A record attempt on the O3400 motor altitude record, currently standing at 75,000 ft. We are projecting 81,000 ft. I am designing and building the fin can to survive Mach 3.5 flight.
Most high-power rocketry fin cans are designed for subsonic or low supersonic flight. At Mach 3.5 the problem set changes fundamentally. The four primary design drivers I am working through are:
Aerodynamic loading. Dynamic pressure at Mach 3.5 produces fin root bending moments and shear loads significantly beyond what standard HPR hardware is rated for. I am sizing the fin geometry and attachment architecture against these loads directly.
Flutter margins. Fin flutter is a aeroelastic instability that can destroy a fin can within milliseconds above a critical velocity. Establishing adequate flutter margins at Mach 3.5 is a primary structural requirement — not an afterthought.
Thermal effects. Aerodynamic heating at Mach 3.5 produces surface temperatures that affect material properties and introduce thermal stresses. Material selection and thickness decisions have to account for the thermal environment, not just mechanical loads.
Structural sizing. The fin can has to be light enough not to hurt performance, stiff enough to maintain flutter margins, and strong enough to survive the full load envelope. These constraints pull in different directions and the co-cured architecture is the solution I have landed on to satisfy all three simultaneously.
OpenRocket flight simulation — O3400 to 81,000 ft
The fin can uses a co-cured carbon composite sleeve bonded directly to the motor tube, with the fins integrated into the sleeve during the cure cycle. This eliminates mechanical attachment interfaces entirely — no bolts, no fasteners, no stress concentrations at the fin root.
The rationale is straightforward: at Mach 3.5, every mechanical interface is a liability. Bolted fin attachments introduce stress concentrations, require precise torque control, and can loosen under vibration and thermal cycling. A co-cured sleeve removes all of that. The fins and the motor tube become a single structural element.
[ As design progresses, add: layup schedule, fibre orientation rationale, tooling approach, cure cycle. ]
[ Co-cured sleeve concept ]
[ Fin geometry — OpenRocket ]
The project is in conceptual design. The vehicle architecture is defined, the motor is selected, and the flight simulation is established in OpenRocket. I am currently working through the structural and aeroelastic analysis that will drive the fin geometry and layup schedule decisions.
This page will be updated as the design matures and hardware is produced.