Why use a motor mount tube?

Thinking of ways to build a little differently in MPR regime. Is a motor mount tube really necessary? Centering rings can align the motor properly. Is there a big issue with the residual heat from the motor burn softening the adhesives?

You are correct, the MMT is not a necessity.

But it makes a handy place to mount a motor retainer

Yes, it is a good place to mount a retainer. Also keep the ejection pressure in mind. I definitely think you can get away without a motor mount tube, but I do think they help clean up an imperfect connection and retain pressure for recovery deployment.

Minimum diameter rockets generally use the body tube instead of an internal motor tube with centering rings.

Also if you were to use say 3 centering rings with the farthest one up around 8" whenever you use a motor smaller then 8" you have the possibility of charring your rings weakening them. Soot would also build up that may make it hard to insert the larger motor. It does sound cool though. I may have to try it sometime. Just use the retention clips on the rear centering ring instead of a retainer.

Using a motor tube makes for a stronger mount.
Having the force against just the aft cr could be a making for a thru and thru motor job.
Motor tubes also serve as stuffers too, in fat rockets.
All said and done..Im sure you know all that...........give it a shot

I like to use motor mount tubes for additional bonding area for through-wall fins. Without the tube, you have significantly less area to attach your fins. But, yes, you could forgo the motor mount tube no problem.