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Basic Training - Motor retention
#1

Basic Training - Motor Retention

I am currently building a LOC/Precision Fantom for my Level 1 Certification attempt in the fall.  John was kind enough to answer some of my early questions, but I thought it would benefit others new to building similar projects to post my questions in the forum.  My current dilemma involves motor retention.  The kit instructions offer the following advice:

"Because of all the different motor combinations available (with varying motor lengths), this kit uses no motor blocks.  Instead, wrap 1/2" wide masking tape around the nozzle end of each motor to a diameter equal to that of the motor mount tube.  This will keep the motor from pushing forward upon ignition.  Friction fit the motor in place by wrapping masking tape around the motor in two places for a snug fit in the motor mount tube.  This will prevent the motor from ejecting rearward upon activation of the ejection charge."

Personally, I've never been comfortable with that method of motor retention - even in low impulse Estes kits.  I see that Aerotech has some nice retention rings, which I might find myself investing in for a Level 2/3 build, but are a little pricey. 

So the question is this:  What would you recommend for motor retention?  Mirror Z-clips have been reccomended as an alternative, but maybe there are other alternatives.  There are a ton of mirror clip products at the local hardware store, many of which might be adapted for this purpose.  Anyone have pictures they can share of their motor retention solutions.

Any advice is appreciated.

Dale
Dale Stoyer - L3
NAR #91256
TRA #13499
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#2
I had the same questions when building my L1 rocket, a LOC IV last year. LOC kits are fine, but don't provide for positive motor retention. I use the method suggested to me by Nat, it is also described in your NAR Member Guide book, page 9. Some motor retainers on the market look real nice & professional, but cost a bundle! The guide book says use 1/4 in. all thread, I've used #8 machine screws. I cut the head off a #8 machine screw (for 29 mm and 38 mm motors), drill a hole just smaller than the screw in the rear centering ring, right next to the motor tube, run a #8-32 tap thru the hole to make some threads, apply some epoxy to the hole. Then I screw the machine screw in until about 9/16 in sticks out past the motor tube. I then epoxy the rest of the mach. screw to the motor tube. When all done, insert the motor, put a #8 nut with washer on it, snug it down (easy!). The motor won't kick out. It's simple, cheap and it works. I like to do it during construction, epoxy the side of the threads inside, also. See the result, picture of my replacement upscale Big Bertha set up with 29 mm motor. David H.
David Haas
NAR #13780
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#3
Another option, similar result to David's design:

Install T-nuts (threaded inserts with a barbed flange on one end) on the back of the rear centering ring (before you build the motor mount, of course!!).  Then you have the option to use a machine screw to attach those screen door clips, or something you've made yourself out of brass strip.

If you go this route:  make 2 points of attachment, at least.  Nothing like having your one retainer fail!

I do like the ease of use of the machined retainers.  Aeropack, Public Missiles, Slimline all carry or are brand names.  Some of the screw on cap types sell just the threaded bits (one per rocket) and the caps interchange, slightly less expensive.  Use JB weld to secure them.

Nat
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#4
I like all of these ideas better than 'wrap it in tape and pray.'  I am leaning towards David's idea, or a variation.  My handicap at the moment is unfamiliarity with reloadable motors.  If I had already bought my motor I could envision retention better.  I like the machined retention caps, but would rather not spend that much on a Level 1 attempt.  When I move up to bigger motors I'll probably splurge for one.
Dale Stoyer - L3
NAR #91256
TRA #13499
Reply
#5
Definitely forget friction fit for anything bigger than Estes. You want that reload case back! Nat's idea excellent too. I've used another variation on my first L2 rocket, a LOC Doorknob kit. I used the tee nuts, just one, but I used a fine tooth hacksaw, clamped the flange in a vise, and cut off about 1/3 the flange. This let me epoxy it close to the motor tube, behind the rear centering ring, inside during construction. Since this kit had a 54 mm mount, I used a #10 tee nut. When done, I put the motor in, and screw a #10 "truss head" machine screw in the tee nut, and snug up to the motor. That works good, too. And there's no screw protruding from the rocket, unless it's prepped for flight. It looks better and doesn't get in the way when the rocket sits around. David H.
David Haas
NAR #13780
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#6
Dave's Bolt system works well, so does tnuts with mirror clips. the only peice of wisdom i would add i spend the extra money on stainless hardware. a winter sitting in storage with spent motor gunk can rust the heck out of the stuff.
Bill Clune L2 | NAR#88583
MARS Club|SRC|METRA|MDRA - Forever
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#7
another 2 cents totally unrelated to motor retention but VERY important, DON'T use the elastic shock cord for any reason. buy a piece of tubular kevlar long enough to go from the mounting location and hang a few inches past the top of the air frame. then attach tubular nylon at least 3 body tubes in length.
Bill Clune L2 | NAR#88583
MARS Club|SRC|METRA|MDRA - Forever
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#8
Thanks, Bill!  I definitely wasn't sold on the elastic either, just not sure what to replace it with.  I'll look into the kevlar/tubular nylon option.  I'm also leaning towards using an eyebolt to attach the nosecone instead of the molded plastic tab on the cone itself.  I'd prefer not to watch the airframe dig a core sample as the molded cone floats over the horizon.  Wink

Dale
Dale Stoyer - L3
NAR #91256
TRA #13499
Reply
#9
  I ended up using Dave's method because of Bill's advice about the stainless hardware.  Not a lot of stainless options available at the hardware stores I checked, but I could pick up some stainless machine screws and cut the heads off.  Not too worried if I can't find stainless washers or nuts, those can be replaced.  Thanks to all for the excellent advice!

Dale
Dale Stoyer - L3
NAR #91256
TRA #13499
Reply
#10
replacing the plastic thingy with an eyebolt is also a good thing, but even with that don't attach the parachute to the nose cone, attach it to the shock cord before the cone and let it hang free.
Bill Clune L2 | NAR#88583
MARS Club|SRC|METRA|MDRA - Forever
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