JPL's Microfluidic Electrospray Propulsion (MEP) thruster design is based on a microfabricated electrospray system with a capillary-force-driven feed system that uses indium metal as the propellant. This architecture provides an extremely compact, modular system scalable to a wide range of applications from micro spacecraft to large, space-based telescopes.
The electrospray system consists of an array of emitters (micro-needles) with integrated grooves, fabricated on a silicon chip by employing state-of-the-art microelectromechanical system (MEMS) design and fabrication. The array is then aligned with apertures of a metal extractor grid. The needles are externally wetted with indium propellant, and when a high-voltage difference is applied between the emitter and the extractor, the high electric field at the needle tip distorts the fluid into a Taylor cone, which emits indium ions from its tip. The electric field then accelerates the ions through the extractor grid apertures. Tapered vias going through the electrospray system are also integrated with the emitter array to allow propellant replenishment from the reservoir located behind the array.
In order to have good control of the indium electrospray, the emitters require micron-scale shape/height control, and the tip part needs to have a very well controlled angle to support the Taylor cone formation. Two well established processing techniques, lithography and etching, are brought together to allow the fabrication of the emitter arrays with complex geometries, and micron-scale precision and uniformity. Reflow photolithography or 3D grey scale e-beam lithography are used to control the shape of the resist patterns and/or the groove geometry, and deep reactive ion etching (DRIE) of the silicon is used to produce the height.
Microfabricated electrospray needle arrays will provide propulsion systems with 10× improvement in mass and volume over SOA for highly compact, scalable, and distributable architectures, for both very large and very small spacecrafts. The microfabricated thruster components, solid metal indium propellant, and capillary-force-driven feed system without valves or a pressurized reservoir enable the development of highly integrated and compact propulsion systems. The micro-needles are also beneficial for biomedical and surgical devices, or microfluidics channels for precise spray-gun applications.
This work was done by Cecile Jung-Kubiak, Nima Rouhi, Frank Greer, Victor E. White, Daniel W. Wilson, Matthew R Dickie, Amanda M. Davenport, Karl Y. Yee, Richard E. Muller, Colleen M. Marrese-Reading, James E. Polk, and John R. Anderson for NASA's Jet Propulsion Laboratory.