A liquid monopropellant MEMS thruster was designed, fabricated and tested. For application on a nanosatellite for orbit control, a liquid propellant MEMS thruster delivers better performance than a solid thruster. Two issues must be addressed for a liquid monopropellant MEMS thruster: high energy content of the monopropellant to overcome the excessive heat loss associated with the small scale of the thruster, and repeatability of generated thrust force. The present study proposed blending 90 wt% hydrogen peroxide with ethanol at an oxidizer to fuel ratio of 30 to enhance the energy content of the propellant. The thruster structure was constructed using glass layers that were individually patterned by wet etching. The decomposition catalyst was separately prepared by wet impregnation of the active material, Pt, on the gamma alumina pellets and inserted into the thrust chamber before the UV bonding process of the glass layers. The firing test of the assembled MEMS thruster was conducted and thrust was measured both with ethanol blended hydrogen peroxide and pure hydrogen peroxide as a reference monopropellant. The measured thrusts were approximately 30 mN for both 1.7 ml min−1 flow rate of blended hydrogen peroxide and 2.0 ml min−1 flow rate of pure hydrogen peroxide. The measured thrust for 1.7 ml min−1 pure hydrogen peroxide was approximately 24 mN. The measured thrust was 40% less than the design thrust for both monopropellants. The uncertainty of the thrust was less with blended monopropellant than with pure hydrogen peroxide.

https://iopscience.iop.org/article/10.1088/0960-1317/24/10/104001/pdf

Design, fabrication and thrust measurement of a micro liquid monopropellant thruster

This paper reports a feasibility study of regenerative micro-cooling channels in a liquid microthruster composed of thermally fragile materials. Glass, which is among the most thermally insulating materials, has been used as microthruster fabrication material to suppress excessive heat loss in micro scale thruster. However, the frangibility of glass has remained a challenge to be solved. To thermally manage the fragile structure, the use of regenerative micro-cooling channels in a microthruster is suggested in this work, and the feasibility was tested through design, fabrication and experimental performance of a glass microthruster with microchannels. Nine photosensitive glass layers were wet etched and integrated to fabricate the microthruster. Before integration of the layers, a fabricated Pt/Al 2 O 3 catalyst was inserted into the chamber of the microthruster for propellant decomposition. Hydrogen peroxide (90 wt%) was used as a monopropellant and served as the working fluid for regenerative cooling. A liquid microthruster with micro-cooling channels was successfully fabricated with a photosensitive glass MEMS process. Experimental performance tests were conducted while measuring the microthruster chamber pressure, chamber temperature, and surface temperatures. The test results showed normal operation of the microthruster, which had an estimated thrust of approximately 48 mN and temperature efficiency of approximately 41%. The decreasing surface temperatures of the microthruster during thruster operation successfully validated the cooling effect of the micro-cooling channels and demonstrated their practicality for the regenerative cooling of liquid microthrusters.

Fabrication of a liquid monopropellant microthruster with built-in regenerative micro-cooling channels

A high performance environmentally friendly ammonium dinitramide (ADN) based liquid monopropellant was used to develop a thruster with improved performance compared to a conventional hydrogen peroxide propellant microthruster. Compared to regular thrusters, microthrusters have high heat loss due to the higher reactor surface area to volume ratio. A high enthalpy propellant needs to be used to increase the decomposition temperature in order to enhance the thruster performance. There is a need to increase the heat tolerance of the catalyst given the increased reactor temperature. A lanthanum doped Pt/Al2O3 catalyst was fabricated and applied to increase the heat resistance performance against the ADN-based propellant with a theoretical adiabatic decomposition temperature of 1630 °C. The microthruster was manufactured using a MEMS process. A total of five layers of photosensitive glass with different shapes were fabricated by wet etching and stacked and adhered, and sensors were assembled to measure the pressure and temperature. A combustion experimentation was carried out. 90 wt.% hydrogen peroxide was injected in advance for catalyst preheating and then the ADN-based propellant was injected. In the experimental results, ignition was observed within the reactor and the combustion temperature was 983 ℃. The thrust and specific impulse performances were obtained. Compared to a microthruster using 90 wt.% hydrogen peroxide, the ADN-based propellant enhanced thrust by 19% while the specific impulse increased by 86%.

High performance microthruster with ammonium-dinitramide-based monopropellant

Microcooling Channel Effect on a Monopropellant Microelectromechanical System Thruster Performance

A novel experimental method to evaluate the ignition characteristics of hypergolic propellants was investigated. Three hypergolic liquid fuels with different ignition characteristics were prepared, and their reaction rates with hydrogen peroxide were measured in a microreactor fabricated using a microelectromechanical systems lithography process. To prevent evaporation and boiling by the reaction heat, a low-concentration of hydrogen peroxide of 34.5 wt % was used as a reference oxidizer. The reaction rates of the hypergolic fuels were obtained from temperature measurements in the microreactor and correlated with the ignition delays that were separately measured by drop tests with high-test peroxide. The reaction rate measurements were fully compatible with the drop test results and proved to be useful as an alternative method for the selection of hypergolic propellants by ignition characteristics.

Measuring the Reaction Rate of Hypergolic Propellants with a Microelectromechanical Systems Reactor

Amicroelectromechanical systemsmonopropellant thruster having a design thrust of 100mNwas fabricated and tested. Hydrogen peroxide (90 wt%), which is a green propellant, was selected as the monopropellant. Platinumwas used to decompose the hydrogen peroxide, and it was coated on Al2O3 via impregnation. Three catalyst loading conditions were tested and experimentally optimized. The optimized catalyst was directly inserted into the thruster chamber to enhance decomposition performance because its surface-to-mass ratio significantly affected its performance. The microelectromechanical systems thruster was composed of eight photosensitive cylindrical-type glasses. An additional reduction process was carried out after the thermal bonding procedure because performance degradation of the catalyst was detected at this stage. The microelectromechanical systems thruster decomposed hydrogen peroxide well. However, chamber instability was observed. This instability resulted from an insufficient pressure drop at the injector. Therefore, it is necessary to increase the feeding pressure and reinforce the injector wafer to decrease chamber instability.

Kim Su-Kyum

Papers

Fabrication of Catalyst-Insertion-Type Microelectromechanical Systems Monopropellant Thruster