Executive Summary, Findings & Recommendations from National Academies Report on Space Nuclear Propulsion

FIGURE 2.1 Photo of a nuclear thermal propulsion (NTP) system from the Rover/NERVA programs (left) and a cutaway schematic with labels (right). SOURCE: M. Houts et. al., NASA’s Nuclear Thermal Propulsion Project, NASA Marshall Space Flight Center, August 2018, ntrs.nasa.gov/citations/20180006514.

Space Nuclear Propulsion for Human Mars Exploration
National Academics of Sciences, Engineering and Medicine
National Academies Press
2021

EXECUTIVE SUMMARY

NASA’s Space Technology Mission Directorate requested the National Academies of Sciences, Engineering, and Medicine to convene an ad hoc committee to identify primary technical and programmatic challenges, merits, and risks for developing and demonstrating space nuclear propulsion technologies of interest to future exploration missions. The particular systems of interest were specified as nuclear thermal propulsion and nuclear electric propulsion systems. The committee was also tasked with determining the key milestones, a top-level development and demonstration roadmap, and other missions that could be enabled by successful development of these systems.

The Aeronautics and Space Engineering Board of the National Academies’ Division on Engineering and Physical Sciences assembled a committee to carry out the assigned statement of task (see Appendix B). The committee members (see Appendix C) held 14 virtual meetings during 2020 and drafted this report based on inputs received during its public meetings, additional documents reviewed by the committee, and the expertise of the members. A list of all of the findings and recommendations that appear in the main body of the report appears in Appendix A.

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 mission—an 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 highassay, low-enriched uranium (HALEU) fuels for NTP and NEP systems as applied to the baseline mission.

FIGURE 2.4 Nuclear electric propulsion (NEP) development roadmap for the baseline mission, with a 2039 launch of the first human 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 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.

Illustration of a spacecraft with a nuclear-enabled propulsion system. (Credit: NASA)

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.

Illustration of a Mars transit habitat and nuclear propulsion system that could one day take astronauts to Mars. (Credits: NASA)

FINDINGS AND RECOMMENDATION

Findings and Recommendations Specific to NTP Systems

FINDING. NTP Fuel Characterization. A significant amount of characterization of reactor core materials, including fuels, remains to be done before NASA and DOE [Department of Energy] will have sufficient information for a reactor core design.

RECOMMENDATION. NTP Fuel Architecture. If NASA plans to apply NTP technology to a 2039 launch of the baseline mission, NASA should expeditiously select and validate a fuel architecture for an NTP system that is capable of achieving a propellant reactor exit temperature of approximately 2700 K or higher (which is the temperature that corresponds to the required Isp of 900 sec) without significant fuel deterioration during the mission lifetime. The selection process should consider whether the appropriate fuel feedstock production capabilities will be sufficient.

FINDING. NTP Storage of LH2 [liquid hydrogen]. NTP systems for the baseline mission will require long-duration storage of LH2 at 20 K with minimal boiloff in the vehicle assembly orbit and for the duration of the mission.

RECOMMENDATION. NTP Storage of LH2. If NASA plans to apply NTP
technology to the baseline mission, NASA should develop high-capacity tank systems capable of storing LH2 at 20K with minimal boiloff in the vehicle assembly orbit and for the duration of the mission.

The NTREES test facility — housed at the Marshall Space Flight Center — safely tests simulated nuclear fuel elements, which reduce risk and costs associated with advanced propulsion technologies. (Credit: NASA/MSFC/Emmett Given)

FINDING. NTP Modeling and Simulation, Ground Testing, and Flight Testing. Subscale in-space flight testing of NTP systems cannot address many of the risks and potential failure modes associated with the baseline mission NTP system and therefore cannot replace full-scale ground testing. With sufficient M&S [modeling and simulation] and ground testing of fully integrated systems, including tests at full scale and thrust, flight qualification requirements can be met by the cargo missions that will precede the first crewed mission to Mars.

RECOMMENDATION. NTP Modeling and Simulation, Ground Testing, and Flight Testing. To develop an NTP system capable of executing the baseline mission, NASA should rely on (1) extensive investments in M&S, (2) ground testing, including integrated system tests at full scale and thrust, and (3) the use of cargo missions as a means of flight qualification of the NTP system that will be incorporated into the first crewed mission.

FINDING. NTP Prospects for Program Success. An aggressive program could develop an NTP system capable of executing the baseline mission in 2039.

RECOMMENDATION. NTP Major Challenges. NASA should invigorate
technology development associated with the fundamental NTP challenge, which 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. NASA should also invigorate technology development associated with 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).

Findings and Recommendations Specific to NEP Systems

FINDING. NEP Power Scaling. Developing a MWe-class NEP system for the baseline mission would require increasing power by orders of magnitude relative to NEP system flight- or ground-based technology demonstrations.

FINDING. NEP Modeling and Simulation, Ground Testing, and Flight Testing. Subscale in-space flight testing of NEP systems cannot address many of the risks and potential failure modes associated with the baseline mission NEP system. With sufficient M&S and ground testing, including modular subsystem tests at full scale and power, flight qualification requirements can be met by the cargo missions that will precede the first crewed mission to Mars. Fully integrated ground testing may not be required.

RECOMMENDATION. NEP Modeling and Simulation, Ground Testing, and Flight Testing. To develop an NEP system capable of executing the baseline mission, NASA should rely on (1) extensive investments in M&S, (2) ground testing (including modular subsystem tests at full scale and power), and (3) the use of cargo missions as a means of flight qualification of the NEP system that will be incorporated into the first crewed mission.

FINDING. NEP Prospects for Program Success. 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.

RECOMMENDATION. NEP Major Challenges. NASA should invigorate technology development associated with the fundamental NEP challenge, which is to scale up the operating power of each NEP subsystem and to develop an integrated NEP system suitable for the baseline mission. In addition, NASA should put in place plans for (1) demonstrating the operational reliability of an integrated NEP system over its multi-year lifetime and (2) developing a large-scale chemical propulsion system that is compatible with NEP.

RECOMMENDATION. NEP Pace of Technology Development. If NASA plans to apply NEP technology to a 2039 launch of the baseline mission, NASA should immediately accelerate NEP technology development.

John Bounds of Los Alamos National Laboratory’s Advanced Nuclear Technology Division makes final adjustments on the DUFF experiment, a demonstration of a simple, robust fission reactor prototype that could be used as a power system for space travel. DUFF is the first demonstration of a space nuclear reactor system to produce electricity in the United States since 1965.

Findings and Recommendations Applicable to Both NTP and NEP Systems

FINDING. Trade Studies. Recent, apples-to-apples trade studies comparing NEP and NTP systems for a crewed mission to Mars in general and the baseline mission in particular do not exist.

RECOMMENDATION. Trade Studies. 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.

FINDING. NEP and NTP Commonalities. NEP and NTP systems require, albeit to different levels, significant maturation in areas such as nuclear reactor fuels, materials, and additional reactor technologies; cryogenic fluid management; modeling and simulation; testing; safety; and regulatory approvals. Given these commonalities, some development work in these areas can proceed independently of the selection of a particular space nuclear propulsion system.

FINDING. Enrichment of Nuclear Fuels. A comprehensive assessment of HALEU vs HEU for NTP and NEP systems that weighs the key considerations is not available. These considerations include technical feasibility and difficulty, performance, proliferation and security, safety, fuel availability, cost, schedule, and supply chain as applied to the baseline mission.

RECOMMENDATION. Enrichment of Nuclear Fuels. In the near term, NASA and DOE, with inputs from other key stakeholders, including commercial industry and academia, should conduct a comprehensive assessment of the relative merits and challenges of HEU and HALEU fuels for NTP and NEP systems as applied to the baseline mission.

FINDING. Synergies with Terrestrial and National Defense Nuclear Systems. Terrestrial microreactors, which operate at a power level comparable to NEP reactors, are on a faster development and demonstration timeline than current plans for space nuclear propulsion systems. Development of microreactors may provide technology advances and lessons learned relevant to the development of NEP systems. Similarly, technology advances within the DARPA DRACO program could potentially contribute to the development of NTP systems for the baseline mission.

RECOMMENDATION. Synergies with Terrestrial and National Defense Nuclear Systems. NASA should seek opportunities for collaboration with the DOE and DoD [Department of Defense] terrestrial microreactor programs and the DARPA DRACO program to identify synergies with NASA space nuclear propulsion programs.