Motor Mount / Motor Length Question

[gregtro]

Looking for advice,
How much longer than the mm can the motor be?
For an example, lets say a 75mm M 1315 which is 31.5 " long.
What might be the min. motor mount length ?

Thanks

Greg

 

[Smokin' Rockets]

No need for the MM to fully enclose the motor. In your example a 24" MM could be used safely. Many large rockets dont even use MM, and just use center rings.

.....Bill

 

[gerbs4me]

I didn't know that bigger stuff use just CR's, like they you learn something new everyday

 

[Marxi]

No need for the MM to fully enclose the motor. In your example a 24" MM could be used safely. Many large rockets dont even use MM, and just use center rings.

Yup!

Johannes Haux always does it this way..:
https://www.raketenmodellbau.org/showthread.php?threadid=1426

Pic: His "Postrakete"

 

[Karl]

Wow, that things built like a tank for sure!!!
Karl

 

[gregtro]

Very nice rocket !!!
Thanks everyone that's what I thought, just wanted to be sure .

Greg

 

[rstaff3]

Most bigger rockets do use motor tubes, its just not required. As long as you don't want to fill the fin can with foam that is

 

[Ryan S.]

https://www.geocities.com/rdh82000/LDRS/Construction.htm

 

[rstaff3]

https://www3.telus.net/public/albertaw/ towards the bottom of the page

 

[llickteig1]

These techniques can be applied to much smaller motors, too. You don't have to have a rocket with a big 3" or 4" diameter motor to build like the examples given.

Also, if you do use a tube for a motor mount, there's no reason the motor can't extend well above the tube. There's not much functionally in many designs that tube is doing. All the thrust is being transferred to the rocket at the aft end, the centering rings, and where the fins meet the MMT and the airframe.

One additional thing to think about, though, is that your motor mount is protecting everything inside your rocket from the high heat of the motor casing and the direct blast of the ejection charge. If your motor extends well beyone your motor mount, adequately protect everything from direct contact which might lead to heat damage.

HTH, --Lance.

 

[rstaff3]

IMHO in most cases, especially the smaller, you might just as well use a motor tube since the COTS rings are generally made for a tube. As discussed in earlier threads, there are multiple purposes for a motor tube other than just holding the motor, including a stuffer to reduce ejection volume.

 

[Mad Rocketeer]

Originally posted by rstaff3
Most bigger rockets do use motor tubes, its just not required. As long as you don't want to fill the fin can with foam that is

I've never used foam in a rocket. Please educate me on how its use relates to using motor tubes. Is this a reference to sealing against ejection gas loss to the rear?

 

[Stymye]

the motor tube acts somewhat as an internal spar, so it provides more ridgity to the rocket compared to no mmt tube at all. plus it ties all of the centering rings together and speads the thrust load along a larger area. on large rockets you often see threaded rods installed to improve the ridgedity of the structure...

without a motor tube all the force is directed against the bottom centering ring... and the chance of kinking the body tube is much greater. without the internal backbone that a mmt provides

 

[rstaff3]

Originally posted by Mad Rocketeer
I've never used foam in a rocket. Please educate me on how its use relates to using motor tubes. Is this a reference to sealing against ejection gas loss to the rear?

It was a joke. Many (some?) people fill the gap between the centering rings and the motor tube with 2-part foam (or canned foam for that matter) to add rigidity to the fin area. With no motor tube you'd have some mess

 

[rstaff3]

Originally posted by stymye
the motor tube acts somewhat as an internal spar, so it provides more ridgity to the rocket compared to no mmt tube at all. plus it ties all of the centering rings together and speads the thrust load along a larger area. on large rockets you often see threaded rods installed to improve the ridgedity of the structure...

without a motor tube all the force is directed against the bottom centering ring... and the chance of kinking the body tube is much greater. without the internal backbone that a mmt provides

True. But if you add that threaded rod and/or glass the tube you may not need the rigidity. I personally would go for the tube.

 

[Mad Rocketeer]

Originally posted by rstaff3
It was a joke. Many (some?) people fill the gap between the centering rings and the motor tube with 2-part foam (or canned foam for that matter) to add rigidity to the fin area. With no motor tube you'd have some mess

D'Oh! I just read it wrong somehow. Yep. Your point is a good one and funny too.

 

[rstaff3]

Heh, have done that one....but have pulled some good ones in my day