Ideas for retaining a motor mount adapter on a 3" PML bull puppy

[plkap74]

I built a 3" diameter PML Bull Puppy that has a 38mm motor mount. Because of the boat tail, I think the idea is just to friction fit for motor retention. But I want to be able to fly it with 29mm motors as well (I want to fly it under 2000' on mid power for the field closest to me). There isn't a ton of stability margin on this rocket, so I don't think moving the motor mount tube farther back would have been an option anyway, but its all glued up and the motor mount is flush with the boat tail end. I have a couple motor mount adapters that are meant to work with screw on retainers, is there any way to make those work with this rocket? Not too concerned about looks, but would rather a solution that can be pulled off for flying on 38mm when a bigger field is available.

 

[Steve Shannon]

Can’t you just recess a standard Aeropack 38mm retainer part way into the tailcone? Otherwise I would look at one of the Giant Leap SlimLine internally threaded or snap ring retainers. I’ve used them inside of tailcones.

 

[plkap74]

The tailcone narrows to be touching the motor mount tube at the bottom and they are epoxied together (there is only one centering ring, so this joint at the base of the tailcone is structural).

 

[Steve Shannon]

The tailcone narrows to be touching the motor mount tube at the bottom and they are epoxied together (there is only one centering ring, so this joint at the base of the tailcone is structural).

Open it up just a little so you can use a Giant Leap slimline retainer. The retaining ring is internal so they minimize the OD of the assembly. You could still epoxy the tailcone to the body of the retainer (I added to my previous post.)

 

[conneryc]

This is what I did on mine back in 2001

 

[plkap74]

Ah I should really update this for future people looking for answers. Above post with the bent metal seems like a good solution, I may end doing that also. Did you have any issue with needing to use very short screws in that boat tail material? (mine also has expanding foam between boat tail and motor mount to hold the screws too).

For me I ended up just using tape to make a very tight friction fit for the 38mm motor (got my L1 cert with this rocket on an H238). I then flew it with a 29mm G125, both great flights. I ended up using tape to friction fit the adapter, and then more tape to friction fit the motor into the adapter. It was maybe a bit sketchy, I'm not sure I'd repeat it, but I wasn't able to easily yank the motor out with my fingers and that seems to be good enough. Overall friction fitting has worked well for me in several mid power rockets, but will make sure to think ahead in future builds to include more positive methods for higher power.

Edit: not related to motor retention, but the G125 flight did suffer from chute not fully opening after chute release triggered. I had it set quite low (300 ft) and the parachute is one of those slightly sticky ones from LOC, not sure the material, but I think it just stuck a bit after chute release let go. Ended up with nose cone a couple inches into wet ground, but no damage. I'm going to see about baby powder or something on those sticky chutes, and set the chute release opening higher next time.

 

[KenECoyote]

From Loc directions:



However, I've built one years ago and from what I recall, I left the motor tube extended out enough for an aeropack retainer and it worked great.

 

[KenECoyote]

However, I've built one years ago and from what I recall, I left the motor tube extended out enough for an aeropack retainer and it worked great.

 

[conneryc]

View attachment 617781

I think this is the overall best option for motor retention on boattails.