Estes Door Knob Motor-Mount Design Question,,,,,,,,,,

[Paul Howard]

Question for those who have built an Estes Door Knob: I fully understand how to follow the instructions for the fin and motor mount assembly - The Question is,,,,,,,,,,,,,,Am I the only one who is scratching my head about Why one of the centering rings is slotted to go through the forward part of where the Fin mounts to the motor-tube and leaving a small bit of wood ahead of it that really can't take much force, and the odd (to me) placement of the Green ring that will go under the forward section of the fin in front of the slotted centering ring???

It's almost as if the whole fin and motor mount were designed to resist force pushing out the tail end of the rocket rather than emphasizing resisting force going upward with the thrust of the motor?!?!?!?! Does anyone else see that???

I do realize that as the instructions and the design call for, it should be just fine, but it seems a little off to me since I've scratch-built enough rockets with Through-The-Wall fins-to-motormount and maximizing connections of fins to body-tube, and fins to motor-mount to handle the thrust going, well, upward and not pushing out the tail. Also, I'm making a (maybe 2) Lite-Ply Centering Ring(s) to either add to, or replace cardboard centering rings for more strength.

I also have a build log going on the same rocket here in the MPR section.

Thanks - Paul

 

[Bruiser]

To help align the fins so you don't necessarily need to use a fin alignment guide?

-Bob

 

[cls]

The Doorknob kit is plenty sturdy as is. Doesn't need any more weight aft.

 

[Paul Howard]

To help align the fins so you don't necessarily need to use a fin alignment guide?

-Bob

Hmmm, Yeah, I could see that. They do have a fin alignment "guide" (not a "jig") that comes with the plans.
Your idea makes sense.

Thanks -Paul

 

[Paul Howard]

The Doorknob kit is plenty sturdy as is. Doesn't need any more weight aft.

Good to know!

I did make the scratch-built Ejection Baffle that will go in, and it doesn't weigh much.

I am adding some weight in the nose with an Apogee twist-lock Electronic Bay Kit to load some lightweight LED lights and lightweight disk batteries (I got those setups from erockets.biz/Semroc and they work great already for LPR's and fit into BT-50 payload sections nicely) into for night launches. The Apogee kit is made from 1/4" plywood which is total overkill weight and strength-wise and either I'll cut a bunch of the supplied wood out, or use it for a template to make a replacrment out of 1/8"Lite-Ply.

So, given the smallish added weight load and it's added inertia load the Motor Mount has to deal with, Do You think the supplied cardboard centering rings and Through-the-body Fin attachment system is still sufficient as long as everything is glued together well and filleted with glue?

Thanks - Paul

 

[cls]

Do You think the supplied cardboard centering rings and Through-the-body Fin attachment system is still sufficient as long as everything is glued together well and filleted with glue?

My girlfriend built hers all stock with titebond glue and has flown it on F, G, and H motors. Doorknob rocks!

 

[wonderboy]

That middle centering ring with the notch that engages on the fins will help align them to be perpendicular to the surface. That, combined with the fin slots makes it really easy to get really nicely aligned fins.

 

[Paul Howard]

My girlfriend built hers all stock with titebond glue and has flown it on F, G, and H motors. Doorknob rocks!

Wow! Hard to argue against it being strong enough with that as evidence!

Thanks - Paul

 

[Paul Howard]

That middle centering ring with the notch that engages on the fins will help align them to be perpendicular to the surface. That, combined with the fin slots makes it really easy to get really nicely aligned fins.

Great! That goes along with someone else's idea about the notched centering ring and makes total sense.

Thanks - Paul

 

[bad_idea]

So, given the smallish added weight load and it's added inertia load the Motor Mount has to deal with, Do You think the supplied cardboard centering rings and Through-the-body Fin attachment system is still sufficient as long as everything is glued together well and filleted with glue?

Yes, beyond question.

 

[Paul Howard]

Yes, beyond question.

Awesome!

Thanks- Paul

 

[Back_at_it]

It's there for alignment only. It doesn't do anything for strength but it doesn't hurt anything either. Some people have build the fin can completely outside the motor tube then slid it in as a using. Having the slotted center ring makes this a touch easier if you choose to do it.

 

[Paul Howard]

It's there for alignment only. It doesn't do anything for strength but it doesn't hurt anything either. Some people have build the fin can completely outside the motor tube then slid it in as a using. Having the slotted center ring makes this a touch easier if you choose to do it.

Interesting, As usual, I'll test assemble fins and motor-mount (without glue) multiple times to find best the method sequences and also for what to watch out for to avoid. One of the things I've considered is the possibility of not-putting the aft-most centering ring in until last so I can get glue on the slotted middle ring in every spot that connects it to the body tube, motor-mount tube and fins with good internal-glue-fillets, let everything dry then put the aft-most centering ring in last with plenty of glue in a ring around the inside of the body tube and side of fins that it will be up against so that it makes more Internal-glue-fillets as the ring is pushed into place - and stand it up vertically so the glue goes with down with gravity to the centering ring/body tube joint and not run elsewhere in any other orientation whilst drying. I would likely pre-glue, let dry, and lightly sand every surface that will get glue so that it's an even better glue joint since I've had good results doing that in the past with rockets and RC Gliders.

I've thought about the "Build the whole Motor/Fin Can Assembly" first outside the rocket since I've never done that and I see the merit to it.

Thanks - Paul

 

[Paul Howard]

Well, since I got tired of dinking around with loose-fitting stock paper rings and I scratch build often, I changed things a bit.

So, please check out my Door Knob Build Log.

Thanks for all the above input, it was still very useful information.

-Paul