Wednesday, November 18, 2009

Motor Mount

While the MLAS MK-70 motors were mounted in a spidery-looking cage, the mount for the µMLAS model is a more conventional 3 x 29mm cluster centered in a 3" phenolic airframe. The centering rings came from Giant Leap and the tubing from PML. The tight cluster didn't leave much room for centering ring web or epoxy fillets to carry the thrust of any individual motor, so I added a single wrap of fiberglass to the motor tubes to transfer loads between them. In the original design, the forward end of the assembled motor mount extended beyond the front of the Boost Skirt through a fiberglass sleeve in the Coast Skirt centering rings to provide support to the base of the CM and keep it from dropping out of the Forward Fairing in flight. Unfortunately, this arrangement also provided a place for the Coast Skirt to bind at separation. I shortened the motor tube after-the-fact and epoxied the section of 3" tubing removed from it into the Coast Skirt sleeve to support the CM. Drawings in the µMLAS package are of the updated design.

Motor retention is provided by a screw and washer centered between the three motors. The screw threads into a brass insert embedded in Epoxy Clay in the cavity between the motors.

The real MLAS' motor nozzles were just visible beneath the edge of the Boost Skirt, so I added four dummy nozzles to the model's aft centering ring to duplicate the look. I cut these from pieces of McMaster-Carr 1" OD, thick-walled phenolic tubing with Estes BT-20 "crayon" nose cones inserted to duplicate the motor nozzles. I could have omitted that last detail, I suppose, but details like that are half the fun of scale models. And besides, I had plenty of weight margin.

So I thought, anyway.

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