Can motor be significantly shorter than the motor mount tube?

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Jimmy D. Jones

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Can you use a short motor in a long motor mount tube?

In particular, the question is what if you have a short Aerotech 38mm 38/120 reloadable in a MMT maybe 10-12 inches long - is that ok as long as the parachute and stuff fits in the rocket ok?

Any chance of the ejection charge busting open the MMT from the inside in a case like that? The charge would be down in the tube a ways in a case like that.

I know its ok for the motor stick out and to be longer than the MMT, but wasn't sure about if its shorter.

Maybe Im thinking about it too much but thought Id ask.
 
In low power, like Estes, the motor typically pushes on a thrust ring at the forward end of the motor mount tube. To use a different size motor, you have to use a spacer. In high power, the motor normally pushes on the aft end of the motor mount tube via a thrust ring that is part of the motor and so the length of motor and motor mount aren't directly related.

Obviously there are exceptions to both setups.
 
Just be aware that if you use motor ejection and short motors, over time you'll get a significant amount of black powder residue in the longer motor tube that can make installing, or removing, a subsequent, lengthier motor difficult. To remove the residue, I regularly swab out soot-encrusted motor tubes with a succession of white vinegar-dampened (not soaked) cloths securely fastened to the end of a dowel rod until they come out clean.
 
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Also, with paper tubes you can get strange effects if the motor mount tube in front of the motor isn't reinforced. I think there was something on this in one of the Apogee newletters a while back.
 
Can you use a short motor in a long motor mount tube?

Yes! Here's my L2 rocket (Design #2 in 38mm). This is a "full stick" (36" of PML phenolic or LOC cardboard) of 54mm tubing cut so I can glue both ends of the motor mount tube. In the picture are (top) 29mm motor mount, lower body tube, upper body tube, 38mm motor mount, nose cone.

01parts.jpg

OR sim.jpg

I just flew the Design #2 in 29mm on a F67. Great flight to 800-900 feet. Just didn't feel like running the rocket down on a cold day. After several flights on shortie motors, you'll probably have ejection grunge to clean out so a longer motor will fit.

The same thing will work for low/mid power rockets.
 
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