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The Lunar Space Tug is a sustainable transportation system able to rendezvous with a target body in Low Earth Orbits environment, assess its current position, attitude and operational status, capture the target and move it to the Cislunar space where the Lunar Orbital Platform-Gateway will be settled. Thanks to the adoption of an electric Propulsion Subsystem based of Hall Effect Thruster clusters, the Lunar Space Tug can save fuel to the detriment of much longer transfer time to deliver the unmanned cargo to the cislunar Gateway. Since the Lunar Space Tug has been conceived to be reusable, one of the main issue is related to the on-orbit refueling to sustain the Lunar Space Tug during its operational lifetime. Each mission scenario shall take into account the effects related to refueling operations according to mission requirements and constraints. Considering the whole mission scenario in which the Lunar Space Tug will operate, this paper focuses on mission analysis and conceptual system design, including typical systems budgets, like transfer duration, Δv, mass and power. Moreover, according to the transfer duration and the needs of the Lunar Orbital Platform-Gateway, a preliminary trade-off analysis has been accomplished to investigate the possibility of using a fleet of Lunar Space Tug instead of a single tug, taking onto account safety, reliability, costs and operations issues. The identified optimal configuration is then compared with the current reference option, i.e. the NASA Space Launch System adopted as cislunar delivery vehicle.

Deep space is the new frontier for human exploration, with Moon and Mars identified as fundamental targets. Improving in-space transportation capabilities has been recognized as one of the critical enabler for sustainable and affordable space programs in Earth proximity and beyond. Envisioning the presence of future deep space infrastructures, cargo transferring becomes a major issue that can benefit from improvements in in-space propulsion technology. Electric propulsion could represent the turning point, thanks to the combination of new system architectures and technology advancements, e.g. cluster architecture and magnetic shielding, and improved capability of on-board power generation. High-power Hall Thrusters are considered the most promising solution for future space exploration, thanks to a favourable thrust to power ratio, higher than Gridded Ion Engines. Reusable platforms, based on Hall Thrusters, could represent a valid alternative to chemical-propelled spacecraft. These systems could be exploited to support human presence in deep space, delivering life support items and providing on-orbit servicing capabilities. In this paper, the typical mission analysis tools have been exploited to analyse the selected scenarios. The analysis highlights possible advantages achievable adopting high-power Hall Thrusters on board reusable platforms. Since the design of these spacecraft envisions the adoption of a 20 kW-class Hall Thruster string, the mass and power budgets are obtained for those subsystems that are most affected by this critical technology. Then, the feasibility of each scenario is assessed considering the needs defined not only by the traffic plan, in terms of loading/unloading cargo and transfer duration, but also by the peculiar mission and physical constraints. Last, the different platform design solutions are compared with respect to their electric propulsion configurations, in order to identify the possible commonalities in terms of architecture and technology, in line with the current trend of modularity and affordability.

According to the Global Exploration Roadmap, a Getaway in the lunar vicinity will enable the human exploration of the Moon, paving the way toward Mars and further in the deep space. A reusable transportation system, the Lunar Space Tug (LST), can be a key support transportation system to achieve a continuous and sustainable connection between the Earth and the orbital lunar outpost. Exploiting an electrical propulsion technology consisting in Hall Thrusters (HTs), this reusable space cargo system can maximize the delivered payload, with a lower propellant consumption with respect to a chemical-based platform. This significant improvement comes with two main throwbacks: a long transfer time between Earth and Moon and a complex multidisciplinary design of the overall LST. The first issue can be overcome with good mission planning in order to reduce the layover times and maximize the use of the LST. However, the peculiarities of the adopted propulsion technology shape the overall mass and power distribution of the space tug, as well as its transfer trajectory. The design complexity introduced by this subsystem can be effectively investigated if the mission analysis, trajectory generation and subsystem sizing are merged together in one software. In fact, the low-thrust transfer trajectories are highly affected by orbital perturbation and eclipse periods. At the same time, the high demand in power of the propulsion system puts constraints on the mass breakdown and power allocations of the LST, therefore on the reachable thrust level. The trajectory and subsystems sizing limitations call for a particular accurate mission analysis in order to succeed. Only combining all those elements, it is possible to define feasible design boundaries without performing countless simulations trying to optimize each element as a stand-alone part. Politecnico di Torino, in collaboration with the European Space Research and Technology Centre (ESTEC), developed a MatLab-based preliminary design tool for electric propulsion space tug missions, called MultidisciplinAry desiGN Electric Tug tOol (MAGNETO). Starting from the mission analysis of the lunar space tug, the tool building blocks and capabilities are presented. Moreover, the build-in trajectory module will be analysed in-depth. The potentiality of the trajectory generation tool in MAGNETO enables a refined design of the initial design envelope of the LST. For this reason, the improved results will be confronted with the previous LST design tool developed by Politecnico di Torino.

The International Space Station is the first space human outpost and over the last 15 years, it has represented a peculiar environment where science, technology and human innovation converge together in a unique microgravity and space research laboratory. With the International Space Station entering the second part of its life and its operations running steadily at nominal pace, the global space community is starting planning how the human exploration could move further, beyond Low-Earth-Orbit. According to the Global Exploration Roadmap, the Moon represents the next feasible path-way for advances in human exploration towards the nal goal, Mars. Based on the experience of the ISS, one of the most widespread ideas is to develop a Cislunar Station in preparation of long duration missions in a deep space environment. Cislunar space is de ned as the area of deep space under the influence of Earth-Moon system, including a set of special orbits, e.g. Earth-Moon Libration points and Lunar Retrograde Orbit. This habitat represents a suitable environment for demonstrating and testing technologies and capabilities in deep space. In order to achieve this goal, there are several crucial systems and technologies, in particular related to transportation and launch systems. The Orion Multi-Purpose Crew Vehicle is a reusable transportation capsule designed to provide crew transportation in deep space missions, whereas NASA is developing the Space Launch System, the most powerful rocket ever built, which could provide the necessary heavy-lift launch capability to support the same kind of missions. These innovations would allow quite-fast transfers from Earth to the Cislunar Station and vice versa, both for manned and unmanned missions. However, taking into account the whole Concept of Operations for both the growth and sustainability of the Cislunar Space Station, the Lunar Space Tug can be considered as an additional, new and fundamental element for the mission architecture. The Lunar Space Tug represents an alternative to the SLS scenario, especially for what concerns all unmanned or logistic missions (e.g. cargo transfer, on orbit assembly, samples return), from Low Earth Orbit to Cislunar space. The paper focuses on the mission analysis and conceptual design of the Lunar Space Tug to support the growth and sustainment of the Cislunar Station. Particular attention is dedicated to the analysis of the propulsion subsystem effects of the Lunar Space Tug design. Main results are presented and discussed, and main conclusions are drawn.