Increasing the Fidelity of Maintenance Logistics Representation in Breakeven Plots
Olivier de Weck
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Breakeven plots are commonly used to conduct trade studies examining different levels of loop closure in environmental control and life support systems for human spaceflight. By showing the mass that must be launched for a given system as a function of mission endurance (time between resupply) for various cases, these charts can help guide technology investment and system architecture decisions. These plots usually capture the emplaced system mass as well as the mass of required consumables such as water, oxygen, and nitrogen. However, the logistics associated with maintenance and repair are not always captured. When maintenance logistics are considered, spare parts mass is often represented as a simplified linear approximation or a certain level of redundancy is applied to cover the risk of failure. These representations do not account for the discrete nature of additional redundancy and/or spare parts, nor do they easily allow for trades with regard to reliability or level of accepted risk. In addition, they do not capture the impact of technology that can reduce maintenance logistics mass requirements such as 3D printing. Maintenance logistics are a very significant challenge for long-duration human spaceflight beyond low earth orbit, and therefore should receive careful consideration during system architecting and development. This paper presents a more detailed implementation of breakeven plots, focusing on increasing the fidelity of maintenance logistics considerations. A case study is presented based on International Space Station-derived data, showing a baseline case and examining the impact of different supportability strategies and levels of risk acceptance. In addition, contour plots showing the impact of variation in both mission endurance and probability requirements are generated. The implications of these results are discussed. Overall, maintenance mass requirements can make up more than half of the total mass required for a mission, and can strongly impact technology selection.