I'm not surprised that John had already done epoxy test with test data results.
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Testing epoxies for use in minimum diameter fin attachment
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I if we determine that a certain set of epoxies is stronger than the materials being bonded (for the normal fillet size), we can safely say they are all sufficient. So I suggest you stick with the standard fillet you use. (For me, that's a gloved index finger, but some people are more precise about it.)
Of course, it's not clear that force in a single direction is what causes max-Q failures. There has been discussion of "fin flutter" at various times, and the failure mode of epoxy could be affected differently in this mode than the components being bonded.
The one time I've had a fin break in a simple way was when a rocket landed with one fin on a rock and broke off cleanly. In that case, the plywood fin itself broke just outside the fillet line.
I had a spectacular max-Q shred, but it's hard to figure out what failed first in these cases. (My guess was fin flutter started the destruct sequence.)
OTRAG shred on 4 M1314s:
Of course, it's not clear that force in a single direction is what causes max-Q failures. There has been discussion of "fin flutter" at various times, and the failure mode of epoxy could be affected differently in this mode than the components being bonded.
The one time I've had a fin break in a simple way was when a rocket landed with one fin on a rock and broke off cleanly. In that case, the plywood fin itself broke just outside the fillet line.
I had a spectacular max-Q shred, but it's hard to figure out what failed first in these cases. (My guess was fin flutter started the destruct sequence.)
OTRAG shred on 4 M1314s:
Wow John !
That was spectacular !
Maybe your rocket is what Venom was singing about in "Flight of the Hydra" ?
I am afraid if I watch one your video one more time, I might turn to stone
-- kjh
That was spectacular !
Maybe your rocket is what Venom was singing about in "Flight of the Hydra" ?
I am afraid if I watch one your video one more time, I might turn to stone
-- kjh
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I do not. I have done fiberglass fins to cardboard but i have never done fiberglass to fiberglass on a rocket.
I've done it a ton on cars but we always used 3M products when bonding fiberglass to pretty much anything but that stuff is crazy expensive.
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That's huge. It would be only slightly smaller than a BT-5 OD on a BT-50. I suspect you're paying a weight and drag penalty.
Posting the info below with the caveat that it is clearly for subsonic bodies. But I still think we tend to overdo fillets in this hobby to the point they are likely a net loss for aerodynamics in many cases. Most of the time, when we care about performance, it's an MD rocket, and most of those end up well below optimum mass at their basic build weight. This means trying to save weight doesn't really matter for a lot of us, a lot of the time. But I still think that the correct size for fillets is whatever provides the optimum or minimum-plus-margin required structural result.
West Systems' manual says to always make the fillet big enough that a bending failure of the joint happens in the material outside the joint. My response is that likely often means the joint is overbuilt and too heavy. I don't see any a priori reason that it has to happen that way. Also, as an ME with a decent amount of design experience, I look at their example T-joint and think that that overall structure is extremely inefficient, if you're talking about boat hulls or airframes. Unfortunately, when we're attaching fins to an MD airframe, that's kinda the joint design we're stuck with.
I think for the present project, it may be productive to, as you suggest, test a series of fillet radii and see where the failure transfers from the airframe or fin to the fillet itself.
On the example above where the short-fiber tube delaminated, I suspect that failure mode would be more or less unavailable with a woven composite. The best I am able to think it through, the short fibers can't transfer stress beyond their ends, so where the stress is concentrated at the edge of the fillet, the loaded fibers with ends very close to that edge are overloaded and separate from the matrix, initiating a separation/tearout, where a long fiber woven or wound indefinitely far amongst other fibers would do far better at transferring that edge load into the matrix and other fibers.
I'm not a composites specialist, but I am an ME and have played a tiny bit with them in the past. Would be very interested in being told I'm all wrong by someone who actually knows what they're talking about.
https://www.rocketryforum.com/threads/the-whys-of-fillet-radius.124576/#post-2422716
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To T88, I have used it with much success on fiberglass and cardboard rockets. Absolutely fantastic stuff
eggplant
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I can see how the pictures of the failure would lead you to believe that the tube was made of "short fibers", but it was a filament-wound CF tube. As is typical for these tubes, the surface was ground down to smooth, which certainly does result in some fibers being cut. Most of the material removed is excess epoxy, so the fibers should be mostly continuous. I agree that it would be cool to see how a convolute-wound tube made from woven cloth compares.
The other thing I'd highlight is that it is definitely not true that most MD rockets are below optimum mass! If they were, you'd see a lot of these builds incorporating substantial noseweight, but instead you see people going to great lengths to reduce mass through measures like extensive use of composites. Many commercial motors have propellant mass fractions of ~0.5, and you typically see an optimal whole-rocket propellant mass fraction of closer to 0.6. As in, you are already heavier than ideal just from the off-the-shelf motor case and closures, before you add a nosecone or fins.
I think the rest by @eggplant uncovered a tube issue, not a fillet issue.
Eggplant, care to share more details about the tube in the test?
EDIT: posted before reading other responses. But my curiosity stands. Alas, I'm angry at filament wound parts these days, so perhaps I'm biased.
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eggplant
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It was a custom wound tube. I expect that a commercially made one might fare slightly better, but the failure mode would probably be about the same. It is important to note that every chain has a weakest link, that doesn't automatically mean that the chain isn't strong enough for some specific application. For example, the instron was applying ~1500 lb of force to the tips of the two fins here when it finally gave out (~750 lbf each!). If flight loads manage to reach that level, yeah, the tube will probably be the part that breaks. That's only if the rest of the rocket somehow survives whatever crazy AoA would cause a load like that! (Hoping I don't jinx my upcoming flight with that
)My post was not supposed to cast doubt on filament wound tubes specifically, but really just to highlight that it is important to design an experiment such that it actually distinguishes between the samples being tested. It would have been a bummer if OP spent a bunch of time and money putting together all the fillet samples only to have them all fail in the tube or plate at about the same load and not tell us anything about the relative strengths of the different epoxies. I'd love to see the test include fillet radius as a variable as well!
Hi, yes, exactly. Proline for the butt joins, fin root to airframe, and for the fillets. That was 15 years ago, though. Wow, time flies. I hope Proline hasn't changed in that time.
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A few things I would point out as you embark on this effort.
First, love to see this! It is awesome when someone wants to put in the time to experiment and obtain empirical data that can help out the whole team.
I think it has already been mentioned, but you need to distinguish between the strength of the epoxy and the strength of the bond.
The strength of the epoxy can be rated by the manufacturer and is usually given by the tensile strength. Most of the epoxies listed so far should have a tensile strength high enough to ensure the epoxy itself is not the weak link, assuming that temps stay within the TG limits of a given epoxy.
The strength of the bond is harder to quantify. I have not seen many manufacturers attempt to quantify bond strength, although System Three deserves credit for publishing their tested strengths with certain materials. For example, you can see that in the TDS of the T-88 product:
https://www.dropbox.com/s/lo9xul8vsx751dq/T-88 Epoxy Adhesive TDS.pdf?dl=0
Although composites are not listed in the materials, the data does show that, in the maple, polyester and concrete tests, the material gave way before the bond. This was also shown earlier in this thread where the body tube gave way before the bond.
Another property that has not been discussed is the brittleness or, more importantly, the flexibility of the epoxies. Generally, the harder the epoxy, the more brittle it is. This has caused me problems in the past. Brittle fillets tend to crack or even shatter and that, I believe, has led to fins popping off the body tube instead of breaking off in some of my MD builds.
For these reasons, I generally use a different epoxy to bond the fin to the body tube than the epoxy I use for fillets.
For bonding the root of the fin, I switched over to T-88 a while ago. There are better and stronger epoxies, but I think, for the cost, T-88 is a strong epoxy and provide excellent bond strength. Once the fin is bonded to the body tube, I put another small bead of T-88 at the root edge of each fin - basically a very small fillet.
For fillets, I have switched over to a flexible epoxy. I tried very expensive epoxies, like Scotch-Weld 2216, but, at $20 an ounce, it isn't very practical. West Systems G-Flex provides a decent tensile strength, excellent bonding strength and is rated for a 32% tensile elongation flexibility, which is super flexible. Once I switched to G-Flex, I have not had a fillet crack, break or fall off. I have also not had a fin pop off.
The way the OP described the testing environment is useful and I look forward to seeing the results, but additional tests at different temps, using different epoxies for the root bonding vs the fillets, and also a test that pulls the fin straight away from the tube would provide additional useful data.
We want to test the limits of the epoxies in this test, so we are not discussing tip-to-tip composites, but, obviously, the point of tip-to-tip composites is to address some of these short comings of epoxies alone.
As with any glue thread, once you go down the rabbit hole, it is a wild ride in wonderland
First, love to see this! It is awesome when someone wants to put in the time to experiment and obtain empirical data that can help out the whole team.
I think it has already been mentioned, but you need to distinguish between the strength of the epoxy and the strength of the bond.
The strength of the epoxy can be rated by the manufacturer and is usually given by the tensile strength. Most of the epoxies listed so far should have a tensile strength high enough to ensure the epoxy itself is not the weak link, assuming that temps stay within the TG limits of a given epoxy.
The strength of the bond is harder to quantify. I have not seen many manufacturers attempt to quantify bond strength, although System Three deserves credit for publishing their tested strengths with certain materials. For example, you can see that in the TDS of the T-88 product:
https://www.dropbox.com/s/lo9xul8vsx751dq/T-88 Epoxy Adhesive TDS.pdf?dl=0
Although composites are not listed in the materials, the data does show that, in the maple, polyester and concrete tests, the material gave way before the bond. This was also shown earlier in this thread where the body tube gave way before the bond.
Another property that has not been discussed is the brittleness or, more importantly, the flexibility of the epoxies. Generally, the harder the epoxy, the more brittle it is. This has caused me problems in the past. Brittle fillets tend to crack or even shatter and that, I believe, has led to fins popping off the body tube instead of breaking off in some of my MD builds.
For these reasons, I generally use a different epoxy to bond the fin to the body tube than the epoxy I use for fillets.
For bonding the root of the fin, I switched over to T-88 a while ago. There are better and stronger epoxies, but I think, for the cost, T-88 is a strong epoxy and provide excellent bond strength. Once the fin is bonded to the body tube, I put another small bead of T-88 at the root edge of each fin - basically a very small fillet.
For fillets, I have switched over to a flexible epoxy. I tried very expensive epoxies, like Scotch-Weld 2216, but, at $20 an ounce, it isn't very practical. West Systems G-Flex provides a decent tensile strength, excellent bonding strength and is rated for a 32% tensile elongation flexibility, which is super flexible. Once I switched to G-Flex, I have not had a fillet crack, break or fall off. I have also not had a fin pop off.
The way the OP described the testing environment is useful and I look forward to seeing the results, but additional tests at different temps, using different epoxies for the root bonding vs the fillets, and also a test that pulls the fin straight away from the tube would provide additional useful data.
We want to test the limits of the epoxies in this test, so we are not discussing tip-to-tip composites, but, obviously, the point of tip-to-tip composites is to address some of these short comings of epoxies alone.
As with any glue thread, once you go down the rabbit hole, it is a wild ride in wonderland
Thanks for the help cls. I have an old, never-built Mongoose 98 from Curtis and I'll use your method of fin attachment when I build it. Soon I hope.
I know that I've changed a lot over the last 15 years. Things hurt more, and more often...
Hopefully the Proline has aged better than I have.
Take care.
Jim
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