Retention questions - short MMT exposure and adaptor use

[Tbird Flyer]

I am currently building a Madcow Super DX3 for my level 1 certification. Although I have used friction fitting for all of my mid-powered rockets, I would like to use a screw on retainer to secure my motor for my certification rocket. However, I ran into 2 issues. 1) My MMT was already secured in my rocket with a coating of epoxy on it before realizing that I needed a 1/2” or so exposed beyond my CR to use this type of retention. I have just UNDER 3/8”. 2) Although my rocket has a 54mm MMT, I had planned on using a 38mm to 54mm adaptor in order to use a 38mm motor on my initial launch. Can I use a 54mm screw on retainer for an engine which is located inside a 38-54mm adaptor? Thanks for any advice!

 

[blackjack2564]

Great solution. Just be sure to have the adaptor on hand . Install it and a motor case to keep things centered when U install the bolt on retainer.
Also helps a great deal to install bolts opposite of one another...like when tightening tire. never next to each other. Install but not tight, after all bolts in place, then torque tight once again...opposite each other to keep flange from deforming.

 

[Cameron Anderson]

1. Should be fine with aprox 3/8. Just prep the MMT well and use good epoxy. Are you using motor ejection or electronic? Motor ejection will obviously put more strain on the retainer that no motor ejection, but I'd still be confident with even 1/4 motor tube to secure the retainer.

2. Depends on your 54-38 MMT. I have a DX3 XL with a 75mm MMT. I have a 54mm adapter kit that I place around the 54mm motor and it fits into the 75mm MMT and I still use the 75mm retainer with it.

https://aeropack.net/motoradapters.asp
I won mine in a raffle so I don't know the cost or where to get one, but it was a high end product and I've used it several times with zero complaints.

 

[Tbird Flyer]

1. Should be fine with aprox 3/8. Just prep the MMT well and use good epoxy. Are you using motor ejection or electronic? Motor ejection will obviously put more strain on the retainer that no motor ejection, but I'd still be confident with even 1/4 motor tube to secure the retainer.

2. Depends on your 54-38 MMT. I have a DX3 XL with a 75mm MMT. I have a 54mm adapter kit that I place around the 54mm motor and it fits into the 75mm MMT and I still use the 75mm retainer with it.

https://aeropack.net/motoradapters.asp
I won mine in a raffle so I don't know the cost or where to get one, but it was a high end product and I've used it several times with zero complaints.

Thanks for your info. I am using a motor ejection with this rocket. I was planning on using JB Weld for my epoxy since I heard it was recommended for motor retentions. I had assumed that I would need a retainer for the motor inside of the adaptor but perhaps the 54mm will cover both based on your assumptions. I will have to look into the dimensions more. Thanks again for the insight!

 

[Cameron Anderson]

Thanks for your info. I am using a motor ejection with this rocket. I was planning on using JB Weld for my epoxy since I heard it was recommended for motor retentions. I had assumed that I would need a retainer for the motor inside of the adaptor but perhaps the 54mm will cover both based on your assumptions. I will have to look into the dimensions more. Thanks again for the insight!

Most motor adapters are just a fiberglass or cardboard insert with limited retention capabilities...both in terms of forward movement against thrust and rearward during ejection. You really just need to see what your adapter can do. But I recommend the Aeropack.

 

[Tbird Flyer]

Ya, the adapter I have is simply a cardboard adapter. I don't see anything built in that is going to help with retention in either direction. I may have to look into a different adapter that will work better with a retainer I find. Thanks again.

 

[kuririn]

I think this was what blackjack was referencing to in post #2.
Bolt on retention plate. Not as convenient as a screw on retainer but works when there is very little motor tube sticking out the aft end.
From Madcow, I think there are other vendors as well.
29mm shown, I'm sure there are other sizes.

 

[jd2cylman]

No. AeroPack offers a bolt on 54mm retainer. Cut off the projecting motor mount and the retainer bolts to the rear centering ring. Then if you get a AeroPack A3854 adapter, it will work perfectly with the retainer. And if you get the set, you can adapt all the way down to 24mm just by nesting the AP adapters. (if'n you had a 24mm motor that could lift the rocket...)

 

[Tbird Flyer]

The retainer diagram was a little confusing to me but I am assuming this is a flanged retainer. This looks to me like a great set up overall with the aeropack just a little more expensive. I want something that I can have confidence in that it will work well and this seems to do the trick. Thanks for the info and the links!

 

[Tbird Flyer]

Thanks for the clarification... I was a little confused by that initial reply. I looked into these, but couldn't find it for a 54mm mount and wasn't sure how it would work with the adaptor.

 

[David Schwantz]

Here is the 54 mm flanged mount. it is bolted onto a thrust plate. Also pics of the adapter you are wanting. Hope this helps.

 

[Tbird Flyer]

Looks great... thanks for the help and the photos. I am not familiar with a thrust plate. Will I need one of these or can I attach the 54mm mount directly into the aft CR?

 

[Steve Shannon]

Looks great... thanks for the help and the photos. I am not familiar with a thrust plate. Will I need one of these or can I attach the 54mm mount directly into the aft CR?

You can attach it directly to the aft CR. In most cases thrust rings are more of a cosmetic design choice than functional. An aft centering ring that’s tight against your fin tabs will never go anywhere. Even a CR that’s epoxied away from the fin tabs is extremely unlikely to budge.
The first time I installed an Aeropack motor retainer I did exactly what you did, left too little motor mount tube extending. I used a sharp chisel and chiseled away the centering ring immediately around the motor mount tube so I could slip the retainer into the gap. It worked great.

Ever since then I build my aft sections around my motor retainer. I put the retainer onto the motor mount tube and mark the tube. Then I build the motor mount, fin assembly, and aft-most body tube. I always have the retainer tight to the aft centering ring and the aft centering ring tight to the fin tabs with internal fillets everywhere I can get to.

 

[David Schwantz]

Yuppers, did that one myself. But I put a couple of notches in the retainer and spun it around the tube to cut into the CR.

 

[Steve Shannon]

Yuppers, did that one myself. But I put a couple of notches in the retainer and spun it around the tube to cut into the CR.

That’s really smart. I didn’t think of that.

 

[David Schwantz]

Sometimes I get lucky

 

[Tbird Flyer]

Thanks again for all of the info. Ya, I will not make this same mistake on a rocket in the future... I guess that is how you learn. Since I am new at the high powered rockets, just curious as to how you put fillets on the fin tabs? I had a slotted tube and was only able to push the fin into those slots to the motor mount. The aft CR was already in place so there would be no access to do this. Do you leave the aft CR off or something until the end?

 

[David Schwantz]

Most of the time I leave the rear CR off to put in fillets. Since it is already glued in you then can drill holes right next to the fin and inject glue and let it run down the fin tab. Then when you do your external fillets this is what will cover the holes that you drilled.