Help with centering ring in wrong position on motor mount tube.

[DrJoe]

I’m building a LOC IV for my L1 cert. and I messed up. After carefully measuring and marking where the 3 centering rings were to be glued to the motor mount tube, I glued the mid ring in place and then glued the forward ring on the wrong end. Reversing the tube the forward ring is correct, but the center ring is too far forward by about 1-1/2” so the forward end of the fins will not mate. I’ve got a laser cutter so making a new centering ring is easy. I can think of 5 options:

1. Add additional centering ring in correct position. - Negative is adds weight and moves CG back.
2. Try to cut around incorrect ring as close to tube as possible with jewelers saw. Negative - lots of effort and may cut or damage tube.
3. Use it as is and not worry about mid centering ring being glued to fins. Sounds crazy, but I don’t think the original IV’s had a mid ring at all.
4. Move motor tube back so center ring is in correct place and cut motor tube down. By far the easiest. Negative is GC moved back. As it is, a 360 motor doesn’t project out the existing tube and a 760 goes way beyond. Cutting down the motor tube the 360 is still completely enclosed.
5. Throw it all away and quit rocketry (just kidding)

(I'm was planning on glueing aft ring in after the fins were glued in so I could get good filets on the interior between fins and tube and fins and frame). Note - I haven't put filets on rings yet - they are glued at on forward end that doesn't show in photo)

Opinions? Thanks!

 

[LGM-25C]

#3

 

[jmasterj]

I'd go with #4. Trimming the motor mount is pretty easy and losing a inch in motor mount length is probably irrelevant. You might want to check that you can still reach down to the forward CR and your recovery mount from the top of the airframe, though (assuming you want it to be repairable).

 

[tsmith1315]

#1, 3, or 4. Or 2. All good solutions. Negligible differences in balance.

I’d take the easy route, too.

 

[Back_at_it]

The fins go through the body tube and mount to the motor tube on the LOC IV so you have plenty of strength. Just add fillets to joints where the fins meet the motor tube and keep building. If you like you can add a very thin cardboard rear ring to clean up the rear of the rocket. With through the wall fins the rear ring doesn't provide anything structural to the build so you can literally make it out of paper if you like.

The LOC IV has a ton of room for error and is an extremely stable rocket so I would not be concerned with additional weight here or there.

Keep going and don't worry about it.

 

[86mustang408w]

#3. I'm sure if you wanted to with a drill bit and a hobby knife kit, holesaw, or small hacksaw blade you could cut off that CR and sand it down, then add the correct one. I use thin CA to mock up parts first, then after about 10 test fits I finally glue/epoxy it.

 

[JimByrne]

Just add a new center ring in the correct place & remove as much of the mislocated ring as you can without damaging the engine tube. The little bit of extra weight won't make a difference in a LOC 4.

 

[Ronz Rocketz]

#2. Doesn't look like a lot yellow glue was used. Score it and push vertical until the glue breaks. Any damage to the tube can be fixed with masking tape and thin CA.

Option 6 is to buy a new tube.
https://www.rocketarium.com/Build/Motor-Tubes/38

 

[jqavins]

1. Add additional centering ring in correct position. - Negative is adds weight and moves CG back.
2. Try to cut around incorrect ring as close to tube as possible with jewelers saw. Negative - lots of effort and may cut or damage tube.
3. Use it as is and not worry about mid centering ring being glued to fins. Sounds crazy, but I don’t think the original IV’s had a mid ring at all.
4. Move motor tube back so center ring is in correct place and cut motor tube down. By far the easiest. Negative is GC moved back. As it is, a 360 motor doesn’t project out the existing tube and a 760 goes way beyond. Cutting down the motor tube the 360 is still completely enclosed.
5. Throw it all away and quit rocketry (just kidding)

6. Buy a new tube, cut two new rings, and start over (credit to Ron)​

Anything except #5. But #2 sounds like a lot more trouble than it's worth. I think, if it were me in this fix, I'd go with #4.

No, that's a lie. I'd tell myself that I should go with #4, then I'd actually go with #1 while telling myself that I'm being unnecessarily cautious and I should have gone with #4, or maybe #3, but what the hell I've already got the extra ring cut so I might as well put it in.

Really, #1, #3, #4, and #6 are all perfectly good solutions. The added mass (#1) and CG alteration (#1 and #4) are utterly trivial. Do what makes you happy.

 

[SolarYellow]

#6.

Anything but getting it right will always nag at you, and it's never easier to fix than before you make it worse.