Improved Efficiency in Nuclear Propulsion

The Multi-Layer NTAC is comprised of a gamma-ray source and various layers of emitters, collectors, and insulators. Ideal emitter materials include elements with high atomic numbers, while ideal collector and insulator materials include elements with low atomic numbers. A high-energy gamma-ray (tens of keV to MeV) is used to liberate a large number of intra-band, inner-shell electrons from atoms within the emitter material for power generation through the primary interactions of photoelectric, Compton scattering, photonuclear, and electron/positron pair production processes. Secondary and tertiary electrons are liberated in the avalanche process as well. If a power conversion process effectively utilizes all liberated electrons in an avalanche mode through a power conversion circuit, the power output is drastically increased. Because power conversion is determined by the absorption rate of high energy photons, increasing power output requires either thicker collector material or a sufficient number of layer structures to capture the high energy photons, leaving no liberated electrons escaping (i.e., minimizing the leak of radioactive rays). The selection of materials, the thicknesses of the emitter, collector, and insulator, as well as the number of NTAC layers required are all determined by the energy of photon source. The thermal energy from radioactive decay can also be converted to electricity using a thermoelectric device to further increase power output. The Multi-Layer NTAC technology can be manufactured using existing semiconductor fabrication technology and can be tailored for small-to-large scale power needs, including kilowatt and megawatt applications.

The NASA patented NTAC-TE system is a promising technology for power generation from radioactive isotopes that generate gamma rays. This will reduce the mass of unusable radioactive waste products, as NTAC-TE can use what is normally waste and harmful in order to generate electric power. The patented integration of Cobalt-59 into fission process, which will be transmuted into Co-60, with NTAC-TE elevates the efficiency well beyond current energy harvesting methodologies considering NTAC-TE can also harness energy from Cesium-137. The gamma radiation energies from these three sources combined together are 7 MeV from the prompt fission reaction, 0.6617 MeV from the Cesium by-product of the fission process, and 1.3325 MeV from Cobalt.

Key subsystems of a scalable PPU for low-power Hall effect electric propulsion have been developed and demonstrated at NASA GRC. The PPU conditions and supplies power to the thruster and propellant flow control (PFC) components. It operates from an input voltage of 24 to 34 VDC to be compatible with typical small spacecraft with 28 V unregulated power systems. The PPU provides fault protection to protect the PPU, thruster, PFC components, and spacecraft. It is scalable to accommodate various power and operational requirements of low-power Hall effect thrusters. An important subsystem of a PPU is the discharge supply, which processes up to 95% of the power in the PPU and must process high voltage to accelerate thrust generating plasma. Each discharge power module in this PPU design is capable of processing up to 500 W of power and output up to 400 VDC. A full-bridge topology operating at switching frequency 50 kHz is used with a lightweight foil transformer. Two or more modules can operate in parallel to scale up the discharge power as required. Output voltage and current regulation controls allow for any of the common thruster start-up modes (hard, soft or glow).

NASAs High Propellant Throughput Small Spacecraft Electric Propulsion thruster offers a propellant throughput capability of greater than 120 kg with a nominal thruster efficiency greater than 50%. The new thruster design combines heritage Hall thruster component design approaches with recent NASA GRC advancements in the areas of advanced magnetic circuit design, robust propellant manifolds, and center mounted cathodes. Prototypes of the High Propellant Throughput Small Spacecraft Electric Propulsion thruster have been fabricated and proof-of-concept has been demonstrated. A significant advancement in the High Propellant Throughput Small Spacecraft Electric Propulsion thruster is NASA's optimized magnetically shielded (OMS) field topology. The new OMS configuration reduces discharge channel erosion rates compared to conventional Hall thrusters, while reducing front pole cover erosion rates compared to traditional magnetically shielded Hall thrusters. This system also includes a largely unibody structure to reduce fabrication cost, increase strength, and optimize thermal management. A coupling plate between the high voltage discharge channel and low voltage thruster body allows more efficient thruster assembly and verification processes. Other design advancements further simplify assembly, improve robustness, and optimize performance.

Innovators at GRC have developed a suite of SSEP technologies for small, low-power spacecraft using Hall effect thrusters including a high propellant throughput small spacecraft electric propulsion thruster (LEW-TOPS-158), a power processing unit for SSEP (LEW-TOPS-157), an anode manifold plug for Hall effect thrusters (LEW-TOPS-159), and additional Hall effect technologies (LEW-TOPS-34). See the Additional Information section at the bottom of the page for more information on each technology suite component. GRC is making these technologies available to U.S. companies through a no-cost*, non-exclusive license agreement and companion Space Act Agreement. Licensees may receive a comprehensive package of design and process documents including issued and pending patents, design drawings, materials specifications, and test data. Licensees will assist in defining system requirements and creating new platforms to use the SSEP technologies. This streamlined, collaborative commercialization strategy helps satisfy NASA exploration and science mission requirements while improving U.S. competitiveness in the global electric propulsion market and improving the success of new electric propulsion developments. Working alongside our licensees, GRC hopes to generate a compendium of SSEP knowledge as a living document, maintained by all users in a consortia-like environment. *Although the license and Space Act Agreement are no cost to the licensees, licensees would be responsible for setting up and maintaining an EAR restricted file sharing space.