Improving Hybrid Electric Propulsion Efficiency

Hybrid chemical motors offer an alternative to traditional liquid or solid motors for spacecraft, missiles, rockets, or other vehicles. The key advantage of a hybrid motor is the capability to throttle the motor via active control, which cannot be done in solid propellant motors. However, rapid throttling presents significant challenges to implement in practice. Here, NASA has combined a deep-throttling hybrid motor previously developed by Utah State University with a fast-acting digital valve design to produce a fast-acting, deep-throttling hybrid. Testing performed to-date using a prototype of the hybrid motor and digital valve design has shown the new hybrid motor to be capable of full-scale throttling twice as fast (1 second throttling compared to 2 seconds) as previous control valve designs. With optimization, there is potential full-range throttling may be further reduced to 0.5 second, a 4x improvement over previous control valve designs for hybrid motors. Additionally, smaller mid-range thrust changes have currently been measured in the 40:1 for relatively small motors (

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.

Flow-restricting features in a Hall thruster anode manifold assembly, typically precision manufactured orifices, can contribute to significant flow non-uniformity if tolerances on the features are not properly controlled. Non-uniformity in flow distribution negatively impacts thruster performance. The anode assembly is usually a complex and expensive assembly to manufacture. Removing the flow restricting elements from the anode manifold structure in favor of modular insertable subcomponents (i.e., plugs) enables the use of more reliable and repeatable precision manufacturing techniques. The resulting components can be tested, characterized, and sorted for acceptance before being installed into the larger anode assembly (i.e., quality control can be performed at the subcomponent level). This may lead to increased performance and yield rate of the final assembly. The flow restrictor plugs can be made in many different ways. The most basic flow restrictor takes the form of a precision hole machined into a cylinder, where the cylinder is then press fit into a hole drilled into the anode base. Alternate embodiments of the flow restrictor include precision machined nozzles, laminar flow elements, or sintered porous metal elements. The flow restrictor can also be made from a different material than the anode base, such as a precision ruby orifice contained in a metal carrier which is installed in a metal anode base. The plugs can be installed in a variety of ways, all of which create hermetic seals. Installation can include a press fit relying on plastic deformation or threading the plug component into the anode base. Welding on the top surface of the anode base can also be done to provide a robust hermetic seal.

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).

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.