Why is JB Weld needed on Aeropac retainers when not using motor ejection?

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dumbquestions

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Motor ejection could try to push a motor casing out the back, so you want your retainer on pretty good.
What about for larger motors where we're not using motor ejection?
Is there really enough gas pressure in the tube that there is a risk of pushing the motor case out the back?
Or is there other reason(s) to use JB Weld to afix motor retainers?
(if it is a heat issue, why not use a higher temp epoxy--JB Weld is better than BSI, but not particularily high temp.
--
DQ
 

3stoogesrocketry

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It's not always the ejection charge . I've lost a few cases due to excessive whipping around durring drouge deployment. Think about how much your motor case weighs when your shock load hit 40 or 50 gees.
 

Bat-mite

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It's the heat of the motor. Plus that is what AP says to use.
Epoxy softens with heat. A known technique to remove epoxy is to heat it up with a torch or heat gun. JB Weld is supposed to maintain integrity up to something like 600F. It was designed for use in car engines.

If you have standard epoxy on your retainer, it will potentially weaken as the motor gets hot. This could cause it to fall off.

Likely? No. Possible? yes.
 

heada

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If you're not using a motor ejection charge, you're using electronics and still have an ejection charge pushing on the motor, just from above it rather than part of it.
 

Andrew_ASC

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Because the heat transfers from the chemical reaction to the motor casing to the motor mount tube to the Aeropac retainer. I would use an I word... but I’ll refrain from that. It had nothing to do with where the ejection charge is and everything to do with heat transfer and melting temperature of epoxy.
 

JohnCoker

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Epoxy has a spec called the "glass transition temperature" (Tg) at which it loses its stiffness and becomes rubbery. This is relatively low for most room-temperature cure epoxies; for example it's 130℉ for West Systems IIRC. It can be increased with an elevated temperature post-cure, or you can use epoxies with a special formulation (such as JB Weld).
 

prfesser

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If the instructions simply said "glue with epoxy", there would be some who would use anything (5 min epoxy? Carpenter's glue?), leading to possible failure. JB Weld is readily available almost everywhere, and it virtually insures that the epoxy won't be the problem if there is a failure of some sort.

In engineering and surgery, "do it the same way every time" is usually a virtue. :)
 

jimzcatz

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If you're not using a motor ejection charge, you're using electronics and still have an ejection charge pushing on the motor, just from above it rather than part of it.

Definitely NOT true. I build all my DD rockets with a solid bulkhead in front of the longest motor I plan to use.
 

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