Ratio of Motor Mount to Motor Casing

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I haven't know of anyone who has a set rule they follow. I would think as long as the motor cant wiggle around it is long enough. Plenty of people run 24" cases in 10" or 12" mounts.
 
Not really, some people even build rockets without motor tubes. The tube only needs to be long enough to keep the motor straight along the axis of the tube. Many of my 29-54mm motor mounts are less than 12" long, and the 75mm and 98mm are sometimes longer due to the fact I like full length fin tabs when possible.
 
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When I run 38mm hardware in my 54mm MMT rockets I use an Aeropack adapter. And it's only about 4" long. With some longer 38mm cases, the supported area is pretty short. Still works great
 
I don't think there's a good rule of thumb. If it seems like enough to be straight and secure, it's good to go. I have used (and seen many others use) only centering rings and no tube. The important part is a good thrust surface at the bottom and a way to secure the motor.
 
I once bought an 11" 38mm MMT tube and cut it in to two 5.5" tubes. Just long enough for the fin tabs and whatever else needs attaching. It's worked great so far.
 
Anything longer than your fin tabs+top centering ring is really just extra weight.
 
Unless you want to cut back on the amount of volume you need to pressurize i.e. stuffer tube.

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Exactly my thoughts too Jim!

A longer tube & CR at the top end will reduce the internal volume needed for pressurization at deployment. Long rockets benefit from a longer MMT..

(It can also help keep the laundry up top (forward), as to not shift back far enough to become a 'CG' worry..)
 
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