A build I'm working on is planned to use a 22" length of BT-70 and the motor mount will be BT-50. Plans are to launch it with D12 and maybe E12. I thought with that large airframe I would extend the motor mount tube to form a stuffer tube and use up some of the extra volume in the tube. I cut the tube to 12" length and made 2 centering rings, the first ring I glued to the front of the tube. So now the plan is to have a 12" long motor mount with cardboard centering ring at each end. I'm wondering if I should have a third ring and locate it near the location of the thrust block. I can do it but it would complicate construction just a bit. I usually like to build things strong so now that I'm thinking about it I probably should go ahead and do it.
stuffer tube / motor mount construction ?
- Thread starter bjphoenix
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To get at the centering ring located at the thrust block I cut both the BT-70 and 50 about 35mm above the thrust ring location. This give me access to fillet the middle ring from te top side and the bottom side. Once glued in place I put the bottom centering ring in place to align the motor tube inside the main tube but don’t glue in place until the glue has set on the on the middle ring. I have previously cut two notches in the lower centering ring, one for the engine clip and other on the opposite side to help pull the ring out with a wire hook for placement of glue on the inside of the main tube and outside of the motor tube before slipping the bottom centring back on. Once in place I then glue the exposed bottom side of the centering ring. The top end of this gets two home made couplers installed. The motor mount and stuffer tube coupler gets a 50mm long coupler glued inplace and the BT-70 gets a 60mm coupler. Once these are dry I glue the rest of the BT-70 back on and then glue the stuffer tube in place and immediately glue the top centering ring in place to maintain alignment.
A stuffer tube is always a good idea. The ejection on black powder motors is all over the place and I've had more than my fair share of weak ejections. For a simple BT70 rocket, I don't think there is any benefit to adding an additional ring near the motor block.
The only time I use additional centering rings is when the stuffer tube extends into multiple body tubes. At which point I'll use the lightest possible centering ring I can find. Often times these are made from file folder material or card stock. They don't really do anything other than keep the motor tube centered. It's really the upper ring that need to be strong.
The only time I use additional centering rings is when the stuffer tube extends into multiple body tubes. At which point I'll use the lightest possible centering ring I can find. Often times these are made from file folder material or card stock. They don't really do anything other than keep the motor tube centered. It's really the upper ring that need to be strong.
I still prefer to have a centring ring made of premium sofa cracker box approximately 1.2mm thick installed at the top of the motor clip. There is a lot of force to transfer from the thrust block to the main frame tube and I don’t want the motor tube disengaging from the main tube and travelling up thru the rocket.
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Thinking outside the ring, consider adding three or four radial “vanes” or spokes , with grain PERPENDICULAR to long axis, along the tail end of the motor tube, say length of the inserted motor segment. These should have a diameter that JUST BARELY fits. Glued and filleted to the motor mount,
Apply glue INSIDE the outer body tube where these will be, and slide in as one piece with the mount. Practice a few times before gluing, and consider white glue or epoxy to avoiding “grabbing”.
Insert. You won’t have an outer fillet, but should be more than enough to transfer force from mount to body tube.
Alternative is just put you three rings on the motor mount, fillet to motor mount. Slide in forward ring, drop a layer of white glue or epoxyinside tail of body tube, and slide it in. Leave rocket nose up.
The middle tube will “push” the glue upward and create its own fillet on top (noseward) of the middle ring. Should be more than strong enough.
If you are really careful you can do this with polyurethane glue glue. But you definitely don’t want this to get OUTSIDE the body tube. It makes a nice internal tube bond.
Apply glue INSIDE the outer body tube where these will be, and slide in as one piece with the mount. Practice a few times before gluing, and consider white glue or epoxy to avoiding “grabbing”.
Insert. You won’t have an outer fillet, but should be more than enough to transfer force from mount to body tube.
Alternative is just put you three rings on the motor mount, fillet to motor mount. Slide in forward ring, drop a layer of white glue or epoxyinside tail of body tube, and slide it in. Leave rocket nose up.
The middle tube will “push” the glue upward and create its own fillet on top (noseward) of the middle ring. Should be more than strong enough.
If you are really careful you can do this with polyurethane glue glue. But you definitely don’t want this to get OUTSIDE the body tube. It makes a nice internal tube bond.
