Trade Study: Lunar Tank Welding Level 0 Design Study

The maturation of in-space manufacturing techniques requires understanding the impacts and shortfalls of their capabilities to inform technology investment strategies. The purpose of this trade study is twofold: to compare advanced manufacturing techniques that could be used to produce lunar storage tanks, and to capture the differences in terms of the benefits and drawbacks of each technique.

This study presents an approach that navigates the numerous difficulties in this area of investigation—the inherent technological uncertainty and need to elicit subject matter expert (SME) knowledge yields results that are qualitative and abstracted, alongside quantitative zeroth- and first-order analyses and estimates for the level of detail available regarding assumptions and background information. The approach is a framework that allows for revisions, refinements, and substitutions of the modules within the study as the uncertainty regarding assumptions and SME knowledge decreases, or the scope and fidelity of analyses and modeling increases.

At the current level of the study, the results are still capable of informing areas of targeted studies for dependent maturing technologies, determining showstoppers—critical disadvantages, insurmountable capability gaps, performance shortfalls—for the various manufacturing techniques, and allow for an initial understanding of the trade space sensitivities. The outcomes of this study are the framework approach and initial findings based on the assumptions, information, and SME knowledge available at the time.

The level of assessment and analysis for this study is rationalized by two main points. Firstly, the extant technology for the advanced manufacturing techniques has not been demonstrated on-premises and at-scale for the expected use case. As such, exact metrics that characterize or analytical models that simulate these processes are not available for comparison. However, SMEs have enough knowledge of these processes and their state of the art to give qualitative evaluations, which were converted to quantitative comparisons via the Analytical Hierarchy Process (AHP) technique. Secondly, capturing launch savings as a meaningful metric of comparison is not straightforward; several intermediate analyses or estimates need to be linked to reach some measure related to launch.

More parameters need to be included within the scope of the trade study for the purpose of defining the nominal use cases to which to size the lunar storage tanks, such that these sized tanks are used to quantify differences between launching whole tanks versus tank materials and components in various form factors, based on launch vehicle volume and mass constraints for a mixed fleet. The set of parameters that enumerate the trade space are a mix of categorical and numerical options, with some of the categorical parameters indirectly defining numerical properties that affect tank sizing. By acknowledging these separate sections of the trade space and intentionally structuring the approach to combine both quantitative and qualitative results in a flexible and systematic manner, the results are a holistic comparison of various advanced manufacturing techniques using the multi-criteria decision analysis method of Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS).

Initial findings evaluated Earth-based, in-space welding, in-space additive manufacturing (ISAM), and in-situ resource utilization (ISRU) for ISAM methods based on the following figures of merit (FoMs) as evaluation criteria: size of tanks launched per launch vehicle, size of any required infrastructure, the infrastructure lunar power demands, degree of manufacturing risk, manufacturing process complexity, intricacy of equipment deployment, rate of production, reliance on low technology readiness level (TRL) capabilities, and size limit of a given process. These FoMs are a set of distinguishing, if not orthogonal, characteristics that describe expected launch savings, process complexity, and capability maturity of each alternative. The current FoMs and their scoring show that, overall, the in-space welding options offer the best balance of launch savings versus maturation and setup difficulty. Weighing the launch FoM as the metric of focus shows that the Earth-based and in-space welding options are the most sensitive to that FoM; it is possible that once super-heavy lift launch vehicles are precluded from the trade space that other manufacturing methods will become more attractive due to their invariance in scoring with respect to launch savings.

Overall, the initial findings are based on scoring from a small group of SMEs and estimates based on broad literature review. Changing sections of the approach—be it the analyses, estimations, scores, or trade space definition itself—will have varied levels of impact on the current results. Relative scoring of each alternative may shift with polling a larger group of SMEs or using their knowledge to revise the set of distinguishing FoMs. Formulating a more detailed tank sizing and payload packing analysis will provide an adjusted scaling of the launch savings FoM with respect to the other criteria but is not expected to drastically shift the scoring. Future iterations of this trade study can redefine the trade space to include more granularity representing ISAM and ISRU techniques, as well as additional tank use cases; the additional trade space parameters and evaluation criteria can be readily adapted into the current approach for the study.