This past weekend, I pulled out all of the kit's main airframe parts, weighed them, and measured them to determine body tube internal lengths and mounting points.
I have settled on Option 2 as outlined two posts back. This means that the model will be powered by a single core motor and a 13mm motor in each of the SRBs. The latter will be fixed to the core and will not jettison during flight.
Keeping it fairly simple.
The model airframe parts and required internal body tubes weighed in at 154 grams total. With the addition of various motor mount parts, launch lug, clear fins, glue, paint, and however much nose weight might be necessary, I estimate the total weight to be in the neighborhood of 198 grams (7 oz.).
According to published motor specs, this is pushing it for even an Estes C5 package.
Therefore, the core section will get fitted with a BT-50 tube, just in case I need to bump up to a 24mm D motor.
The inside diameter of the core section measures 37.5mm, while the SRBs measure 14.5mm, just a tad bigger than a BT-5.
Cutting the tubes...
Using a crude shop-built version of the 'Kuhn Cutter', I ended up with a 6.3cm section of BT-55 for a parachute compartment, a 25.9cm BT-50 core motor tube, and two 50.8mm BT-5 sections to fit in the SRBs.
After dry-fitting the custom cut tubes, I had to hit the plastic core halves with a Dremel tool to knock down the top edges of some of the internal alignment pins/sockets to allow the BT-50 tube to properly center. The two forward pins had to be removed entirely to make room for the BT-55 parachute compartment.
Assembling the internal tube system...
The front end of the BT-50 was mated to the BT-55 via a 5055 centering ring.
Two centering rings for the motor tube are hand cut from 1/16 basswood stock. Both are positioned and glued in place.
The forward ring is placed so that it butts up against the plastic square posts which are the future attachment points for the SRBs.
Rather than cut another thin ring to center the section of BT-55, I chose to glue on several 3/32 balsa strips instead. These should do the job nicely.
Finally, a motor block and a .020 internal music wire motor hook is installed into the aft end of the motor tube.
Once all the glue is completely dry, it's time for a dry-fit test of the completed assembly.
Perfect. The model airframe halves fit together nicely, and the internal tubes are snug and well centered.
Next post: gluing it all up...