Researchers at NASA’s Jet Propulsion Laboratory (JPL) are developing a novel microthruster that could provide easy-to-control propulsion during spaceflight. Using solid silver as the fuel source, this innovative microthruster provides thrust via electrospray without heating the fuel reservoir or transporting liquid metals. Instead of transporting a molten metal, this design transports metal ions via a solid electrolyte film.
NASA’s need for highly controllable microscale thrusters for in-space propulsion prompted the development of this innovative concept. Because it is easier to control currents and voltages than chemical reactions, aerospace engineers have been exploring electrospray-based microthrusters.
Traditional designs use capillary forces to transport molten metal from a fuel reservoir across a substrate to the tip of an electrospray emitter. The emission creates a plume that provides a small amount of thrust. Such designs pose several challenges, including having to heat the fuel reservoir and the substrate, successfully control the molten metal, and construct appropriate and consistent emitter tip geometry.
JPL’s concept addresses the limitations of other microthruster designs. As shown in the image, the JPL design uses a simple, container-free reservoir of solid silver. A covalently bonded chalcogenide solid electrolyte film transports the silver ions to the emitter tip via an applied electric field, creating the thrust plume. (This same approach has been used to transport silver ions between electrodes in low-power, nonvolatile memory systems.)
The concept offers several important advantages. Compared to using capillary forces to move molten metal, transporting metal ions via electrochemical potential is fast and easy to control, without the need to heat the fuel. Alloying the solid silver fuel source with nonmobile metals (e.g., copper, aluminum) provides an easy means to enhance self-diffusion of the silver. The vacuum-compatible electrolyte film rapidly transports ions with liquid-like conductivity at near room temperatures (25 °C vs. >150 °C in conventional systems).
Fuel transport is not dependent upon emitter tip geometry, which reduces the variability in thrust from one tip to another. With an ion-to-electron emission ratio of 1:1 — a ratio that cannot be achieved with the clusters of ions and droplets in liquid-metal microthrusters — this system offers extremely high mass-to-charge functionality.
This simple, highly controllable design consumes less energy than liquid-metal microthrusters and offers several other significant advantages. Because electrospray thrusters are more efficient than ion and Hall electrostatic thrusters, this concept is expected to provide a unique capability for in-space precision propulsion for a wide range of space missions.