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    Mars: NASA’s Kilopower Reactor and the Path to Higher Powered Missions




    The U.S. space nuclear program has found considerable challenges in developing a flight qualified fission reactor for NASA missions over the past half century. In fact, the 1960’s SNAP (Space Nuclear Auxiliary Power) program was not only the last time the U.S. has flown a space reactor, the 1965 launch of SNAP 10A, but is also the last time that the U.S. has completed a nuclear powered ground test for any space reactor. Without speculation, it is clear that a successful program will need to have clear advantages over current technologies, be affordable, and be efficiently executed by a qualified team. NASA has partnered with the Department of Energy’s National Nuclear Security Administration to recruit specific talents in reactor design, fuel manufacturing, and criticality testing from the Los Alamos National Laboratory (LANL), the Y-12 National Security Complex and the Nevada National Security Site. This Kilopower team will hopefully overcome the historical challenges and successfully complete a nuclear ground test in 2017 that will provide crucial information about the reactor neutronics and verify if the design can power the future of space exploration.

    January 18, 2018 | Reuters #ScienceNews
    U.S. tests nuclear power system to sustain astronauts on Mars

    WASHINGTON (Reuters) - Initial tests in Nevada on a compact nuclear power system designed to sustain a long-duration NASA human mission on the inhospitable surface of Mars have been successful and a full-power run is scheduled for March, officials said on Thursday. Months-long testing began in November at the energy department’s Nevada National Security Site, with an eye toward providing energy for future astronaut and robotic missions in space and on the surface of Mars, the moon or other solar system destinations.


    Sec. 7. Human Exploration Missions  (See: Page 8)


    NASA and its commercial partners are focused on putting humans on Mars within the following two decades as the next great step in human exploration. The Mars Design Reference Architectures (DRA) [2] have base-lined fission power as the primary power system for surface operations and have recently established the 10kWe Kilopower reactor as the leading technology.

    There are two main phases of the Mars program that require new power system technology. The first phase requires a power system that will autonomously deploy and supply an In-Situ Resource Utilization (ISRU) plant. The ISRU plant will separate and cryogenically store the oxygen from the Martian atmosphere for Ascent vehicle propellant. The second phase requires the same autonomous power system to support the human crew that arrives after completion of the necessary propellant phase. The power requirements for both phases are directly linked to the number of astronauts arriving and the science missions involved during the stay. NASA DRA studies have settled on 40kWe as the required power level to support early Mars missions with a crew of 4-6 astronauts. In 2016, the Human Exploration and Operations Mission Directorate (HEOMD) commissioned the COMPASS team to further evaluate the fission vs. solar trades for Mars [7]. The study looked at the requirements for both the ISRU and crewed phases of the mission with several different power architectures. Rucker et al reported the results [11] along with further evaluation on the subject. A brief summary is included here for discussion purposes.

    Phase 1: ISRU Demonstrator Launch Vehicle: Delta IV Heavy Payload Mass to Mars Surface: 7500 kg Location: Jezero Crater, 18°51′18″N 77°31′08″E Propellant Production: 4400 kg of LOX (1/5 scale) The study took 3 different approaches to the solar architecture design including 1A: daylight only operation at 1/5 production, 1B: around the clock operation at 1/5 production, and 1C: daylight only at 2/5 production. All three designs used the ATK UltraflexTM arrays that were designed to operate at 120V DC with a conversion efficiency of 33%. The arrays were mounted on a gimbal that would track the sun and perform dust mitigation by sloping to 45 degrees. Array and battery sizing changed with architecture options with contingencies for a 120 day global dust storm and an average of 10 hrs/Sol of daylight. Lithium ion batteries were used for energy storage at 165 W-hr/kg. The fission option used a slightly oversized 10 kWe Kilopower unit with a permanent radiator attached to the top of the lander. The reactor operated 24 hours a day at 6.5 kWe (65% capacity) with no interruptions or power loss from dust storms or landing locations. Power conversion was performed by (8) 1,250 We Stirling engines in the dual opposed configuration. Most lander subsystems were identical between the two power systems with some discrepancy in the thermal control systems. Comparison between the solar and fission power system ISRU demonstration mission are shown in Table 3 with conceptual drawings in Figures 9 and 10.

    The ISRU 1/5 scale demonstrator favors solar in terms of mass but requires more time to produce the needed liquid oxygen. Option 1C offers the best balance between propellant production time and mass given the studies assumptions but does not adequately address the follow on energy storage requirements of a crewed mission and cycles on and off every day. For this reason, option 1B is a better technology demonstrator as it accomplishes both the ISRU and crew phase needs with minimal start/stop cycles. Trading option 1B with fission provides a more apples to apples comparison with minor differences in mass and propellant production time.

    Phase II: Crewed Mission
    Launch Vehicle: Space Launch System
    Year: 2038; Crew: 4-6; Landed Mass: TBD
    Locations: Jezero Crater, 18.9°N 77.5°E
     Columbus Crater 29.8°S, 166.1°W
    Propellant Production: 23000 kg of LOX 

    According to the NASA Design Reference Architecture 5.0 there will initially be three expeditions of 4-6 astronauts going to Mars for a stay of approximately 500 days for the conjunction class missions. Each expedition will land at a different location on Mars to adequately explore the diverse geological and environmental terrain. Each expedition will incorporate a pre-deploy mission architecture that allows a lower energy trajectory and larger payload masses with several key parts. First to arrive at the surface is the cargo landers, which house the autonomous power system, ISRU propellant production, and Mars Ascent Vehicle. The power system will initially be used to convert the Martian CO2 atmosphere into oxygen where it will then be cryogenically cooled and stored in the Mars Ascent Vehicle (MAV). After the required ascent propellant has been produced and stored in the MAV and the Mars orbiting habitat fully checked out, the crew will leave earth and rendezvous with the Mars Transfer Vehicle (MTV) in LEO and begin the 175-225 day fast-transit trajectory to Mars. After arriving in Mars orbit, the crew will rendezvous with the habitat and begin the entry, descent, and landing to the pre-deployed cargo landers to start their surface mission. Rucker et al. [11] analyzed the ISRU COMPASS results to accommodate the crew phase logistics using the same technologies and general lander architectures to further evaluate the trade between solar and fission. The results in table 4 give a brief summary of the power system comparison with insight into the differences between the crewed and un-crewed ISRU portions of the mission. The 50kWe fission system, 4+1 spare 10kWe Kilopower units, is delivered on the first lander and provides all three expeditions the required power with a design life of 12 years. The reactors performance would not change based on global location or dust storms, and could be permanently attached to the lander or offloaded for strategic arrangement.

    The solar architecture is analyzed at both the Jezero and Columbus crater sites to give insight into the locational constraints of the solar insolation on Mars. The main difference between the ISRU un-crewed mission and the crewed mission is the necessity for energy storage overnight and the additional requirements for crew keep alive power during the global dust storms. This energy storage and power management addition can be seen in the mass of the first lander of each expedition and in the subsequent landers, closely matching option 1B from the ISRU study. These initial results show that the fission system for crewed expeditions is roughly half the mass of a comparable solar system, even at favorable solar latitudes. The rarely debated advantage of using fission surface power systems on Mars is their tolerance to dust storms and their ability to produce abundant power at any point on Mars. Another advantage that does not receive enough awareness is the potential for long power producing lifetimes beyond mission requirements. The Kilopower reactor’s thermal output in relation to the core’s total fissionable energy is small, which reduces the fuel burnup significantly. With controlled reactivity insertion throughout the lifetime of the reactor it is possible to achieve full power production for several decades. Although this advantage is attractive it cannot be easily tested in ground demonstrations and will require an extended space mission to fully prove. The disadvantage of fission is the produced radiation, requiring shielding to protect equipment and crew. The mission architectures will likely have astronaut keep out zones and radiation safety protocols that would not be required with solar systems. For non-human rated systems, such as the ISRU demo or other mechanical/electrical systems, radiation hardened components will greatly reduced the amount of shielding required and thus leading to mass benefits. The advantages of solar are their simplicity, redundancy, and flight heritage that have been proven with many successful missions. The challenges for solar missions to Mars have remained numerous regarding dust accumulation on solar panels, limited solar insolation from dust storms, and available sunlight at northern and southern latitudes. These very reasons supported decisions to move away from solar powered rovers such as Spirit and Opportunity and replace them with a nuclear powered Multi-Mission Radioisotope Thermoelectric Generators (MMRTG) as seen on the Mars Science Laboratory.



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