Crewed missions beyond low Earth orbit (LEO) require more advanced propulsion than chemical engines. The Nuclear Thermal Rocket (NTR) is a promising solution that uses nuclear reactors to heat hydrogen propellant for thrust. The melting temperature of solid fuels limits traditional NTRs. This study presents a liquid-core NTR design with High-Assay Low-Enriched Uranium (HALEU) and uranium-manganese (UMn) fuel, capable of operating up to 3500°C. This fuel enables better reactivity control and longer reactor lifetime by removing neutron-absorbing fission products. Core calculations using the Serpent 2.20 code confirmed criticality, with over 92% of fissions in the thermal/epithermal spectrum and a reactor mass of 2250 kg. Moderators such as synthetic diamond, graphite, and BeO optimize neutron moderation and performance. This concept enhances NTR performance, safety, and proliferation resistance, paving the way for future deep-space missions.