Modified Wildman DD - advice?

[mparker59]

So, I got this Wildman Dual Deploy kit in a kinda weird deal and I'm modifying it a bit.

I left out the motor mount, cut the fin tabs down and am tip to tip glassing them so that I can fly it on 75mm. I cut the booster tube down and put in a coupler and bulkhead so that I can run a bolt down from the bulkhead into a threaded forward closure for motor retention. Put in a little section of new tube to provide a payload bay for the drogue and the rest of it is stock.

But then I was looking at the motor mount and the tailcone retainer that the previous owner had bought. So, I think I oughta be able to make a 54 mm adapter for it and given the retainer comes with a 54/38 adapter maybe even go there if the weight works out.

The plan would be to put a kevlar harness on the adapter just long enough to reach a bolt on the bulkhead at the front of my booster. This is just to retain the adapter while the adapter retains the motor. I'm pretty sure it'll all be OK on the recovery end. But, during the "up" part all of the force of the motor will bear on the inner ring of the retainer which will transfer it via the retainer's threads to the tailcone, which will bear on the back of the rear body tube. Seems like there's a couple of points of failure there and a high thrust motor might find a way to fly up inside the booster body tube and ruin my day. I could cut a section of 3" coupler just the right length to bear on the forward centering ring and transfer the force to the coupler at the front of the booster. It adds weight and cost, but is a fair bit more bullet proof.

What do you all think? Do I need a photo or can you picture an Aero Pack tailcone retainer transferring all of the thrust of a 54mm motor to the body tube without benefit of glued in centering rings? You have to admit that as designed I'll have all of the thrust of a 75 mm motor delivered to the back of the body tube by the rear closure of the motor - is it so different?

Thanks,

Mike

 

[troj]

Transferring the thrust to the tube itself, rather than a centering ring, is a much better method. If the airframe can't handle the thrust, you've got serious issues, whereas a centering ring is reliant on a mechanical bond between the ring and the epoxy, to transfer the thrust to....the airframe.

It sounds like you have no retainer on the main body of the rocket, so what I'd suggest is two centering rings on the rear of your adapter, sandwiched together. The upper one has an OD that matches the ID (75mm) of the airframe. This helps align things. The rear one has an OD that matches the OD of the airframe itself. This way, you know you've got a bearing surface across the center bottom of the airframe, rather than just the inner lip.

Butt your retainer up against that CR stack, and you have direct thrust transfer from the motor, though the retainer, to the rear CR to the airframe.

-Kevin

 

[rfjustin]

Mike,

Send me a PM with your email address, and I can send you some pics of my 75mm minimum diameter glass bird. The pics will show a workable solution to your 75mm motor retention and 54mm adapter question.

Justin
hprjf AT aol DOT com