Motor mounts - flush recessed vs protruding

[snrkl]

So, what's the thinking on motor mounts.

I'm trying to decide if I recess or flush mount.




Design considerations I'm assessing are:

- Fin protrusion and motor tube accessibility
- CG differences (albeit small)

Other than that, is there anything else to be taken into account?

 

[OverTheTop]

Robustness on landing. Depending on how hard it lands have a think about what hits the ground first (most of the time anyway!).

 

[Rex R]

in addition, motor retention. if you want to tape the motor in, you'll need to get to the tube, also an 18mm mount in a bt50 tube may require some of the tube to protrude to allow the motor hook enough room to flex far enough to allow you to get the motor in/out of the tube. there is also the question of tail cones...but you may already thought of that .
Rex

 

[snrkl]

in addition, motor retention. if you want to tape the motor in, you'll need to get to the tube, also an 18mm mount in a bt50 tube may require some of the tube to protrude to allow the motor hook enough room to flex far enough to allow you to get the motor in/out of the tube. there is also the question of tail cones...but you may already thought of that .
Rex

Yeah. I have. I settled on recessed slightly as it allows the rear CR to be filleted but allows a tape wrap on the MMT/motor intersection.

This is for my kraken copy, so the only retention in use will be a tape ring on the motor end and a tape wrap to the MMT.

 

[Mr Rocket]

(Edit: I guess you can pretty much ignore this post since you replied while I was typing that you are tape mounting )

What type of retainer are you using?
* Hook
* Screw on
* Tape

Hard to tell from the pics. It looks like it could be setup for a screw on type. If so, even if mounted flush, the cap will still hit first (Good thing)

Aesthetics also play a role in how much of the tube or retainer you want to see.

 

[rharshberger]

I rarely use the flush type anymore since I leave out the motor blocks and no motor hooks usually, then with the 1/4" or so protruding I harden the tube with CA and use either tape thrust rings and friction fit to hold the motors with a wrap of tape as insurance. Plus the motor is usually first thing to hit depending on fin configuration.

 

[rstaff3]

When I do flush recessed as you show, I like to add an extra layer of tube inside the recess to minimize dings to the tube. Especially if the fins are upswept. You can use a sectioned strip of about any similarly sized tube.

 

[Banzai88]

One thing that I've been doing on the recessed design is putting in a piece of coupler to 'double up' the airframe. Haven't had a single ding since.

 

[BABAR]

Tip on tape motor retention. After finishing your rocket, put a piece of clear/cellophane tape around the protruded segment of the motor mount, where your retention tape will attach/overlap motor and tube. Thus your retention tape will attach directly to the cellophane tape and not to your paint. It will hold the motor just as tight, but retention tape will peel off easier and not hurt the underlying finish.

If you don't mind (or even want) a splash of color, Mylar tape will do the same job.

 

[GlenP]

or option C. - Boat Tail !!!

 

[Micromeister]

So, what's the thinking on motor mounts.

I'm trying to decide if I recess or flush mount.

Design considerations I'm assessing are:

- Fin protrusion and motor tube accessibility
- CG differences (albeit small)

Other than that, is there anything else to be taken into account?

Yes:
Motor mount Hooks and straight internal friction fit motor retaining leave a bit to be desired. I've seen motor hooks fail to keep the spent casing in the model and seen countless examples of friction fit motors being ejected rather then popping the Nose cone.

Because of these possible problems I always project my motor mount tubes (including clusters) at least 3/8" from the aft end of the body tube to allow a wrap of 1/2" masking tape around the motor hook or friction fit motor. Since adding this little extra bit of adhesive tape I have not had a single ejected motor in more the 20years in sport or competition model and MPR size rockets.