• WISCONSIN • FUSION • TECHNOLOGY • INSTITUTE D-3He Magnetic Fusion for Space Propulsion J.F. Santarius January 1992 UWFDM-875 Presented at the US-USSR Workshop on D-3He Reactor Studies, 25 September – 2 October 1991, Moscow. FUSION TECHNOLOGY INSTITUTE UNIVERSITY OF WISCONSIN MADISON WISCONSIN D-3He Magnetic Fusion for Space Propulsion J.F. Santarius Fusion Technology Institute University of Wisconsin 1500 Engineering Drive Madison, WI 53706 http://fti.neep.wisc.edu January 1992 UWFDM-875 Presented at the US-USSR Workshop on D-3He Reactor Studies, 25 September – 2 October 1991, Moscow. D-3He Magnetic Fusion for Space Propulsion John F. Santarius University of Wisconsin, Madison, WI 53706 Abstract This paper gives a brief overview of the reasons why magnetic fusion energy using the D-3He fuel cycle appears attractive for space applications—particularly space propulsion. The paper also surveys some fusion reactor configurations, examines the capabilities of these reactors, and explores the implications for space development. Based on several conceptual designs of fusion reactors for space propulsion, such systems could enable the efficient, large-scale exploration and development of the Solar System. 1. Overview Modern developments in fusion reactor science and technology now make possible the realization of a thirty-year old idea: that D-3He fusion reactors can provide very attractive space propulsion systems [1]. The key reason for this attractiveness is that high exhaust velocity (specific impulse) can be achieved at sufficient thrust-to-weight ratios for both fast, human transport and large-payload, cargo transport throughout the Solar System. Advantages for D-3He fusion over chemical or nuclear fission reactors in space include: • No radioactive materials are present at launch, and only low-level radioactivity is present after operation; • Conceptual designs project higher specific power values (1–10 kWthrust/kg) for fusion than for nuclear-electric or solar-electric propulsion. • Fusion gives high, flexible specific impulses (average exhaust velocity), enabling efficient long-range transportation. • D-3He produces net energy and is available throughout the Solar System. • D-3He fuel provides an extremely high energy density. The fusion power density in a D-3He plasma is smaller than that of a D-T plasma, and, for this reason, the D-T fuel cycle is more commonly investigated for terrestrial fusion reactors. These two main fusion reactions, plus the important 'side' D-D reactions are shown in Table 1. The charged fusion products from D-3He reactions can be guided to provide direct Table 1. Key fusion fuels, including main secondary reactions. D + T → n (14.07 MeV) + 4He (3.52 MeV) D + 3He → p (14.68 MeV) + 4He (3.67 MeV) D + D → n (2.45 MeV) + 3He (0.82 MeV) D + D → p (3.02 MeV) + T (1.01 MeV) thrust or electricity, whereas 80% of the energy from D-T reactions is produced as neutrons and requires more massive shielding, thermal cycle energy conversion at relatively low efficiency with larger radiator mass for waste heat rejection, and an intermediate system to convert the resulting electricity into thrust. The D-T fuel cycle also requires a tritium breeding blanket, adding a great deal of complexity to the system. Depending on the configuration, the plasma exhausted from the core and used to provide thrust can range from ~10 eV to ~1 MeV. For hydrogen, this corresponds to ~4 × 10^4 m/s to ~10^7 m/s. The high end of that range is not useful within the Solar System, so matter must be added to the exhausted plasma to decrease velocity and increase thrust. Traditionally, the average exhaust velocity is normalized to Earth's surface gravity and termed the specific impulse, Isp = vex/g0. A detailed discussion of rocket dynamics and so-called "low-thrust" trajectories may be found, for example, in Ref. [2]. In comparing D-3He fusion to other options, the specific impulse and the thrust-to-weight ratio (T/W) for the propulsion system are useful indicators of performance. These are plotted in Fig. 1 [3]. Fusion's capabilities lie in a niche important for propulsion throughout the Solar System: Isp ~ 10^4–10^5 s and T/W ~ 10^-3. Figure 1: Comparison of D-3He fusion with chemical, nuclear thermal, and nuclear electric propulsion systems. [Plot showing Specific Impulse (s) vs. Thrust-to-Weight Ratio with regions for Chemical, Fission Thermal, Gas-Core Fission, Fission Electric, and D-3He Fusion]. In addition to the technical advances in fusion energy research, two factors increase the timeliness of studying space fusion propulsion: (1) U.S. President Bush's Space Exploration Initiative [4] provides a framework and incentive for long-range space development, and (2) the identification of major resources of 3He on the Moon [5] solves, in principle, the problem of the scarcity of 3He on Earth. A review of research on terrestrial and lunar 3He resources is contained in Ref. [6]. Figure 2. Some potential D-3He magnetic fusion propulsion configuration options [7, 16, 14, 3]: Tandem Mirror, FRC (Field-Reversed Configuration), SCIF (Spherically Convergent Ion Flow), and Dipole. 2. Potential Configurations Several studies of space fusion propulsion have been performed, although all have been small in scope. They have included reactors of many configurations, such as: • Simple mirror [1], • Tandem mirror [7], • Dipole [3], • Field-reversed configuration (FRC) [8, 9, 10], • Colliding compact toroids [11], • Spheromak [12], • Spherical torus (ST) [12], • Bumpy torus [13], and • Spherically convergent ion flow (SCIF) [14]. A discussion of some of these concepts, including a brief historical overview may be found in Ref. [15]. Figure 2 shows some configuration examples, and illustrates the diversity of fusion propulsion options. 3. Mission Capabilities Magnetic fusion propulsion gains greatly in comparison to other options by the ability to optimize the thrust parameters as a function of time along the trajectory. The advantages of tunable systems have been known for some time [2], but a typical nuclear electric system, for example, is limited in its range of exhaust velocities. Figure 3 shows optimized Isp and T/W over a three-month trajectory from Earth to Mars [2]. Note, from Fig. 1, that these parameters fall within the projected range for D-3He magnetic fusion propulsion systems. Such performance leads to a continuum of capabilities from fast, human transport to large-payload, cargo transport. For these alternatives, D-3He fusion propulsion is compared to chemical propulsion for one-way missions from Earth to Mars and to Jupiter in Figs. 4 and 5. Figure 3. Specific impulse and thrust-to-weight values during an optimized, three-month trajectory from Earth to Mars. Figure 4. One-way flight time at the same payload fractions for chemical and D-3He fusion missions from Earth to Mars and to Jupiter. [Plots showing Earth-Mars 33% payload and Earth-Jupiter 7% payload flight times comparing Chemical vs Fusion]. Figure 5. Payload fractions for same flight time for chemical and D-3He fusion missions from Earth to Mars and to Jupiter. [Plots showing Earth-Mars 260 days and Earth-Jupiter 1000 days comparing Chemical vs Fusion payload fractions]. These capabilities will enable large-scale development of the Solar System. Furthermore, D-3He fusion systems will enable scientific outposts analogous to Antarctic bases—with groups of scientists and technicians in orbit around the gas giant planets, for example. Fusion will be able to provide propulsion, space power, beamed power to surface bases, and materials processing. In particular, there presently appear to be no alternatives to D-3He fusion for the attractive ranges at very high specific impulse and modest thrust-to-weight ratios. 4. Conclusions Magnetic fusion for space applications, particularly propulsion, appears attractive from both generic arguments and from preconceptual design studies. The key performance parameters are specific power ~1–10 kW/kg, specific impulse ~10^3–10^6 s, and thrust-to-weight ratio ~10^-1–10^-5. Thus, D-3He magnetic fusion power can provide safe, efficient transport of humans and cargo, and can enable large-scale settlement and exploration of the Solar System. Acknowledgement This research was funded by the Grainger Foundation and the University of Wisconsin. References [1] G.W. Englert, "Towards Thermonuclear Rocket Propulsion," New Scientist 16:16, 307 (1962). [2] Ernst Stuhlinger, Ion Propulsion for Space Flight (McGraw-Hill, NY 1964). [3] E. Teller, A.J. Glass, T.K. Fowler, A. Hasegawa, and J.F. Santarius, "Space Propulsion by Fusion in a Magnetic Dipole," Lawrence Livermore National Laboratory Report UCRL-JC-106807, to be published in Fusion Technology (1992). [4] T.P. Stafford, et al., America at the Threshold (U.S. Government Printing Office, Washington, D.C., 1991). [5] L.J. Wittenberg, J.F. Santarius, and G.L. Kulcinski, "Lunar Source of 3He for Commercial Fusion Power," Fusion Technology 10, 167 (1986). [6] L.J. Wittenberg, E.N. Cameron, G.L. Kulcinski, J.F. Santarius, G. Sviatoslavsky, I.N. Sviatoslavsky, et al., "A Review of Helium-3 Resources and Acquisition for Use as Fusion Fuel," to be published in Fusion Technology (1992). [7] J.F. Santarius, "Lunar 3He, Fusion Propulsion, and Space Development," in Lunar Bases and Space Activities of the 21st Century–II (Lunar and Planetary Institute, Houston, held 1988, to be published in 1992). [8] R. Chapman, G.H. Miley, and W. Kernbichler, "Fusion Space Propulsion with a Field Reversed Configuration," Fusion Technology 15, 1154 (1989). [9] N.R. Schulze, G.H. Miley, and J.F. Santarius, "Space Fusion Energy Conversion Using a Field-Reversed Configuration Reactor," Proc. NASA Space Transportation Propulsion Technology Symposium (1991). [10] H. Momota, et al., "Conceptual Design of D-3He FRC Reactor 'Artemis,'" these proceedings (1991). [11] V.E. Haloulakos and R.F. Bourque, "Fusion Propulsion Systems," 25th Joint Propulsion Conference, paper AIAA 89-2629 (1989). [12] S.K. Borowski, "A Comparison of Fusion/Antiproton Propulsion Systems for Interplanetary Travel," 23rd Joint Propulsion Conference, paper AIAA-87-1814 (1987). [13] J.R. Roth, W.D. Rayle, and J.J. Reinmann, "Fusion Power for Space Propulsion," New Scientist, p. 125 (April 20, 1972). [14] R.W. Bussard, "Some Physics Considerations of Magnetic Inertial-Electrostatic Confinement: A New Concept for Spherical Converging-Flow Fusion," Fusion Technology 19, 273 (1991). [15] J.F. Santarius, "Magnetic Fusion Energy and Space Development," 24th Intersociety Energy Conversion Engineering Conference, Vol. 5, p. 2525 (IEEE, NY, 1989). [16] W.C. Condit, et al, "Preliminary Design Calculations for a Field-Reversed Mirror Reactor," Lawrence Livermore National Laboratory Report UCRL-52170 (1976).