# 2.56" Tube with 54mm MMT - motor retention?

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#### DAllen

##### Well-Known Member
I hate starting a thread like this but I can't find an answer I like in the searches I've done...The title says it all - how do you do this? Slimline is nice and all but I'd rather explore other options first before spending all that \$. I have thoughts of taking t-nuts and cutting slots in the body tube with wood blocks to hold in the nuts but I'd like to see what other people have done first.

-Dave

#### GregGleason

##### Well-Known Member
I have a similar project. I have a removable MMT on my DD project that has a 2.56 ID (I plan to fly it on 29, 38, 54 motors), it slides into the airframe and a coupler slides in and butts up against the aft CR. For the 38/360 motor, the coupler will have some stops (I'm thinking 3) that contact and hold the aft closure. The coupler has a hole in it that the aft launch button passes through, holding things securely. The coupler also has a wrap or two of FG on the inside to make it a bit stronger, though I am not sure that it was necessary. You have to have a different coupler for each MMT, but it is one way to have motor retention.

Greg

#### rocketsmith

##### Well-Known Member
I have a 3" to 54mm with a boat tail that goes down to the outside of the 54mm motormount. I built in a wood block between the motormount and boat tail and mounted a metal hook on the side of the boat tail that wraps around and retains the motor case. It's just a flat piece of 18 gauge steel bent in a vice @ 90 degrees with 2 holes drilled in it. Just make sure you mount it out of the way of the rail buttons or launch lugs.

#### kandsrockets

##### Well-Known Member
Dave,
I had this with the booster of a 2 stage. Everything I could come up with reqiured mounting something to the out side of the body tube. I ended up just getting a aeropack retainer. One thought was to embed some 6/32 all thread into the fin fillets and use nuts and washers or wire to hold the motor in.

#### WillMarchant

##### Well-Known Member
TRF Supporter
You might think about using a motor casing with a threaded forward closure for forward motor retention. And with the Aerotech and Cesaroni new offerings you can now do this with motor ejection...

#### DAllen

##### Well-Known Member
You might think about using a motor casing with a threaded forward closure for forward motor retention. And with the Aerotech and Cesaroni new offerings you can now do this with motor ejection...
That's a great idea but I want to fly it on 29/360 motors every now and then...so...hmmm...It's only going to be 36" long or so.

As much as I don't want to do it I think that I am going to have to go with the old 6/32 threaded rod glued to the fin fillet between the MMT and body tube. I would go with the removable MMT as suggested but I've already committed to to a motor tube with fins and shock cord already glued to it.

-Dave

#### WillMarchant

##### Well-Known Member
TRF Supporter
You can still do forward motor retention on your 29->54 adapter and then have something to retain the motor casing into that...

Gluing a piece of all-thread into a fin fillet sounds like a great solution, too.

#### Handeman

##### Well-Known Member
TRF Supporter
This pic may be hard to see, but on my El Juan, I use a piece of Hex coupler ? (not sure of terminology) with 6-32 internal thread. I had split the BT so I could install the fin can. The bottom of the fins were 1" from the bottom of the BT. Instead of filling in the slots, I epoxied two hex couplers right below opposite fins.

This was a 2.25 OD with 38mm MMT.

#### blackjack2564

##### Crazy Jim's Gone Banana's
TRF Supporter
A simple solution for minimum space and very useful for retrofitting motor retention in built rockets.

I call them barrel nuts, don't know what proper term is.

They are brass, can be found in the hardware drawers at the usual places, Lowes, HD, Ace etc...

Drill hole so they fit easily when screwed in. They have a slot for screwdriver on end, but the tabs will break off easily if the hole is too tight.
Pack some Vaseline or grease in the inside threads so when you put some CA on the external ones to hold in the centering ring you don't get it in the threads!

While at the store pick up screws or allen bolts of the correct size and some mirror clips or cable holders. Plain washers will work too to hold motor case in.
This size will fit in 3/16in.hole, you should have about 1/4 in to work with on each side if mm is 2.1 and airframe is 2.5. Close but should work. They have many different sizes of these nuts, this just happens to be what I have.

The entire set up will cost approx. 5.00 and will do several rockets. Barrel nuts 3 pack 1.00...... Clips 3pack 1.95. bolts .......35 cents @

This is a Dual Deploy AT Arreaux with them mounted. I fly this with H-motors and don't want to lose the cases!
There is 1/4 inch from MM to airframe, why I picked this pic.

They are especially handy where there is not enough room for a T-nut.
They come in various sizes and I have used them for 29mm to 75mm retention without issue.

You can also epoxy a small section of threaded rod into the CR and use clips on it to hold motor in.

Hope this helps.

Last edited:

#### JDcluster

##### Well-Known Member

JD

A simple solution for minimum space and very useful for retrofitting motor retention in built rockets.

I call them barrel nuts, don't know what proper term is.

They are brass, can be found in the hardware drawers at the usual places, Lowes, HD, Ace etc...

Drill hole so they fit easily when screwed in. They have a slot for screwdriver on end, but the tabs will break off easily if the hole is too tight.
Pack some Vaseline or grease in the inside threads so when you put some CA on the external ones to hold in the centering ring you don't get it in the threads!

While at the store pick up screws or allen bolts of the correct size and some mirror clips or cable holders. Plain washers will work too to hold motor case in.
This size will fit in 3/16in.hole, you should have about 1/4 in to work with on each side if mm is 2.1 and airframe is 2.5. Close but should work. They have many different sizes of these nuts, this just happens to be what I have.

The entire set up will cost approx. 5.00 and will do several rockets. Barrel nuts 3 pack 1.00...... Clips 3pack 1.95. bolts .......35 cents @

This is a AT Arreaux with them mounted. There is 1/4 inch from MM to airframe, why I picked this pic.

They are especially handy where there is not enough room for a T-nut.
They come in various sizes and I have used them for 29mm to 75mm retention without issue.

You can also epoxy a small section of threaded rod into the CR and use clips on it to hold motor in.

Hope this helps.

#### Pantherjon

##### Well-Known Member

JD
Yep..I have used them on few rockets as well..I had a hard time locating them at our local Lowes..But found them in the Hardware section, and then they had a small section for 'Furniture hardware', which is where I found them..

#### JDcluster

##### Well-Known Member
The better ones are made of brass & found in hobby shops.
The Dubro ones are smaller & can fit into tighter spots.
Installation is a bit tricky at times because you have to use the corresponding machine screw to install it, I use a dab or JB weld type of epoxy.

JD

Yep..I have used them on few rockets as well..I had a hard time locating them at our local Lowes..But found them in the Hardware section, and then they had a small section for 'Furniture hardware', which is where I found them..