BMS 3" School Rocket
 

Last night I started on a BMS 3" School Rocket. I'm planning to have the kids in my rocketry project group build them this year, and I hate to have them build something I haven't built. I have to say, it's a very solid feeling kit with good instructions.

The only place I've deviated from the instructions is that, instead of using masking tape for engine hook retention, I'm using upholstery thread and yellow glue. It's my new favorite trick for sturdy engine hook attachment; beats a mylar strip for strength, and defeats masking tape hands down.

Also learned how to tie a bowline knot from this kit... never really knew I needed it before, but now that I've done it, I plan to do it again.

Tonight I put the engine mount I assembled last night into the body tube, and then attached the TTW fins. When you have laser-cut fin slots on a rocket, and the fins trail behind the body tube. there is one really easy way to get those fins to dry straight... use straight edges and clips. The rubber band holding the fin tips to the body was included in the kit (holding the bundle of fins together).



No, as you can see, I don't sand the edges of the fins before assembly. I have a problem with curving the edges when I do that, whereas I seem not to do so when sanding the fins on the rocket.

Could you take a picture from the aft end when it's dry and you've released the clamps? I'd like to see how orthogonal the pairs of fins are. Please.

It's a special interest of mine :-)

Here you go:



Not perfect, but probably about as good as I could have eyeballed it, and I didn't have to mess around much getting it done.

Well, it looks like yours are a lot more perpendicular than my last set were.

I like the 3" size - I can fit my hand in.

Something to keep in mind...

Often, the fins can be lined up perfectly, but when you remove the clamps, pent up forces, due to glue shrinkage, will pull them out of alignment. Or, if the glue wasn't fully cured when the jig was removed, subsequent curing may pull them out of alignment.

This applies mainly to yellow/wood glue.

Several times I've had what I thought was a perfect alignment only to see some misalignment a couple days later.

Doug

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YEP, I've seen that too. Pisses me off every time it happens.

When that happens, heat it up with a heat gun or hair dryer and it will relax the glue enough to get them straight.

And next time, use 30 minute epoxy, at least for fillets. I've seen folks bring their rockets to launches where the temperature is high. They bring their rockets out of the car where they have been lying on their sides in the heat, and all of their fins are out of line; they had used aliphatic glue (yellow) or something akin to Elmer's.
Not a huge problem when using epoxy, at least for me. The tiny bit of extra weight is well worth it to me.

Allen

+1 to what Allen just said.
EPOXY for fillets.
I have even switched to that for 18mm LPR fillets.
They are perfect single-pass fillets EVERY time.
Fins never break off the tube either.

Who is telling who what to do? :D