NTP Engine System Design and Modeling

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Aerojet Rocketdyne (AR) has long had a vision for providing propulsion that permits exploration and extensive travel capability across the Solar System. AR’s history and current efforts include providing propulsion and power for NASA’s far-reaching exploration goals and science missions. These include propulsion for the return to the moon with the Space Launch System (SLS), battery and power systems for the International Space Station (ISS), propulsion for Mars landers, and power for Mars rovers (e.g., Perseverance) and propulsion to power deep space missions like New Horizon. AR has continued the race in developing advanced propulsion systems that help the USA and NASA advance their goal of getting humans to Mars by working on a High-Assay Low Enriched Uranium (HALEU) Nuclear Thermal Propulsion (NTP) system. Current mission studies are focused on Mars missions in the late 2030’s and beyond. NTP engine requirements (e.g., thrust size, Isp) have been connected to those current studies that have been on-going since 2019. Those studies have shown a wide range of thrust sizes (e.g., 12,500 to 25,000-lbf) which can close the architecture and vehicle design using a nuclear fuel material that is capable of high temperature (e.g., peak temperatures between 2,800 to 3,000-deg K) operation to achieve a specific impulse (Isp) at or above 900 seconds. Although Mars missions dominate where NTP shows high pay-off, cis-lunar missions where NTP can support faster 2-3 day missions and more cargo mass also show payoff versus typical chemical propulsion systems. The HALEU NTP designs continue to use hydrogen propellant as the coolant and can use either a Ceramic-Metallic, Ceramic-Ceramic, or Carbide based fuel. The new approach in 2021 for the NTP fuel arrangement utilizes discrete cylindrical assemblies in the moderator block. The grouping is optimized for achieving criticality, which produces the required power level but is similar to the previous approach of using hexagonal (prismatic form) fuel elements with hydrogen flowing in channels within the fuel assembly. Current work has extended the fuel and core designs beyond NTP fuels initially analyzed between 2017 to 2020 and have started looking at other fuels with Carbide material approaches since 2021. Engine design trades are still on going to identify the optimum core/engine system operating characteristics. The NTP design trades that are continuing rely heavily on thermodynamic cycle modeling that includes the neutronic design attributes of the fuel and how it operates within a reactor core design. This paper presents a discussion on the methods for NTP modeling of the engine system for both steady state and transient operation, examining start, shutdown and post-cool down operations. In addition to steady state and transient modeling architectures, the implications of capturing component design influences on the NTP design is discussed.