Adjustable 75mm MD Motor Retainer

[Scott C]

I have been working on an idea I had and I'm looking for some Suggestions/Advice/Input/ etc etc, good or bad. With my design, you can adjust to the case length you are going to use (without needing additional adapters to use shorter cases), use in multiple rockets, check/replace your shock chord.... Its definitely still in the prototype stage and adjustments are still being made.

So far, there are 2 configurations, Single ER (Expansion Ring) or Dual ER. When tightening the bolt, it will expand the center ER and contact the inner walls thus clamping into place (like rear drum brakes). The eyebolt on the top is either pinned in place or JB welded to a bolt or All-thread... anything to keep it from spinning. I'm still working out any problems with it spinning while tightening. So far in the prototypes I have made, this doesn't seem to be a problem but i know better than to trust that design. As far as materials, I plan on 6065 Aluminum for the end plates and the center ER will be 3D Printed TPU, although the PETG ER on the prototype grips hard enough that smacking the end of the all-thread rod with a 5lbs hammer didn't even move it.

I added some pics of the CAD and a prototype (not the final materials). The bolt used isn't the correct size either but its just a prof of concept at the moment.

Any Input is appreciated and welcome.

 

[pugachu]

I have a similar design that I've used for 54mm MD rockets. It's a modified tube expansion plug from McMaster Carr. It works well.

 

[Scott C]

I have a similar design that I've used for 54mm MD rockets. It's a modified tube expansion plug from McMaster Carr. It works well.

That's kinda what made me designed my own. Just couldn't find one large enough, that i trusted or $$.

 

[pugachu]

this is what I used for the 54mm version, I had to trim the OD on the lathe, but it's very cheap https://www.mcmaster.com/2645K42/
They have a 3" version that would probably work for 75mm, with some mods.

 

[Scott C]

Might have to order one and check it out. Definity will need modified a little. Doesn't look like you can just order the rubber piece unfortunately.

 

[SolarYellow]

this is what I used for the 54mm version, I had to trim the OD on the lathe, but it's very cheap https://www.mcmaster.com/2645K42/
They have a 3" version that would probably work for 75mm, with some mods.

That appears to be this unit, available for $3 less and with free shipping:

https://www.amazon.com/Pipe-Plug-Mechanical-Size-270229/dp/B000R84XB2/?th=1

 

[mtnmanak]

Test plugs are great. Useful for making paint can shakers too:

I also use them in a similar manner as the paint can shaker, but make them in various sizes and glue sand paper to the outside of the tube - makes a great tool for sanding the inside of fiberglass tubes.

 

[davidb123]

That appears to be this unit, available for $3 less and with free shipping: https://www.amazon.com/Pipe-Plug-Mechanical-Size-270229/dp/B000R84XB2/?th=1

I see that these have a max rating of 17 psi. I wonder if that rating is for leakage or holding strength? Ejection charges could exceed that, but it may still hold up fine. Definitely a cheaper alternative to Aeropack’s MD retainer!

 

[NateLowrie]

I really like this idea! Really you just need to account for the force of the motor because the motor lip is going to handle the recovery force.

Have you measured the force at which it starts slipping in the tube?

 

[Scott C]

I really like this idea! Really you just need to account for the force of the motor because the motor lip is going to handle the recovery force.

Have you measured the force at which it starts slipping in the tube?

Not yet. I still need to build a finished part with the materials I would like to use.

 

[manixFan]

I've been using the test plug method since @Adrian A mentioned it in a post about minimum diameter rockets several years ago. So far I've only used it in 38mm rockets and I've never had an issue. As long as the body/motor mount tube has sufficient hoop strength, it seems pretty bullet-proof. I took the easy way out and just bought two of the test plugs and used the two sides that matched. The smaller side of the 1.5" plug works perfectly for a 38mm tube. @NateLowrie – I've never had one slip, and it would have to slip the entire length of the motor/body tube. And the recovery force is really no different than using an eyebolt screwed into the forward closure, which is a very common technique.

Below is a pic on a AT/Monster 1320 case – I've also used it a fair number of times with the Loki K627 in the 1200 case. In some rockets, I do attach the recovery harness to the bolt, but I use a different method than an eye bolt.


Tony

smaller sides of two plugs: