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Casting KNSb grains for 8" static test motor

In February 2020, I obtained a 26' section of 8" ID commercial filament wound fiberglass pipe rated to 600 PSI continuous, 1200 peak rated from my favorite aerospace surplus yard.  I designed two 7 grain motors and a 4 grain motor and with the assistance of the Mt. SAC rocket team, cut the pipe into the appropriate sections.  The 4 grain case was to be used for static testing to prove that the pipe could serve as an effective motor casing for ~550 PSI KNSb motors. 

After a lengthy break due to COVID-19 work commenced on the static test motor in Mid June.  While I have made numerous 4" and 5" KNSb motors, 8" is a larger scale.  The motor was designed to use a standard Bates grain configuration.  Sono tube was sourced that fit loosely in the motor tube which would allow for a Kevlar liner to be bonded to the casting tube.   This technique had been under development since 2017 and tested with repeated success with 4" and 5" motors in 2019-2020.


In this next photo, the sono tubes have been cut to length using a sled for the vertical band saw and are laid out next to the static test tube with the raw material for the bulkhead, nozzle and retaining rings. 

Since the sono tube is a fairly thin casting tube, a jig was designed to support the tubes and keep them round during the pour.  This jig was laser cut and assembled with hot glue. Acrylic disks that were a tight fit to the tube were laser cut with a 1/4" hole in the center to provide a flat, non-stick surface for the bottom of the grain. 
The core was inserted and a piece of 1/4-20 threaded rod ran through the center to tension and center the core mandrel.   A 5/16" thick piece of hardboard was cut to provide a strong base so that the grains could be moved after they were poured.  

Once the grains were complete it was time to pour them.  All of our large propellant fabrication is done at the FAR site where we have access to appropriate safety equipment and a explosive magazine to store the finished propellant. Rick Maschek supervised the formulate on and mixing of the propellant while I kept 5 cookers running.  With the help of a number of FAR members including Davie Bauld and Eric Beckner, we poured 135lbs of propellant in just under 5 hours. 
Here Eric and the Author are pouring a 8" grain.  KNSb when prepare properly pours like a thin milkshake making it very easy to work with.  Each grain was weighed with a target of 97% of theoretical density. 

Once the grains were poured, they were carefully moved to the explosive magazine to cure for one week.  The next step would be to create all the motor hardware and install the Kevlar liner. 






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