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Executive Summary In 2020, the National Academies of Sciences, Engineering, and Medicine convened the ad hoc Space Nuclear Propulsion Technologies Committee to identify primary technical and programmatic challenges, merits, and risks for maturing space nuclear propulsion technologies of interest to a future human Mars exploration mission. Through interactions with experts from across the space propulsion community, the committee assessed the present state of the art, potential development path, and key risks for (1) a nuclear thermal propulsion (NTP) system designed to produce a specific impulse1 of at least 900 s and (2) a nuclear electric propulsion (NEP) system with at least 1 megawatt of electric (MWe) power and a mass-to-power ratio that is substantially lower than the current state of the art. As requested by NASA, each system was assessed with regard to its ability to support a particular baseline missionâan opposition-class human exploration mission to Mars with a 2039 launch date.2,3 For both NEP and NTP systems, efforts to mature the requisite technology and mitigate key technical risks were integrated into a top-level development and demonstration roadmap. Infusion of technology results, expertise, and synergy with other government programs and missions was also examined. In the near-term, NASA and the Department of Energy (DOE), with inputs from other key stakeholders, including commercial industry and academia, should conduct a comprehensive assessment of the relative merits and challenges of highly enriched uranium (HEU) and high- assay, low-enriched uranium (HALEU) fuels for NTP and NEP systems as applied to the baseline mission. For NEP systems, the fundamental challenge is to scale up the operating power of each NEP subsystem and to develop an integrated NEP system suitable for the baseline mission. This requires, for example, scaling power and thermal management systems to power levels orders of magnitude higher than have been achieved to date. While no integrated system testing has ever been performed on MWe-class NEP systems, operational reliability over a period of years is required for the baseline mission. Lastly, application of a complex set of NEP subsystems to the 1 Specific impulse is the thrust of a rocket (or electric thruster) divided by the weight flow rate of the propellant. The unit for Isp is seconds. 2 Opposition-class missions to Mars have shorter mission times but require a more capable propulsion system than the alternative: conjunction-class missions. 3 The human exploration mission in 2039 would be preceded by cargo flights beginning in 2033. PREPUBLICATION COPYâSUBJECT TO FURTHER EDITORIAL CORRECTION 1
2 SPACE NUCLEAR PROPULSION FOR HUMAN MARS EXPLORATION baseline mission requires parallel development of a compatible large-scale chemical propulsion system to provide the primary thrust when departing Earth orbit and when entering and departing Mars orbit. As a result of low and intermittent investment over the past several decades, it is unclear if even an aggressive program would be able to develop an NEP system capable of executing the baseline mission in 2039. NTP development faces four major challenges that an aggressive program could overcome to achieve the baseline mission in 2039. The fundamental challenge is to develop an NTP system that can heat its propellant to approximately 2700 K at the reactor exit for the duration of each burn. The other three challenges are the long-term storage of liquid hydrogen in space with minimal loss, the lack of adequate ground-based test facilities, and the need to rapidly bring an NTP system to full operating temperature (preferably in 1 min or less). Although the United States has conducted ground-based testing of NTP technologies, those tests took place nearly 50 years ago, did not fully address flight system requirements, and recapturing the ability to conduct necessary ground testing will be costly and time-consuming. Furthermore, no in-space NTP system has ever been operated. Despite recent work in fuel development, this area remains a challenge, particularly for NTP systems. A comprehensive assessment of HALEU versus HEU for NTP and NEP systems that evaluates a full set of critical parameters as applied to the baseline Mars mission has not been performed. Similarly, a recent apples-to-apples trade study comparing NEP and NTP systems for crewed missions to Mars, in general, or the baseline mission in particular, does not exist. The committee recommends that NASA and DOE, with inputs from other key stakeholders, including commercial industry and academia, conduct a comprehensive and expeditious assessment of the relative merits and challenges of HEU and HALEU fuels for NTP and NEP systems as applied to the baseline mission. The committee recommends that the development of operational NTP and NEP systems include extensive investments in modeling and simulation. Ground and flight qualification testing will also be required. For NTP systems, ground testing should include integrated system tests at full scale and thrust. For NEP systems, ground testing should include modular subsystem tests at full scale and power. Given the need to send multiple cargo missions to Mars prior to the flight of the first crewed mission, the committee also recommends that NASA use these cargo missions as a means of flight qualification of the space nuclear propulsion system that will be incorporated into the first crewed mission. NEP and NTP systems show great potential to facilitate the human exploration of Mars. Using either system to execute the baseline mission by 2039, however, will require an aggressive research and development program. Such a program would need to begin with NASA making a significant set of architecture and investments decisions in the coming year. In particular, NASA should develop consistent figures of merit and technical expertise to allow for an objective comparison of the ability of NEP and NTP systems to meet requirements for a 2039 launch of the baseline mission. PREPUBLICATION COPYâSUBJECT TO FURTHER EDITORIAL CORRECTION