Technology survey and performance scaling for the design of high power nuclear electric power and propulsion systems

Abstract

High power nuclear electric propulsion systems have the capability to enable many next-generation space exploration applications. To date, use of electric primary propulsion in flight systems has been limited to low-power, solar electric missions. There is a need for a large-scale research and development effort to field systems capable of meeting the demands of future high-power electric propulsion missions, especially missions utilizing nuclear power plants to power electric propulsion systems. In formulating such an effort, it is first important to identify the likely requirements around which such a system might be designed. These requirements can be effectively cast in terms of required thruster lifetime, thrust, specific impulse, output power, and power plant specific power. Projected requirements can be derived based on the mass characteristics of space-borne nuclear power plants, and the optimized trajectories of spacecraft missions enabled by the use of megawatt-level nuclear electric power systems. Detailed mass modeling of space-based Rankine cycle nuclear power plants is conducted to evaluate the achievable specific power of these systems. Based on the figures for specific power so obtained, mission modeling is next conducted using the Mission Analysis Low-Thrust Optimization software package. Optimized thrust, specific impulse and lifetime figures are derived for several missions of interest. A survey of available electric propulsion thrusters is conducted and thruster configurations presenting the lowest developmental risks in migrating to high thruster output power are identified. Design evolutions are presented for three thrusters that would enhance or enable operation at the megawatt level. First, evaluation of projected lifetime for dual-stage gridded ion thrusters is conducted using the CEX2D simulation tool to evaluate the utility of multi-stage gridded ion engines in obtaining the required thruster lifetime for operation at high specific impulse. Next, to evaluate the utility of Hall thrusters operating at high propellant mass flow rate, a numerical thruster model is developed that incorporates the effects of the neutral fluid in predicting thruster performance. Using this code, numerical simulations are conducted to investigate the effects of variations in propellant mass flow rate, magnetic field topology, and thruster channel geometry on achievable performance. Finally, the effects of variations in the channel contour of magnetoplasmadynamic thrusters on performance and efficiency are evaluated using the MACH2 software package. Incremental variations in thruster channel contour are implemented, and the effects of these variations on the performance onset condition, and electrode current distributions are observed. Conclusions regarding the utility of each of these three design evolutions in developing thrusters for multi-megawatt electric propulsion systems are discussed. Contributions stemming from this research include, first, the establishment of an appropriate requirements space for the design of advanced highpower electric power and propulsion systems. This design space is comprised of projected requirements for power plant specific power, derived from power plant mass modeling, and thruster output power, specific impulse and lifetime derived from mission modeling. Additionally, this work provides evaluation, using state-of-the-art simulation suites, of several electric thruster design evolutions of potential utility in developing electric propulsion systems designed to operate at the megawatt level.