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MD motor retention - hiding a rod in the fillet?

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ActingLikeAKid

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I'm thinking I saw this technique somewhere here and my search mojo is off tonight.
The idea was something like this: To retain a motor, "hide" a threaded rod in a fin fillet, so that it's completely encased in epoxy. The rod extends maybe 1/4 to 1/2" past the bottom of the rocket, then you just slip a washer onto it to hold on the motor and a nut to hold on the washer.

Am I remembering this right? Is there anything more to it? Would it be a good idea to have >1 rod? How thick a rod would you use?
 

AfterBurners

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I'm thinking I saw this technique somewhere here and my search mojo is off tonight.
The idea was something like this: To retain a motor, "hide" a threaded rod in a fin fillet, so that it's completely encased in epoxy. The rod extends maybe 1/4 to 1/2" past the bottom of the rocket, then you just slip a washer onto it to hold on the motor and a nut to hold on the washer.

Am I remembering this right? Is there anything more to it? Would it be a good idea to have >1 rod? How thick a rod would you use?
I'm assuming it would be an internal fillet?

If it was an external fillet, not sure how that would work? I mean say you have a 4" diameter rocket. How big of a washer would you need to hold the motor down?? I guess that would also depend on the diameter of the motor, but for example if you were using a 38mm, that's about an inch and a half, that is still another 2 and half inches from the OD of the motor tube to the OD of the air frame. Thats a big washer.
 

fyrwrxz

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I've done that and ground the o.d. down on the airstream side and cleared the nozzle side with a rotary bit so it clamps without obstruction on the case rear closure. You really don't need much on a MD bird.
 

ActingLikeAKid

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I've done that and ground the o.d. down on the airstream side and cleared the nozzle side with a rotary bit so it clamps without obstruction on the case rear closure. You really don't need much on a MD bird.
I'm not sure I follow...ground down the OD of what?
 

Salvage-1

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Yup - done a couple like this, hide the allthread in the fillet on one side of the fin, with just enough sticking out that I can put a washer and nut on the end. If / when I can dig out one of the rockets, I will post pic.
 

ActingLikeAKid

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Yup - done a couple like this, hide the allthread in the fillet on one side of the fin, with just enough sticking out that I can put a washer and nut on the end. If / when I can dig out one of the rockets, I will post pic.
Thanks! What diameter motor were you using, and how big an allthread?
I finally found one of the places I'd seen this technique:
http://archive.rocketreviews.com/reviews/all/pml_cirrus.shtml
and they reported that the allthread bent under ejection load.
 

fyrwrxz

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I'm not sure I follow...ground down the OD of what?
Okayyy....I ground down the o.d. of the washer I used on 10-32 allthread on the outside (hanging in the wind) and then with a aluminium nut I measured how far I had to tighten down the nut/washer combo to hold the reload casing in. I relieved the nozzle side so the washer wound up looking like a deformed circlip. The ejection charge could not act with enough energy to deform the allthread as I trimmed off the excess to about 2 threads after the washer was tightened down on the reload rear closure. I have the flu right now and if you really are still confused, I'll try to dig out the rocket tomorrow and post a pic. It's really pretty simple.
 

Salvage-1

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Steve Shannon

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The first time I saw the threaded rod embedded in an exterior fin root or fillet was by a very good rocketeer named John Smolley back during the Rocketry Planet days. I visited him in Denver on my way through one time and he showed it to me. It worked very well. The rod he used was about like a thin bike spoke.
 

dhbarr

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Why not six embedded actual titanium bike spokes?
 

ActingLikeAKid

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Okayyy....I ground down the o.d. of the washer I used on 10-32 allthread on the outside (hanging in the wind) and then with a aluminium nut I measured how far I had to tighten down the nut/washer combo to hold the reload casing in. I relieved the nozzle side so the washer wound up looking like a deformed circlip. The ejection charge could not act with enough energy to deform the allthread as I trimmed off the excess to about 2 threads after the washer was tightened down on the reload rear closure. I have the flu right now and if you really are still confused, I'll try to dig out the rocket tomorrow and post a pic. It's really pretty simple.
Nope, got it, thanks. Feel better!
 

ActingLikeAKid

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Why not six embedded actual titanium bike spokes?
The only reasons I can think of:
1. Even titanium isn't THAT strong at such small dimensions, it might bend.
2. Tightening six tiny nuts to hold in your motor at every launch sounds like a pain.
3. You'd have to be incredibly precise six times to make sure all the rods were perfectly vertical.

I think it could be done but I think I'd go nuts trying to get it right.
 
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