Sizing a parachute for a target descent rate
A rocket under a parachute reaches terminal velocity almost immediately after deployment — the point where drag exactly balances weight and the descent rate stops changing. That relationship gives us a direct way to size a canopy for a chosen landing speed.
In design, you already know the descent rate you want and need to solve for canopy area instead:
Target descent rates are largely a function of what you're protecting: roughly 15–20 ft/s (4.5–6 m/s) is a common target for a durable, all-up rocket recovering as one piece, while a fragile electronics bay or a rocket recovering in two heavy sections is often sized for a gentler 10–15 ft/s (3–4.5 m/s).
Choosing a drag coefficient
Cd depends on canopy shape — but published values for the "same" shape can vary by more than double between sources, because manufacturers and researchers don't always use the same reference area (the flat cut area of the fabric vs. the smaller inflated/projected area). Always check which convention a value assumes before using it.
| Canopy type | Typical Cd range | Reference area convention |
|---|---|---|
| Flat circular (standard hobby chute) | 0.75 – 0.9 | Flat cut area |
| Hemispherical / dome | 1.4 – 1.75 | Projected (inflated) area |
| Cruciform / cross-form | 0.6 – 0.85 | Flat cut area |
| Toroidal / annular | 1.4 – 1.6 | Projected area |
Sizing an ejection or separation charge
A black powder ejection charge needs to raise the pressure inside a sealed section (payload bay, parachute compartment) enough to overcome friction and shear the nose cone or airframe joint free. The standard approach uses the ideal gas law, treating the charge as a source of a fixed volume of hot combustion gas:
For FFFFg black powder specifically — the standard choice for ejection charges — using P in psi, V in cubic inches, R = 22.16 ft·lbf/(lbm·°R), and T = 3307 °R, the equation simplifies (after unit conversion) to a form you can use directly:
Target pressure depends on how the section is held together: a friction-fit nose cone typically needs roughly 10–13 psi, while a system using shear pins is sized to the pins' rated shear force instead — divide the total force the charge needs to produce (pressure × cross-sectional area) by each pin's rated shear strength to find how many pins your charge and section size can support.
Shear pin sizing
Shear pins hold the nose cone or payload section on during flight and are designed to snap cleanly when the ejection charge fires, rather than relying on friction alone. Sizing them is a straightforward force balance:
Round down, not up — using more pins than the charge can reliably shear is a common cause of a "no separation" failure that looks identical to an undersized charge, but is actually an oversized pin count.
Shock cord: length and material
A common rule of thumb is a shock cord at least 4–6 times the airframe's length, giving enough separation between the nose cone and body tube that they don't tangle or collide on the way down.
- Tubular nylon webbing — the standard choice for low-to-mid power: cheap, strong, and slightly elastic, absorbing some deployment shock.
- Kevlar cord — used near the motor/ejection end where heat resistance matters, since nylon can melt or weaken from hot ejection gases.
Field note, preventing "zippering": a nylon shock cord attached directly to the airframe wall can cut a long slit down the tube if it goes taut hard during deployment. A short Kevlar leader (12–18") between the motor mount and the main cord, plus a sliding standoff near the coupler, spreads that shock load and is standard practice above L1 airframes.