Redundancy Testing and Cost Assessment for Environmental Control and Life Support Systems 2009-01-2495

Environmental control and life support systems are usually associated with high demands for performance robustness and cost efficiency. However, considering the complexity of such systems, determining the balance between those two design factors is nontrivial for even the simplest space missions. Redundant design is considered as a design optimization dilemma since it usually means higher system reliability as well as system cost. Two coupled fundamental questions need to be answered. First, to achieve certain level of system reliability, what is the corresponding system cost? Secondly, given a budget to improve system reliability, what is the most efficient design for component or subsystem redundancy?

The proposed analysis will continue from previous work performed on series systems by expanding the scope of the analysis and testing parallel systems. Namely, the online and offline redundancy designs for a Lunar Outpost Mission are under consideration. At the current stage, components in parallel are still considered non-repairable. However, our analysis does consider the fact that the integrated system will have different reliability functions if it uses subsystem redundancy (for example, parallel control systems for atmosphere control) versus components redundancy (for example, standby OGS). Several analytical reliability prediction approaches are used, including the utilization of components' MTTF (Mean Time To Failure) for predicting system reliability over time. In addition, reliability predictions are made using a stochastic simulation tool which generates life testing data and derives system level reliability functions with maximum likelihood estimations. The outcomes can provide quantitative information regarding the difference between the selected approaches. In previous work it has been shown that the difference between these techniques can be significant since conventional approaches do not readily address the impact of buffering capacity in environmental systems. This work is also coupled with a newly developed tool for evaluating the cost associated with different system designs. This paper will seek to identify the qualitative and quantitative tradeoffs between reliability and cost for ECLSS.