Knowledge of system development models used by NASA partners for systems engineering and integration can prevent miscommunication and unfulfilled expectations within NASA insight and oversight organizations.
The NASA Engineering and Safety Center (NESC) evaluated systems engineering and integration (SE&I) processes and functions currently used in space exploration programs. Systems engineering practices and processes following the traditional waterfall development model differ from practices and processes incorporated in the spiral development model. Differences in development models accentuate areas of concern.
The NASA waterfall model consists of a sequential review process, including Systems Requirements Review, Preliminary Design Review and Critical Design Review, as the system progresses through the development life cycle.
The design of a system following a spiral model, however, evolves with each pass through the spiral. Subsequent spirals may change or eliminate capabilities verified in earlier spirals. The incremental approach of the spiral model presents a unique challenge for engineers who may have to re-verify the entire system at each spiral completion to ensure affected system functions and interactions are properly captured and understood.
NASA Engineering and Safety Center CKO Dan Yuchnovicz on the importance of this lesson learned:
Robust systems engineering and integration (SE&I) capabilities and processes are important throughout the system life cycle to ensure safe and reliable system operations. They should facilitate systems thinking, provide insights into unexpected system-level interactions, provide an understanding of critical flight and ground system functions, and support management of system margins necessary for safe human spaceflight.
NASA is engaging in partnerships with commercial companies who use SE&I approaches that are different from the “waterfall” approach traditionally used by NASA programs and projects. This has led to miscommunication and unfulfilled expectations by NASA insight and oversight teams charged with evaluating system development.
Because of these differences between NASA SE&I requirements and those used by commercial partners, the NASA Technical Fellow for Systems Engineering led an NESC team tasked to determine how the SE&I functions of NASA partners compare to those used typically by NASA in the “waterfall” approach to SE&I.
The team developed a baseline model of nearly 200 SE&I functions to which other developers’ SE&I processes can be compared. The SE&I processes included:
- Requirements definition and analysis
- Systems analysis and margin management
- Risk management and system safety definition and maintenance of natural and induced environments
- Integrated testing and analysis among others
This ‘reference’ model assists in identifying equivalent functions and gaps in SE&I development models, e.g., waterfall or spiral. The team’s key takeaway, captured as a lesson learned, was that programs using the life cycle models described in NASA policy and guidance references must be aware the partner’s life cycle model may not conform to the NASA waterfall model.
When a NASA program accepts any system development model other than the traditional waterfall model reflected in NASA policy and guidance documentation, the program should tailor its technical insight processes, verification and validation methods, and system review milestones to compensate for this underlying difference between development models. Tailoring is necessary to ensure proper understanding of the maturation of the partner’s system throughout the development life cycle.