A quantitative, probabilistic approach to human-rating of space systems

This paper presents the quantitative, probabilistic approach to human rating proposed by the authors, together with the current status of this issue at NASA. The authors suggest that decisions on human rating must be made based on the probability of crew survival. The probability of crew survival, in turn, is calculated by applying probabilistic risk modeling to the space system. Not only does this yield the desired quantification for use by the decision-maker, but it results in a dynamic tool for use throughout the system life cycle to identify and focus available risk reduction resources on the system´s primary risk drivers. In conclusion, while human rating can be achieved by the application of classical engineering design techniques including redundancy, fault tolerance, and safety margins, it can be measured only in terms of the probability of crew survival. This ability to measure crew survival probability through probabilistic risk assessment (PRA) methods provides the framework for: 1) trade studies by which to optimize crew survival among the many competing system design variables, and 2) the most resource-efficient application of the classical engineering design techniques to achieve crew survival goals