This SBIR will develop passive reactor criticality control technology for Nuclear Thermal Propulsion (NTP) identified by Ultra Safe Nuclear Corporation (USNC) in Phase 1. This technology will allow NTP systems to start up by rotating the control drums to a single predetermined location and remain there for the duration of operation for the majority of the burns associated with a Mars mission. Passive technology will greatly simplify the control of NTP systems and increase their overall performance during operation. USNC's passive criticality control technology works by -Employing advanced burnable neutron poison to completely remove the need for control drum movement during a full power burn. -Tuning the hydrogen density in the tie-tubes to ensure a consistent start-up position for the control drums. -Enhancing the fuel temperature reactivity feedback mechanism to ensure the stability of the reactor and reduce the burden for active control. This work addresses noted research needs so that NTP systems can help enable human exploration to Mars and other destinations. USNC's Phase 2 work will be a substantial improvement over the state-of-the-art and increase the overall knowledge of NTP control. At the end of Phase 2 USNC will: -Produce a NTP transient code (named the "TRICORDER" code) capable of modeling NTP systems through start up to the end of a burn. -Develop passive criticality control technology rigorously with TRICORDER -Design and then fabricate a new NTP burnable neutron poison (named the "BORGalloy" alloy) and test it in prototypic NTP environments. -Deliver NTP LEU Cermet, LEU Graphite Composite, and HEU Graphite Composite NTP system designs that showcase passive criticality control for human Mars missions.