DMS doesn't fit the hole.

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gary7

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For the second time now I have an Aerotech DMS K535 that will not go fully into a 54mm Loc motor tube. The motor will only go in about 4 inches. Reload cases are a little snug but go in without any problems and only a little extra effort.
I have removed the label, cleaned off the glue from the label and I have sanded the motor tube as much as I want to. Can I sand the motor case? How much?
 

Steve Shannon

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Since you have had the problem before I would fix the motor mount tube. I wouldn’t remove material from the motor case. A drum sander on an extension might make short work of it.
 

Tractionengines

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I've carefully sanded the ID of Motor mount tubes with a fine brake cylinder hone.
It helps stay mostly round, spreads the work area out, etc.
Bare cardboard : be very gentle.
Epoxied Cardboard : works well.
Fiberglass : works very well.
 

gary7

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I've carefully sanded the ID of Motor mount tubes with a fine brake cylinder hone.
It helps stay mostly round, spreads the work area out, etc.
Bare cardboard : be very gentle.
Epoxied Cardboard : works well.
Fiberglass : works very well.
I used a sheet of 120 wrapped long ways around a 1 1/4 inch pvc. Kept (tried) keeping it round, continuously moving around inside the motor tube. After very little effort and only a 1-2 minutes of sanding that K535 slid in rather easily.

Thanks!
 

Steve Shannon

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I used a sheet of 120 wrapped long ways around a 1 1/4 inch pvc. Kept (tried) keeping it round, continuously moving around inside the motor tube. After very little effort and only a 1-2 minutes of sanding that K535 slid in rather easily.

Thanks!
Sometimes it’s as simple as a small glob of glue that ends up in the MMT.
Good job fixing it.
 

jimzcatz

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Get ya a flap wheel or sanding drum, sand the hell out of the MMT until it fits, I have had to do that several times. Don't worry about removing too much material, you wont. Its your MMT that's off size, not your motor.
 

rharshberger

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I like using a piece of 2" diameter material and cut a slit parallel to the long axis with a table saw (can be done with a handsaw though), insert a piece of sandpaper into the slit and make a long thin wedge to lock sandpaper into slot, a 1/2" dowel centered in one end allows the thing to be chucked up in a drill and the large diameter help keep the tool somewhat centered in the bore of the MMT to prevent making the tube egg shaped or taking to much material from one area.
 

GlueckAuf

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If you've used motor ejection extensively with shorter motors than the ones you're using now, there could simply be burnt black powder residue in the motor tube. Swabbing it out with a lightly vinegar-dampened paper towel or rag attached to a dowel will clean up the tube so that its restored internal diameter will again accommodate the longer motors. Be careful not to saturate the motor tube, though, if it's cardboard, Blue Tube, or other porous tubing that can be damaged by excessive wetness.

This is an important maintenance chore, too, in the air frames of piston-assisted recovery systems. Excessive black powder residue along its path can jam the piston and prevent it from pushing out the recovery system.
 

SDramstad

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I have found that if one of the centering rings is a bit tight it kind of crimps the motor mount tube making the motor bind when it gets to it.
 

prfesser

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Don't worry too much about the MMT. A couple weeks ago Peter Tarle had a 10" dia rocket with no MMT, just thick centering rings. It flew perfectly on an M motor. Recovered and flew again on the same day with a hybrid motor.

Best -- Terry
 

bad_idea

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Don't worry too much about the MMT. A couple weeks ago Peter Tarle had a 10" dia rocket with no MMT, just thick centering rings. It flew perfectly on an M motor. Recovered and flew again on the same day with a hybrid motor.
It's drifting off topic a bit, but I have wondered about this for a while. Why do most rockets (aside from min. di.) use motor mounts? I could see where it would brace the inside of the centering rings, but I wonder how important that is on lower power rockets, or even if the centering ring fits a larger motor tightly enough. Is it just hobby-wide inertia, or is there a logical reason?

I've thought about dispensing with the motor mount in a low/mid power build as an experiment to see if I learn anything the hard way, but that's a good way from the top of my project list at this point.

EDIT: I started a new thread about this question.
 
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Steve Shannon

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It's drifting off topic a bit, but I have wondered about this for a while. Why do most rockets (aside from min. di.) use motor mounts? I could see where it would brace the inside of the centering rings, but I wonder how important that is on lower power rockets, or even if the centering ring fits a larger motor tightly enough. Is it just hobby-wide inertia, or is there a logical reason?

I've thought about dispensing with the motor mount in a low/mid power build as an experiment to see if I learn anything the hard way, but that's a good way from the top of my project list at this point.
People have just used centering rings, without a motor mount tube, for high power rockets. Personally I like having the fin tabs epoxied to the motor mount tube on TTW construction. Then the fin tabs, centering rings, and MMT all form a pretty strong structure to withstand thrust, but an argument could probably be made that the upper and lower centering ring on either side of the fin tabs makes a pretty strong structure.
Try it. That’s not a bad experiment.
 

GlueckAuf

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Why do most rockets (aside from min. di.) use motor mounts?
In non-minimum diameter rockets, the motor tube distributes the motor's thrust evenly along its full length to all the centering rings (I use 3 here), to the fin tabs, and in turn to the slots in the air frame.

1636763695049.png


Remove the motor tube (sizing the centering rings accordingly) and the motor's thrust ring concentrates thrust against the aft-most centering ring alone, unless another manner of better distributing that thrust is thought up and applied.

1636763916969.png


Thus, the motor tube adds great strength to the rocket at little cost in added weight. It helps to seal off the aft end of the rocket so that ejection charge pressure isn't bled off inefficiently. And, as I did with the design illustrated here, a longer-than-usual version of it can be employed as a stuffer tube to allow reducing the rocket's pressurized volume, and hence the quantity of ejection charge media that would otherwise be required to adequately pressurize the air frame to exit the recovery gear.

Some talented designers and engineers have developed safe, workable alternatives to the traditional way our sport has adopted the motor tube to elegantly solve the problem of distributing thrust as efficiently as possible to the entire rocket. But the motor tube seems to perform this task pretty efficiently, and the parts makers provide us a wide variety of sizes of centering rings, tubes, and related hardware like motor retainers to make it an easy method to employ.
 

bschultz32

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GlueckAuf commented about piston-assisted recovery systems requiring diligent removal of black powder residue. He's right, it's important you include that in post-flight cleanup. That prompts me to comment about an item that should be in every rocket enthusiast's range box, but usually isn't: powdered graphite.
DSCN3201 LOWRES.JPG

Readily available at any hardware store for less than 5 bucks, and takes up very little room in your box. After cleaning the inside of the body tube, spritz a little powdered graphite around, then move the piston up and a few times. It's amazing how well it works.
Plus, powdered graphite is helpful in other areas as well, like a nose cone that fits too tight.
Bob Schultz
 

Five

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For the second time now I have an Aerotech DMS K535 that will not go fully into a 54mm Loc motor tube. The motor will only go in about 4 inches. Reload cases are a little snug but go in without any problems and only a little extra effort.
I have removed the label, cleaned off the glue from the label and I have sanded the motor tube as much as I want to. Can I sand the motor case? How much?
It has happened to me a couple times. Once you finally get the motor in the tube, get ready for the hard part and thats getting the motor out.
Here’s a pic of my brother in law trying to help me get it out. My sister thought it was funny as hell so she had to take a picture.
 

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Donnager

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It has happened to me a couple times. Once you finally get the motor in the tube, get ready for the hard part and thats getting the motor out.
Here’s a pic of my brother in law trying to help me get it out. My sister thought it was funny as hell so she had to take a picture.
It's definitely funnier if its not you trying to un-stick it.

I'm glad there wasn't a camera to capture some of my methods of unsticking motors or couplers.
 
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