Assessing the Feasibility of a Martian ISRU Propellant Plant

In-situ resource utilization (ISRU) is the process of extracting local resources to provide consumables for propulsion and regenerate resources for life support systems rather than transporting consumables from Earth. Numerous multidisciplinary technologies are required to interact and synergize to develop efficient ISRU systems. Cost-effective techniques to extract consumables from the environment is a critical element of the supply chain that enables a sustained presence on the Martian surface. To assess the feasibility of a full-scale Martian ISRU architecture, the Systems Engineering and Integration (SE&I) ISRU Modeling and Analysis (SIMA) project developed the Mission Analysis and Integration Tool (MAIT), a modeling tool capable of calculating the mass, power, and volume requirements of an ISRU plant based on location, production requirements, Concept of Operations (ConOps), and environmental conditions. This modeling tool works by combining subsystem models created by government, industry, or academia into a standardized framework to integrate all subsystem models into a single model. This allows for the interactions between all subsystems in an ISRU process to be parametrically assessed and optimized rather that individually analyzing each subsystem.

The Martian ISRU system model investigated in this paper produces liquid methane (CH4) and oxygen (O2) propellant by extracting carbon dioxide (CO2) from the atmosphere and water from mining ice embedded in the Martian surface. The ISRU plant is broken into four sub-processes, denoted by background color in Fig.1: (1) water supply (blue), (2) electrolysis (orange), (3) methane production (green), and (4) storage (pink). The water supply sub-process consists of a Rodwell mine, two transport vehicles, and a feed steam generator. The Rodwell mine pumps heat into a frozen ice sheet to melt water and remove for processing. This water is transported to the main ISRU plant where it is combined with recycled water from other subsystems (electrolysis and methane production) and heated to produce steam. The electrolysis sub-process consists of a series of heat exchanges (HX) and a Solid Oxide Electrolysis (SOE) assembly. The heat exchangers preheat the steam entering the SOE, and the SOE uses a stack of cells to electrolyze the incoming water. A condenser downstream of the SOE anode separates the unreacted water from hydrogen (H2) gas. The methane production sub-process consists of a series of CO2 pumps, reverse water gas shift (RWGS) reactors, condensers, and methanation reactors. Atmospheric CO2 is pulled into the system using a scroll pump. The RWGS reactors shift the incoming carbon from CO2 to carbon monoxide (CO) to improve the efficiency of the downstream methanation reactors. The storage sub-process involves liquefying and storing the O2 (from SOE) and CH4 (from methanation). Both gases are liquefied using a 90 K cryocooler and stored in insulated aluminum tanks with a maximum length constraint of 3 meters.

To assess the feasibility of a Martian ISRU plant, a trade study was created to determine the mass, power, and volume of two plants capable of producing 30 and 300 metric tons (mt) of propellant, based on an O2 to CH4 fuel mixture (by mass) of 3.5:1, in one Martian year. For each propellant production goal, the model was run with a parametric sweep of five variables to assess the sensitivity of the model to various design parameters. The results of these parametric sweeps were used to identify the critical design parameters. The power system was adjusted to determine the trade-off of using fission surface power (FSP) compared to solar arrays and batteries. The ratio of CO2 to H2 in the methanation subsystem was adjusted to determine the optimal ratio from a system perspective. The layers of insulation, numbers of stacks in the SOE, and efficiency of the condensing radiators were also assessed.

The ISRU system assessed in this paper was found to be a feasible and advantageous method of producing liquid methane and oxygen on the Martian surface. The ISRU system provides a significant return on landed mass for both plant sizes investigated in this study over the course of a Martian year (~687 Earth days). The system met a return on landed mass even when adjusted to create a fully redundant system with mass growth allowances as specified in the American Institute of Aeronautics and Astronautics (AIAA) standard “Mass Properties Control for Space Systems” (AIAA S-120A-2015). The return on landed mass was found to grow at larger production targets indicating that the system becomes more efficient (in terms of landed mass) at higher production rates.