Systems Engineering
4. Requirements Types
4.1 Performance
Performance requirements are measures of the primary intended function of a system. Every system must have at least one performance measure for what it does, and often there are a number of them. As an example, the design capacity for space transport systems is often expressed as a Mission Model. The mission model quantifies the system performance in terms of multiple parameters like dates, flight rate, payload dimensions and mass, mission duration, destination orbit, type of cargo, and maximum g-level. For a space habitat, performance might be measured in number of crew supported, levels of atmosphere, food supplies, and gravity, and total living volume. An industrial system might have requirements for Throughput, in tons per day of materials processed, and Efficiency in terms of (theoretical energy required)/(actual energy used). The particular performance measures which matter will vary by system.
An example mission model for the Apollo program might have started out as follows, with more detail added as the project progresses. Even in this early version, it lists a number of different performance measures that the design needs to meet:
Cargo characteristics:
- Number of crew to the lunar surface: 2/mission
- Maximum Stay time: 4 days/mission
- Additional science equipment: 250 kg/flight
- Lunar samples returned: 100 kg/flight
Mission Schedule:
- First Flight: as early as possible but before Jan 1, 1970
- Flight quantity: 10 to lunar surface (this was the original plan)
- Flight rate: 4 flights/year
Performance requirements only address what a system does when it is operating as intended. It does not address what happens outside that context, such as
- In between active operation, such as the 80 days between Lunar missions in the mission model above.
- When the system fails, as did the Apollo 13 mission,
- Before and after the 5 years of manned missions.
- Interactions external to the program, such the supply of technical personnel for the project, environmental impact of the launches, or return of Lunar germs to Earth. The last turned out to be a needless worry, but the quarantine system for returning astronauts and moon rocks is not something covered in performance measures.
So performance alone does not encompass the entire system over the entire life cycle, and other requirement types are needed.