Shock Cord Sizing & Recovery Harness Guide
General guidelines for typical shock cord sizes, lengths, and recovery hardware.
Typical Shock Cord Guidelines
Each shock cord or recovery harness should be sized based on the entire rocket, not just airframe diameter. These recommendations are a strong starting point for most builds.
| Airframe | Shock Cord | Shock Cord Length | Swivel | Eyebolt |
|---|---|---|---|---|
| < 2.2" | 1/8" Kevlar / Nylon Combo or Tubular Kevlar |
Booster: 8 to 10 ft Payload: 6 to 8 ft |
225 lbs | #8 or 3/16" |
| 2.2" / 2.6" | 3/16" or 1/4" Tubular Kevlar | Booster: 15 to 20 ft Payload: 10 to 15 ft |
350 lbs | #10 or 3/16" |
| 3" | 1/4" or 3/8" Tubular Kevlar | Booster: 20 to 25 ft Payload: 15 to 20 ft |
600 lbs | 3/16" or 1/4" |
| 4" to 5" | 3/8" or 1/2" Tubular Kevlar | Booster: 20 to 30 ft Payload: 15 to 25 ft |
1000 lbs | 1/4" or 5/16" |
| 6" to 8" | 1/2" or 9/16" Tubular Kevlar | Booster: 35 to 40 ft Payload: 30 to 35 ft |
1500 lbs | 5/16" or 3/8" |
Shock Cord Configurations
Why a third loop?
A third loop helps prevent the parachute from tangling with the nose cone or airframe during deployment, while also allowing the parachute to rotate during descent.
- Booster Section (3-loop): Attach one end to the motor mount or booster attachment point.
- Position the swivel or third loop about 1/3 of the way down the shock cord for the drogue parachute.
- Payload Section (2-loop): Attach one end to the avionics bay hardpoint.
- Attach the other end to the nose cone hardpoint.
- Attach parachute to the nose cone or third loop, about 1/3 down the cord.
Deployment Logic
- At Apogee: The ejection charge pushes the drogue and booster downward while the payload moves upward.
- Weight and drag differences help ensure proper separation.
- During Descent: The drogue helps keep sections stable in an inverted “V” shape.
- Main Deployment: The nose cone pulls the main chute away from the booster for cleaner recovery.
Drogue Chute Setup:
The drogue stabilizes descent and keeps rocket halves in an inverted “V” shape.
Too large: Sections may hang down, increasing entanglement risk.
Too small (preferred): Sections spread wider in a “V,” helping the nose cone point away from the booster for cleaner main deployment.
The drogue stabilizes descent and keeps rocket halves in an inverted “V” shape.
Too large: Sections may hang down, increasing entanglement risk.
Too small (preferred): Sections spread wider in a “V,” helping the nose cone point away from the booster for cleaner main deployment.
Best Practices
- Place the third loop or swivel about 1/3 down the shock cord.
- Use swivels to reduce line twist.
- Use quick links for easier removal and maintenance.
- Inspect lines and knots before every flight.
- Booster cords should generally be longer than payload cords.
- Nylon stretches more than Kevlar and can be useful when space allows.
- Use a Madcow shock cord protector, especially with nylon.
- With nylon, go up one size and increase length by 20 to 30%.
- Choose drogue size for control, not just drag.
- Avoid drogue-less recovery, which may cause tumbling and tangles.