Motor mount length question

I’m new to high power. I did a successful Level 1 flight last weekend with a LOC cyclotron, my first 38mm rocket. I flew on a Aerotech H130W. When I assembled the motor, the instructions said to use as much of the black powder as you need for your rocket. I figured it’s better to have to much than not enough, so I used all of it.

The cyclotron came with a 15” motor mount tube and a 3” X 34” body tube. That left me with around 16 inches between the end of the motor mount tube and the shoulder of a transition piece to put recovery gear. One my first (and only) flight, using all the black powder that came with the AT H130W, everything worked. (Whew)

I’m starting to build a LOC Shadowhawk and the Shadowhawk, like the Cyclotron, came with the same LOC 3” body tube, 34” long.

The Shadowhawk however, came with a 20” motor mount tube. 5 inches longer than the cyclotron motor mount tube (remember, same body tube). That would only leave me with 10” between the end of the motor mount tube and a coupler bulkhead for recovery gear. So I assume I’m supposed to cut the motor mount tube down to size... yes?? (and I'm a little nervous putting recovery gear so close to the end of the motor mount, even with nomex)

My question is, roughly how long should the motor mount tube be? Should it be longer that the biggest motor you plan to use? By how much?

Since I can pick how long the motor mount tube is, that means I can pick the size of the space I need to pressurize with the black powder charge. Are their guidelines for the size of space a given black powder charge can pressurize?

I would just use it as included in the kit, as long as you have room for your chute. The motor mount does not have to be longer than your longest motor. It just has to extend beyond your forward most centering ring. In large diameter rockets sometimes and extended MMT has a centering ring mounted at the forward end to reduce the volume that must be pressurized by the ejection charge. That configuration is referred to as a stuffer tube.
There are online calculators to help calculate how much BP you need to pressurize a given volume.

[emoji1010] Steve Shannon [emoji1010]

blackwing94 said:

My question is, roughly how long should the motor mount tube be? Should it be longer that the biggest motor you plan to use? By how much?

Click to expand...

It only needs to be longer than the fin roots plus centering ring thickness. It does not limit your motor length, the motor just sticks out the top no big deal.

Binder Design said:

It only needs to be longer than the fin roots plus centering ring thickness. It does not limit your motor length, the motor just sticks out the top no big deal.

Click to expand...

The length of the booster tube is what limits the length of your motor. All the MMT does is keep the motor centered and stationary.

Although you technically only need two centering rings, it's sometimes advantageous during the build to use a longer motor mount tube and have a third ring at the top end of the mount. You glue on the top ring and the ring just above the fins first and leave off the back ring, then glue the motor mount assembly into the body tube. This gives you access to the fins' root joints during gluing, so you can fillet the roots and the inside body tube-fin joints... after you're done, you glue on the end ring and the motor retainer. If you're going to be flying above mach, this is a really good idea... things tend to come flying off at about 1000 ft/sec or so.

Found BP calculator. Thanks.


Sounds like I can make the motor mount tube as short or long as I want assuming the forward centering ring is in front of the fin tabs. Even though I use nomex, I like to throw a handful of dog barf in, so I'll make the mount an inch longer than the biggest motor I plan on using for this rocket and let the forward centering ring be the dog barf shelf.


I'm still using motor ejection. I'll start playing with electronics this fall.

I will say if one has a tall rocket and uses a long motor tube, if there is a major mishap sometimes there's enough left to cut down and do a coupler repair to save a fincan. Appropriately spaced centering rings add strength.
I have one 4" LOC tubed rocket that's on it's second "salvage" job after this particularly wild flight: [video=youtube;hMvT9BKzbUg]https://www.youtube.com/watch?v=hMvT9BKzbUg[/video]

I'm glassing a short segment of replacement tube to use on yet another coupler repair that should be more resilent. New upper bay needs to be constructed and the ebay had the softest landing it ever had under a 6 foot chute.
The camera was on the upper bay and that tube completely zippered off the ebay when the main blew. It is seen fluttering down. Stop frame review caught shots of the sustainer going in with the broken shockcord wrapped around it.
The fincan survived intact but the end of the longneck tube of course was damaged. The 38mm, 33" long motor tube gave me plenty of room to cutback the damage and make a new bulkhead for a shockcord anchor. Kurt