The Vision for Space Exploration plans for eventual outposts on the lunar surface, requiring large amounts of mass launched from Earth. Each kilogram of oxygen or fuel produced on the Moon using lunar regolith or potential polar ice would reduce Earth launch mass by 5 to 8 kg. NASA is developing the architecture for lunar missions, and the benefits and requirements of a lunar-based in situ resource utilization (ISRU) system need to be quantified.
The interaction of components and subsystems will be critical to the overall design of this complex system. A team at the NASA Glenn Research Center has created a system-modeling tool to capture these interactions and to help understand optimization at the system level. The initial effort focused on the baseline concept of lunar regolith ISRU, which requires an excavation subsystem, a reaction subsystem, and an oxygen liquefaction subsystem, as shown in the following figure.
Simplified lunar regolith ISRU system.
Long description of figure 1.
The excavation subsystem excavates and transports regolith to the reaction site. The subsystem model evaluates either a single roverlike vehicle or a combination of specialized vehicles, and it sizes the vehicles to successfully navigate the lunar soil.
The reaction subsystem converts the metal oxides in the regolith to water within the reactor and then converts the water to oxygen using an electrolyzer. The initial model assumes a fluidized bed batch reactor at 600 to 1000 °C using hydrogen to both fluidize and convert the oxides to water. The reactor is sized according to fluid-mechanics and chemical-reaction-rate requirements. Multiple reactors can be used in parallel to allow more time for reaction and heat-up of the regolith. The electrolyzer converts the water to hydrogen and oxygen, accounting for the thermodynamics and the electrical characteristics. Then the hydrogen is recycled back to the reactor, and the oxygen is sent to the liquefaction subsystem. Other components in the model include pumps and compressors, heat exchangers, and condensers.
The liquefaction subsystem model analyzes the power required to liquefy and maintain the liquid oxygen as well as the volume and mass of the tanks and cryocoolers.
At the system level, users can select which components are in the system and how these components will be arranged. This allows for great flexibility in both updating individual modules as new data become available and in incorporating entirely new components, such as alternative reactor processes. The optimization feature converges on the user-specified production rate while solving for a user-specified goal (such as minimum system mass) within sets of equality and inequality constraints.
The system model has produced both qualitative and quantitative results, and development continues. For example, the batch-based nature of the hydrogen reactor results in highly variable flow rates for downstream components. Operating with parallel reactors increases the reactor mass but reduces the overall system mass by 20 percent.
More quantitatively, parametric studies have been performed, such as varying the oxygen production level. The following plot shows mass and power results for an equatorially located system where the regolith is heated via electrical power. Nuclear power has lower peak power demands because of 365-days-per-year operation, whereas solar power can operate only during daylight hours (about half time).
Estimates of overall mass and peak power for different oxygen production levels for an equatorial lunar regolith ISRU system.
The system model is being refined with a focus on component and system validation. The goal is to be flexible in regards to other system approaches while maintaining consistent assumptions and an overall framework. This will enable a tool that can be used for tradeoff studies and in preparation for future hardware development.
Glenn contacts: Diane Linne, 216-977-7512, Diane.L.Linne@nasa.gov, Joshua E. Freeh, 216-433-5014, Joshua.E.Freeh@nasa.govLast updated: December 14, 2007
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