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Motor mount question

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Horizon

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Hi all,

I recently purchased a kit and it has a "short" motor tube. I was wondering if it was best practice to have a motor tube that will be the length of the largest motor I plan to put in it?

Is it better for stability purposes to be able to support the entire motor rather than to have a large portion hanging out of the motor tube?

There is plenty of room in the body to support a longer tube.

Thanks in advance!
 

rharshberger

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All a longer tube will do is reduce internal volume, add weight and add bit more stiffness to the outer airframe. As long as the motor is supported properly and secured a longer motor tube is not necessary. Some members have even built rockets without motor tubes just using centering rings.
 

Bat-mite

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A longer MMT will put your top CR higher in the booster, and thus you have less harness exposed to the ejection charge. If you are not using motor ejection, then it is a moot point. Probably a moot point, anyway.
 

Coop

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Depends on the diameter of the airframe for how useful it is. Moving the top CR of the motor mount forward may be of benefit on airframes where getting to the shock cord can be a challenge... or one might want to have the chute compartment smaller so that a smaller charge can be used to separate... Or they just want a long motor completely enclosed in the MMT. There's little reason for the latter, other than it making the flier feel better, but just because you have a 16" motor, doesn't mean you can't fly it in your 9" long MMT, provided you have the extra room...


Later!

--Coop
 

blackjack2564

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I 've flown a rocket with no motor mount [not a minimum diameter] just used correctly sized centering rings.

No need for full length support of motor case.

The motor mount tube only needs to be long enough to glue through the wall fins too.... the majority of the time.
 

cbrarick

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What are you trying to "support" with the longer MMT? When the rocket is in flight, or sitting on the pad waiting, the longer tube itself won't do anything but add weight and slow you down. It's "support" while stationary will be to exert it's weight on the motor retention system(such as the aeropack). When it's flying it exerts a pressure - we'll call thrust - which is carried by the tube thru the centering rings to the outer surface of the rocket, right? after burn-out, thru ejection and recovery it's just exerting it's weight on the motor retention system again, right? One might make a argument that the ejection event may spike the amount of force on the retention system, but that's just splitting hairs. So, as you see axial loading arguments are difficult (unless you have a rocket horizontal, but that's not normal). even then you might be surprised by how strong even loc tubing is (search rocketmaterials.com probably need the wayback machine). you can calculate that load and the change per unit length of the mmt (when horizontal) but you'd be surprised by the number compared to that of the burst pressure of even a small length of mmt.

so a the end of my ramble, the answer is emphatically "no you don't even need to lengthen it" My mega wildman doesn't use motor tubes, just centering rings. That is the usual rig for 6 inch motors, cause they generally don't fit tubes unless the OD is machined down.
 

AfterBurners

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All a longer tube will do is reduce internal volume, add weight and add bit more stiffness to the outer airframe. As long as the motor is supported properly and secured a longer motor tube is not necessary. Some members have even built rockets without motor tubes just using centering rings.
exactly plus 1
 

AfterBurners

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Depends on the diameter of the airframe for how useful it is. Moving the top CR of the motor mount forward may be of benefit on airframes where getting to the shock cord can be a challenge... or one might want to have the chute compartment smaller so that a smaller charge can be used to separate... Or they just want a long motor completely enclosed in the MMT. There's little reason for the latter, other than it making the flier feel better, but just because you have a 16" motor, doesn't mean you can't fly it in your 9" long MMT, provided you have the extra room...


Later!

--Coop
I usually feel best when the chute comes out for a perfect deployment and the crowd cheers loudly as they run like a herd of cattle to retrieve my rocket.
 

T-Rex

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I have flown a 29/360 casing from a 5" MMT. No adverse effects to the model or the motor. I was a little concerned about the hot motor casing being in contact with my 'chute, but the nomex burrito prevented that from occurring.
 
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