than 40 years. See what happens to fin glue joints when you subject yellow glue to 150 degrees heat for a while.
To put this discussion to bed, one would need the MSDS for each product. Can we get those?
Allen

I was going to say this, but Micromeister beat me to it: use a bigger parachute!

But Bob also had the right take - fins popping off are rarely the fault of the adhesive, unless you use something totally out of whack, like rubber cement or Testor's. The only time that I have broken a fin off of a rocket in recent memory was when I had a recovery failure (chute deployed but didn't open) with my Gauchito, and it came in on a flat spin from about 250 feet. The fillets held, but the leverage of the fins against the ground from the hard landing caused three of them to pull off, along with a layer or two of BT-58.

There are a few rockets that are notorious for popping off fins upon landing though; the Estes X-Prize Rubicon is one that immediately comes to mind. Part of the blame for that one seems to be how the instructions describe attaching the plastic fins, and what adhesive is commonly used on them. But fortunately, they seem to be easy to reattach.

It may be true that, as Bob says, it is better that the fin take the stress and pop off than let something else take the full force of the landing, something that would be harder to repair. But if that's the worry, then the fix is to have the rocket make a softer landing by switching to a larger parachute. If conditions at the field make you reluctant to use a bigger chute, because you are afraid of having the rocket drift away, then either use a smaller motor that won't boost it as high, or else don't fly it that day.

(It might be relevant to note, as an aside, that many hobby-level water rockets, or at least the ones that are sold as kits, feature break-away fins that are meant to pop off upon landing. They do this as a shock-absorbing technique.)

Damage from the initial impact of landing (as opposed to damage that occurs if the rocket gets dragged on the ground after it is down) to our type of rockets, at least, is almost always due to either a recovery failure or to choosing too small (or the wrong kind) of a recovery device.

Almost all of us have had something get damaged because, in a freak accident, it happened to strike something hard on or near the ground, and if not for hitting that object, it would have landed undamaged. But those are, by definition, freak, uncommon, incidents.

Sorry - I meant this to be a short post.

Mark \\.

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There are those of us who live in parts of the country where such a scenario would never happen...

Mark \\.

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I did mention not to use 5min epoxy for minimum diameter rockets in my original post...
By the way, JB Weld, which is an "industrial" strength epoxy has a heat resistance to over 600F. I have seen it used to repair 2 and 4 stroke motorcycle exhaust pipes that get really HOT, and it lasts for years. That would be my choice for min-diameter rockets that would be exposed to casing heat. 5-min epoxy has horrid heat resistance; one can get it to soften with a SHORT application of a heat-gun.

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Yes, there is such a thing as NORMAL, if you have to ask what is "NORMAL" , you probably aren't !
Failure may not be an OPTION, but it is ALWAYS a POSSIBILITY.
ALL systems are GO for MAYHEM, CHAOS, TURMOIL, FIASCOS, and HAVOC !

I've seen that product and it made me pick up a pkg of Mighty Putty on clearance thinking it might be useful for fillets. But I haven't got around to trying it yet, has anybody used Mighty Putty for fin fillets?

Mighty Putty - As Seen on TV

I'll second that... I had read about double glue joints in G.Harry Stine's "Handbook of Model Rocketry" years ago (in a pre-BAR age) and thought it was interesting but too much trouble... impatience of youth...

Last two rockets I put together I used double glue joints using Elmer's yellow carpenter's glue, and it worked TERRIFIC!!! It doesn't take a lot longer to do, in fact if you go on and get your fins done and your tube ready and do them first, you can be letting the first layers of glue on the tube lines where the fins are going to go and the fin roots be drying while you're doing the motor mounts or something else like that and not lose ANY time waiting for the first layers to dry... besides, the dry tube and balsa fin root 'suck' the water out of the glue pretty darn quick. Then, a light layer of glue on the fin root for the second pass and stick the fin carefully on the dried pre-glued tube line. Make sure the alignment looks good, hold it perpendicular 20-30 seconds and BAM the glue is already set enough you can let go of the fin and it not 'dink' over. COOL! Almost as fast as CA but MUCH stronger!

Try it... you'll like it! OL JR

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I think that this is exactly the same stuff as plumber's epoxy putty, which has been around for a long time. If this is indeed the case, then it would be way too heavy to use on anything but a high power rocket, and HPR builders have plenty of other, better options.

Mark \\.

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