Prepping filament wound fiberglass.

mtnmanak

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How do you deal with a three CR mmt? I suppose I could use a rubber mallet!

Steve has you covered if you are starting from scratch, but did you already adhere all the CRs to the motor tube? If so, you can try this (it is a bit tedious, but if the CRs are already epoxied on, it may be your best bet):

- Line the first CR up in the body tube and mark off the three spots it is going to hit the fin slots
- Take a sanding block and lightly flatten out the CR at those three marks until it slides into the body tube and past the fin slots
- Repeat with the other CR, if necessary

Using the method Steve described is the best way (in my opinion) when you have everything loose, but if the CRs are already epoxied to the motor tube, the flattening technique works best to minimize the amount of gaps you are going end up with between the CRs and the body tube.

Basically, you will end up with CRs that look like this (flattened areas exaggerated for effect):

1657944453931.png
 

Steve Shannon

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Steve has you covered if you are starting from scratch, but did you already adhere all the CRs to the motor tube? If so, you can try this (it is a bit tedious, but if the CRs are already epoxied on, it may be your best bet):

- Line the first CR up in the body tube and mark off the three spots it is going to hit the fin slots
- Take a sanding block and lightly flatten out the CR at those three marks until it slides into the body tube and past the fin slots
- Repeat with the other CR, if necessary

Using the method Steve described is the best way (in my opinion) when you have everything loose, but if the CRs are already epoxied to the motor tube, the flattening technique works best to minimize the amount of gaps you are going end up with between the CRs and the body tube.

Basically, you will end up with CRs that look like this (flattened areas exaggerated for effect):

View attachment 527928
Great advice.
 

CaptHaywire

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If you sand the forward centering ring so that it passes freely through the fin slots then it may be rather loose on the other side. Some force applied is normal.

I find sanding a slight bevel on the forward CR helps.
 

mtnmanak

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Nothing has been put together yet! I'm very grateful for all your advice!

Very cool - follow Steve's advice. The pictures and discussion in these Darkstar threads on my site may help - I pretty much used the technique Steve described above on all of them, but, with split fins on the Darkstars, you are dealing with 4 CRs per rocket and I injected the internal fillets on a number of them, so there may be some things on those builds that will give you more ideas. Lots of photos to guide you:

 

DeltaVee

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So as mentioned I have three CRs. Two at the fore and aft end of the fin tabs, one near the front of the motor tube. As mentioned nothing has been epoxied yet. As the main body is 48" long, it's not clear to me how to get the forward ring in (assuming I epoxy the middle ring in) in a precise location and not having it move during curing. Of course the harness will be attached to that ring and I guess I could lower it in via the harness and tie it off. I was half-joking about the mallet but I'm wondering.... since it seems possible to push the rings in so long as you apply enough force to them then perhaps I just need to use a bit of brute force on the MMT to push it into place... not necessarily a mallet but a a sustained continuous force applied from the top of the airframe tube (i.e. put beads of glue in the airframe, then lower it onto the mmt (in an upright position) and then apply significant weight to the top of the airfram.)

Have you guys ever had things get so stuck out of position that it wasn't possible to get the mmt completely into position?

Oh and what do you use to mark the tube(s)? I'm finding pencil or ballpoint doesn't quite cut it! Felt tip perhaps?
 

Steve Shannon

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A long tube that’s smaller in diameter can be used to help push centering rings into position in a long body tube.

Definitely look for build threads by other people building the same model to see what issues they had and how they have overcome them.

I have always gotten the MMT into the right position, but I do a lot of test fitting as I go. I have epoxied fins in slightly off and not noticed it at first. I dry fit everything until I’m sure and I practice the order of assembly until I know it’ll work. You might laugh (I wouldn’t blame you) but I have even written instructions for myself for how I see things must be assembled, especially when the kit builder hasn’t or when I disagree with the kit builder. Also, I very seldom use 5 minute epoxy unless it’s just to hold something temporarily until I can get some 30 minute epoxy on it.

I use Sharpies to mark everything. Graphite from pencils inhibits epoxy adhesion. Ball point ink is hit or miss, but Sharpies work every time.
 
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DeltaVee

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Now that I've cleaned off the graphite markings from the motor tube (it was sanded down so pencil marked it easily!)... I had gone ahead and marked the tube and it was at that point I thought to ask since graphite does make a good lubricant after all...

I think I'm sold (more or less!) on your technique of epoxying one of the forward rings in place and shimmying the mmt into position. One interesting thing seems to be that I can pretty easily get the ring that is supposed to rest on the forward edge of the fin tab into position at the front of the slots fairly easily from the front. All rings when placed at the forward end of the lower airframe slide right down into the middle of the slots. One of them went all the way to the floor but I could see it tilt somewhat when it hit the slots. This makes me think I could put the two forward rings in place and drop the mmt in from the front then dump epoxy on the forward cr. In all cases they are a bit loose so I *think* that enough epoxy will oozed in between the ring edge and airframe to form a reasonably good joint. Unf. I would have to drill some holes to get fillets on the ring/tube joint between the rings but that's certainly doable.

Finally irony btw: If I take ANY of the rings, and slide them in canted into the airframe ahead of the fin slots, straighten them so they are horizontal, will slide FAR more easily thru the fin slots than if I try to push them in from the back end... iow they seem to have a ridiculously easier time sliding in from the front to the back than the reverse.... seems bizarre to me!
 

Steve Shannon

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I have also slid forward rings in dry, poured in epoxy, and spun the rocket to let inertia (“centrifugal force”) distribute the epoxy along the joint between the BT and CR. Any port in a storm!
 

jmasterj

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Finally irony btw: If I take ANY of the rings, and slide them in canted into the airframe ahead of the fin slots, straighten them so they are horizontal, will slide FAR more easily thru the fin slots than if I try to push them in from the back end... iow they seem to have a ridiculously easier time sliding in from the front to the back than the reverse.... seems bizarre to me!

That matches my experience too. I think epoxying the front and middle centering rings to the motor tube and then sliding it in until the middle one is right in front of the fin slots is the way to go. You'll be able to get epoxy on both of those rings afterward and you'll be confident that the motor tube is in straight because you'll have two rings on it.
 
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