Early-Stage Design of Nuclear Thermal Propulsion Systems

Nuclear thermal propulsion (NTP) is a promising technology for enabling faster and more flexible crewed missions between Earth and Mars, offering significantly higher specific impulse than chemical propulsion while maintaining comparable thrust levels. However, the strong coupling between neutronics, thermal-hydraulics, and propulsion performance makes NTP system design challenging, and high fidelity simulations are too computationally expensive for early-stage design exploration. This work presents two coupled computational tools for the preliminary design of high-assay low enriched uranium (HALEU) NTP systems: a Monte Carlo neutronic model based on OpenMC, and a system-level expander-cycle model. Both tools operate on a shared parameterised description of a heterogeneous hexagonal-lattice reactor and are iterated to obtain a self-consistent solution for the power distribution and thermal state. The framework is demonstrated on a reference reactor sized for a 100 kN thrust and 850 s specific impulse. The results show a critical and thermodynamically feasible configuration, highlighting the effectiveness of reduced-order, system-level modelling for early-stage NTP design. The main limitation of the current approach lies in the assumption of uniform power distribution across fuel-element channels, which affects the accuracy of wall temperature predictions.