Estes Motors a smidge too big to fit the motor mount tubes?!?!?!?

Paul Howard

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Hi, This "too-big-motor" seems to be a recurrent problem, especially with the 13mm "Mini-T" size motors but I also have had 18mm A, B, C and 24mm C11 & D with the same problem. Most D's and A, B, C's fit just fine Most of the time, so it's not the motor mount (I don't think). I also routinely clean any crud out of the motor mounts and the rest of the rocket.

They're all/only the paper-wound casing Black Powder motors (not composites with the plastic casing).

The same "too-big-motors" don't fit any brand of kit (Estes, Semroc, Custom Rocketry, Discount Rockets US, Starlight, etc.) consistently so it's NOT the brand of rocket. Some motors fit just fine, and those fit just fine in any rocket brand.

I am storing them in sealed freezer bags grouped by motor types (such as a bag full of only C6-3/5's and C5-3's) in a closet in the middle of the house not near any exterior walls.

I don't think it is an atmospheric moisture problem, I live in humid wet-in-the-winter Oregon, but we have a de-humidifier we use often, and in the very dry summers and fall times of the year, the "too-big-motors" do NOT fit any better at those times either.

I do have a solution that fixes the problem with each "too-big-motor", it's just a chore to do it and it does not lead to CATO's.

Q1: -How many other people are having this same problem?

Q2: -Is anyone changing something in their Motor Mounts during construction to avoid this? and if so, how much "slop" do you get with motors that fit like they normally would otherwise (and are you having to shim your motors in with tape to fill the gap)?

Thanks - Paul
 

Paul Howard

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I have had to peel the A10 paper casings in order to fit the BT-5 motor tubes.
Evidently they have been manufactured a smidge too large in diameter.
Just peel enough to fit, not too much.
This can happen with any size motor due to humidity variations.
That's sorta like what I'm doing but with a rasp and sandpaper for even reduction. I didn't say it since there's usually some one makes a fuss about it.
Thanks -PTH
 

Scott_650

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Couple of pieces of 100 grit sandpaper in my range box and in the mini-tote box I use for “dog barf” at launches - just in case I have case that needs de-cased 😆
 

PDawg

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What SolarYellow said. If you look at the edges of the motor tube it's flared up a little at the ends. A super light sanding of just the lip makes insertion and removal easier. You also reduce your chances of damaging the end of your engine mount tube.
 

Paul Howard

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Yep, I always bevel the top ejection end of a batch motors as part of my prep the night before launches(or even sooner) for all the reasons mentioned above, and I do sand and/or rasp down the diameter to get them to fit. I didn't mention that part in my original post since I've had some rather rule-bound people make a big fuss about it in the past.

It sounds like "motors-too-big" is Very Common and everyone is likely doing the same thing - reducing diameter by reducing some of the outer casing surface.

Thanks -PTH
 

dpower

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Hmm, it's not common for me, but it does happen from time to time, and usually it's an entire pack. Just rolling or sanding the edges typically solves the problem, but occasionally I get a pack of 13mm or 24mm motors (not 18mm, no idea why) that are clearly larger than nominal. For 24mm motors, I've been known to peel a thin layer off the case with no ill effects. For 13mm it's tough to peel just a little without making the case too thin, so I just set those aside for things like saucers where diameter often isn't critical.

I do always put a little CA on the inside of a MMT tube when building, then sand, which prevents the tube end from getting munged and adding to the problem.
 

lakeroadster

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Q1: Funny you mentioned this... I'm building a 18mm (5) motor cluster. I pulled some used motors to install into the mount as temporary fixtures and they wouldn't fit in the 18mm tubes. I then pulled some new 18mm motors out and they fit just fine, and are even a bit sloppy.

BP motors, since they are, as you already pointed out, wrapped in a cardboard tube will swell if subjected to a high moisture environment.

Sand to fit, kick the tires, light the fires.

But (that's a big but) if your new rocket is something near and dear to you... buy new motors.
 
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Tractionengines

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One other thing, if you build your motor mounts / centering ring assembly with white/wood glue etc. It shrinks as the glue dries. This can lead to tight motor tubes in general.
Nice big multi-layer glue fillets between CR and MMT, and all of a sudden ... tight motors. (Not as big of a deal with epoxy, as it cures, not dries.) YMMV.
 

Paul Howard

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One other thing, if you build your motor mounts / centering ring assembly with white/wood glue etc. It shrinks as the glue dries. This can lead to tight motor tubes in general.
Nice big multi-layer glue fillets between CR and MMT, and all of a sudden ... tight motors. (Not as big of a deal with epoxy, as it cures, not dries.) YMMV.
Interesting, that thought crossed my mind about tightening of the motor mount tube through shrinkage with water-based glues. I might try epoxy on my motor mount & centering rings next time. Also, I do tend to "overbuild for durability" and have wrapped some of my motor mounts with nylon or cotton fabrics applied with wood glue - and "that" certainly has some shrinkage potential.
 
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