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Space Nuclear Propulsion


Scope Title:

Nuclear Electric Propulsion

Scope Description:

Space Nuclear Propulsion (SNP) is a subtopic that develops low-TRL systems that use fission energy, rather than combustion, for propulsion. Nuclear Thermal Propulsion (NTP) uses fission energy in a heat exchanger to directly heat a propellant for thermal expansion through a traditional nozzle. Nuclear Electric Propulsion (NEP) uses a fission reactor electric power system to run electric thrusters.

NEP is a propulsion concept being investigated by the SNP project. A nuclear fission reactor heats a working fluid. A power conversion system transforms the working fluid heat to electricity, and the radiator disposes of the waste heat. An electric thruster uses the electric power to accelerate ionized propellant to high velocities and provides continuous thrust. NEP is a concept that can be utilized for cislunar missions, Mars missions, and outer solar system science missions. Reference 1 recommended NASA raise the TRL of NEP technologies to allow informed trades for future missions. NASA has identified five technology areas that must be integrated for a complete NEP system. The five technology areas are the Reactor Subsystem (RXS), Power Conversion Subsystem (PCS), Power Management and Distribution (PMAD), Electric Propulsion Subsystem (EPS), and Primary Heat Rejection Subsystem (PHRS). This subtopic seeks to advance the TRL of specific elements within three of these subsystems with targeted research and development. This subtopic focuses on technologies specific to high-power NEP systems but does not include the electric thruster itself.

All NEP subsystems and components must be designed to withstand launch load environments and space environment effects. Specific technologies being sought include:

  1. PHRS: High emissivity leads to smaller radiator area. Radiators for NEP significantly contribute to the total mass of the vehicle. NASA seeks high-emissivity coatings and/or surface treatments for space radiators operating at temperatures up to 750 K with lifetimes of 25,000 hr. The coating or surface treatment process needs to be able to be applied to an individual modular radiator panel (~15 m2). Coatings or surface treatments should also be resilient and experience a minimal loss of properties within the relevant environment, i.e., long-duration, high-temperature operation in a vacuum, cold soak, and exposure to sunlight, radiation, and the exhaust of the EPS. Coatings need to be compatible with radiator substrate material to include titanium and carbon-carbon composites.
  2. PMAD: NEP systems use high-power electricity and need rapid switching for management of fault conditions and redistribution of power. These need to be high-power, flight-weight, vacuum-rated switches (mechanical contactors) capable of switching 1,000 amps AC (frequency of 1-2 kHz) at 1,000 volts (i.e., holding off 1,000 V in the open state and conducting 1,000 A in the closed state). The switch should be capable of closure within tens of milliseconds and operating for at least 100 close/open cycles.
  3. EPS: A power-processing unit (PPU) efficiently (>90%) converts the polyphase AC power coming from the generator to low-ripple DC power required for the electric thrusters (for magnetoplasmadynamic (MPD) thruster, ~80 V and 12,000 A; for Hall thruster, ~600 V and 160 A). NASA seeks high-power PPU components for a 1MW MPD thruster or a 100kW Hall effect thruster: components to include rectifying diodes and solid-state switches. Input polyphase AC power can be assumed to be 1,000 V; 1,000 to 2,000 A; and at a frequency of 1 to 2 kHz.

Expected TRL or TRL Range at completion of the Project: 3 to 5

Primary Technology Taxonomy:

  • Level 1 01 Propulsion Systems
  • Level 2 01.2 Electric Space Propulsion

Desired Deliverables of Phase I and Phase II:

  • Prototype
  • Hardware
  • Research

Desired Deliverables Description:

Desired deliverables for this technology would include research that can be conducted to determine technical feasibility of the technology during Phase I and show a path toward a Phase II hardware demonstration. Testing the technology in a simulated (as close as possible) NEP operating environment as part of Phase II is preferred. Delivery of a prototype test unit at the completion of Phase II allows for follow-up testing by NASA.

Phase I Deliverables: Feasibility analysis and/or small-scale experiments proving the proposed technology to develop a given product (TRL 2 to 3). The final report includes a Phase II plan to raise the TRL. The Phase II plan includes a verification matrix of measurements to be performed at the end of Phase II, along with specific quantitative pass-fail ranges for each quantity listed.

Phase II Deliverables: A full report of component and/or breadboard validation measurements, including populated verification matrix from Phase I (TRL 3 to 5). Also delivered is a prototype of the proposed technology for NASA to do further testing if Phase II results show promise for NEP application. Opportunities and plans should also be identified and summarized for potential commercialization of the proposed technology. Unique government facilities can be used as part of Phase II.

State of the Art and Critical Gaps:

The NEP concept is a much larger scale system than any SEP system to date. The larger scale NEP has significant technology gaps to the required subsystems. Current space radiators do not operate at the required high temperatures needed for NEP. The kind of switch gear required for high-power NEP has been used for terrestrial systems but does not meet the requirements needed for use in space. PPUs for low-power EPSs have been used in the past, but there are no high-TRL PPUs for a high-power EPS. This scope is only addressing a few of the NEP gaps.

Relevance / Science Traceability:

STMD (Space Technology Mission Directorate) is supporting the SNP project to investigate and mature critical technologies needed for NTP and NEP.

Future mission applications:

  • Human missions to Mars.
  • Science missions to the outer planets.
  • Planetary defense.

Some technologies may have applications for fission surface power systems.




  1. “Space Nuclear Propulsion for Human Mars Exploration,” A Consensus Study Report of the National Academies of Sciences, Engineering, and Medicine, February 2021.
  2. “Independent Assessment of the Technical Maturity of Nuclear Electric Propulsion (NEP) and Nuclear Thermal Propulsion (NTP) Systems,” NASA Engineering & Safety Center, June 2020.
  3. NEP Technology Interchange Meetings (TIM), 2020-2021. NASA Technical Memorandum in process; notes for individual TIMs available through the Space Nuclear Propulsion (SNP) Project, NASA MSFC.
  4. NEP Technology Maturation Plan (TMP) will be made available through SNP, NASA MSFC; likely publication date is late 2022/early 2023.

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