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Motor Retention

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prowlerguy

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I have my first 29mm rocket, and I have a problem. My son started to build it, and didn't really position the centering rings well. Long story short, I don't have blind nuts installed for clips, and there isn't enough engine mount to put one of the Great Leap type retainers.

I found this unit, but it is a Canadian company that doesn't seem to eager to ship out of the Great White Up.

http://www.coastrocketry.com/Motor Retention.htm

Does anyone stateside sell a similar item, or this item? Or any other suggestions on what I can do?
 

shreadvector

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I'm very good at friction fitting and I used masking tape on the borrowed casing I used for my Level 1. No problem.

For models with a standard motor hook, I use a nylon wire tie to secure it to the clip (or to secure the clip and prevent it from deflecting).

You should be able to find all of the harware parts for your model at a GOOD hardware store. Finding one of those is the difficult part in some towns/cities.

Good luck.
 

AKPilot

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You can use screen door/mirror clips, found at Lowes and Home Depot. They work just fine.
 

AHansom

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You can make someting similar out of a piece of wood or alumminum. I made one out of a piece of scrap 1/8" plywood left over from cutting out a set of fins. I have over a dozen flight on this retainer including my level 1 flight.

motret.JPG
 

MarkII

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How much space do you have on either side of the motor tube? I was able to outfit a 29mm motor mount with positive motor retention (PMR) with only 3/16" of clearance on either side of the tube.

You may not be able to put blind nuts on the back side of the lower centering ring, because it is already glued in, but you can install the mount in "backwards." Simply drill a pair of holes into the rear entering ring on opposite sides of the motor tube, right next to the tube. Then obtain two short lengths of all-thread (threaded rod - 4-40 or 6-32 should be sufficient), about 4" long, and epoxy them into the holes while bonding them to the sides of the motor tube. Use a good quality epoxy, like JB Weld, that can handle the heat. Leave enough projecting out of the centering ring so that there will be about 1/2" to 3/4" of rod extending below the lip of the reload case or the end of the SU motor when it is installed. Slip some suitable retaining clips onto the posts, and fasten them with locknuts.

Screen door window clips will work, but for something so close to the motor, I have used another piece of screen door hardware called a "button." (It doesn't actually look like a button, but rather like a small tab. It is actually part of the latch mechanism, I think. There will be nylon ones and metal ones; pick out a metal one that looks suitable.) You'll find it in the same drawer as the window clips. Another type of clip that I have used for tight spaces is a speed nut. You also have the option of obtaining some brass strip from your local hobby shop and fashioning your own clips that are sized just right for your mount. If you can find them in the right size, you could even use "T"-shaped (or "narrow") blind nuts, screwed onto the posts upside down, with a locknut on behind each one to retain it. The flange of the blind nut would act as your clip. Whichever type of clip you use, always retain it with a locknut, even if it screws onto the post. And also get a nutdriver that is the right size for the locknut; it can be much easier to use than a wrench when you are at the field.

Hope this helps.

MarkII
 

Pem Tech

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Friction fitting is my choice as well, but if you are set on mechanical retention the above posts are right, go to a hardware store for the parts and build your own.
 

hardinlw

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The device pictured is very similar to the older PML (Public Missiles Limited) PMR style retainer. The heart of the system is a brass insert which has machine screw threads on the inside and a coarse thread on the outside. You drill a hole through the centering ring and thread in the insert. On the PML web site, look under the hardware heading. You may be able to find the inserts in a woodworker's supply place or maybe even Home Depot.
 

Bazookadale

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Friction fitting is my choice as well, but if you are set on mechanical retention the above posts are right, go to a hardware store for the parts and build your own.
The local TARC teams are using PML "HAMR" retainers this year. They are lightweight, sleek aluminum rings similar to the "Slimline" retainers. But at $25 a pop too expensive for me - so I went to the local hardware store to see if I could make something similar. I found some PVC pipe fittings that I cut down and made a thread on retainer that should work just fine. Now mine are big,clunky looking things that are heavier than the HAMR retainers but only cost a little over $2 for the set!
 

vdotmatrix

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do you have a picture of that PVC fitting and would you still use this?
 

quickburst

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Cut a piece of coathanger wire. Bend a loop on one end and a hook on the other then secure with a screw.

Very easy, I've done it hundreds of times.
 

Bazookadale

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do you have a picture of that PVC fitting and would you still use this?
I've used the PVC on almost all of my 29mm. Build can be found here. http://www.rocketreviews.com/featured/tip_featured17.shtml
I'm glad Mikeyd posted that link because my camera is out of service right now. The pictures posted on his link are the same as my method except I only use about half the threaded portion. It is big ugly and heavy but then so am I.:roll:
 

Stradivari

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I use positive retention on reload casings, but one of my tricks to friction fitting is to use cheap store brand masking tape, not the nice 3M stuff. The cheap stuff is thinner and is easier to get just the right fit. You can use some brass stock to make Kaplow Klips. I think if you google Kaplow Klips you will find them. It's better to put the blind nuts on the back side of the centering ring, but you can J-B Weld them to the bottom side as well if you want, They should hold fine.

Dwight
 

plano-doug

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I use positive retention on reload casings, but one of my tricks to friction fitting is to use cheap store brand masking tape, not the nice 3M stuff. The cheap stuff is thinner and is easier to get just the right fit. You can use some brass stock to make Kaplow Klips. I think if you google Kaplow Klips you will find them.
I use that trick, too :) The thin does indeed help to get it dialed in just right.

One trick I learned about HPR is to use both friction fit and positive motor retention. In this case, you don't need perfect friction fitting, just enough to keep the case from sliding out. This way, in case of an extreme event, if the motor should move aft enough to break the positive motor retention, the tape will still hold it in.

By the time the retainer gets broken, most of the separating energy will be dissipated, so the tape only has to hold the weight of the motor but not any added g loads.

Plus, with HPR, a separated motor case is pretty scary. So some redundancy is hardly overboard.
...

In addition to brass, I've played around with stainless from the hobby store. Here are some hooks I did:


Doug

.
 
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MarkII

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This is semi-related. I built my Nova and Oso with 24mm mounts and E9-length motor clips. When they made their maiden flights (on C11s with spacers) last year, both rockets fully deployed their 'chutes right at the expected times, but when I retrieved each one, I noticed that the motors were missing. I build my black powder mounts with 1/4" of underhang, so in subsequent flights I wrapped masking tape around the hook and protruding end of the motor. This prevents the hook from being sprung out by the ejection charge recoil and allowing the motor to kick. I had no more incidents of kicked motors after I started doing that. I attributed the successful deployments in the first two flights to just plain luck and I certainly didn't want to tempt fate again.

I didn't fly my Oso again until a couple of weeks ago (on a D12 this time along with the usual spacer ). I loved that flight! It was one of the highest that I have ever made, and again it had perfect deployment and a long, gentle recovery with touchdown just a shortish walk from the pad. But you can probably guess the rest. When I picked it up I discovered that yes indeed, I had forgotten to tape down the hook again and the motor was gone.

This is getting weird. My 24mm Estes motors get kicked out of my rockets if their hooks aren't taped down, yet even then they never fail to push out the laundry. I don't know if it matters, but all recoveries were on Top Flight Recovery standard mil nylon parachutes. I certainly can't complain, yet I feel that my streak of successful deployments despite kicked motors is very strange.
 

MarkII

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Oh, and BTW, I can't believe that as recently as February 2009 I was still describing the technique of gluing all-thread to motor tubes and using them as posts upon which to attach motor retention. (See post #5 above.) I would never recommend doing that now, and I hadn't mentioned it in what I thought was a very long time. Whenever I construct homemade motor retention devices now that use screen door clips, washers, etc., I fasten the clips to the rear centering ring with machine screws that screw into the rocket rather than attaching hard mounting studs that protrude out of the rocket. This sometimes requires a bit more ingenuity, but it's the right way to do it. Bob Kaplow described the correct method to me a couple of years ago or more.
 

vdotmatrix

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All these ideas are pretty cool if you haven't installed the motor mount and becomes problematic in a retro fit.

I still plan to cut a flap in the Quest BIG DOG body tube and installing a threaded rod like in the following link:

http://www.apogeerockets.com/images/Threaded_Engine_Retainer.jpg

Does anyone have any thoughts why this on this would be good or bad?

thanks in advance
 

judo

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Oops. That must have been where I got that idea. There just isn't that much room between the tubes for much else.

3xJav09.jpg


3xJav37.jpg
 

MarkII

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In my Eos and Maverick, I installed "blind nut + machine screw" style motor retainers, but it was work in both cases. Each rocket has an ST-16 (BT-60) lower airframe, and a 29mm motor mount. If you do the math, the difference between the OD of the motor tube (1.22") and the ID of the lower airframe (1.6"), divided by 2, doesn't leave room for very much (about 3/16" on either side of the mount). The smallest blind nuts that any hardware store carries are 4-40 or 6-32. The barrel of a 4-40 blind nut is approximately 3/16" in diameter and the flange is 3/8" wide, so that size won't work. But I obtained 2-56 blind nuts online from Micro Fasteners. They had a barrel width of 1/8" and a flange width of 1/4" and were the smallest blind nuts that I could find anywhere.

With a great deal of care I drilled out a pair of 1/8" holes in a plywood CR- 11560 from Semroc. On either side of each hole was 1/32" of plywood and that was all that kept the ring from snapping in half. I clipped off the prongs of the two blind nuts and gently epoxied them into the holes. I traced another CR-11560 onto a piece of 1/64" plywood, cut that out and drilled a pair of 3/32" holes in it, just large enough for 2-56 machine screws to pass through. I glued that onto the front of the centering ring with the blind nuts to strengthen it. Then I used my Dremel to grind off the excess flange that extended over the inner and outer edges of the ring.

Once the rings were bonded onto their motor tubes and then epoxied into the airframes, they are plenty strong enough for the job. I use 2-56 socket screws and conduit washers or bushings for the actual retention. These resemble steel centering rings, with an OD of 1.5" and an ID of 1.125". I just drill a pair of 3/32" holes in one and fasten it over the end of the 29mm motor, with the nozzle poking through the center hole. When I use my 24mm to 29mm adapter, I slip a smaller bushing with an outer diameter of 1.25" and an inner hole that is 0.875" in diameter under the larger retainer before I screw it down over the end of the adapter. The smaller bushing can't get pushed through the center hole of the larger one, and the 24mm diameter motor can't get pushed through the center hole of the smaller bushing. The 2-56 screws are more than strong enough to hold down the retainers. I just need to be sure to bring a 5/64" hex key with me to launches so that I can install and remove them.

DSCF1029.jpg


DSCF1063.jpg


Holes for PMR.JPG


DSCF1068.jpg
 

bradycros

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Mark II, that's the best post that's been on the TRF in more then a week. Thank you for the useful information.
 

judo

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That's exactly what I was dealing with in the post above yours: a 29mm MMT in an ST-16 airframe. I seriously doubt that I have the skills, resources, or patients to pull off what you did. Hat's off!
 

Hangar 11

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This is the 38mm 3 mounting hole version.... I make a 38mm 4 hole version

2.99 each.....

 
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Handeman

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That's exactly what I was dealing with in the post above yours: a 29mm MMT in an ST-16 airframe. I seriously doubt that I have the skills, resources, or patients to pull off what you did. Hat's off!
Something I did and found quite easy was to embed a piece of paper clip in the fillets of fins. Then use a wire tie to hold the motor in place.

Here one clip is in the fillet and the other has a single layer of glass over it to hold it to the BT. I put a number of kinks in the wire to make sure it couldn't pull out of the fillet, but I think I over did it some. Just a few slight bends should do quite well.
 

qquake2k

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Something I did and found quite easy was to embed a piece of paper clip in the fillets of fins. Then use a wire tie to hold the motor in place.

Here one clip is in the fillet and the other has a single layer of glass over it to hold it to the BT. I put a number of kinks in the wire to make sure it couldn't pull out of the fillet, but I think I over did it some. Just a few slight bends should do quite well.
What is the white plastic thing on the one fin?
 
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