Motor retention for Wildman Mini?

Title says it all. About to start in on building a Wildman Mini kit. I'd love to avoid the weight and expense (OK, mostly the expense) of an Aeropack retainer, but there isn't a ton of room between the motor mount OD and the body tube ID (if my math is right, about 3.5mm; it's downstairs right now and I'm tired.) Wildman's prescribed DIY retention method for their stuff is to glue nuts to the top of the lower CR, but that won't work here. I looked and a 4-40 nut is 1/4" wide, too big for the gap; I'm not sure that I'd feel confident in anything smaller being strong enough to hold the motor on. So that won't work.

Right now, the best idea I have off the top of my head is to - after assembly - VERY carefully drill a hole through the aft CR, (of a size that I could tap with a #8 screw), then get a long #8 set screw (i.e. threaded, but with no head and an indentation for an allen wrench at the top) from McMaster. Drive the screw into the CR, epoxy it in place once it sticks out half an inch or so from the bottom of the rocket, then, to launch, add a washer and a nut to retain the motor.

But even with that idea, I'd have to be damn careful not to get the screw too close to the mmt tube, lest I not be able to get the motor fully inserted.

Yeah, the more I think about it, unless someone has something brilliant, I think I just need to buy a retainer from Wildman.

I've got a "Wildchild" (38mm airframe, 29mm MMT) setup for dual deployment and it's got an Aeropack on it, no problem. Almost flush with the airframe and sure makes the motor retention piece easier. Giant Leap Slimline could also be a more slim and cheaper retention option.

https://www.apogeerockets.com/Build...etainer_38mm?zenid=juau8vcvp5ngfh9e8b7d3n9vu3

Or 29mm version, whichever your rocket needs.

Aeropack is best, but...
Friction fit will work great. The 29mm motors don't have a whole lot of kick, so as long as your nosecones aren't glued on, you're okay.
Otherwise the setup you described should work just fine. The CRs on the minis are hefty, so there's lots of 'grab' for screws.

CzTeacherMan said:

Aeropack is best, but...
Friction fit will work great. The 29mm motors don't have a whole lot of kick, so as long as your nosecones aren't glued on, you're okay.
Otherwise the setup you described should work just fine. The CRs on the minis are hefty, so there's lots of 'grab' for screws.

Click to expand...

With friction, would I just do a wrap of tape around the thrust ring and the bottom of the mmt? Or literally just "stuff it up there and cross my fingers that it doesn't pop out"?

Update: I realized I have the ring for a 29mm Aeropack, so I should be able to just order another body - $13 isn't bad.

If it is close to the same size as a Hurricane 38, I believe it is, I used an Estes retainer on my Hurricane.

ActingLikeAKid said:

With friction, would I just do a wrap of tape around the thrust ring and the bottom of the mmt? Or literally just "stuff it up there and cross my fingers that it doesn't pop out"?

Click to expand...

No, LoL! Standard friction fit is strips of masking tape on the motor until it's tight enough to hold, it's a finesse... Too tight and it'll be a bear to get out, too loose and you lose your case.
Definitely use the Aeropack if you can. It fits like a glove on the minis, the OD of retainer is basically the exact same OD as the airframe, so it's virtually a part of the airframe.

ActingLikeAKid said:

Title says it all. About to start in on building a Wildman Mini kit. I'd love to avoid the weight and expense (OK, mostly the expense) of an Aeropack retainer, but there isn't a ton of room between the motor mount OD and the body tube ID (if my math is right, about 3.5mm; it's downstairs right now and I'm tired.) Wildman's prescribed DIY retention method for their stuff is to glue nuts to the top of the lower CR, but that won't work here. I looked and a 4-40 nut is 1/4" wide, too big for the gap; I'm not sure that I'd feel confident in anything smaller being strong enough to hold the motor on. So that won't work.

Right now, the best idea I have off the top of my head is to - after assembly - VERY carefully drill a hole through the aft CR, (of a size that I could tap with a #8 screw), then get a long #8 set screw (i.e. threaded, but with no head and an indentation for an allen wrench at the top) from McMaster. Drive the screw into the CR, epoxy it in place once it sticks out half an inch or so from the bottom of the rocket, then, to launch, add a washer and a nut to retain the motor.

But even with that idea, I'd have to be damn careful not to get the screw too close to the mmt tube, lest I not be able to get the motor fully inserted.

Yeah, the more I think about it, unless someone has something brilliant, I think I just need to buy a retainer from Wildman.

Click to expand...

If you were to do this, just skip the set screw part and use a Allen socket head bolt and washer. Just screw the bolt in each time...

MikeyDSlagle said:

If it is close to the same size as a Hurricane 38, I believe it is, I used an Estes retainer on my Hurricane.

Click to expand...

I have used an Estes 29mm retainer on a 38mm diameter rocket. The Estes retainers are certainly the most economical commercial retainers on the market. The link below will give you a visual comparison of the Estes and Aerotech retainers. I didn't have an Aeropac retainer at the time I did these photos.

https://www.rocketryforum.com/threads/lpr-mpr-motor-retention-comparison.138476/#post-1660395

Tape retention works pretty well for 29mm. But don't do "friction fit", it's tricky and I've seen motors get spit even when they were very tight. First build up a thrust ring (if the motor doesn't have one) with narrow strips of good ol' sticky tan masking tape. Then wrap a couple of layers of the same sticky tape around the bottom of the MMT and the protruding part of the motor. Do *not* use blue low-tack masking tape, and I'd also avoid vinyl electrical tape. Competition people do this method with mylar tape for everything up through 24mm and it's considered the most reliable lightweight retention.

Something else to try:
Embed a #6 or #8 thread coupler in one of your fillets. Then just use a small spacer, machine screw and washer.
On my Viper I notched two fins to accommodate rod couplers and the fillets pretty much covered them. It would be easy to do it you haven't built the rocket yet. I have (had) pictures in my Viper thread if they are still there, I can produce some of need be. If done right the couplers blend in the fillet really well.

Thanks for all the suggestions!
I had forgotten the threads-in-the-fillet trick, may try that.