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Well-Known Member
Dec 16, 2016
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About to start on a LOC IV with 38mmt. The airframe is 34", the mmt is 27", leaving 7" for parachute and shock cord.
There are 2 centering rings with this kit, for 27" of mmt. Should I add a CR in the middle? The mmt is fairly rigid,
just seems like it could use support for that length.
27" seems excessively long to me. I don't remember mine being that long.
I just measured mine, and it's 14-1/2" from the forward centering ring to the top of the body tube. I'm going to say the motor tube is 15" long.



It does seem to be a pretty long stuffer tube. I had a mishap a few weeks ago on my LOC IV due to a bonus delay (user error). It popped the nose about 20 feet off the ground and crunched two fins and the airframe. The nose and motormount survived just fine! Just needed to sand the glue off. Emailed LOC and they sold me a new airframe and new fins. She will hopefully fly this weekend, if the weather cooperates. But no issues on the motor tube with two centering rings at each end. The only thing I did differently is putting a 1/4 in eyebolt in the top centering ring for shock cord mount instead of the loop expoxied to the side of the airframe (although it held up ok). I also tossed the elastic and went with tubular nylon for a shock cord.
Here is what mine looks like unassembled. Looks to be a 4"x4" area for the chute. IMG_2413.jpg
Measured mine and it is about 14 and 3/4" from the top to the centering ring.
Cut it down 24% and you'll have a stubby little motor tube for something else! At least you won't have to worry about too much internal volume for the charge!

Also, the 2 rings are fine. The thrust should.mostly be transferred through the the aft ring and fins and buckling the tube is highly.unlikely
I would cut it closer to 50%. I don't see any reason to leave it so long. A 38/720 case is less than 14" long.
Yep. Cut it in half. Then you have another 38mmt for your next project. Unless the next project is 54mm... :dark:
Whichever length you settle on make sure you check the CG of course (obvious point I know but worth restating). LOC are pretty generous with their motor mount tubes. My LOC Fantom (another 4" / 38mm combination) came with an MMT that was originally 20", which as qquake2k says, is more than enough, even if you were going to fly a CTI Pro 38 GXL casing or similar. After a cato I built a shorter booster for it at 14". In your case it wouldn't seem to be a mix up in the factory with another kit because there's no LOC rocket I'm aware of that would need such a long 38mm MMT anyway - maybe yours was a Friday afternoon job and you just got lucky:)!

I added another thinner CR to box in the fin tabs, but I don't think it added any strength to speak of. LOC fibre board has a good combination of toughness and lightness that can survive some pretty hard landings.
I wouldn't cut it in half. The reason the tube is long is to reduce the amount of body tube to pressurize. I would cut it to make sure you have enough room for recovery but it doesn't need to be all that much. I'd cut the tube so that the upper ring is placed where you want your forward rail guide.
You already have the motor tube!

That's what I meant. If I cut it in half, it leaves plenty of tube for another project, but leaves 20" inside the LOC IV. If I just cut off 7", it will have
14" like everyone else, but only 7" for the next project. Decisions decisions....
So cut it at 15" like mine is. Then you'll have a 12" motor tube left over. I've never had a problem with mine deploying, even with G motors. This is on a G69 Skidmark.