Install motor mount before or after fins?

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neil_w

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Instructions to the Solar Warrior show building the motor mount, then gluing the fins, then installing the motor mount. Is there a particular reason for this sequence? It would seem that installing the motor mount would be easier and safer without having to worry about the fins.

I was thinking maybe the instructions were sequenced that way was so that you could glue the fins while the motor mount was drying, but I'm not concerned about that.
 
For me it depends on the model.
I just finished the Estes Lynx and glued the engine mount in after it was painted.
The instructions said to put the hook on top next to the white rudder.
I put it underneath. I thought it looked better there and there is less chance of nicking the rudder when removing an engine.
 
The glue for rhe motor mount may change the tube shape a bit for cardboard so I would tend to install the motor mount first.
 
May be off topic, but on all my scratch builds, mid and high power I glue the Rings to the mmt, glue fins to mmt, and slide that assembly into the main body tube.
 
I mostly build my MMT the test fit it. Usually install it after the fins are done.
 
For model rockets, which seldom have TTW fins, I like to add the motor mount last, and after the rocket is painted, if possible. Often makes it easier to hold the rocket on a dowel or rolled up paper for painting. Also, if I decide to paint the end of the motor mount tube (usually flat black), this is most easily done before the motor mount is installed.
 
I almost always use the traditional method of installing the motor mount first. Easier to push it in when there are no fins in the way and then painting can be done using a stand with old expended motor casings holding the rocket up.
 
Needless to say, with TTW fins you have to install the MMT first. If not TTW, then do whatever is easier for you.
 
I usually glue the fins on first and then the motor mount. I like the engine retainer hook lined up evenly with one of the fins and this method allows me to do it easier.
 
If you use, the Estes fin alignment jig, would can either slide the motor mount in without gluing it in place or have it glued in place first. This helps you align the model in the jig. I almost always glue the mount in after the model has been built unless it has through the wall fins. I almost never use a commercial jig to build my prototypes with. For some models it is nice to be able to set the base of the body tube on a table while the glue dries. If there is a motor hook sticking out you can't do this. Also some instruction manual drawings come with a full size top view of the fin placement. This allows you to place the model over this drawing to align the fins. What I've come to relies as a designer for Estes is I present the build in the instruction manual a way I know the model can be built correctly. It certainly isn't the only way a model can be assembled since EVERY MODELER has their way of doing things, and that's ok. So you most likely will never get a consensus on how to build any part of a model rocket. You will in time do it your way.

John Borren
 
Thanks, definitely some considerations that hadn't occurred to me before. I'll feel my way through this one and see how it goes.

I've never used a jig of any kind for fin attachment, eyeballs all the way.
 
Neil,

I almost always put the motor mount on after the fins. On one of the first models I build the body tubed deformed where the centering rings were glued, probably too much glue. After that I started putting the fins on first then putting the motor mount in later. I've not had the issue since and I think I use a lot of glue. It my be coincidence. Like you I eyeball the fins and the motor mount hook sometimes becomes a visual distraction to getting the fins on straight.

Thanks
Joe
 
Motor mount 1st. always did it that way and nary a problem. Besides, some of my designs require me to get the motor mount done first anyway. Fins get in the way. I've used engine hooks once, in the last 30+ years. Usually I rely on an engine block and friction fit, but when I did my upscale Astron Cobra, I opted for a mount for 3 - 24mm motors. Granted, the old instructions for the Cobra was to use tissued glue to stuff between the motor tubes. Since the 3 - 24mm motor mounts were going in a BT 70, I opted for a motor mount kit that had engine hooks and plywood rings. I followed the instructions on the motor mount, (and I really wish I hadn't) which positioned the aft CR about 3/4" up inside the airframe, and I went with the engine hooks as well. (wish I hadn't done that either.) Had a great flight. Chute deployed...yadda yadda. When my brother retrieved the rocket, this is what the motor mount looked like.

Our regional rocket geek/scientist/brainiack, suggested that, the recessed CR may have created a vacuum, causing the exhaust to invert and burn up the motor tubes. The motors were missing as well. Apparently, the heat flexed the hot engine hooks and the motors shot out at ejection. The repair was finished with my USUAL aft flush CR installment. And no stinking engine hooks!

DSCF3542.jpg
 
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LPR and some MPR, mount first. I think it supports the tube and as someone mentioned I often paint with a spent motor and dowel installed.

On HPR, which usually has TTW fins, mount less reat CR, then fins, then internal fillets then rear ring.
 
Motor mount 1st. always did it that way and nary a problem. Besides, some of my designs require me to get the motor mount done first anyway. Fins get in the way. I've used engine hooks once, in the last 30+ years. Usually I rely on an engine block and friction fit, but when I did my upscale Astron Cobra, I opted for a mount for 3 - 24mm motors. Granted, the old instructions for the Cobra was to use tissued glue to stuff between the motor tubes. Since the 3 - 24mm motor mounts were going in a BT 70, I opted for a motor mount kit that had engine hooks and plywood rings. I followed the instructions on the motor mount, (and I really wish I hadn't) which positioned the aft CR about 3/4" up inside the airframe, and I went with the engine hooks as well. (wish I hadn't done that either.) Had a great flight. Chute deployed...yadda yadda. When my brother retrieved the rocket, this is what the motor mount looked like.

Our regional rocket geek/scientist/brainiack, suggested that, the recessed CR may have created a vacuum, causing the exhaust to invert and burn up the motor tubes. The motors were missing as well. Apparently, the heat flexed the hot engine hooks and the motors shot out at ejection. The repair was finished with my USUAL aft flush CR installment. And no stinking engine hooks!

Hopefully Bob Krech will chime in on this one, for some reason it doesn't sound quite right to me. The motors are at least flush with the bottom of the rocket, usually the effect you have is iirc called the Krushnic Effect, if the base of the rocket was really close to the blast deflector then a Bernoulli Lock could have held the rocket to the pad long enough for the exhaust to have caused the damage you received. These are not areas I have great experience in but may have contributed the damage.
 
The rule of thumb for avoiding a lot of performance due to Krushnik is to recess no more than one caliber in. That doesn't mean that a vacuum did cause that damage. However, I have recessed a bunch of mounts and never saw that. :shrug:
 
Not counting the time my 4" Archer was flat on the deflector and was sucke onto the deflector. Toasty.
 
So the question is whether the rocket was sitting on a flat deflector...if not, no Bernoulli Lock.
 
Well, before we let speculation fill it's hungry gut, and allow my rocket damage to side line this thread, let me just say, that my Cobra left the pad quick enough that the round blast deflector couldn't have had a hand in it's demise. It just didn't sit there long enough. This is the only rocket I have, that this has happened to, and it had an unusually deep recessed CR. I still buy Tom Binford's (Rocketjunkie), analogy. It seems to make more sense, that the burn got somewhat inverted on the way up and quite long enough to heat those motor hooks enough to weaken the strength. They just didn't hold. And I'm certain I haven't lost my collective building skills of 55 years to poorly mount 3 lousy motor hooks. :rant:

Now, it's been refitted with a new mount, which btw, the fins only got in the way with removal of old mount, cleaning up the innards and installing a new one. The proof will be in the pudding when I get another chance to launch it. I was going to do that last month at Bayboro, but there was just too much wind.
 
I had a five engine cluster model that I've flown many times. One time I flew it all five motor tubes burned up just like yours did. Never could figure out why it happened other then sometime STUFF happens.


John Boren
 
Maybe the instructions are written this was due to the tailcone? Usually I install the motor mount before the fins but I did it after on this build. I didn't install it until after painting.
 
I glue the motor mount first in case the glue wants to pull the body tube in a little bit. That can be corrected when the fins go on and the glue for the fins can fill in the tiny gap under the root edge of the fins. Also, for perfectly aligned fins, I don't want the body tube getting sucked in after I attached the fins and possible shifting them a very tiny bit.
 
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