Tradespace Exploration of Space Settlement Architectures Using Long-term Cost and Benefit Metrics

Space systems development choices made decades ago, such as decisions for the Space Shuttle and the International Space Station (ISS), still influence and constrain human spaceflight technology development in the present day. Similarly, upcoming decisions in NASA's Artemis program and other Moon / Mars programs will likely influence the development of human spaceflight capabilities for subsequent decades. Given this, we argue that space systems architects require a new approach suitable for tradespace exploration across decades, and we have developed a new metric and tools to illustrate this approach. Typical tradespace exploration approaches trade off metrics for cost and benefit. The main families of metrics are monetary, utility and physical. Unfortunately, dollar, mass or utility metrics are fraught with weak assumptions when applied to decades-long timelines and to space settlements that will benefit from reusable rockets and in-situ resource utilization. Past work by the authors had proposed a new physics-based cost metric suitable for multi-decade architectures termed Lifetime Embodied Energy. Here, we propose a new benefit metric which may be of interest to diverse stakeholders and suitable for the comparison of alternative long-lived architectures: the Sustainable Long-term Growth Rate (SLGR). The SLGR has two components, a sustainability test to be defined by the architect and an average growth rate for key internal and external Figures of Merit (FOM). We show the application of the SLGR metric to a (future) long-term Mars base campaign using only Apollo-derived notional elements without ISRU, and to a case of a human settlement on Mars using future in-situ resource utilization technologies. The key FOM in both scenarios are habitable volume, stock of life support equipment and stock of consumables, as well as their derivative, which is carrying capacity of humans. We use a System Dynamics model to simulate the long-term growth, decline and stability behaviors of these FOM variables under each design option. For this model, we defined the SLGR's sustainability test as avoidance of collapse, meaning that the key FOM's per capita must remain above predefined dynamic thresholds at all times within the multi-decade model horizon. Architectures which collapse are deemed potentially fragile and discarded. For the remaining, potentially robust architectures, the growth rate of each of the key FOM across the multi-decade model horizon is its SLGR. We conclude with an assessment of SLGR compared to other metrics, a discussion of potential pros and cons, and a framing of the SLGR methodology in terms of pathfinding as opposed to traditional tradespace exploration.