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Priorities in Space Science Enabled by Nuclear Power and Propulsion 2 Engineering and Technical Issues NUCLEAR REACTORS AND HUMAN EXPLORATION Propulsion Several studies over the past few decades have recognized the need for advanced propulsion to support human exploration. As early as the 1960s, Wernher von Braun and others recognized the value of a nuclear rocket for sending humans to Mars. The great distances, harmful cosmic radiation, and physiological response to zero gravity all supported the concept of using a nuclear rocket to decrease mission time. These same needs have been recognized in later studies.1-5 Exploration missions by humans beyond the Moon must cope with radiation levels between 1 and 2 centiSievert (cSV) per week from galactic cosmic rays.6,7 Astronauts will also face the substantial decalcification of bone that occurs in a zero-gravity environment. A high-thrust nuclear propulsion system with a high specific impulse would mitigate these threats by reducing exposure time. Power Nuclear reactor power systems can support human exploration at surface outposts and onboard spacecraft. A nuclear reactor on the surface of the Moon or Mars can be a source of reliable power to provide life support, to replenish fuel cells for mobile systems, and to supply the large power demands of facilities processing materials. A continuous source of power is needed for life support onboard a spacecraft. Power levels for surface and shipboard life support systems are approximately equivalent. However, the environments in the two applications can vary dramatically. Shipboard systems will need to radiate all waste heat to the vacuum and will need to have a low specific mass (i.e., kg/kW), which calls for operation at higher temperatures and the use of more exotic materials. Specific mass is typically less important for surface systems—as opposed to actual mass, which must be minimized for ease of transportation—and so more common materials can be used, and it will be easier to address radiation shielding issues. However, in the case of Mars or other surfaces possessing an atmosphere, the materials used may be dictated by the chemical interactions between, for example, hot radiators and atmospheric gases. In addition, radiators designed for shipboard use—i.e., to function most efficiently in the vacuum of space—will need to be modified to function efficiently in an atmosphere. Thus, surface power systems are, in practice, likely to be very different from shipboard reactors.
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Priorities in Space Science Enabled by Nuclear Power and Propulsion Testing and Reliability Nuclear reactor power and propulsion systems for human exploration missions must be qualified to a much higher level of reliability than power and propulsion systems that are intended for use by robotic missions. In the past, chemical systems such as the Space Shuttle Main Engines were tested more than 400 times on 30 engine systems to establish operational safety margins. These tests included operation at full power or greater, and for full lifetime duration or greater. Nuclear systems will likewise need to be validated for reliable operation at full power and for mission lifetime. The latter factor is especially important because some potential applications—e.g., missions using nuclear-electric propulsion (NEP) to the outer solar system—require continuous operation without maintenance or repair for periods as long as 10 to 20 years. No high-power-density reactor has, however, ever been operated on Earth, without maintenance shutdowns, for any period longer than one order of magnitude or more below such a duration. CANDIDATE SOURCES OF ELECTRICAL POWER AND PROPULSION Nuclear-Electric Propulsion Currently, NASA’s Project Prometheus is pursuing the development of NEP systems. In these systems, the heat from the reactor is carried away by a coolant in a closed loop to a power-conversion system, where it is used to generate electricity. The electricity then powers one of a variety of different electric propulsion technologies that accelerate ions and eject them from the thrusters at high velocities. JPL has demonstrated ion thrusters with a specific impulse of some 3,100 seconds in space on the Deep Space 1 mission and more than 6,000 seconds in the laboratory, which greatly reduces the amount of propellant required for the mission as compared to chemical propulsion systems. The electric power from an NEP system is also available for spacecraft instrumentation. Typically the power requirements for the mission may range from ~100 kWe, the nominal requirement for JIMO, to many megawatts of electricity (MWe) that might be required for human and cargo missions to Mars and beyond. The leading reactor concepts employ liquid-metal coolants (either actively pumped or circulated in heat pipes), liquid-metal-to-gas heat exchangers, and Brayton-cycle conversion systems. An inherent difficulty with NEP systems is that only 10 to 20 percent of the thermal energy generated in the reactor is converted into thrust. Thus, massive radiators are required to reject the waste heat into space. In addition, electric propulsion systems provide low thrust, so NEP systems must operate at near 100 percent duty for long durations to provide high delta-V. The low thrust also complicates navigation, maneuvering the spacecraft and trimming its orbit in a complex gravitational field (e.g., when orbiting a satellite of one of the giant planets). In addition, the NEP systems currently under development for robotic missions may not scale up to provide the tens of megawatts necessary for expedited human missions to Mars. In 2004, NASA conducted a limited set of parametric studies to examine the utility of the basic JIMO spacecraft design for other missions. The results of such studies are highly dependent on the assumptions made about the system and mission performance, which are, in turn, tightly linked. Figure 2.1 shows the results of a typical set of calculations to determine how the flight time and launch mass for a Neptune mission vary for different power values for the NEP system and the specific impulse of its ion thrusters. Representative solutions from such calculations can be combined and plotted to show the scaling relationships between mission metrics such as transit time, total launch mass, reactor power, propellant mass, and delta-V (Figure 2.2).a These parametric studies had mixed results. The JIMO design envisioned when these calculations were performed in 2004 did appear suitable for some missions—e.g., a multiasteroid sample-return mission and a Saturn-Titan mission. But missions to other important solar system destinations—e.g., Neptune—did not appear feasible because the transit times were excessively long and launch masses excessively large. Studies performed in a The committee cannot verify the results of these calculations because specific assumptions—e.g., mass-to-thrust ratio, spacecraft mass, power-conversion efficiencies, and payload mass—were not provided.
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Priorities in Space Science Enabled by Nuclear Power and Propulsion FIGURE 2.1 Calculations showing how the flight time and launch mass for a Neptune mission vary for different values for the power of the nuclear-electric propulsion (NEP) system and the specific impulse of its ion thrusters. Thus, for example, to achieve a transit time of 13 years requires a 300-kWe NEP system powering ion thrusters with a specific impulse of 6,000 seconds, and the resulting spacecraft has a launch mass of almost 55,000 kg (i.e., almost 10 times the mass of Cassini). The results of such calculations are highly dependent on assumptions made about the design characteristics of the spacecraft, its propulsion system, and its mission. Thus, the assumptions inherent in these particular calculations appear to be incompatible with a transit time of less than 12 years or a launch mass of less than 20,000 kg. The results plotted here assume performance characteristics derived from a particular design for the Jupiter Icy Moons Orbiter spacecraft, and they assume that the spacecraft is launched from Earth with a C3 = 0 km2s−2. Solutions to the right of the fundamental limiting curve are consistent with the technology under consideration for JIMO. No allowance is made for performance enhancements such as planetary gravity-assist maneuvers. Representative solutions from families of such calculations can be combined and plotted to show the scaling relationships between important mission metrics, such as launch mass, for a broad variety of missions, as indicated in Figure 2.2. Illustration courtesy of Lennard Dudzinski, NASA. 2005, which were based on the expected performance of the Prometheus Baseline (PB) 1 spacecraft design and included performance enhancements (e.g., Jupiter gravity assists) not previously considered, revealed a somewhat better performance. This performance is, however, still below that of conventional chemical propulsion systems (Table 2.1). Roughly speaking, the performance of a nuclear propulsion system can be measured by the ratio α of the initial mass in orbit to the power embodied in the outgoing propellant. For the NEP system proposed for JIMO and follow-on mission, α is between ~250 and 300 kg/kW. For a long mission to the outer solar system, the transit time varies roughly as the cube root of α.8 The effects of this relationship can be demonstrated by looking at three examples: To reduce the transit time to Neptune from 13–15 years to 10 years (as required for the Neptune-Triton System Explorer mission described in Box 6.5) will require reducing α from ~250–300 to ~75–140. This improvement in efficiency is probably achievable with current technology.9
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Priorities in Space Science Enabled by Nuclear Power and Propulsion FIGURE 2.2 Results of parametric studies (such as those shown in Figure 2.1) can be combined and plotted to understand how the velocity change (delta-V) needed to undertake four representative missions to the outer solar system scales with important mission metrics such as (clockwise from upper left) transit time, launch mass, propellant mass, and reactor size. The solid lines circumscribe solutions characterized by representative fast and slow transit times. These plots clearly show that the particular spacecraft design characteristics embodied in these calculations are not well suited to distant outer solar system missions because of excessively large launch masses, long transit times, etc. The illustrations are simplified versions of plots supplied by Lennard Dudzinski, NASA.
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Priorities in Space Science Enabled by Nuclear Power and Propulsion
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Priorities in Space Science Enabled by Nuclear Power and Propulsion TABLE 2.1 Transit Times of NEP Spacecraft to Representative Objects in the Outer Solar System Objective NEP Transit Time 2004 Study (years) NEP Transit Time 2005 Study (years) Minimum/Maximum Transit Times Achieved So Far Using Conventional Propulsion Systems (years) Jupiter 7–8 5–6 1.6 (Voyager 1)/6.3 (Galileo) Saturn 9–10 7–8 3.3 (Voyager 1)/6.5 (Pioneer 11) Neptune 18–20 13–15 12.0 (Voyager 2) Kuiper Belt object 22–24 17–19 [9.5 to 15.0 (New Horizons)]a 200 AU ~30 n/ab n/a aAssuming a successful Jupiter flyby in February 2007 and a Pluto flyby in July 2015. bThe 20+ year transit time exceeds the PB 1 design lifetime, and the mission’s delta-V requirements may demand that the spacecraft be redesigned to increase the size of the xenon tank and/or the ion thruster’s throughput capacity. To reduce the transit time for a mission to the outer heliosphere from 30 years to 15 years will require reducing α from ~250–300 to ~30–40. Although such a specific mass is below levels offered by current technologies (~70 kg/kW), such a reduction might be achievable.10,11 To reduce the heliospheric flight time still further to 10 years will require a propulsion system with an α of ~10. This is probably beyond the bounds of existing technology and may well require a radically new technical approach. Nuclear-Thermal Propulsion Nuclear-thermal propulsion (NTP) systems produce propulsion by exhausting reactor coolant directly through a nozzle. These systems typically employ cryogenic hydrogen as the propellant and must be designed to operate at an exhaust gas temperature >2,200 K. However, NTP systems typically have short operation times (under 2 hours for human missions to Mars, for example). NTP systems have higher temperature requirements for fuel and structural materials than do NEP systems, but they have the potential to provide the high thrust needed for faster human missions to Mars.12,13 NTP systems may also be suitable for some robotic missions, but few studies of the relevant trade-offs have been performed comparing NTP to NEP for robotic missions, and development efforts for robotic missions are focused on NEP systems. Roughly 95 percent of the thermal energy generated in the reactor of an NTP system is vented in the exhaust, which eliminates the need for large radiators. The United States built and ground-tested several NTP systems during the NERVA/Rover program in the 1960s (see Appendix A for details). More than 23 engine tests were performed at the Nevada Test Site demonstrating a specific impulse of 850 seconds (Figure 2.3), peak temperatures of 2,550 K, operation for more than 1 hour, multiple restarts, and the safety of various accidental failure modes. It should be noted that the open-air testing of an NTP system and the associated venting of radioactive materials are no longer permissible. An NTP variant is an indirect system that employs a heat exchanger to transfer thermal energy from the reactor primary coolant to the hydrogen propellant. A potential advantage of such a system is that the exhaust plume is far less likely to be contaminated with fission products, which is an issue with a direct system. Bimodal Systems Bimodal systems produce both propulsion via the normal direct hydrogen exhaust method and electricity via a separate power-conversion loop. The engines tested in the Rover/NERVA programs had design features allowing a secondary closed-loop, electric power production system, although electric power was never generated in the tests. Compared with either a pure NTP or NEP system, the bimodal concept places the greatest demand on the nuclear fuel, because the fuel in a bimodal system must operate in a high-temperature mode for a short duration for
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Priorities in Space Science Enabled by Nuclear Power and Propulsion FIGURE 2.3 This sequence shows (clockwise from top left) the assembly and test firing of the Pewee nuclear-thermal rocket engine in 1968. The Pewee had 500-MW thermal power and would have had a thrust of 15,000 pounds at a specific impulse of 850 seconds. The Pewee engine was intended for use on a tug that would ferry supplies between Earth orbit and lunar orbit to support a lunar base. Courtesy of Stephen Howe, Los Alamos National Laboratory.
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Priorities in Space Science Enabled by Nuclear Power and Propulsion “rocket” thrusting, and then in a low-temperature mode for a long duration for “NEP” thrusting and/or while supplying power for the avionics, communications, and life-support systems. The bimodal operation offers the fastest trip times for high delta-V missions but will require significant testing and demonstration to validate fuel performance. Surface Power Systems Surface power systems are those designed to produce electricity for use on the Moon and Mars. Electrical power requirements may range from a low of between several kilowatts and tens of kilowatts for robotic missions, to between 50 kilowatts and 100 kilowatts for human outposts. Surface power systems may share much technology with NEP systems but must be designed to undergo entry, descent, and landing and to withstand gravitational, chemical, atmospheric, and thermal environments very different from those to which in-space systems are exposed. Power-conversion concepts include thermoelectric, Stirling, Brayton, Rankine, and a variety of other options that are more technologically advanced, but less mature. THE IMPORTANCE OF STUDIES OF TRADE-OFFS NASA missions enabled by nuclear propulsion will, in general, require high delta-V, fast transit times, or large amounts of electric power at the destination. To determine the benefits of nuclear propulsion for such missions requires that trade-off studies be conducted on a “level playing field.” Various propulsion options need to be compared using metrics such as initial mass in low Earth orbit, launch requirements, or transit time to primary destination. Without such a series of comprehensive studies of trade-offs, it is not possible to demonstrate quantitatively where nuclear systems would be most enabling. Trade-offs between chemical systems, NEP systems, NTP systems, and bimodal systems are needed for missions requiring a wide range of delta-V. These studies should consider both robotic missions to the outer planets and human missions to the Moon or Mars. Furthermore, these studies should incorporate the impact on the systems of requiring risk mitigation through the use of redundant subsystems and extensive ground testing. The estimated fiscal cost of developing a nuclear propulsion system is viewed as significant and could impact currently planned science missions.b However, nuclear propulsion will clearly enable missions in the future that cannot even be considered now. The identification of the benefit versus cost and the best technology development path to follow requires “apples to apples” comparisons among different technologies. POTENTIAL TECHNOLOGY ISSUES TO BE ADDRESSED IN A PHASE II STUDY The committee intends to investigate many potential technology issues associated with space nuclear reactors during Phase II of its study, including the following: Testing and validation. Reactors will be expected to operate as designed with a significant margin of safety and reliability. Although NTP and NEP systems would be operated only after they are launched into a safe orbit, the reactors must be ground-tested at full operational parameters (e.g., power and lifetime) to establish reliability. Existing concepts allow NTP systems, which will operate intermittently for short periods of time during an operational mission, to be tested for their full operational lifetime. In addition, NEP systems can be tested at full power. However, because NEP systems will operate continuously during multiyear missions, cost and schedule considerations will require that some type of accelerated life tests be developed, validated, and approved. b Despite repeated requests to NASA for information on projected costs for JIMO and other Prometheus-related activities, none were forthcoming. One estimate places the cost of JIMO at $10 billion. During testimony to the House Science Committee on June 29, 2005, NASA Administrator Michael Griffin commented that JIMO was “at $11 billion and counting for cost estimates before we got off the drawing board….” For details see, for example, <www.spaceref.com/news/viewsr.html?pid=17157>, last accessed December 16, 2005. The committee cannot confirm or refute these estimates.
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Priorities in Space Science Enabled by Nuclear Power and Propulsion Reliability and unattended operation. Reactors must operate under conditions in which communications are delayed from minutes to several hours, and be appropriately engineered for reliability and long-term drift in instruments. Testing and evaluation. There are potential gaps in the U.S. infrastructure for research, manufacture, and testing of space reactor systems. Areas of concern include the following: Fabrication process facilities for highly enriched uranium fuel and for lithium hydride components for the radiation shield subsystem; Irradiation facilities that can adequately simulate the fast neutron energy spectrum of space reactors; Facilities for integrated ground tests of space reactors; and Liquid metal facilities for component development and testing. In addition, the presence of a nuclear reactor may introduce new issues in relation to the assembly, test, and launch operation processes that NASA currently uses: High-temperature materials. Mission requirements for higher performance and lower vehicle mass can be met only by high-temperature (>1,200 K) materials such as refractory alloys. These types of materials are not used in commercial nuclear power plants or research reactors. The database to support the design and qualification of these materials is only partially complete. Availability of radioisotope power systems. The long-term availability of plutonium-238 for radioisotope power systems for base electric power is assumed for mission planning but may become an issue if a proposed new production facility at the Idaho National Laboratory is not realized.14,15 Radiation shielding. The protection of spacecraft electronics from the neutron and gamma-ray emissions of the reactor requires a complex radiation shield subsystem consisting of hundreds of components fabricated from lithium hydride, depleted uranium, and tungsten. The ease of component fabrication and the engineering design of the shield are issues to be addressed. Studies undertaken in the context of the SP-100 program (see Appendix A) suggest that the shield design was rather straightforward, but that misrepresents the actual situation. Nuclear fuel development. Development of the specialized nuclear fuels required for space reactors may be expected to require long time periods, and the research and development facilities and human expertise infrastructure will have to be rebuilt. Launch considerations. Reactor systems must be capable of being accommodated within the volume of launch-vehicle shrouds and must withstand the vibrations and acceleration forces associated with launch. Reactors must also be designed to mitigate the effects of severe accidents such as launch-pad explosions or events such as reentry into Earth’s atmosphere. Lunar and planetary entry and surface operations. Issues with lunar and planetary entry might include heat management and rejection (for planets with an atmosphere) and ambient radiation from Van Allen belts (Jupiter), as well as gravity issues. Mars’s oxidizing atmosphere may preclude the use of the refractory alloys currently under study for reactor systems for propulsion and lunar applications. The environment for lunar or planetary surface operations may also involve large variations in temperature and ubiquitous dust. Systems integration. Much more than with previous space nuclear power and propulsion development programs, the effective integration of systems will be a fundamental aspect of accomplishing a comprehensive program of robotic and human exploration. Issues that must be addressed include integration between and among the following: Reactor, power conversion, and propulsion systems; All spacecraft systems; Hardware and software systems; Astronauts and other human operators; The totality of individual missions; and Related missions.
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Priorities in Space Science Enabled by Nuclear Power and Propulsion REFERENCES 1. W. Mendel, ed., Manned Mars Mission, NASA M001, NASA, Washington, D.C., 1986. 2. T. Paine et al., Pioneering the Space Frontier: The Report of the National Commission on Space, Bantam Books, May 1986. 3. A. Cohen, Report of the 90-Day Study on Human Exploration of the Moon and Mars, NASA, Washington, D.C., November 1989. 4. T.P. Stafford, America at the Threshold: Report of the Synthesis Group on America’s Space Exploration Initiative, U.S. Government Printing Office, Washington, D.C., 1991. 5. D. Cooke et al., Human Exploration of Mars: The Reference Mission of the NASA Mars Exploration Study Team, NASA Special Publication 6107, NASA Johnson Space Center, Houston, Tex., March 1997. 6. T.P. Stafford, America at the Threshold: Report of the Synthesis Group on America’s Space Exploration Initiative, U.S. Government Printing Office, Washington, D.C., 1991. 7. Radiation limits for human exploration outside low Earth orbit have not yet been established. Radiation limits for astronauts in low Earth orbit for blood-forming organs are 25 cSv per month, 50 cSv per year, and 40 to 300 cSv per career (depending on age and gender). See, for example, National Council on Radiation Protection and Measurement, Radiation Protection Guidance for Activities in Low-Earth Orbit, NCRP Report No. 132, NCRP, Bethesda, Md., 2001. 8. See, for example, P. Dimotakis, F. Dyson, D. Eardley, R. Garwin, J. Goodman, D. Hammer, W. Happer, J. Katz, C. Max, and J. Vesecky, “Interplanetary Travel: Nuclear Propulsion Prospects,” presentation based on JASON-01-752 Summer Study for the NASA Office of Space Science, JASON Program, The Mitre Corp., McLean, Va., 2001, p. 8. 9. L.S. Mason, “A Comparison of Brayton and Stirling Space Nuclear Power Systems for Power Levels from 1 Kilowatt to 10 Megawatts,” NASA-TM 2001-210593, NASA Glenn Research Center, Cleveland, Ohio, 2001. 10. See, for example, P. Dimotakis, F. Dyson, D. Eardley, R. Garwin, J. Goodman, D. Hammer, W. Happer, J. Katz, C. Max, and J. Vesecky, “Interplanetary Travel: Nuclear Propulsion Prospects,” presentation based on JASON-01-752 Summer Study for the NASA Office of Space Science, JASON Program, The Mitre Corp., McLean, Va., 2001, p. 8. 11. L.S. Mason, “A Comparison of Brayton and Stirling Space Nuclear Power Systems for Power Levels from 1 Kilowatt to 10 Megawatts,” NASA-TM 2001-210593, NASA Glenn Research Center, Cleveland, Ohio, 2001. 12. S.K. Borowski et al., “Nuclear Thermal Rocket/Vehicle Design Options for Future Missions to the Moon and Mars,” AIAA-93-4170, American Institute of Aeronautics and Astronautics, Reston, Va., 1993. 13. S.D. Howe, B. Travis, and D.K. Zerkle, “SAFE Testing Nuclear Rockets Economically,” Proceedings of the 20th Space Technology and Applications International Forum (STAIF-2003), M.S. El-Genk, ed., American Institute of Physics, Melville, N.Y., 2003. 14. W.J. Broad, “U.S. Has Plans to Again Make Own Plutonium,” The New York Times, June 27, 2005, pp. A1 and A13. 15. See, for example, Office of Nuclear Energy, Science and Technology, U.S. Department of Energy, Draft Environmental Impact Statement for the Proposed Consolidation of Nuclear Operations Related to Production of Radioisotope Power Systems, DOE/EIS-0373D, U.S. Department of Energy, Washington, D.C., June 2005. Available at <http://www.consolidationeis.doe.gov/>, last accessed February 2, 2006.
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