3D Printed 38mm Threaded Retainers

I'm attempting to make threaded plastic retainers for 38mm motors similar to the estes ones. https://www.hobbylinc.com/htm/est/est9750.htm?source=froogle&gclid=CMCQ1pfMmrwCFUYOOgodPjYAxQ

Since the metal HPR cases are a lot hotter, I incorporated a thin pieces of motor mount into the cap to act as an insulator for the plastic. Initial blowtorch testing indicates the insulator works well.

The current prototype only works with the thin external thrust rings that loki and AMW use. I can make a cap that fits cesaroni cases as well (they all have plastic shoulders?)

I'd like to make the retainer work for aerotech cases too, but I am not sure how hot they get. Because of their shoulder geometry, I can't use the same style of insulator I use for the loki and AMW cases.

I'm looking for any suggestions for improvement as well as a few beta testers to see how this works on an actual rocket.

I'm estimating final costs for the loki/AMW style to be $15/set, with the threaded section that attaches to the tubes to be 2/$10 and additional caps to be $10. Of course, this all assumes it works as expected.

Landru said:

I'm attempting to make threaded plastic retainers for 38mm motors similar to the estes ones. https://www.hobbylinc.com/htm/est/est9750.htm?source=froogle&gclid=CMCQ1pfMmrwCFUYOOgodPjYAxQ

Since the metal HPR cases are a lot hotter, I incorporated a thin pieces of motor mount into the cap to act as an insulator for the plastic. Initial blowtorch testing indicates the insulator works well.

The current prototype only works with the thin external thrust rings that loki and AMW use. I can make a cap that fits cesaroni cases as well (they all have plastic shoulders?)

I'd like to make the retainer work for aerotech cases too, but I am not sure how hot they get. Because of their shoulder geometry, I can't use the same style of insulator I use for the loki and AMW cases.

I'm looking for any suggestions for improvement as well as a few beta testers to see how this works on an actual rocket.

I'm estimating final costs for the loki/AMW style to be $15/set, with the threaded section that attaches to the tubes to be 2/$10 and additional caps to be $10. Of course, this all assumes it works as expected.

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I would be interested if you could get the AT figured out
Nice!!!

Pm sent

codysmith said:

I would be interested if you could get the AT figured out
Nice!!!

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I'll send you one if you will try it our for me in the not too distant future.

Landru said:

I'll send you one if you will try it our for me in the not too distant future.

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I would like to try it out. Though it would be a while before I could. Since I already have my Level 1 build done ( with retainer installed) I can't use it on that rocket. Buuut, to be able to get the motors I want without bugging my mentor, I am trying to get my mom to do a build so she can get her level one. I would love to try it on that!
:handshake:https://www.rocketryforum.com/images/smilies/shakehands.gif

Thanks for sending me one for cti . So if I'm reading right . You think cost would be $15.00 a set ? That's half the price of a areopac. More build 38mm Woohoo

You can make the AT ones work by simply having zero contact between metal and 3d-printed plastic. If the forward end of the thrust ring touches the MMT directly, then there's no immediate thermal path to conduct heat. Likewise, the aft end would be insulated by the bit of motor tube you have incorporated into the design.

CarVac said:

You can make the AT ones work by simply having zero contact between metal and 3d-printed plastic. If the forward end of the thrust ring touches the MMT directly, then there's no immediate thermal path to conduct heat. Likewise, the aft end would be insulated by the bit of motor tube you have incorporated into the design.

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Not entirely. It works to use a small piece of tube in a thrust ring system because the ring is so far up the motor. If it were done on an aerotech motor, for one I think it would have to be considerably longer (~0.25"), and it would need to stick out likely to either the base of the nozzle or beyond, which would increase heat flux considerably.

For AT I'd recommend building a groove into the aft end of the retainer and provide a buna o-ring. They cost $5 for 100, so that cost would be pretty much negligible, and if the customer ever looses it they could just buy a new set of 100 for a couple bucks. Carlo did make the good point that the fore end of the thrust ring sits against the MMT, and the sides don't "need" to touch the retainer, as it can be made extra wide.



Brown: Motor
Silver: O-ring
Black: Retainer


Quick and dirty CAD, but it shows that it can work no problem in theory. Just up to you to apply it to the real world. And it should work fine for snap ring motors, just add the sliver of tubing back in. You could probably step it somehow.

In any case it's late, and I have some machining to do tomorrow

I would also use one if you could make it work for aerotech cases. Is there anyway to print it using different colors? I would use it on my Junior Cert DX3 build that still needs motor retention.

And one question Landru, what is your max force before the plastic breaks? I ask because the thickness and diameter of the plastic section are both very small, and I would be concerned with a motor ejection that doesn't blow the chute, but instead pushes most of the force against the motor. It would put a lot of energy in to a very thin ring of plastic.

What ever happened with this project? Just curious because I was thinking about something similar for my 38 mm Loki stuff.