Acta Astronautica 210 (2023) 82–94 Available online 25 April 2023 Research paper Nuclear fusion powered Titan aircraft Michael Paluszek a,∗, Annie Price a,1, Zoe Koniaris a,2, Christopher Galea a, Stephanie Thomas a, Samuel Cohen b, Rachel Stutz a,3 a Princeton Satellite Systems, 6 Market St. Suite 926, Plainsboro, NJ, USA b Princeton Plasma Physics Laboratory, 100 Stellarator Rd, Princeton, NJ, USA ARTICLE INFO Keywords: Fusion propulsion, Titan, Aircraft, Nuclear propulsion, Mars, Space power ABSTRACT This paper discusses a system for Titan exploration enabled by nuclear fusion power. Titan is one of the most interesting locations in the solar system with a thick atmosphere, surface oceans, under-ice oceans and complex terrain. This paper provides a conceptual design of a fusion-powered system to explore many parts of Titan and enable the use of high-power instruments. The design includes a fusion-powered orbital transfer vehicle and an electric Titan science aircraft. A Direct Fusion Drive (DFD) propulsive engine could bring a sizable spacecraft to Titan orbit in less than two years. A second fusion reactor, configured as a closed-loop power generator, would be used for an electric Titan science aircraft. Both reactors are based on the Princeton Field-Reversed Configuration (PFRC) concept which combines an FRC with a magnetic mirror. PFRC uses a novel radio-frequency plasma heating system and deuterium-helium-3 fuel. A lower temperature plasma flows around the closed-field FRC region removing the fusion products. In the DFD propulsive configuration, this secondary flow permits direct and variable thrust and exhaust velocity. The science aircraft would do a powered entry to Titan and then have the capability to fly anywhere on the moon at subsonic speeds. The DFD-powered transfer vehicle would allow the in-orbit transfer stage to change inclination as needed to cover different areas of the surface. 1. Introduction Recent work by Gajeri, Aime, and Kezerashvili [1], demonstrated that a spacecraft powered by Direct Fusion Drive could reach Titan in less than 2.6 years with a 2 MW power plant for a payload mass of 1000 kg. This paper extends that work to include as a payload a Titan aircraft that can enter the atmosphere and fly around Titan for years. The fusion-powered electric aircraft is propelled by ducted fans. Entry does not use any thrust. The fusion-propelled transfer vehicle stays in orbit with its scientific payloads and acts as a communications node for the Titan aircraft. The Titan aircraft would have over 100 kWe for Titan science, several orders of magnitude greater than will be available to Dragonfly. [2]. 2. Titan mission 2.1. The Titan environment A composite infrared image of Titan from the NASA Cassini mission is shown in Fig. 1. The complexity of the geography is evident. The surface of Titan [3] is obscured by a hazy atmosphere. It has standing bodies of liquid, including rivers, lakes, and seas. Titan may have volcanos with liquid water lava. Extensive regions of dunes stretch across Titan in the equatorial areas. Titan does not have its own magnetic field, but it does orbit within the magnetosphere of Saturn [4]. Its thick atmosphere protects the surface from ionizing radiation. 2.2. Dragonfly science objectives Dragonfly's primary goal is [5] to study Titan prebiotic chemistry and to look for chemical bio-signatures of water-based life. The quadcopter is shown in Fig. 2. Dragonfly's target is the 80 km diameter Selk Crater shown in Fig. 3. Its location on Titan is shown in Fig. 4. Dragonfly's major instruments are shown in Table 1. The entire spacecraft is about 450 kg and the power produced by the MMRTG is about 70 W. [6]. Table 1: Dragonfly science payload. - Dragonfly mass spectrometer: A linear ion trap mass spectrometer - Dragonfly gamma-ray and neutron spectrometer: Measure bulk elemental composition in the shallow subsurface - Dragonfly geophysics and meteorology package: Measure atmospheric temperature, pressure, wind speed and direction, methane humidity, hydrogen partial pressure, crustal seismicity, electric field, surface dielectric properties, surface temperature, and ambient sound - Dragonfly camera suite: Dragonfly carries eight scientific cameras Acronyms: - FRC: Field Reversed Configuration - JIMO: Jupiter Icy Moon Orbiter - DFD: Direct Fusion Drive - MeV: Million electron volts (6 × 10^-14 J) - MMRTG: Multi-Mission Radioisotope Generator - NIAC: NASA Innovative Advanced Concepts - PFRC: Princeton Field-Reversed Configuration - PSS: Princeton Satellite Systems - RF: Radio Frequency - RMF: Rotating Magnetic Field 2.3. Science objectives This mission expands on the Dragonfly mission and incorporates elements of previously proposed Titan missions. Dragonfly aims to study Titan’s atmosphere, surface, and subsurface ocean by sampling various sites around the moon, and it is one of the first NASA missions since the Viking landers to explicitly include a search for signs of life [5]. This mission would continue that search with the Titan Aircraft. In particular, the Titan Aircraft will greatly expand the area of Titan’s surface that could be studied in depth. The Huygens probe was only able to transmit a few hours’ worth of data about Titan’s atmosphere, including only one point on the surface of the moon [7]. The Dragonfly quadcopter will be able to move on Titan’s surface, but its mobility will still be limited by its power and speed. The Titan Aircraft would have a longer and more extensive mission due to fusion-enabled speed and power. This would make it ideal for using imaging or radar techniques to map out an unprecedented amount of Titan’s surface. The mission could also support additional science payloads. A submarine to explore Titan’s oceans more in-depth has received a Phase II NIAC grant [8], and NASA/ESA’s proposed Titan Saturn System Mission (TSSM) included a Montgolfière balloon intended to circumnavigate Titan and image its surface [9]. Payloads similar to either or both of these proposed instruments could be included in this mission, possibly being deployed from the Titan Aircraft. The Titan submarine has a maximum power requirement of 842 W and a mass of 100 kg while the balloon has a power requirement of 1.8 kW and mass of 25 kg, both of which are only fractions of the available power (500 kW) and mass (1000 kg) available for science missions on the Titan Aircraft. Experiments such as these or others intending to image or sample Titan’s surface would bring a new depth of knowledge about Titan’s properties and potential to harbor life. Table 2: RF sounder power requirements. - Subsurface sounding: 10000 W - Magnetospheric sounding: 10–10000 W - Ionospheric sounding: 100–500 W - Local resonances: 10000 W The science goals of this mission would follow those set out in the TSSM report [9]: • Titan: an Earthlike System. How does Titan function as a system; to what extent are there similarities and differences with Earth and other solar system bodies? • Titan’s Organic Inventory. To what level of complexity has prebiotic chemistry evolved in the Titan system? 2.4. High power instruments The NASA Jupiter Icy Moon Orbiter mission (JIMO) had a 100 kWe nuclear fission reactor for power and propulsion. Instruments were proposed that could take advantage of that power level. An advanced radio sounder instrument was proposed that could be used on a wide variety of outer planet missions [10]... Table 2 gives the power requirements. A National Research Council (NRC) study [11] lists a large number of missions enabled by nuclear power systems... JIMO also planned to use 1000 W for its communication system [12]. JIMO allocated 45 kWe to the science payload [13]. Hartman lists 250 kWe as a future goal for a nuclear powered spacecraft. Table 3 lists the JIMO instruments. Four require power greater than 1 kWe. All four would be potential Titan instruments. Hart [14] gives a peak power demand of 25269 W. The Multiple Instrument Distributed Aperture Sensor (MIDAS) for remote sensing [16] is an example of an instrument enabled by the availability of high power (Fig. 5). Another is the high-capability Planetary Advanced Radio Sounder (PARS) [17]... 2.5. Mission economics Table 4 gives the economics for several scientific missions. A simple metric is dollars per watt for the science mission... The Jupiter Icy Moon Orbiter (JIMO) which would have used a nuclear reactor is seen as a real bargain. Operations costs range from $15M/year for New Horizons to $50M/year for other spacecraft. 3. Mission plan The mission plan is: 1. Launch on a heavy lift launcher such as SLS, Falcon 9 Heavy, or the Delta Heavy 2. Direction insertion into a heliocentric orbit 3. Constant acceleration to Saturn 4. Entry into Saturn orbit 5. Rendezvous with Titan 6. Entry into Titan Orbit 7. Separation of the aircraft; transfer vehicle remains in orbit 8. Unpowered aerodynamic entry of aircraft into the Titan atmosphere 9. Separation of the ducted fan shrouds 10. Fly to the first landing zone 11. Powered landing on Titan 4. Fusion power plant The Princeton Field-Reversed Configuration (PFRC) and Direct Fusion Drive have been previously presented at the IAC conferences in 2012 [18,19], 2014 [20], and 2015 [21]... Table 3: JIMO instruments [14,15]. - Super high-res camera: Mass 65 kg, Avg Power 100 W, Peak Power 100 W (Scan platform) - High res telescope: Mass 20 kg, Avg Power 5 W, Peak Power 5 W (Scan platform) - Mapping camera: Mass 5 kg, Avg Power 5 W, Peak Power 5 W (Scan platform) - Wide-angle camera: Mass 3 kg, Avg Power 5 W, Peak Power 5 W (Scan platform) - Hyperspectral imager: Mass 25 kg, Avg Power 15 W, Peak Power 15 W (Scan platform) - Thermal imager: Mass 11 kg, Avg Power 14 W, Peak Power 14 W (Scan platform) - SAR topographic mapper: Mass 150 kg, Avg Power 200 W, Peak Power 1400 W (Bus-mounted on boom) - Interferometric SAR: Avg Power 1683 W, Peak Power 7923 W - Ice penetrating radar: Mass 50 kg, Avg Power 2700 W, Peak Power 13000 W (Bus-mounted) - Laser-illumination spectrometer: Mass 250 kg, Avg Power 2500 W, Peak Power 2500 W (Bus-mounted) - Laser altimeter: Mass 44 kg, Avg Power 1400 W, Peak Power 1400 W (Bus-mounted) - Plasma wave spectrometer: Mass 10 kg, Avg Power 7 W, Peak Power 7 W (Bus-mounted) - Magnetometer: Mass 3 kg, Avg Power 3 W, Peak Power 3 W (Bus-mounted on boom) - Ion and neutral mass spectrometer: Mass 10 kg, Avg Power 28 W, Peak Power 28 W (Turntable) - Heavy ion counter: Mass 3.3 kg, Avg Power 7 W, Peak Power 7 W (Turntable) - Energetic particle detector: Mass 11 kg, Avg Power 10 W, Peak Power 10 W (Turntable) - Plasma spectrometer: Mass 13 kg, Avg Power 11 W, Peak Power 11 W (Turntable) - Dust detector: Mass 5 kg, Avg Power 6 W, Peak Power 6 W (Turntable) - Auxiliary science package: Avg Power 375 W (Bus-mounted) Table 4: Mission capability cost is $/Watt. - JIMO: Cost $16B, Power 100,000 W, Transfer 5 Years, $160,000/W - New Horizons: Cost $0.78B, Power 202 W, Transfer 9 Years, $3,861,386/W - Juno: Cost $0.7B, Power 435 W, Transfer 5 Years, $1,609,195/W - BepiColombo: Cost $0.7735B, Power 420 W, Transfer 7 Years, $1,841,666/W - Dragonfly: Cost $1B, Power 70 W, Transfer 9 Years, $14,285,714/W Table 5: Velocity changes using the continuous thrust plan from [1]. - Earth departure: 7.56 km/s - Interplanetary acceleration: 14.50 km/s - Interplanetary deceleration: 36.03 km/s - Saturn orbit insertion: 18.90 km/s - Titan orbit insertion: 1.57 km/s - Total: 78.57 km/s Table 6: Vehicle power and payloads. - Fusion power: Transfer vehicle 2.4 MW, Titan aircraft 1.15 MW - Thrust power: Transfer vehicle 1.4 MW, Titan aircraft N/A - Electrical power: Transfer vehicle 0.1 MW, Titan aircraft 0.5 MW - Reactor mass: Transfer vehicle 2531 kg, Titan aircraft 1006 kg - Payload mass: Transfer vehicle 3500 kg, Titan aircraft 2000 kg 4.1. DFD summary The PFRC employs a radio frequency (RF) plasma heating method, odd-parity rotating magnetic field (RMF) heating, first theorized in 2000 [22] and demonstrated in the PFRC-1 experiment in 2006 [23]. Experiments are ongoing with the second-generation machine, PFRC-2 (Fig. 6)... When scaled up to achieve fusion parameters, PFRC would result in a 4–8 m long, 1.5 m diameter reactor producing 1 to 10 MW... The PFRC is specifically designed to have ultra-low radioactivity. Being a “high-β” machine, that maintains a high ratio of the plasma pressure to magnetic pressure (β), the PFRC should achieve the high temperatures necessary to burn helium-3 (3He) and deuterium (D) in a compact machine. The reaction equations are: (1) D + 3He → 4He (3.6 MeV) + p (14.7 MeV) (2) D + D → T (1.01 MeV) + H (3.02 MeV) (3) D + D → 3He (0.82 MeV) + n (2.45 MeV) (4) D + T → 4He (3.5 MeV) + n (14.1 MeV) Simply burning 3He-D at high temperatures in a large FRC reactor could result in an increase in neutrons due to the above D-D and D-T side reactions. PFRC has three features that greatly reduce the number and impact of the neutrons from this baseline: one, the small size of the reactor (~25 cm plasma radius); two, operating fuel ratio of 3He:D adjusted upwards to as much as 3:1; three, rapid elimination of tritium (T) produced by D-D side reactions, preventing D-T reactions. Fig. 8: Maximum DFD thrust as a function of jet power leads to a practical lower exhaust velocity limit of 70 km/s assuming a 0.4 power to thrust conversion efficiency. 4.2. PFRC-2 experimental results ARPA-E funding was provided to upgrade the PFRC-2, required for entry into the theoretically predicted ion heating regime [30]. The target operating parameters are 0.1 T central magnetic field, 200 kW forward RF power, and operating frequency below 2 MHz. The PFRC-2 has been operating at 1.8 MHz since November 2022. 4.3. Reactor power and mass breakdowns - Fig. 9: Power model for the transfer vehicle engine (Fusion 2.38 MW -> Thrust 1.42 MW, Gas Box 0.95 MW, Heat Engine 1.86 MW, etc.). - Fig. 10: Mass breakdown for transfer vehicle engine (2.4 MW / 2531 kg): Radiators 28% (719 kg), Shielding 22% (559 kg), Magnets 16% (417 kg), Cooling 14% (345 kg), Conversion 9% (230 kg), Structure 6% (156 kg), Heating 4% (105 kg). - Fig. 11: Power model for aircraft engine (Fusion 1.14 MW, Exhaust 0.68 MW, Gas Box 0.45 MW, Heat Engine 1.49 MW, Electric Output 0.50 MW). - Fig. 12: Mass breakdown for aircraft engine (1.1 MW / 1006 kg): Radiator 40% (407 kg), Cooling 19% (195 kg), Conversion 18% (181 kg), Shielding 9% (91 kg), Magnets 9% (91 kg), Heating 4% (41 kg). Table 7: Mission requirements for the titan aircraft. - Cruise speed: 100 m/s - Takeoff distance: 500 m - Cruise altitude: 1 km 5. Titan aircraft design 5.1. Mission requirements The design was driven by key requirements (Table 7). Constant electric power of up to 1 MW comes from the fusion reactor during subsonic flight. Dry mass of aircraft is assumed constant at 2000 kg (no fuel burn). Mass of engine is constant 1000 kg. Hypersonic entry considerations: Heat transfer qw = -kw (∂T/∂y)w (Eq. 5). Blunt nose alleviates heating on the body during hypersonic re-entry. 5.2. Wing loading and geometry selection Titan surface gravity is 1.352 m/s^2 (Eq. 6). Kinematic equations: (7) s_TO = 0.5 * a * t^2 (8) t = sqrt(2 * s_TO / a) (9) v_final = v_initial + a * t (10) = v_TO (11) v_stall = v_TO / 1.2 (12) V_stall = sqrt((W/S) / (rho * Clmax)) (13) W/S = sqrt(V_stall * Clmax / rho) Master equation (14) for Tsl/WTO and power-to-weight... Cruise thrust loading (Eq. 15): Tsl/WTO = (beta/alpha) * [ k * beta / q * (WTO/S) + Cd0 / (WTO/S) * (beta/q) ] Takeoff thrust loading (Eq. 16) Climb thrust loading (Eq. 17) Relation to power (Eq. 18): T/W = (500 * eta_prop / V) * (hp / W) A wing loading value W/S = 25 was chosen, yielding wing area S = 108.16 m^2 and wingspan b = sqrt(A * S) (Eq. 20). Wing geometry relations: (21) Croot = 2S / [b(1 + lambda)] with taper ratio lambda = 0.25 (22) Ctip = lambda * Croot (23) Cmean = (2/3) * Croot * (1 + lambda + lambda^2) / (1 + lambda) (24) Ybar = (b/6) * (1 + 2*lambda) / (1 + lambda) 5.3. OpenVSP sketching and analysis The delta-wing configuration incorporates six ducted fan engines placed near the rear fuselage to take advantage of Boundary Layer Ingestion (BLI). An initial NACA 0010 airfoil was evaluated, giving L/D a little over 13 at alpha=3 deg (Fig. 19). The final design transitioned to a symmetric double-wedge airfoil to handle supersonic/hypersonic entry conditions and reduce wave drag. Fig. 20: Final iteration of the Titan aircraft, with a delta wing, six propellers, and a canard. Fig. 21: Cl, Cd, and Cl/Cd vs Mach Number. Fig. 22: Cl, Cd, and Cl/Cd vs Angle of Attack. 5.4. Tail configuration and sizing Vertical tail volume coefficient CVT = (LVT * SVT) / (bw * Sw) = 0.07 (Eq. 25). 5.5. Flight envelope Max Cl/Cd is 18.74 at alpha = 4.44 deg. 5.6. Distributed electric propulsion Six ducted fans connected in parallel to the fusion power plant. 5.7. Sizing Speed-power coefficient Cs = Vcruise * (rho / (P * n^2))^(1/5) = 2.0097 (Eq. 26) with n = 4000 RPM, cruise power 28 kW per fan. NACA 640 propeller charts: advance ratio J = 0.8, pitch 15 deg, fan efficiency eta = 0.82. Diameter D = Vcruise / (n * J) ≈ 5 ft (Eq. 27). Power coefficient Cp = P / (rho * n^3 * D^5) = 0.004 (Eq. 28). Thrust coefficient CT = eta * Cp / J = 0.996 (Eq. 29). Thrust per fan T = Afan * Nfans * CT = 117.4 N (Eq. 30). Table 8: Aircraft parameters. - Mass (m): 2000 kg - Wing span (b): 13.56 m - Root chord (Croot): 12.8 m - Wing area (s): 108 m^2 - Vertical tail area (SVT): 11.34 m^2 - Aspect ratio (A): 1.9 - Number of engines: 6 ducted fan - Power per engine: 160 kW - Takeoff distance (stakeoff): 500 m 6. Aircraft optimal entry and landing Equations of motion (Eqs. 31-34): vr_dot = vt^2 / r - mu / r^2 + ar vt_dot = - (vt * vr) / r + at r_dot = vr a = (T + D + L) / m Aerodynamic force equations (Eqs. 35-41): q = 0.5 * rho * (vr^2 + vt^2) u = [vr, vt]^T / sqrt(vr^2 + vt^2) D = -cd * q * s * u L = cl * q * s * [0, 1; -1, 0] * u cl = cl_alpha * alpha cd = cd0 + cl^2 / (pi * A * epsilon(M)) T = [cos(alpha), -sin(alpha); sin(alpha), cos(alpha)] * u Fig. 26: Oswald efficiency factor epsilon(M). Fig. 27: Knudsen number for Titan. Fig. 28: Optimal landing trajectory and velocity profiles. Fig. 29: Titan atmosphere model (98% nitrogen). 7. Conclusions A nuclear fusion-powered transfer vehicle and aircraft could open new avenues for Titan scientific exploration. The paper presents a conceptual design-level analysis for both the orbiter and the aircraft. Transit time is less than two years, and the high power enables instruments and mission operations orders of magnitude beyond current capabilities. PFRC-3 will advance the technology in its 5-year life, with full development expected in 10–15 years. Acknowledgments: Previous work supported by NASA NIAC (Grant NNX16AK28G), NASA STTRs NNX17CM47P and NNX17CC74P, ARPA-E OPEN 2018 (Grant DE-AR0001099), and ARPA-E GAMOW (Grant DE-AR0001372). References: [1] M. Gajeri et al., Acta Astronaut. 180 (2021); [2] T. Talbert, Dragonfly's Journey to Titan (2020); [3] NASA Titan (2021); [4] S. Simon et al., Ann. Geophys. 24 (2006); [5] J.W. Barnes et al., Planet. Sci. J. 2 (2021); [6] R.D. Lorenz et al., Johns Hopkins APL Tech. Digest 34 (2018); [7] NASA Huygens (2021); [8] S. Oleson et al., NIAC Phase II (2020); [9] NASA/ESA TSSM Joint Summary Report; [10] J.L. Green et al., IAC (2004); [11] NRC Space Science Priorities (2006); [12] J. Downey (2004); [13] C. Hartman (2003); [14] M. Hart, Aerospace Corp Tech Report (2004); [15] JPL Prometheus Report (2005); [16] J. Pitman et al., Proc. SPIE (2004); [17] Space Science Lab PARS; [18-21] PFRC/DFD papers (2012-2015); [22-25] S.A. Cohen, C. Galea et al. (2000-2023); [26] M.A. Paluszek et al., U.S. Patent 10,229,756 (2019); [27-48] Further aerodynamic, plasma, and propulsion literature.