3 motor cluster retention

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PokerJones

PokerJones

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Are there any new techniques for motor retention on a cluster of 3-38's in a 4" tube? Aero Packs are out because the tubes are too close together. Thinking of just using a blind nut in the center of the MM, then using a machine screw and a washer large enough to cover the corners of all 3 motors. Ideas?
 
You could also add three shared ones on the outside just to be sure. Or just use those, and omit the center one.

Gerald
 
Funny you should ask... I'm working on the same thing.

Here's a poor rocksim 3D rendering. I plan on using a simple aluminum disc that threads over the three motors.

6%20inch%203x38_001_zpsri88k8xt.jpg
 
PokerJones said:
Are there any new techniques for motor retention on a cluster of 3-38's in a 4" tube? Aero Packs are out because the tubes are too close together. Thinking of just using a blind nut in the center of the MM, then using a machine screw and a washer large enough to cover the corners of all 3 motors. Ideas?
Click to expand...
On Polish Thunder III which is 3 54mm motor tubes. I did the blind nut in the center. Worked great.
 
Worsaer said:
Funny you should ask... I'm working on the same thing.

Here's a poor rocksim 3D rendering. I plan on using a simple aluminum disc that threads over the three motors.



That's about what I was thinking Bill. The problem I have is trying to fit a blind nut in the center, I have about 3/8th of an inch to work with between the tubes. I may just run a piece of all thread between the two MM's and use a 1" washer/wingnut to hold the motors. Or I could use 3 blind nuts on the outside of the motor tubes. I'm leaning toward the 1/4" all thread, might even give it a little more strength.

20150702_074305.jpg
Click to expand...
 
I see your point. I'll be going without mmt tubes. The base thrust plate will be bored to fit the motors. In my case it's a 6 inch rocket with a 98 mm motor mount, so there's room to play. Looking at your photo, I'd probably go with three retaining clips.
 
If you don't have room for the blind nut, allthread is your next simplest answer. It only gets in the way of standing the rocket up for display, etc. I have to be careful to not ding up the rod(s) on my cluster rockets. I've had 100% success with this type of retention.
-Ken
 
There are some brass, threaded inserts available that may be smaller outside diameter than the blind nuts. You drill a recommended sized hole and then these inserts self tap into the rear centering ring. I usually use epoxy on them. The center is a machine screw size just like the blind nuts. You can get these anywhere from 2-56 on up to 1/4-20. Maybe even bigger.

Follow this link and you can see what i am taking about. Easier than taking and posting a photo. I've used these for the same thing on 29mm clusters. You don't have the rod sticking out.

https://www.amazon.com/s?ie=UTF8&page=1&rh=n:16310091,k:brass threaded insert
 
JoeG said:
There are some brass, threaded inserts available that may be smaller outside diameter than the blind nuts. You drill a recommended sized hole and then these inserts self tap into the rear centering ring. I usually use epoxy on them. The center is a machine screw size just like the blind nuts. You can get these anywhere from 2-56 on up to 1/4-20. Maybe even bigger.

Follow this link and you can see what i am taking about. Easier than taking and posting a photo. I've used these for the same thing on 29mm clusters. You don't have the rod sticking out.

https://www.amazon.com/s?ie=UTF8&page=1&rh=n:16310091,k:brass threaded insert
Click to expand...

I think that is the winner Joe. I have used the brass inserts in the past and had good luck with them in smaller rockets. Totally forgot about these, I guess that's what old age does to you ;)
 
An excellent option for close proximity clusters is threaded lamp pipe found in the electrical section at Lowes, Menards, etc. Cheap and effective. Put in the middle of your motor mounts and you get motor retention with a single washer and nut. Better yet, buy a long piece and use the open piping to thread airstart matches back to your av-bay (well, not likely to use with at tri-cluster, but nice for even numbers).

$_35.JPG
 
Though it was 18mm in a BT60 tube, I ran a short piece of all thread in the middle and nutted a washer onto that to secure all 3.

Are the motors going to be exposed to the ejection charge forces? If so, I would ad 3 more around the outside. If not, I would go with just the center one.
 
As much as like the idea of a center retaining point for this build there is just not enough room for anything adequate enough in the center to securely hold 3 motors. I ended up going with 3 blind nuts on the outside which should do the job. I like the idea of the lamp thread, I'll keep that in mind for larger builds in the future.

20150707_091156.jpg
 

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