The detail that jumped out at me is the casing wall: roughly 0.08 inches of carbon fiber holding together an M-Class motor with 7.392 lb of propellant on board. That is a very thin structure for the power involved, and Andrew Reilley pulled it off. The total mass came out at 10.6 lb, which means the propellant mass fraction landed at 0.73, clearing the stated goal of getting above 0.7.
The build itself is a serious piece of work. The casing was produced on an automated filament winding apparatus and then post-cured at 130 C. Up front there is a CNC-machined aluminum closure, and the nozzle side pairs a brown phenolic retainer with a black graphite insert. The propellant was cast using a custom 83% solids APCP formula, and the core geometry, a four-fin star, was designed in OpenMotor before any mixing happened.
The firing was done upright in a steel test stand, and the numbers look strong: a peak thrust of 417.7 lbf over a burn of 4.976 seconds, adding up to 7298 N s of total impulse. After the burn the graphite throat was still glowing, which says a lot about how hard that little insert worked.
One open item: the maker noted the results did not line up with the simulation. The motor clearly performed, but the mismatch between the OpenMotor predictions and the measured numbers is the loose end I would want to see chased down in a follow-up.
Overall this is an ambitious step from 2 inches up to 3 inches for more thrust and impulse, and the execution shows.
Project by Andrew Reilley. Watch it on their YouTube channel: