In the fall of 2018 I was tasked with leading the redesign of UVic Rocketry’s (UVR) stabilizing fins in order to undergo supersonic flight aboard the rocket Hyak-1. Hyak-1 was UVR’s first entry into the 30,000 ft commercial-off-the-shelf (COTS) category of the Spaceport America Cup, and so would be undergoing significantly higher loading conditions than UVR’s rockets had previously seen, including a maximum air speed of Mach 2.
The old fin design consisted of a 3D-printed PLA core over-wrapped with one layer of carbon fibre composite material for stiffness. This offered very short lead times for manufacturing, in addition to flexibility surrounding the shape of the fin and ease of attachment since the fin bolted externally to the rocket. A major focus of the redesign was ensuring that the new fin design did not fail under the increased loading and flutter that can occur at higher velocities, in addition to preserving the modularity and ease of assembly of the previous design.
Following a literature review and benchmark testing of the old fin design under bending conditions, it was decided to keep the general fin design but undergo modifications to make the fins stiffer and stronger through additional layers of over-wrapped carbon fibre and stiffness inserts in the center of the 3D-printed mold. Extensive bending and stiffness testing of the fins was performed to validate the design against expected aerodynamic loading and flutter failure modes. This testing was necessary since the use of unconventional materials such as carbon fibre composites and 3D-printed PLA made FEA simulations difficult and inaccurate.
Unfortunately, due to manufacturing defects in addition to the insecure fastening method, the new fin design failed during the powered flight phase of Hyak-1’s launch. It appears that the front of the fin did not sit flush to the fuselage, which resulted in an unforeseen high aerodynamic force that ripped the fins out of their fasteners. Once the fin fell off the rocket, the flight became unstable and the rocket underwent a rapid unplanned disassembly.
The lessons learned from this failure were applied to the redesign of the fins for Hyak-2. These second-generation supersonic fins consisted of a 1/4″ carbon fibre plate fastened to a machined aluminum foot. This provided a much more secure connection between the fin and fuselage, in addition to a tight fit between the foot and fuselage. This design was able to easily survive the aerodynamic and aeroelastic loading during flight, in addition to the ground impact loading upon landing.
