A Comparison of Five ISPP Systems for a Mars Sample Return Mission

Introduction The most direct method of reducing the cost of a mission to Mars is to reduce the mass of payload that is launched from Earth. If the propellants for the return phase of the mission are produced on Mars, the total Earth-launch mass could be reduced significantly. The upcoming Mars Sample Return (MSR) mission will demonstrate the feasibility of in-situ propellant production (ISPP) but it is not clear what type of ISPP system is the best choice for Mars. Five candidate ISPP systems for producing two fuels and oxygen from the Martian atmosphere are considered in this study: 1) Zirconia cell with methanol synthesis, 2) Reverse water gas shift with water electrolysis and methanol synthesis, 3) Sabatier process for methane production with water electrolysis, 4) Sabatier process with water electrolysis and partial methane pyrolysis, and 5) SabatierlRWGS combination with water electrolysis. These systems have been the subject of Regardless of the type of craft or the payload for the Mars-Earth return trip, the overwhelming majority of the vehicle launch mass will be the propellant required for the launch and insertion into the transit trajectory. If the propellant for the return phase of the mission were to be produced on Mars, the Earthlaunch mass could be reduced significantly, thereby reducing the cost of the Earth-launch system. A strong case has been made by many mission planners that l however, the major attributes of a Mars-based system will be its long-term reliability, autonomous operation, and stable performance. numerous previous analytical studies and laboratory demonstrations. In this investigation, the systems are objectively compared on the basis of thermochemical process models developed with a commonly used chemical plant analysis software package. The realistic effects of incomplete chemical conversion and nonideal gas phase separator performance are included in these models. The factors considered in the system trade study include system reliability, cost to produce flight hardware, system mass and volume, power consumption, production rate, ancillary equipment needs, fuel choice, and system scalability. These categories are weighted to reflect the importance of each to mission success. Within each category, the ISPP systems are ranked according to the quantitative model results and current state of the system’s development. This study focuses on the chemical processing and product separation subsystems. The CO2 compression upstream of the chemical plant and the liquefaction/storage components are not included hereTo provide a basis for this trade study, the candidate ISPP systems are sized to meet the needs of a Sample Return Mission. A complete mission assessment is outside the scope of this analysis; so, a simple approach is used to estimate the propellant quantities needed for Earth-return trip. We will assume that a single-stage vehicle is launched from the surface and is transferred from Mars orbit to Earth orbit. In this case, a basic rocket equation is used to provide a first-order estimate of mission propellant needs: The simple Sabatier and Sabatier with methane pyrolysis systems are found to be the best design choices for Mars ISPP operations. Mdry + M~el _ AV M exp dv L I &Is, (1) where Md,.,, = vehicle dry mass = 140 kg (assumed), M&l = total propellant required, AV= sum of all flight velocity changes, g, = 9.807 m/see2 Isp = rocket engine specific impulse For the purposes of this study, the total AV required for returning to Earth is specified as 9 km/set ’ Member AIAA Copyright