A CO₂-driven main parachute separation mechanism for high-power rocketry — designed to reduce pressurized volume, increase separation force, and improve reliability through a custom HDPE sliding blast disk.
Duke AERO's previous recovery system had struggled with weak and unreliable main parachute separations. The root issue was that the CO₂ ejection system had to pressurize a relatively large internal volume before generating enough force to shear the main coupler pins.
To address this, I designed a custom HDPE blast disk positioned between the aft avionics bulkhead and the main parachute. By reducing the volume that must be pressurized, the system converts more of the CO₂ expansion into useful force acting on the coupler, improving separation performance without increasing charge energy.
The blast disk also had to integrate directly into the recovery system architecture. It includes a sealed cable gland for the shock cord and a breakaway VGA connector for avionics wiring, both of which needed to maintain pressure integrity while allowing clean separation during deployment.
Improve main parachute separation reliability by reducing pressurized volume and efficiently transferring CO₂ pressure into shear pin failure, while maintaining sealing and mechanical compatibility with recovery system interfaces.
Defined system-level requirements including expected CO₂ pressure range, shear pin failure loads, allowable volume, and integration constraints with avionics and recovery subsystems. The target pressure bounds were estimated between 21.5 psi and 180 psi, representing the pressure required to break the main coupler shear pins and the maximum achievable pressure in the reduced volume.
Designed a sliding disk geometry with a lip and shoulder interface. The lip seats on top of the main parachute coupler tube while the shoulder sits inside it, allowing pressure from the CO₂ Raptors to push the disk into the coupler and shear the retention pins during separation.
Incorporated two required pass-throughs: a shock cord routed through a submersible cable gland and avionics wiring routed through a breakaway VGA connector. Both interfaces had to preserve pressure sealing while avoiding interference with the separation event.
Selected HDPE for its low-friction sliding behavior against the avionics body tube and sufficient yield strength under expected pressure loads. FEA was performed at a 60 psi pressure load with a 1-inch disk length, excluding the shoulder, resulting in a factor of safety of 1.95.
Planned fabrication from a 6-inch OD, 3-inch long HDPE disk. The outer diameter is turned to 5.947 inches, with a 0.5-inch shoulder turned to 5.8 inches. A miniature separation test with pressure instrumentation was planned to experimentally determine maximum pressure and refine final disk length.
FIG. 1–3 — CAD assembly, FEA stress plot (60 psi), and machined HDPE prototype. Drop your real files in at these paths.
FIG. 4 — Fusion 360 toolpath showing machining strategy for blast disk geometry.
Mechanical function: When the CO₂ Raptors fire, gas rapidly fills the reduced volume between the aft avionics bulkhead and the blast disk. The resulting pressure load is transferred through the disk into the main parachute coupler tube, shearing retention pins and initiating separation.
Material selection: HDPE was chosen for its low friction interface with the avionics body tube and adequate yield strength under expected pressure loads. Low friction was critical because the disk must slide cleanly during deployment without binding against the tube wall.
FEA validation: The blast disk was analyzed under a 60 psi internal pressure load with a 1-inch disk length, excluding the shoulder. Results showed a factor of safety of 1.95, confirming structural integrity within the initial design case.
Pressure uncertainty: The system operates within an estimated pressure range of 21.5–180 psi. Because these bounds depend on difficult-to-model separation conditions, a miniature separation test with pressure sensing was planned to validate the real pressure environment and refine final disk dimensions.
Interfaces: The design integrates a sealed cable gland for the shock cord and a breakaway VGA connector for avionics wiring between the avionics bay and aft control stack. These features preserve pressure integrity while allowing the rocket sections to separate cleanly.
Early design discussions focused on generating more separation energy, but the more effective solution was reducing the volume being pressurized. This shifted the problem from simply increasing output pressure to designing a better mechanical interface for force transfer.
FIG. 5 — CO₂-driven separation test demonstrating shear pin failure and disk motion.
Designing the blast disk required accounting for real machining constraints. Features that were trivial in CAD had to be simplified or reworked to be manufacturable on a lathe while maintaining tolerances and proper fit.
Early coordination with the avionics team directly impacted what was possible. Switching from a U-bolt to an eye bolt created the space and routing needed for the blast disk to function within the recovery system.
While FEA validated structural integrity, the actual pressure during separation depended on transient gas behavior and volume effects that were difficult to model. Instrumented testing was required to understand real performance.
Improving separation was not about generating more pressure, but about reducing the volume being pressurized. This allowed the CO₂ system to more efficiently translate energy into shear pin failure.