As the Structures Lead, I own the end‑to‑end structural development of a 10k ft Commercial‑Off‑The‑Shelf hybrid rocket headed to the International Rocket Engineering Competition (IREC) in June 2025. Since team formation in November 2024 we have delivered a flight‑ready prototype, secured sponsorships, and sourced manufacturing partners.
| Sub‑system | Material / Method | Why | Source |
|---|---|---|---|
| Airframe (4″ Ø) | G12 filament‑wound fiberglass tube | Radio‑transparency | Wildman Rocketry |
| Nose cone | 3:1 Fiberglass | Optimized σ/weight & k/weight | Madcow Rocketry |
| Bulkheads | Birch Plywood | High modulus | CNC Laser Cutting |
| Fins | Birch Plywood | Easy Manufacturing | CNC Laser Cutting |
| Centering rings | PETG 3‑D printed (100% infill) | Rapid iteration | 3D Printed |
| Bonding | West System 105/206 epoxy | Aerospace pedigree | West Systems |
Primary Objective – Achieve the smallest absolute error between predicted and actual apogee while remaining within 30% of the 10k class target. OpenRocket simulations predicted apogees between 10 459 ft and 11 155 ft AGL depending on wind shear.
A three‑fin clipped‑delta layout in filament‑wound fiberglass was selected after a Pugh matrix scored options against stability, drag, ease of manufacture, and cost. Three fins posed no stability nor aerodynamic performance changes when compared to 4 fins, so the team opted for the simpler design of 3 fins. Fiberglass beat carbon because the hybrid's nitrous vent antenna needed RF transparency and the team lacked an autoclave for high‑temperature cure.
Commercial G12 FWFG tubes from Wildman were adopted as the primary structure. Because the vendor publishes only basic tensile data, the laminate was reverse‑engineered: ±45° balanced ply assumption, 80 / 20 fibre‑to‑resin, E‑glass with West System 105 / 206 epoxy. Ideally we would utilize coupon tensile tests (ASTM D3039) to confirm if the laminate's strength properties agreed within predictions, validating the CLT model. However, due to time constraints, the team opted to use the vendor's published data.
Each material choice closed a specific requirement: radio transparency for avionics, manufacturability inside the six‑month window, and safety factor under 256 lbf max thrust.
First‑pass hand calculations sized the fins and body tube using the Barrowman method and Euler buckling respectively. With q = 6.6 kPa (Mach 0.92 at 3 700 m) each fin sees 99 lbf normal load, yielding a bending stress of 15 MPa—well below the birch allowable of 34 MPa. To simplify normal force acted on each fin, we set the following assumptions: air flow is inviscid, composed of an ideal gas, subsonic, and laminar, angle of attack is sufficiently small (≤10°), quasi-static loading is justified since rate of change of aerodynamic forces and angle of attack do not change rapidly, steady-state flight conditions apply at a single instance during steady-angled flight (no significant acceleration or angular velocity change during instance), fins act as thin flat plates and its wall boundaries are smooth, primary aerodynamic loads occur normal to fin surface.
Nf = q · A · CNα · α ≈ 440 N
Moving from the ground-proven 3.34 g (main) and 2.12 g (drogue) to 4.5 g and 4 g lifts chamber pressure by ≈ 35 % and ≈ 90 %, enough to cover altitude, cold-soak, and binding uncertainty while staying well below coupler stress limits. The pressures produced by 4.5 g/4 g translate to bulkhead forces several times the value of our nylon plastic shear pins, so the pins still fail cleanly without overstressing the airframe. Redundancy-wise, we loaded 6 g backup charges for both bays; even if a primary mis-fires, the secondary delivers more than the required force to guarantee deployment.
Build operations crammed into forty‑eight hours on‑site after dust storms shredded the range schedule. Rapid joints used medium‑cure USComposites 3∶1 epoxy. An internal epoxy drip between aft tube and coupler later blocked motor insertion; the defect was corrected by diamond‑reaming and finish‑sanding with 800 → 1200 grit.
Thickness checks across thirty points showed the body-tube walls averaged 0.064 in—about 6.25 % thicker than the manufacturer’s nominal value and within our ±1⁄16 in measurement tolerance—while the couplers held at 0.060 ± 0.004 in. Because the tubes and couplers came from different vendors and the tubes ran slightly heavy, we had to sand the couplers extensively to slide smoothly without creating an airtight seal that could trap a vacuum. Future builds will eliminate this mixed-vendor mismatch by single-sourcing all airframe stock.
Go / No‑Go matrix enforced green status on avionics continuity, motor retention, and telemetry lock before pad departure.
Top open risks at final review were: (1) coupler tolerance in desert grit, (2) nitrous vent icing, (3) team fatigue. Mitigations included oversize sanding, water‑trolley vent visualisation, and a two‑shift operations roster. Spare shear‑pinned nose module and redundant igniters were on‑site.
Launch was scrubbed minutes before L‑0 when range closed at 12:00 due to severe dust–storm gusts. No telemetry beyond pad diagnostics; therefore trajectory comparison is not available.
Scrubbing the first flight was disappointing but invaluable. The coupler fit issue, vent‑hole placement, and motor length mis‑match exposed systemic gaps in BOM control and schedule float. Next cycle priorities are:
TL;DR – Built a 4″ hybrid to 10 k ft spec, scrubbed by desert storm; lessons learned embedded above, next flight scheduled Fall 2025.