JB Weld - So, What Did We Decide?

Did we decide that it is OK to use JB weld to attach fins to motor mount tubes or not?
I don't recall.
Not having any of the "stuff I should use" handy, I have the two part Hobby Lobby stuff that I've used up till now, JB Weld, and could get some other 2 part stuff in the double barrel syringe things.
Could always mix a bit of Kevlar pulp and/or chopped carbon into it.

Greg

I'd stick with epoxy for gluing the fins and epoxy mixed with CF, milled fiber or colloidal silca for fin fillets. It's what I've always used and I've yet to have fins come lose due to "softening" of epoxy...and that includes long burn motors. Keep in mind that the NFPA code only permits the outside of the motor case to reach ~ 200 deg F; if it got hotter than this, the motor couldn't be certified. The motor tube, even paper, further dissipates the heat so the temperature of the epoxy of your fin fillets should never approach a temperature where your epoxy begins to melt. If you are wanting to use JB Weld because of a heat concern, I say you don't need to. I am not aware of a single flier who I fly with where this has happened and AFAIK they all (or at least the vast majority) use epoxy, not JB Weld, to attach fins, CRs, etc.

Public Missile LTD recommends JB weld for attaching there HAMR motor retainer as it is a high heat epoxy over other forms of epoxy. I have used JB weld for years on motorcycles for over 35 years including loose valve guides and it has worked well. Now as far as fins I do not know but do not see why it would not work.

You can, however it's a waste of money.

Also, I think you will find the iron powder filler in JBWeld is a Beyatch to sand so don't even consider it for fillets.

Tensile strength on JB weld is comparable to most other epoxies however shear is a bit less.

It sounds to me like it's not so much a case of you-shouldn't-use-JB-Weld as it is JB-Weld-would-be-overkill. I'd imagine if you didn't have anything else handy, you could use it, but doesn't it have a LOT LONGER cure time than most epoxies?

How sandable is cured JB-Weld? I don't think I'd necessarily want to use it for exterior fillets without knowing I could sand it.

(Wow, three replies over a 4 minute period.)

I agree with the others that it is more expensive and for most uses I would use other epoxy. JB weld has the original then they all so have JB Kwik that bonds in 4 minutes and cures in 4 hours.

I keep JB Weld on hand, but I've never had an occasion to use it on rockets. Like all adhesives, JB Weld has it's place where it works better then most anything else. If retainer manufactures recommend it, I would use it, but other then that, I don't use it on rockets except to add some extra strong grip to threaded inserts and other small metal parts I want to stay in place.

As for using JB Weld to attach fins to MMTs, I see that like using 10d nails to put up drywall. They will work, but it's a lot cheaper and easier to use drywall screws which will do just a good of job. That said, if you have one small piece of drywall and only have 10d nails, well......

Not so much worried about the temp.
I have some stuff that I've used the Hobby Lobby stuff for and where I have left the epoxy uncovered, it has turned brown over time.
Not sure what that means.
Not that it is a negative, but pretty sure it is not a positive.
Know what I mean?
As for sandability...
JB Weld?
Not all that sandable as I recall.
JB Weld with Kevlar pulp mixed in?
Jeesh.
But, a Dremel will do the job.

TWRackers said:

It sounds to me like it's not so much a case of you-shouldn't-use-JB-Weld as it is JB-Weld-would-be-overkill. I'd imagine if you didn't have anything else handy, you could use it, but doesn't it have a LOT LONGER cure time than most epoxies?

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The standard stuff...yes, much longer cure. I think about 18 hrs.

How sandable is cured JB-Weld? I don't think I'd necessarily want to use it for exterior fillets without knowing I could sand it.

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It is, but it ain't easy. Most who use it use it for the fin-MMT attachment and internal fillets, not external fillets. I wouldn't recommend using the standard epoxy fillers with it if you intend to sand.

JB weld has the original then they all so have JB Kwik that bonds in 4 minutes and cures in 4 hours.

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Do NOT, under any circumstance, use the Quick JB for attaching the HAMMR or similar retainer; those who have used the quick stuff have lost motor cases because the bonding is not as strong or as temperature resistant as the standard, long cure. The motor retainer IS directly exposed to the exhaust gases from the motor, hence the reason for JB required for attaching retainers; fins are not exposed to this same heat.

I find it to be a waste of money for fin attachment since you really are not reaping any benefit from using and it is significantly more expensive than standard epoxies.

BRAVO...as for your "miscolored" JB, I'm not sure what exactly has occurred (probably some form of oxidation) or how it will affect performance. When it doubt, chuck it.

I don't have any problem using JB Weld for external fillets. These fillets came out this smooth without any sanding. Proper techniques for applying is essential. I can sand it just fine whenever I have to as well.

View attachment IMG_1421.JPG

MarkM said:

BRAVO...as for your "miscolored" JB, I'm not sure what exactly has occurred (probably some form of oxidation) or how it will affect performance. When it doubt, chuck it.

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Not the JB Weld that discolored.
It was the stuff from Hobby Lobby, which I suspect is probably about the same as any 5 or 30 minute, two part, one to one hobby stuff.

Not that I really need to worry about it for a while.
I still need to cut four rings and I use the router and Jasper guides.
With the heat and humidity right now, the dust that'd be created would stick to sweat like feathers to tar.
Would not be good.

As a back-up visual reference, any one remember the Cheers episode where Woody was telling about bailing hey bareback out in the summer sun and how the chaff and the sweat would mix with the back hair into a nasty nest.
And it turned out he was talking about his grandmother.

MarkM said:

Do NOT, under any circumstance, use the Quick JB for attaching the HAMMR or similar retainer; those who have used the quick stuff have lost motor cases because the bonding is not as strong or as temperature resistant as the standard, long cure. The motor retainer IS directly exposed to the exhaust gases from the motor, hence the reason for JB required for attaching retainers.

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Yes I agree as the Kwik is only good up to 300 degrees and the original is good up to 500 degrees. To install my HAMR engine retainer I will be using the original JB Weld.

Also, is the 200 deg F in the TRA/NAR rules for motor certification...

1) Applied to when the motor is firing?

2) Any time, including after motor burnout?

I ask because the latent heating of the motor casing may be the thing that saves many certifications. I've had some very hot cases...

After burnout, the bond joint could increase in temperature for quite some time until failure occurs...although by the time it is weakened the rocket is probably not going too incredibly fast.


As a side-note:
For anyone who wants to experience something awesome... mix some JB weld together and then get a magnet and put a blob on the end. It will totally cover the magnet like THE MASK!

Graham Orr said:

As a side-note:
For anyone who wants to experience something awesome... mix some JB weld together and then get a magnet and put a blob on the end. It will totally cover the magnet like THE MASK!

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Tell me you didn't discover this yourself!

Graham Orr said:

Also, is the 200 deg F in the TRA/NAR rules for motor certification...

1) Applied to when the motor is firing?

2) Any time, including after motor burnout?

I ask because the latent heating of the motor casing may be the thing that saves many certifications. I've had some very hot cases...

After burnout, the bond joint could increase in temperature for quite some time until failure occurs...although by the time it is weakened the rocket is probably not going too incredibly fast.

Click to expand...

According to the NAR testing manual (https://www.nar.org/SandT/docs/ST-MotorTestingManual.pdf)
under section 8.6 Thermal Requirements it states:

8.6 Thermal Testing Requirements.
The external casing temperature will be measured on at least one motor of a given type during initial certification testing, to ensure that this temperature does not exceed 200 C during or after firing, as required by NFPA 1125.

It is 200 C, not 200 F.

Personally when I've tested motors I've been able to hold them immediately after firing and even with heat soak from the nozzle I can still hold them. They make nice hand warmers in the winter.

Edward

edwardw said:

8.6 Thermal Testing Requirements.
The external casing temperature will be measured on at least one motor of a given type during initial certification testing, to ensure that this temperature does not exceed 200 C during or after firing, as required by NFPA 1125.

It is 200 C, not 200 F.

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I looked high and low for a copy of NFPA to quote this. Thanks for your find. The point is that it is quite a bit hotter and could potentially weaken standard epoxy. I have no doubt that most motors never reach this temp on the exterior but some could. 200 C is still the temp on the outside of the motor on the other side of the motor tube. But depending on the epoxy used that could be an issue. JB Weld is probably overkill. I still use it on my front and rear centering rings due to my perception that these have higher heat loads than the middle rings.

I do not mount fins with JB Weld becuase with TTW I have multiple bonds. The motor tube bond could soften but by then the flight will have slowed and the outside and inside filets on body should keep things attached.