Depending on the lifetime or class of the mission, different redundancy strategies can be applied. The most common strategies to increase reliability are:
- Single-string: The most basic strategy, where only one component is used for each function.
- Block redundancy: A group of components is used to perform the same function. If one of the components fails, the other block can take over.
- Cross-strapping: The components are connected in a way that allows them to be used interchangeably.
SMAD, Figure 14-1: Different Types of Redundancy
The table below shows the typical redundancy strategy based on the lifetime of the mission1:
| Lifetime | Redundancy |
|---|---|
| <1 year | Single-string |
| 1–3 years | Selected redundancy |
| 3–5 years | Block redundancy |
| >5 years | Fully-cross strapped |
Single-String Systems
- Suitable for short missions
- Any failure generally leads to mission loss
- Benefits:
- Simpler testing process
- Can be very reliable with thorough testing and careful part selection
- Examples: NASA’s Small Explorers and AMSAT spacecraft (10-20 years operation)
- Limitations:
- Vulnerable to single-point failures
- Limited graceful degradation options
Redundancy Implementation
- Consider redundancy for missions beyond 1 year
- Priority areas for redundancy:
- Hard-to-test components (deployments, actuators, propulsion systems)
- Complex circuits with analog components (e.g., power converters)
- Types of redundancy:
- Block-redundant: Switching all components to alternate side
- Cross-strapped: Individual components can be switched independently
- Functional redundancy: Different components providing same function
Footnotes
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J. R. Wertz and W. J. Larson, “Space Mission Engineering: The New SMAD,” Microcosm Press, 2011, page 400. ↩