THE AEROSPACE CORPORATION Overview of Space Nuclear Propulsion and Power (SNPP) Greg Meholic Launch and Propulsion Concepts, Civil Systems Technology December 2024 Approved for public release. OTR 2024-01042 © The Aerospace Corporation, 2024 Outline • All information is UNCLASSIFIED and public release • Does not contain contractor proprietary information Sections • What Is Space Nuclear Propulsion and Power (SNPP)? • Nuclear Fission Basics • Nuclear Thermal Propulsion (NTP) and Space Nuclear Propulsion (SNP) Definitions • Advantages and Challenges (NTP and SNP) – NTP Characteristics – SNP Critical Technology Elements (CTEs) • What Is Different Now? • Government Support of SNPP • History and Current Projects (NTP and SNP) • General SNPP Challenges and Considerations • Closing Statements Acknowledgments: Randy Bell, Jeff Laube, and Torrey Radcliffe What Is SNPP? SNPP is a category of space propulsion and power where the thermal effects or energetic particles from nuclear reactions are used to do work. • Radioisotope—utilizes particle emissions from element isotopes (typically plutonium-238) to create electricity or heat – "Decay-based" power, radioisotope power systems, and radioisotope thermoelectric generators (RTGs) – Typically, only a few hundred watts of electric power (We); always "on" – Space application heritage (more than 40 launched; Galileo, Cassini, Voyager, New Horizons, etc.) • Nuclear Fusion—releases energy from merging light atomic nuclei to form heavier nuclei under extreme conditions – Creates other energetic nuclei fragments that combine in the same way – Sustained fusion yet to be achieved; active research and development efforts • Nuclear Fission—releases energy from splitting of heavy atomic nuclei to form lighter nuclei by the absorption of additional neutrons – Neutrons that are released continue the process – Decades of terrestrial use; small-scale test reactors flown [Images: Voyager Spacecraft RTG (NASA); Artist Concept of Research Fusion Reactor (Oak Ridge National Laboratory); Terrestrial Nuclear Fission Reactor (Shutterstock)] Generalized Fission Process Physics Inside the Nuclear Fission Reactor 1. Uranium-235 (most common fission fuel) absorbs a neutron and becomes uranium-236 2. Uranium-236 immediately overcomes binding energy and splits (fissions) into various fission fragment combinations • 2−3 more neutrons are produced, as well as gamma rays • Energy density (heat release per volume) is 10^5−10^7 times that of Hydrogen gas oxygen combustion—significant heat flux • Reactor materials can approach or exceed 3,500 K (~6,000 °F) 3. Released neutrons cause additional fissions (some are lost) 4. Process "goes critical" and becomes self-sustaining if reaction density is high enough • Activity is controlled using neutron-absorbing, reflecting, and moderating materials (e.g., boron, beryllium, etc.) ‒ Modulates power and thermal output ‒ "Control rods" or "control drums" • Want enough neutrons to be released to maintain stable chain reaction • Do not want supercritical, where fission proceeds at increasing rate, runaway reaction (nuclear weapons) • Process remains "off" and inert until directed to begin ‒ Different than decay-based systems like RTGs that are always "hot" [Images: Idaho National Laboratories Advanced Test Reactor; Nuclear chain reaction diagram (Shutterstock)] NTP and SNP • NTP: Heat from fission is used to thermally energize a propellant, which is then expelled through a rocket nozzle – Looks and operates like a traditional rocket engine (pumps, valves, feedlines, nozzle, etc.) – Has few subsystems – Has "short" operation periods; high thrust – Has "very high" reactor temperatures (~3,000 K at outlet) • SNP: Heat from fission is coupled to a power-generation system to produce electricity – Several CTEs (red boxes), options, and integration aspects; becomes a powerplant in space ("system of systems") o Nuclear electric propulsion (NEP) utilizes the electricity produced from a nuclear source to power electric propulsion (EP) thrusters – Many subsystems – "Long" operation periods, low thrust (if coupled to EP) – "High" reactor temperatures (~1,200 K at outlet) [Diagrams: NTP Systems vs. SNP Systems; Artist Concept of NTP Spacecraft (NASA); Artist Concept of NEP Spacecraft (NASA)] NTP Advantages and Challenges Compared to Traditional, Chemical Propulsion Advantages: • Is nearly 2‒3x more thermodynamically efficient (higher specific impulse [Isp], 900 s vs. 470 s) • May enhance delivery and/or performance capabilities for large-payload missions (deep space/planetary) • Uses a single propellant (usually liquid hydrogen [LH2]) • Has same sizes and thrust classes as upper-stage engines • Has integration similar to traditional rocket engines • Can use multiple inert propellant types • Heritage programs (Nuclear Engine for Rocket Vehicle Applications [NERVA], Space Nuclear Thermal Propulsion [SNTP]) Challenges: • Needs significant alterations to spacecraft subsystems and components (~3x volume of LH2 for same energy [Δv]) • Needs very high-temperature materials for reactor (hotter than LH2/liquid-oxygen [LOX] flame temperature) • Is radioactive during and after use; requires shielding (added mass) • Has low thrust to weight (~10 vs. 120+) • Cannot [currently] be ground tested • Must follow regulatory considerations (launch/reentry of nuclear material) • Has extended start and shutdown transients (30‒60 s vs. 3‒7 s) [Images: Conceptual NTP Spacecraft for Crewed Mars Mission (Polzin, K., 2022); Notional NTP Demonstration Vehicle (Houts, M., 2020)] NTP Characteristics Many similarities between existing upper-stage engines and NTP concepts. Comparison: RL10B-2 (Delta IV Upper Stage) vs. Notional NTP Engine (Mars Transport) • Propellant: LH2/LOX vs. LH2 • Cycle: Expander vs. Expander • Thrust: 24,750 lbf vs. 25,000 lbf • Weight: 750 lb vs. ~11,000 lb • Flow Rate: 52 lb/s vs. ~28 lb/s • Chamber Pressure: 633 psi vs. ~1,000 psi • Nozzle Area Ratio: 285 vs. ~300 • Isp: 465 s vs. ~900 s • Time to Full Thrust: <3 s vs. 30–60 s • Dimensions: - RL10B-2: 500 cm height, 200 cm diameter - Notional NTP Engine: ~750 cm height, 220 cm diameter NTP offers the same thrust and ~2x the efficiency (half the flowrate) but can be ~20x heavier. [Diagrams: RL10B-2 engine; Notional NTP Engine diagram; NERVA Reactor Cross Section and Fuel Segment Cluster (Houts, M., NASA)] SNP Advantages and Challenges Compared to RTG, Solar or Battery-Power Systems Advantages: • Scalable to a broad power range (100s kWe or MWe) • Continuous power, regardless of solar alignment/exposure • Many options for technology, power cycle, and integration • Possibility to enhance payload and/or mission capabilities (as supplement to other power sources) • Manageable reactor temperatures • Heritage programs (System for Nuclear Auxiliary Power [SNAP]-10A, Jupiter Icy Moons Orbiter [JIMO]/Prometheus, Russian Thermionic Experiment With Conversion in Active Zone [TOPAZ]) Challenges: • Integration and operation of multiple complex systems for power production • Significant alterations to spacecraft subsystems and components (traditional power systems may still be required) • Possible employment of multiple working fluids • Radioactive during and after use; requires shielding (added mass) • Regulatory considerations (launch/reentry of nuclear material) [Images: JIMO With Prometheus Reactor; Conceptual Fission Surface Power (FSP) System; Conceptual Lunar Surface Power (LSP) System (NASA)] SNP (CTEs) • CTEs must be integrated to form the complete SNP system: – CTE-1: Reactor (Uranium fuel type, enrichment level, "fast" vs. "thermal" design) – CTE-2: Power Conversion Subsystem (PCS) (Brayton, Stirling, Rankine, Thermionic, Thermoelectric) – CTE-3: Power Management and Distribution (Switching systems, transformers, etc., driven by end application needs) – CTE-4: EP (gray)—only if propulsion is considered – CTE-5: Primary Heat Rejection (thermal management) (Pumped fluid loop or heat pipe radiators, liquid metal or nonliquid metal working fluids) • Each CTE has options that must be traded to determine the best fit for mission needs – No "one-size-fits-all" solution/combination for the integrated system – Each option/combination must be traded against others and against conventional power systems – Individual challenges and technology maturation considerations • Maturing each CTE could be considered a standalone technology development effort – Long timelines, lengthy qualification, and substantial investments • Regardless of the end application(s), every SNP system will have the same CTEs that enable power generation [Diagram: SNP CTEs - Notional Brayton PCS architecture] What Is Different Now? • U.S. Department of Defense (DoD) and civil and commercial space enterprises recognize the need for alternatives to traditional propulsion and power options that may enable novel missions or enhance mission-unique capabilities – Renewed interest in Mars, cislunar spacecraft operations, surviving the lunar night • SNPP has become a viable consideration based on decades of advancements in: – Uranium fuel form development, enrichment, and processing – Reactor design and manufacturing (materials, additive manufacturing) – Nuclear industrial base growth (new companies, capabilities, facilities) – Interests in space applications across government and commercial communities Evolution (60+ years): Prior Space Reactors: • Highly enriched uranium • Uranium nitride chips embedded in graphite • Tie-rod prismatic reactor • Material and structural issues from temperature mismatches • Little modeling available Modern Space Reactors: • High-assay low-enriched uranium (HALEU)―easier to obtain and nonweaponizable but less efficient • Advanced fuel forms—tristructural isotropic (known as TRISO), accident tolerant, production history (terrestrial and Naval) • Multiple core configurations—moderator block, tie rod, particle/pebble bed • High-temperature materials and manufacturing techniques—additive manufacturing, welding, ceramic composites • Validated physics modeling and design tools—reduces cost/risk, improves design Advanced fuels and reactors, novel missions, relaxed regulatory processes, and potential benefits have enabled fission-based SNPP to become tradable options. [Images: Fuel Element/Reactor schematic; Russian Moderator Block Reactor] Government Support of SNPP • The potential benefits of SNPP have led to the creation of government directives and documents (since 2019) that specifically address in-space nuclear systems for propulsion or power Document Summary Table: • Document Title: Energy for Space | ID Number: - | Publication Date: Jan 06, 2021 | Subject: Exploration, Infrastructure, Nuclear | Originator: Department of Energy | Document Type: Report • Document Title: Promoting Small Modular Reactors for National Defense and Space Exploration | ID Number: EO 13972 | Publication Date: Jan 05, 2021 | Subject: Nuclear | Originator: President, Nat'l Space Council | Document Type: Policy Directive • Document Title: National Strategy for Space Nuclear Power and Propulsion | ID Number: SPD-6 | Publication Date: Dec 16, 2020 | Subject: Nuclear, Exploration | Originator: President, Nat'l Space Council | Document Type: Policy Directive • Document Title: Launch of Spacecraft Containing Space Nuclear Systems | ID Number: NSPM-20 | Publication Date: Aug 20, 2019 | Subject: Nuclear | Originator: President, Nat'l Space Council | Document Type: Memo Note: SPD-6 provides a good summary of national strategy needs. • Government agencies have adapted strategies for including and addressing SNPP in their operations – Multiple agencies with regulatory authority over various aspects of the launch/reentry of fission-based nuclear material that has yet to be practiced (Federal Aviation Administration [FAA], U.S. Environmental Protection Agency, Department of Commerce, etc.) – Establishment of several interagency safety review boards (NASA, DoD, etc.) – Orbital debris of SNPP systems also considered Processes for safety assurance and cross-agency coordination (launch approval, processing, operations) need to be considered early in project development and mission planning. NTP History and Current Projects • Nuclear Energy for the Propulsion of Aircraft and Aircraft Nuclear Propulsion (1946–1961) – Nuclear reactors driving turbojets – Successful full-scale ground tests – Reactor shielding tested aboard NB-36H aircraft (1956) • Project Pluto/Tory (1957–1964) – Air-breathing nuclear ramjet for supersonic intercontinental ballistic missile – Successful full power ground tests (~500 MW) • NERVA (1960–1972) – Most prolific NTP project – Phoebus 2A: 4 GW, 210-klbf thrust, highest power reactor ever tested – Successful full-power ground tests, various thrust classes • Reactor In-Flight Test (1962–1964) – Intended as test bed for NERVA engine – Potential growth to Saturn class • SNTP (1987–1994) – High-performing, lighter NTP engine with pebble-bed reactor (Timberwind) • NASA SNP (2016–2023) – High-temperature materials and fuel development for Mars mission systems – Three near-Preliminary Design Review NTP reactor designs (from industry) • Defense Advanced Research Projects Agency (DARPA) Demonstration Rocket for Agile Cislunar Operations (DRACO) (2019–present) – DARPA-led, NASA as partner (2023) – Launch in 2027, may be first NTP to fly in space [Images: NERVA at Test Site; Kiwi/Phoebus configurations; NERVA Cutaway; DARPA DRACO rendering] SNP History and Current Projects • SNAP (early 1960s) – SNAP-10A has been the only U.S. flight reactor – 0.5 kWe at 1.4% efficiency • Soviet Space Reactors: Radar Ocean Reconnaissance Satellite (known as RORSAT), TOPAZ (1967–88) – ~1-kWe demonstration systems – Over 30 reactors launched; some have reentered • SP-100 (1983–1992) – 100 kWe, 2.5 MW, 1,350 K reactor outlet temperature – Growth to 500-800 kWe for LSP – Silicon-germanium thermoelectric conversion with growth to Brayton and Stirling • NASA JIMO/Prometheus (2000–2005) – 200 kWe, 800 kW, 1,150 K reactor outlet temperature, multiple helium-xenon Brayton power conversion systems – Significant research and development by industry and government • NASA Constellation (2007–2012) – 40 kWe at 875 K temperature, Stirling conversion • Kilopower Reactor Using Stirling Technology (KRUSTY) (2015–2018) – ~0.8 kWe from two (of eight possible) Stirling systems – Ground demonstration only; built from today’s technology; not intended for space • NASA FSP (2005–present) – Exploring growth of KRUSTY to ~40 kWe for planetary surface power • Air Force Research Laboratory Joint Emergent Technology Supplying On-Orbit Nuclear Power (JETSON) program for Spacecraft NEP (2022–present) – JETSON Low: 1–5-kWe RTG-based systems – JETSON High: ~40-kWe fission-based systems [Images: SNAP-10A; Artist Concept of SP-100; JIMO/Prometheus Spacecraft] General SNPP Challenges and Considerations • HALEU Processing – Significant investment needed to develop facilities for enrichment of low-enriched uranium to HALEU to meet prospective needs • Development of Very High-Temperature Materials and Assembly Processes – Required for NTP where internal reactor temperatures can exceed 3,500 K (~6,000 °F) – Will also benefit SNP systems that have much longer operational lifetimes • Reactor Ground Testing Not (Currently) Possible – No facilities exist in the United States that can support reactor ground testing (larger than KRUSTY) – NTP testing requires that engine exhaust be scrubbed of radiologics before being released o Could result in substantially large, prohibitively expensive facilities that take years to build and qualify – SNP systems require reactor testing with partially and/or fully integrated CTEs • Complex System Engineering and Integration (SNP) – CTEs must autonomously interact to ensure stability and control across the power-demand range – Must be packaged and qualified for space applications – Multiple, lengthy, expensive test/qualification programs • Commercial Launch Operator Must Obtain License to Launch Nuclear Material – SNPP asset must undergo several multi-agency reviews to assess safety assurance – FAA license application process can take 12–24 months – License might also cover debris and/or disposal, depending on mission • Modifications to Launch Vehicle, Pad, and Ground Systems to Support SNPP – May require requalification, thus driving schedule and cost – May result in substantial investments for unique, infrequent, one-off missions The investments and time needed must be considered when trading SNPP against traditional alternatives and mission needs. Closing Statements • In addition to past programs, decades of studies have been and continue to be performed by industry, academia, and government on fission-based SNPP – Missions and applications, technology development/concepts, performance comparisons, policy, maturation plans, technology readiness level assessment, etc. • Since SNAP-10A, the United States has not flown another fission-based SNPP system – Regulatory issues, obviated need, cost/time to develop flight-ready prototypes – Foreign interests in SNPP may influence the United States – DRACO may be the world’s first NTP engine • None of the prior SNPP projects mentioned were canceled due to safety reasons, technology failures, or heightened risk of radioactive hazards – Terminated mainly due to lack of need, cost, change in national priority, or “something better came along” • Mars exploration and lunar night survivability (NASA), as well as space mobility and logistics near earth (DoD), have renewed the interest in nuclear alternatives • Recent advances in policy and in proposed use of HALEU fission fuel have enabled SNPP to generate broad interests and positive support within DoD, civil, and commercial space sectors – Government recognition of SNPP need and issuance of supporting directives • Standards are being drafted to address SNPP operational considerations – Part of the American Society for Testing and Materials Commercial Spaceflight Committee (F47) – Multi-agency participation • Adopting SNPP systems will require careful consideration of their cost, development time, benefits, and challenges when performing mission-level comparisons to current systems Technology advancements, novel missions, and potential benefits have enabled fission-based SNPP systems to become viable options tradable with traditional systems. THE AEROSPACE CORPORATION Thank you!