Construction question on the BSD Little John

[Murrill]

I recently purchased the BSD Little John and am in the very early stages of construction. I'm confused (a frequent state) about the placement of the centering rings on the MMT, and about the location of the fin slots on the airframe. I'm hoping some of you have built this rocket before and can help me out.

The instructions call for the rear centering ring to be placed 1/4" from the rear of the MMT. The ring itself is 1/4" plywood; that means that with the ring in place, the distance from the rear of the MMT to the forward face of the centering ring is 1/2".

The instructions next call for the fin slots to be cut on the airframe to within 3/16" of the rear of the tube. Then, the MMT is epoxied into the airframe with the rear centering ring flush with the rear of the fin slots.

So, if I follow the instructions, the rear centering ring will not fit entirely inside the airframe; instead, the last 1/16", as well as the entire rear 1/4" of the MMT, will project out the rear of the airframe.

Just about every HP rocket I've ever built had the rear of the MMT flush with the rear of airframe, and certainly every HP rocket (and every model rocket, for that matter) had the centering rings entirely inside the airframe.

I feel stupid asking this question, I'm certain I'm just mis-reading something and I'll feel like an idiot when someone points out my mistake. But I've built a few HP rockets before, I would like to think I know what I'm doing, and I really can't figure out what I'm missing.

Please show me the error of my ways.
Thanks in advance.
John

 

[Ryan S.]

you can build it your own way. If you want top put the motor mount flush, just put it 1/8" up the MMT. If you do this, I would recomment a good way to retain you rmotors, because an eighth inch of BT isnt much for the tape to grab to

 

[Mike Fisher]

I know these kits used to come with 3/16" centering rings so that probably what the instructions were written for. If you indeed do have 1/4" centering rings, then following the instructions exactly will be a problem.

If you do indeed have 1/4" centering rings, only slot the airframe tube to within 1/4" of the rear of the tube. Then the aft ring will sit flush.

Don't worry about the extra bit of motor tube that sticks out, that's the way it is supposed to be. In fact, I leave an extra 1/2" sticking out in case I want to bond a slimline or aeropack style motor retainer to it later.

Hope that helps.

Mike Fisher

 

I ran in to the exact same problem with my 3" BSD THOR, Murrill. Only I didn't catch it beforehand like you did! I followed the directions exactly, cutting in the fin slots at 3/16" from the end of the tube.

I deviated a bit and left the rear centering ring off so I could do internal fillets. When I went to install the rear centering ring, that's when I discovered the problem, and that the ring was 1/4" thick.

To fix it, I ended up cutting dado slots in the fin-side of the rear centering to let the fins in to the ring and allow it to sit flush with the end of the BT.

Don't feel stupid. Your asking the right kind of questions!