Semroc MMX build: some observations

[graylensman]

I am currently building the Semroc MMX Cherokee and Blue Bird Zero. Some observations about the builds so far:

The fin jigs. At first I was skeptical, but quickly realized the value. The hardest part has been sliding the fins into place; the jig slots are loose on the body tube but really tight on the fins. For the small forward fins on the Blue Bird Zero, I set all three in place on the mark with one jig, then slid the second jig into place, to line up the front ends of the fins. I'm using white glue, so I had some leeway before the glue set. Naturally, I figured this technique out last; I wish I thought of it at the beginning of assembly. One last point: it's helpful to have at least three jigs to work with - two for fin placement and a third to hold up the far end of the body tube.

The centering rings. These are made of layers of paper, but not in the way larger Estes-type rings are made. I had to sand them down in order to fit into the body tubes, and one ring came apart, requiring an application of glue to restore it. This has been the only disappointment in these kits.

Shock cord attachment. This made zero sense to me, not in a philosophical way but from an assembly standpoint. I just couldn't figure it out. I opted to attach them in the Fliskits fashion.

 

[jeffk813]

Does this illustration help at all with the shock cord mounting technique? It does seem to be a bit tricky to comprehend without an illustration...

You basically tie one end around the forward centering ring, glue it on the MMT and then install the whole thing in the body tube. The kevlar is thin enough to do this without too much trouble.

 

[hcmbanjo]

The Semroc MMX instructions have you tie a "double" overhand knot in the end of the Kevlar line.
The knot is set into the motor mount tube. The engine block is glued in, the end flush with the end of the tube.
This leaves the knotted Kevlar line held at the top of the engine mount with the knot under the low end of the engine block.

 

[jeffk813]

Then I guess my illustration wouldn't be too useful then.

 

[kitbasher99]

Don't worry Jeff. I'll let it slide this time.

 

[jeffk813]

LOL thanks Phil. I was actually thinking of another, older MMX kit I have that has you do the shock cord that way.

 

[graylensman]

hcmbanjo: Thanks for the clarification. I think part of what didn't make sense to me was placing the kevlar shock cord right there against the motor; that seems like you're asking for the cord to burn through. Maybe MMX ejection charges aren't that potent?

 

[hcmbanjo]

I've had Kevlar burn through before, even on MMX rockets.
The MMX ejection charges are strong enough, I guess.

I'll be doing a build of the Semroc MMX Blu Bird Zero on my blog soon.
Some of your observations are the same as mine.

 

[jeffk813]

Even with kevlar, I still use a little bit of wadding. Kevlar may be flame resistant, but plastic streamer material isn't.