Motor Tube Length vs Motor Length

[Sleestak]

Hey guys, just wondering if it is OK to have a motor that is slightly longer than the motor tube. Specifically for 38mm and 54mm MMT's. I assume it is OK, from a motor safety perspective, but could lead to extra crispy nomex/chutes, etc if using motor ejection. I never heard this discussed or asked before.

 

[Titan II]

Yes. There are numerous threads on this issue.

 

[rharshberger]

Hey guys, just wondering if it is OK to have a motor that is slightly longer than the motor tube. Specifically for 38mm and 54mm MMT's. I assume it is OK, from a motor safety perspective, but could lead to extra crispy nomex/chutes, etc if using motor ejection. I never heard this discussed or asked before.

Not uncommon, the motor tube can be much shorter than the motor.

 

[Sleestak]

Yes. There are numerous threads on this issue.

Thanks...first post...didn't try searching :-(

 

[jimzcatz]

MMT length is pretty much irrelevant, as long as its centered and attached securely. Another reason to never use a motor block/hook.

 

[Rocketjunkie]

Just enough to hold the motor straight. I usually make it long enough to sandwich the fin tabs between the centering rings.

 

[gary7]

One reason to have a longer motor tube on some rockets is that your body tube will otherwise require more 4F. That is, if you also have a CR at the forward end of that motor tube.

 

[dhbarr]

I think the lowest I would go would be about a motor tube diameter or three. But I suppose a single centering ring would be fine if the attachment and motor retention were stout enough.

 

[prfesser]

Built a Gizmo and flew it on a J570; the MMT is only about 8" long. The forward end of the motor extended into the nose cone. Flew just fine (twice).

 

[beeblebrox]

A shorter tube will not get charred inside making it difficult to put a bigger motor in there later. You test fly on an F a few times then you try to put that H in there... A tube fitting around the outside of the MMT for a stuffer tube would be a good idea if you wanted to reduce the pressurization volume. Being larger it would not matter if it got charred. This is a moot point if you are using electronics for ejection in the payload.

 

[DaHabes]

The motor can be as.long as you want, just leave room for the laundry....

 

[sharkbait]

I have done some of my scratch builds without any motor tube, just used centering rings to hold motor straight