Design and testing of a microelectromechanical-system-based high heat flux vaporizing liquid microthruster

Abstract

This paper presents the design of a low power (8 W) water-fed vaporizing liquid microthruster that operates in the Leidenfrost boiling regime. The direct thrust measurements show the proposed design is able to produce one of the best specific impulse amongst all similar microthrusters reported in the literature. One of the important features that enables the superior performance is the high heat flux miniature molybdenum heating elements fabricated inside the vaporization chamber in a single silicon device package of dimensions 35 mm ✕ 35 mm ✕ 1.05 mm. The molybdenum heating elements are capable of producing a heat flux output of 370 kW/m2 resulting in a sustained operational pressure of 2 bar and temperature of 446 K. The specific design details, such as choice of configurations and sizing, are given in this paper. In particular, the choice of material and the fabrication process carried out at the in-house nanosystem fabrication facility are described. The experimental setup and uncertainty analysis of the mN-range hanging pendulum direct thrust measurement stand are presented. Then, the evaporation of liquid water propellant is investigated in detail. The occurrence of Leidenfrost effect inside the vaporization chamber is captured with a high-speed camera at a frame rate of 5000 Hz. It can be seen that the rapid vaporization of propellant in the Leidenfrost boiling regime results in a high specific impulse amongst other microelectromechanical-system-scale vaporizing liquid microthrusters. The direct thrust measurements show the current design is capable of producing a specific impulse ranging between 25.9–51.1 s at a continuous thrust level of 1.00–2.00 mN; considering contemporary energy storage technologies, the estimated thrust per device dry mass and specific impulse per device dry mass levels are 4.0–8.0 mN/kg and 103.6–204.0 s/kg respectively.