Parachute sizing ensures that the rocket descends at a safe landing velocity, preventing airframe damage while avoiding unnecessarily large canopies. For EuRoC, correct sizing is essential for both recovery and compliance with competition safety rules.
At terminal velocity, the drag force balances the rocket’s weight:
W=21CdρAvd2,
where:
- W = weight of the rocket (N)
- ρ = air density (≈ 1.225 kg·m⁻³ at sea level, decreasing with altitude)
- Cd = drag coefficient of the parachute
- A = parachute area (m²)
- vd = target descent velocity (m·s⁻¹)
¶ Drift and Deployment Altitude
Deploying the main parachute directly at apogee (3 km or 9 km for EuRoC categories) causes the rocket to remain under canopy for several minutes. Even light winds can result in kilometres of horizontal drift, potentially carrying the vehicle outside the recovery zone, posing a safety hazard and complicating retrieval.
To reduce drift, teams generally use either a dual-deployment system, or
a reefed parachute for which full inflation is delayed.
Dual deployment uses two distinct parachutes:
- Drogue parachute: A small, robust chute deployed at apogee to stabilise the rocket and limit descent to around 20–30 m·s⁻¹.
- Main parachute: Deployed a few hundred metres above ground to slow the final descent to 5–8 m·s⁻¹.
This approach sharply reduces drift while keeping recovery loads manageable. It does, however, require reliable altitude-sensing electronics and redundant ejection systems.
A reefed parachute is a single canopy that is partially constrained during initial inflation. A reefing line or cord restricts the skirt diameter, limiting both airflow and canopy expansion. After a preset time or altitude trigger, the reefing line is cut or released, allowing full deployment.
- Faster initial descent: The smaller effective drag produces a descent rate similar to that under a drogue, reducing time aloft and thus horizontal drift.
- Lower opening shock: Gradual inflation spreads the deceleration over a longer period, reducing stress on the parachute and recovery hardware.
- Drag coefficient: When reefed, the parachute’s effective drag coefficient changes, because canopy shape and airflow are altered.
- The parachute must still be sized for the fully deployed phase, which governs the final descent velocity and landing safety.
- During modelling, separate Cd values should be used for the reefed and fully open stages to estimate descent profiles accurately.