Engine Retention

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tjkopena

Rocketship Games
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Location
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@kuririn, I saw your comments on MMX engine retention in the current wishlist thread over in the General Discussion forum. Do you have a lot of MMX motors pop out? My little MMX fan group has lost extremely few motors. I can only definitively recall two out of dozens of flights the past few months, and one was a downsize of an old glider design for which that was intended behavior. I was really surprised that we don't see more motors ejected but theorized that the ejection charge just isn't strong enough if there's any other way for the pressure to escape. If you see it fairly often, I'm curious if that's a product of slight differences in tube tolerances across manufacturers.
 
Since friction fit or tape wrap is the only method available that's what I've used.
The problem for me isn't motors popping out but the tediousness of cutting little strips of scotch tape to get just the right fit. Too much and it won't go in. And scotch tape works better in my experience than masking tape (too thick). Not to mention removal. So I was looking for an easier way, like a wire hook.
Will be doing some experiments and fabrication.
 
I tried music wire. It's too stiff. I could bend it into the right shape but it doesn't flex to allow mounting of the motor.
Brass and copper wire are too soft. They don't spring back after bending.
I need a wire made of spring steel. Something that holds its' shape while allowing enough flex to insert the motor.
 
I tried music wire. It's too stiff. I could bend it into the right shape but it doesn't flex to allow mounting of the motor.
Brass and copper wire are too soft. They don't spring back after bending.
I need a wire made of spring steel. Something that holds its' shape while allowing enough flex to insert the motor.
Safety wire won’t spring back either. I’m going to try to use the wire from a paper clip.
 
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https://www.mcmaster.com/9495K76/
304 stainless so it shouldn't rust much.
Listed as "bounce back after bending" so should hold it's shape after bent into a hook to retain the motor
just about 1/32" (0.031") diameter so shouldn't add much mass
96ft for $9 plus shipping should give enough supply for all the MMX kits currently built (by everyone)
might have to soften it (heat it red hot and allow to air cool), bend it into shape and then re-heat treat (heat up red hot, quench in oil, heat up to "gold" or straw color and then let cool) I'd try as-is before going this route
 
https://www.mcmaster.com/9495K76/
304 stainless so it shouldn't rust much.
Listed as "bounce back after bending" so should hold it's shape after bent into a hook to retain the motor
just about 1/32" (0.031") diameter so shouldn't add much mass
96ft for $9 plus shipping should give enough supply for all the MMX kits currently built (by everyone)
might have to soften it (heat it red hot and allow to air cool), bend it into shape and then re-heat treat (heat up red hot, quench in oil, heat up to "gold" or straw color and then let cool) I'd try as-is before going this route

If McMaster-Carr ever goes out of business, my rocket days are over. :)
 
Okay, so I had some thin steel wire from ksmetals, purchased from Hobbylinc.
Don't ask me for particulars cuz I cut off the label to open it long ago.
Seems like it's spring steel.
Anyhoo I bent it into a motor retainer with pliers.
It can flex and still retain its' shape.
I made the top bend long enough to go through both sides of the motor tube.
This is to prevent twist/torsion to keep the aft bend centered on the nozzle end.
Then I incorporated it into the Snikk motor mount assembly.
When cut with a wire cutter the end is a chisel point, and the wire is so thin that it's like a needle.
Don't ask me how I know.
So if you do this sand down the cut end with a needle file.
The acid test will be if it holds in the motor when I fly it.
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Okay, so I had some thin steel wire from ksmetals, purchased from Hobbylinc.
Don't ask me for particulars cuz I cut off the label to open it long ago.
Seems like it's spring steel.
Anyhoo I bent it into a motor retainer with pliers.
It can flex and still retain its' shape.
I made the top bend long enough to go through both sides of the motor tube.
This is to prevent twist/torsion to keep the aft bend centered on the nozzle end.
Then I incorporated it into the Snikk motor mount assembly.
When cut with a wire cutter the end is a chisel point, and the wire is so thin that it's like a needle.
Don't ask me how I know.
So if you do this sand down the cut end with a needle file.
The acid test will be if it holds in the motor when I fly it.
View attachment 489479

View attachment 489480
View attachment 489481

Maybe think about snipping the hook part a little shorter? That looks like it's going to interfere with your igniter tip at first glance.
 
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