FAR December 2nd 2017

Over the years of propellant and motor manufacturing I’ve normally utilized either BATES or moonburner grain configurations. I wanted to try something different and make a monolithic-finocyl grain in order to maximize propellant volume loading and quite frankly, it just seemed cool.

Using Burnsim I developed a geometry for a 98mm motor with a total propellant length of 22.75″ cast directly into the phenolic liner. After I was satisfied with the numbers, I designed a break-apart mandrel using Delrin strips for the fins, an aluminum rod for the central core and a 3D printed base that would position everything during casting.

 

CAD design of mandrel.  Assembled on shaker table.  Cast finocyl grain.

The motor was installed in a 98mm minimum diameter rocket and despite Burnsim showing a low initial mass flux, the motor had an erosive spike at liftoff which produced a nice punch off the rail. The flight was arrow straight and burned for eight seconds due to the small core and large propellant web. Recovery was nominal using my typical single-break dual-deploy using a line cutter. The propellant liner was toast near the fin sections but the case looked good. Future revisions will use a XX phenolic liner or will have a casting tube glued in the lower section of the liner where the fins are to provide added insulation.

 

IMG_2524Liftoff! Soft desert landing. Big Red Bee GPS data.Forward insulator proved to be a little on the thin side.

 

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