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Space Nuclear Propulsion for Human Mars Exploration (2021)

Chapter: 4 System and Programmatic Issues

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Suggested Citation:"4 System and Programmatic Issues." National Academies of Sciences, Engineering, and Medicine. 2021. Space Nuclear Propulsion for Human Mars Exploration. Washington, DC: The National Academies Press. doi: 10.17226/25977.
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Suggested Citation:"4 System and Programmatic Issues." National Academies of Sciences, Engineering, and Medicine. 2021. Space Nuclear Propulsion for Human Mars Exploration. Washington, DC: The National Academies Press. doi: 10.17226/25977.
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Suggested Citation:"4 System and Programmatic Issues." National Academies of Sciences, Engineering, and Medicine. 2021. Space Nuclear Propulsion for Human Mars Exploration. Washington, DC: The National Academies Press. doi: 10.17226/25977.
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Suggested Citation:"4 System and Programmatic Issues." National Academies of Sciences, Engineering, and Medicine. 2021. Space Nuclear Propulsion for Human Mars Exploration. Washington, DC: The National Academies Press. doi: 10.17226/25977.
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Suggested Citation:"4 System and Programmatic Issues." National Academies of Sciences, Engineering, and Medicine. 2021. Space Nuclear Propulsion for Human Mars Exploration. Washington, DC: The National Academies Press. doi: 10.17226/25977.
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Suggested Citation:"4 System and Programmatic Issues." National Academies of Sciences, Engineering, and Medicine. 2021. Space Nuclear Propulsion for Human Mars Exploration. Washington, DC: The National Academies Press. doi: 10.17226/25977.
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Suggested Citation:"4 System and Programmatic Issues." National Academies of Sciences, Engineering, and Medicine. 2021. Space Nuclear Propulsion for Human Mars Exploration. Washington, DC: The National Academies Press. doi: 10.17226/25977.
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Suggested Citation:"4 System and Programmatic Issues." National Academies of Sciences, Engineering, and Medicine. 2021. Space Nuclear Propulsion for Human Mars Exploration. Washington, DC: The National Academies Press. doi: 10.17226/25977.
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Page 72
Suggested Citation:"4 System and Programmatic Issues." National Academies of Sciences, Engineering, and Medicine. 2021. Space Nuclear Propulsion for Human Mars Exploration. Washington, DC: The National Academies Press. doi: 10.17226/25977.
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4 System and Programmatic Issues NTP AND NEP ARE DIFFERENT TECHNOLOGIES Both nuclear electric propulsion (NEP) and nuclear thermal propulsion (NTP) systems show great potential to facilitate the human exploration of Mars with significant advantages relative to chemical propulsion. The two systems, however, have very different heritages. The development of high-power NTP systems benefits from the robust ground-based testing of many NTP reactors during the Rover/NERVA programs, but NTP systems require reactor operating temperatures about 1500°C higher than NEP systems. NASA, the Department of Energy (DOE), and the Department of Defense (DoD) are currently supporting substantive NTP research and development programs. Even so, an NTP system has never flown in space. In contrast, advanced electric propulsion (EP) technologies are deployed routinely in operational spacecraft. Such systems have demonstrated long life and high reliability, but only at power levels far below those needed for a megawatt electric (MWe)-class system, and only in a solar-powered mode. Over the past decade, there has been very little advancement in NEP technology at the scale and power- level required for the baseline mission. Given this imbalance in technology maturity, system trades are difficult to make. NTP systems and NEP systems (which include a chemical propulsion system) are composed of many technologies, including the following:  NTP and NEP  Nuclear reactors  Shields  Cryogenic fluid management  NTP specific  Turbomachinery, valves, and pipes  Nozzles  Long term storage of liquid hydrogen (LH2) PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION 65

66 SPACE NUCLEAR PROPULSION FOR HUMAN MARS EXPLORATION  NEP specific  Power conversion  Heat rejection  Power management and distribution  Electric propulsion  Chemical propulsion (for application to crewed Mars exploration missions)  Long-term liquid oxygen (LOX)/liquid methane storage For those technologies that are used in both NEP and NTP systems, the engineering challenges are very different because of different operating temperatures, operational lifetimes, startup regimens, and requirements for integration with other system elements. For example, while both concepts use a nuclear reactor, as shown in Table 1.3 the operational requirements and design specifications for an NTP reactor are very different than those for an NEP reactor. As a result, different approaches may be needed to address some safety assurance requirements (see the discussion of safety assurance, below). Similarly, propellant storage temperature requirements greatly vary: 20 K for LH2 (NTP), 110 K for LOX (NEP), 90 K for liquid methane (NEP), and supercritical storage of xenon (NEP). The propellant mass of an NTP system will far exceed the propellant mass for the electric thrusters in an NEP system; the latter, however, will need to store a sizeable mass of propellant for its ancillary chemical propulsion system. System complexity is another consideration. NTP systems have a smaller number of subsystems to integrate, whereas the nature of NEP enables initial subsystem separability for ground testing. Given the above circumstances, meaningful and objective trade studies will require expertise in all the above technologies as they apply to NTP and NEP systems scaled to meet the needs of the baseline mission. 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 nuclear electric propulsion and nuclear thermal propulsion systems to meet requirements for a 2039 launch of the baseline mission. DEVELOPMENTS COMMON TO BOTH NTP AND NEP SYSTEMS Despite the many differences between NEP and NTP systems and subsystems, there are some areas of synergy, including the following:  Nuclear reactor fuels. Ongoing work to develop advanced fuels, such as TRISO particles and high-assay, low-enriched uranium (HALEU), may be applicable to both NTP and NEP reactors.  Materials. High temperature materials play a role in many aspects of reactor designs, and such materials are often developed agnostic of the application. NTP and NEP systems have very different reactor operating temperatures, interface requirements, and operational considerations. Even so, there is a general need for high-temperature PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION

SYSTEM AND PROGRAMMATIC ISSUES 67 materials, and materials applicable to NTP systems may be useful for NEP systems (though not necessarily vice versa). This includes the commonality of high-temperature materials for fuels, cladding, cermet and cercer fuel matrices, and moderators (if included in the reactor design), reflectors, and neutron absorbers for reactor control and criticality. A second class of common materials lies in high-temperature, radiation-hardened sensors and electronics, which are needed for either system to assure controllability, safety, and reliability, and life.  Additional reactor technologies. Reactor designs for NTP and NEP systems share common components such as shielding, actuators for control drums or rods, and instrumentation. Both design principles and materials may be common in these areas, although the specific designs will ultimately address different conditions of operation.  Cryogenic fluid management technology. Technologies developed for long-term storage of LH2 (most challenging) may also be applicable to the long-term storage of LOX and liquid methane.  Modeling and simulation (M&S). Validated M&S tool significantly reduce the number of costly physical tests of NTP designs and accelerate component and integrated level qualification schedules. Modeling of reactor core neutronics, fluid flows through reactor coolant channels, and dynamic codes to model startup and other transient behaviors share some common fundamentals. The adequacy of M&S tools to accurately capture the rapid system dynamics of NTP designs needs to be examined. Given the exponential growth in computer power and similar advances in multi-physics flow modeling, the potential for high-fidelity coupled simulations of the thermal and fluid flow in power systems, including flow structural interactions, may be possible. Such integrated simulations can provide insight into component interactions and transient and feedback effects.  Testing. NTP and NEP systems share commonality in separate effects testing of fuels and materials (including coupon and fuel element testing) and for some reactor subsystem testing, although the fuel temperature requirements are different. However, the recommended full-scale ground test facilities for an NTP reactor that is about 500 MWth and must capture the engine exhaust would be much more extensive than facilities for an NEP reactor that produces about 3 to 10 MWth and is a closed cycle.  Safety assurance. Safety assurance for nuclear systems is essential to protect operating personnel as well as the general public and Earth’s environment. Safety assurance policies and practices are inherent in all U.S. nuclear endeavors conducted by or for NASA, DOE, and other federal agencies. Safety goals are generally achieved by a combination of system design and operational safety measures. Such safety measures include, for example, (1) launching reactors with fresh fuel before they have operated at power to ensure that the amount of radioactivity onboard remains as low as practicable at launch,1 (2) ensuring safe, reliable in-space system operation while providing adequate shielding for the crew and radiation-sensitive spaceflight hardware, (3) restricting reactor startup and operations in space until spacecraft are in nuclear safe orbits or trajectories that ensure safety of Earth’s population and environment, and (4) ensuring that reactors remain in a safe state in the event of a launch failure. Additional policies and practices need to be established to prevent unintended system reentry during return to Earth (after 1 An NTP or NEP reactor only builds up appreciable fission products when operated at power for a period of time. PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION

68 SPACE NUCLEAR PROPULSION FOR HUMAN MARS EXPLORATION reactors have been operated for extended periods of time). The safety analysis and launch approval process for the baseline mission will be similar for either an NEP or NTP system. Relevant functional design and operational safety criteria have been identified and applied to prior U.S. space reactor programs.2,3 Incorporating lessons learned from these programs is vital to ensure adequate safety for operational NTP and NEP systems.4,5,6  Regulatory approvals. Presidential memorandum (NSPM-20), which was released in August 2019, provides the most recent guidance on the launch approval process for space nuclear systems. This memorandum addresses safety issues such as potential inadvertent criticality stemming from a launch or reentry accident.7 NSPM-20 also instructs NASA to develop guidance for safe nonterrestrial operation of nuclear fission reactors. These guidelines can be applied to either NEP or NTP systems. If HALEU fuels are adopted for NEP or NTP systems, regulatory issues will also be common. NEP or NTP systems will also face common regulatory requirements related to indemnification and to the construction and transportation of systems before launch. 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. HEU VERSUS HALEU The decision between HEU (in this context, uranium with an enrichment greater than 90 percent)8 and HALEU (less than 20 percent enrichment) fuel involves more than feasibility and system performance. No such comprehensive assessment that compares the fuel types head-to- 2 Marshall, A.C., Bari, R.A., Brown, N.W., Cullingford, H.S., Hardy, A.C., Lee, J.H., Niederauer, G.F., Remp, K., Rice, J.W., Sawyer, J.C., and Sholtis, J.A. Jr., Nuclear Safety Policy Working Group Recommendations on Nuclear Propulsion Safety for the Space Exploration Initiative, NASA Technical Memorandum 105705, Final Report of the Joint NASA/DOE/DoD Nuclear Safety Policy Working Group, National Aeronautics and Space Administration, April 1993. 3 Sholtis, J.A. Jr., “Proposed Safety Functional Guidelines for Space Reactors,” paper presented at the ANS 2005 Space Nuclear Conference (SNC-05), 5-9 June 2005, San Diego, CA, in Proceedings of the ANS Embedded Topical Meeting - Space Nuclear Conference 2005, ISBN: 0-89448-696-9, ANS Inc., LaGrange Park, IL, June 2005. 4 Sholtis, J.A., Jr., Winchester, R.O., Brown, N.W., Connell, L.W., Marshall, A.C., McCulloch, W.H., Mims, J.E., and Potter, A., “U.S. Space Nuclear Safety: Past, Present, and Future,” Chapter within A Critical Review of Space Nuclear Power and Propulsion 1984-1993, American Institute of Physics (AIP) Publishing, New York, NY, ISBN 1-56396-317-5, pp. 269-303, 1994. 5 Nuclear Power Assessment Study – Final Report, Johns Hopkins University – Applied Physics Laboratory Report# TSSD-23122 under NASA Contract NNN06AA01C, Task NNN13AA17T, Chapter #4, February 4, 2015 (Released June 1, 2015). 6 Marshall, A.C. (Editor), and Haskin, F.E., Usov, V.A. (Co-Editors), Space Nuclear Safety, ISBN-13: 978-0- 89464-061-2 and ISBN-10: 0-89464-061-5, Krieger Publishing Company, Malabar, FL, 2008. 7 NSPM-20, available at https://www.whitehouse.gov/presidential-actions/presidential-memorandum-launch- spacecraft-containing-space-nuclear-systems/. 8 HEU refers to uranium enriched to the point that it contains at least 20 percent uranium-235. HEU fuels used in space nuclear propulsion and power systems would likely be enriched to greater than 90 percent uranium-235. PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION

SYSTEM AND PROGRAMMATIC ISSUES 69 head (as distinct from a standalone assessment of the feasibility of an HALEU system) for either an NTP or NEP system was available to the committee. Key factors to be included in a comparative assessment of HEU and HALEU for both systems are as follows:  Technical feasibility and difficulty. A HALEU reactor has never been built, tested, or flown for either NTP or NEP applications, and there are no experimental data on the behavior of HALEU NTP reactors to benchmark modeling and simulation codes. In contrast, HEU NTP reactors have been built, tested, and benchmarked using prior M&S software. Technical feasibility and difficulty considerations favor HEU for NTP systems, but they do not clearly favor one fuel enrichment level over the other for NEP systems.  Performance. Fuel enrichment affects the performance of the system. For example, the relative mass and size of NTP and NEP systems (including shielding) is a function of fuel enrichment and other parameters such as each reactor’s power level and neutron spectrum (fast vs. moderated). Data from the Rover/NERVA programs provide insight into the operational performance of HEU NTP reactors; equivalent data does not exist for HALEU reactors for NTP or NEP systems. Performance considerations do not clearly favor one fuel enrichment level over the other.  Proliferation and security. HEU fuel, by virtue of the ease with which it could be diverted to the production of nuclear weapons, is a higher value target than HALEU, especially during launch and reentry accidents away from the launch site. As a result, HEU is viewed by nonproliferation experts as requiring more security considerations. In addition, if the United States uses HEU for space reactors, it could become more difficult to convince other countries to reduce their use of HEU in civilian applications. Proliferation related concerns also affect other factors such as cost, schedule, the ability of the commercial space sector to participate in reactor development, and the extent to which domestic politics becomes a factor in obtaining launch approval. Proliferation and security considerations favor HALEU.  Safety. The selection of fuel enrichment, in conjunction with the reactor’s neutron spectrum, can affect the design approach and difficulty in preventing inadvertent criticality events during launch and reentry accidents. This may require different emergency planning, accident response, and recovery protocols, even if there are no radiological consequences to the public. Safety considerations are design dependent, and do not clearly favor one enrichment level over the other.  Fuel availability. It may be possible to obtain HEU from DOE’s National Nuclear Security Administration stockpile. Producing HALEU would either require down- blending HEU from the stockpile or enriching lower enriched uranium. The latter would require new infrastructure. DOE is investigating production of HALEU to support near- term terrestrial power reactor needs, but there are concerns about the long-term availability of HEU. Overall, fuel availability considerations do not clearly favor one enrichment level over the other.  Cost. The costs of HEU and HALEU systems differ because of factors such as safeguards and physical security, facilities, fuel procurement and fabrication, and system development. From a launch approval perspective, HEU systems require Presidential approval. While this may have schedule implications, it may not have cost implications as the cost of launch approval will likely be dominated by the safety analysis, which will PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION

70 SPACE NUCLEAR PROPULSION FOR HUMAN MARS EXPLORATION be similar for HEU and HALEU systems. Cost considerations do not clearly favor one enrichment level over the other.  Schedule. Use of different enrichment levels will affect the design, development, testing, and launch preparations schedule. Possible locations for test facilities may be more limited for HEU due to the different security requirements, which could protract schedule, but there is a more historical data on HEU reactors. Schedule considerations do not clearly favor one fuel enrichment level over the other.  Supply chain. The use of HEU would restrict the number of private-sector organizations which are able to participate in system development and manufacture. HEU would limit participation to DOE laboratories and the small number of private companies with licenses to work with HEU. Use of HALEU, on the other hand, would permit the involvement of a larger number of private companies and enable a variety of public- private partnerships. Supply chain considerations favor HALEU. While there is some clarity on each of the criteria above, they are not equally important. Performance, security, and safety concerns are significantly more important than those related to the supply chain. This weighting must be considered prior to making a fuel enrichment decision. FINDING. Enrichment of Nuclear Fuels. A comprehensive assessment of HALEU versus 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 highly enriched uranium and high-assay, low-enriched uranium (HALEU) fuels for nuclear thermal propulsion and nuclear electric propulsion systems as applied to the baseline mission. INDUSTRIAL BASE A growing number of private-sector companies are developing system concepts for space nuclear systems. These concepts include applications for orbital maneuvering, deep space exploration, and planetary surface electrical grids. No single entity—public or private—has all the requisite expertise or facilities to develop a space nuclear propulsion system. As has been demonstrated in recent space launch initiatives, NASA can leverage private-sector expertise interests and investments, along with DOE and NASA facilities, to spur the development of necessary technologies. Several engine manufacturing and launch services providers have developed or are developing LOX/LH2 engines for in-space propulsion. Many of the needed non-nuclear engine components have heritage from these product development and demonstration efforts, but additional investment is required to convert these systems for application to an NEP or NTP propulsion system. PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION

SYSTEM AND PROGRAMMATIC ISSUES 71 Cryogenic fluid management, which is critical for both NEP/chemical and NTP systems, has primarily been a government-led development effort. Private-sector fuel tank and pressure vessel manufacturers exist, but the technically challenging nature of multiyear containment of cryogenic hydrogen (necessary for NTP) will require sustained government investment in the design, fabrication, and testing of these systems. Very few private-sector entities have the capability to develop nuclear reactor fuels, cores, shields, and control systems. However, several are investing in these capabilities and can be expected to contribute directly to the design, manufacturing, and assembly of space nuclear propulsion systems. If efforts to develop a space nuclear system are scaled up on an accelerated timeline, there may be shortfalls in the workforce needed for such systems. A significant space nuclear power development effort would benefit from concomitant efforts to enhance relevant aspects of the science, technology, engineering, and mathematics educational pipeline, particularly nuclear engineering. This pipeline faces the following three principal challenges: 1. The sector suffers from a lack of gender and ethnic diversity. 2. Non-aerospace technology companies compete with the aerospace sector for talent, especially in information technology fields. 3. Export control regulations and the classified nature of some of research and technologies preclude non-U.S. citizens from participating, constraining the size and quality of the pipeline. LESSONS LEARNED FROM THE HISTORY OF DEVELOPING SPACE NUCLEAR SYSTEMS Since 1961, the United States has launched 47 radioisotope power systems of eight different types in support of 30 navigational, meteorological, communications, and space science satellites, spacecraft, and planetary landers and rovers. In contrast, the United States has launched only one fission power system—the 500 We SNAP-10A reactor power system was launched in 1965 as an experimental test of an NEP system concept. At least a dozen other programs have been initiated to develop fission systems for space applications. While none of these additional programs launched a nuclear reactor, several lessons have emerged from these efforts that are worth incorporating in future space nuclear propulsion development efforts.  Need must be compelling. Development and testing of space nuclear propulsion systems are expensive and time consuming relative to non-nuclear propulsion technology. Ambitious robotic and human exploration programs have succeeded without the need for space nuclear propulsion systems. Operational space nuclear propulsion systems are only likely to be developed and deployed if they are enabling or strongly enhancing for a particular mission of national importance.  Mission and product focus are critical. Once the need for space nuclear propulsion systems is clearly established, having a specific mission with a clear customer, adequate funding, well-defined requirements, and a firm schedule serves as the best stimulus for development of an acceptable product that will be delivered on time and within cost. Mission-pull also ensures that technology development is focused on the critical need. PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION

72 SPACE NUCLEAR PROPULSION FOR HUMAN MARS EXPLORATION  Limit technical risk impacts early in program. Identification of the highest technical risk areas and selection of necessary technologies in need of maturation must be completed early. The program must consider the benefits of existing and emerging technology options and trade technical, schedule, and cost risks. During the development process, it is critical to maximize hardware production and testing at each level of integration to obtain key validation data (test-as-you-fly, fly-as-you-test). Once demonstration is complete, additional enhancements to system performance, reliability/life, and utility for a greater range of missions would require only incremental tests to validate the enhancements. KEY TECHNICAL RISKS As detailed in Chapters 2 and 3, there is uncertainty regarding the ability to predict whether a complete space nuclear propulsion system can be developed in time to launch cargo missions to Mars beginning in 2033 and to execute the baseline mission in 2039. The level of uncertainty is presently lower for NTP than for NEP. Each system is characterized by a small number of significant risks (see Table 4.1). The fundamental NTP 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 fundamental NEP challenge is to scale up the operating power of each NEP subsystem and to develop an integrated NEP system suitable for the baseline mission. PROGRAMMATICS The roadmaps of Section 2.6 and 3.6 show the key milestones necessary to execute the baseline 2039 human Mars mission preceded by cargo missions beginning in the 2033 opportunity. These roadmaps assume that NASA accelerates development decisions and maturation of the requisite technologies through an aggressive and focused development program, beginning in 2021 (18 years before the planned departure of the first crew and 12 years prior to the flight of the first full-scale cargo mission). NASA previously demonstrated greater expediency from less of a technical base in the successful Mercury, Gemini, and Apollo programs (e.g., Mercury was announced in October 1958 with a first successful crewed flight 31 months later; Apollo was announced in 1961 with the first human lunar landing 8 years later, including programmatic recovery from a major failure that resulted in the death of three astronauts). The International Space Station (from Freedom proposal in 1984 to first sustained crew presence 16 years later) provides another comparable. The committee believes that should the federal government choose to invest aggressively in this space nuclear propulsion technology, there is sufficient schedule to achieve the baseline mission. In addition, as presented in Figure 1.2, over the 17-year synodic cycle, 9 of 10 Earth-departure opportunities are feasible within the propulsive capability of a space nuclear propulsion system sized for the 2039 opportunity. As such, the next opportunities of 2042, 2045, and 2047 provide fallback potential and schedule mitigation for the chosen path. PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION

SYSTEM AND PROGRAMMATIC ISSUES 73 TABLE 4.1 Major Challenges for Developing Nuclear Electric Propulsion (NEP) and Nuclear Thermal Propulsion (NTP) Systems for the Baseline Mission Category NTP NEP Reactor Core Fuel  High reactor fuel operating and Materials temperature (more than 2700 K) System  Rapid system startup to full  Long system operational Operational operating temperature reliability (4 years for power Parameters (preferably in 1 min or less) generation, 1 to 2 years for thrust) Scale  Power conversion and thermal subsystem tests conducted to date have been at power levels orders of magnitude below that required for baseline mission  Limited full scale, short duration electric propulsion subsystem testing at power levels an order of magnitude below that required for baseline mission Ground-Based  Need to capture and process  No fully integrated system Testing engine exhaust (resulting in testing experience high cost)  Facility preparation time (stresses baseline schedule)  Little integrated system testing experience; none of it recent  Last relevant-scale tests were nearly 50 years ago In-space  Long-term storage of liquid  Parallel development of a Propulsion hydrogen in space at 20 K with chemical propulsion systems Technology Needs minimal loss System  Highly complex: six NEP Complexity subsystems and a chemical propulsion system PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION

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Space Nuclear Propulsion for Human Mars Exploration identifies primary technical and programmatic challenges, merits, and risks for developing and demonstrating space nuclear propulsion technologies of interest to future exploration missions. This report presents key milestones and a top-level development and demonstration roadmap for performance nuclear thermal propulsion and nuclear electric propulsion systems and identifies missions that could be enabled by successful development of each technology.

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