Vehicle wide optimization of subsystem trade study option selection

As demand for highly reliable complex systems increases, engineers are being forced to consider the risk implications of design decisions earlier in the conceptual phase of projects and with greater accuracy. In highly mass constrained systems, in order to achieve increased reliability, buying down risk with mass cannot be considered from a `stove-piped´ stand point, but rather it must be approached from a global system perspective. The Altair lunar lander design team was able to implement a process of Risk Informed Design utilizing the Valador Reliability Tool [1] (VRT) in order to achieve high reliability. This tool is able to quickly and accurately produce estimates of the risk of Loss of Mission (LOM) and Loss of Crew (LOC) and provide insight to the designers as to how their decisions will impact overall mission success. During a design analysis cycle, the VRT can analyze isolated subsets of this risk for trade studies in order to produce alternate low-risk options. The results of these independent studies produce a database of options for various trade studies scored on the basis of LOM, LOC, and DeltaMass required to implement each option. Using this information, the Risk Reduction Efficiency (RRE), or reliability improvement per unit mass, can be calculated as the change in Risk between the baseline and the option divided by the cost of selecting this option. For any given delta-mass to spend globally reducing risk there exists an optimum set of options to select across the range of trade studies that will maximize the reliability of the entire system. The Optimizer Tool (OT) employs ad hoc optimization techniques based upon the RRE to produce a Pareto Frontier of the set of options which combine to create the highest reliability system for each incremental delta-mass. This curve is extremely useful for identifying the point of diminishing returns for spending mass to improve reliability. Moreover, the OT can help identify where the design team may have ´u- - nder-invested´ in a trade study (passed on a more massive option) by comparing the set of vehicle design team choices to the optimizer choices at the same mass. Similarly, the tool can identify where the design team ´over-invested´ in an option to reduce risk by comparing the design team choices to the optimizer choices with the same risk. These exercises add value by challenging the design team to provide rationale for each decision that does not follow the optimized selections of the OT and thus helps to identify and document when a decision was made for reasons outside the scope of reliability. By examining the evolution of the optimizer selected option sets as mass increases, trends can be discerned which speak to the relative value of various options. In the end, the Pareto Frontier serves as a benchmark to judge the efficiency of the design team in producing a highly reliable system. The Pareto Frontier, combined with other Lunar Campaign aspects can also be used to calculate the Expected Mass Delivered to the moon per year. Overall, the OT is not meant to make design decisions, but rather to get the design team to attack the risk in the system from a global perspective and to encourage thoughtful discussion concerning design decisions.