# MEMS Vaporizing Liquid Microthruster: A Comprehensive Review
## 5 Concluding Remarks  
The present work provides a review of vaporizing liquid microthruster (VLM) technology for application to secondary propulsion systems on-board nano- and pico-satellites. The major advantages of VLMs were identified in the simplicity of their working principle, which relies on micro-resistojets operating with liquid propellants. They allow to satisfy the requirements of thrust (from a few micronewtons up to a few millinewtons), specific impulse (above 100 s), and impulse bit (ranging between nNs and mNs), in combination with light and compact propellant management systems using a microvalve system placed downstream the propellant tank to control the feeding pressure and, hence, the mass flow rate. The main drawback is the need for propellant vaporization and over-heating, which requires the integration of a heating chamber and a power cost. An introductory analysis explored all the existing VLM concepts and their novelty, with a particular focus on the heating chamber configuration (single-channel or multiple parallel channels), the MEMS technology in terms of materials and manufacturing processes (silicon-based or ceramics-based), and the heating system (resistive heating or induction heating). The advancements experienced in the last twenty years have allowed increasing the technology readiness level (TRL) of VLMs up to four, as confirmed by the water-propelled VLM demonstrator for in-orbit pico-satellite application, designed at TU Delft and recently launched on-orbit in the PocketQube platform Delfi-PQ. Figure 12: Active control of flow boiling using pulsed heating: temporal profiles and normalized spectral density of the heat flux and the wall temperature on a microchannel at different pulsating frequencies as measured in [52]. However, the actual development of VLMs has shown several limitations which jeopardize practical use on-board small spacecrafts. In fact, among all VLM concepts developed, experiments have measured \(\tau\) below 0.3 mN/W and an overall efficiency parameter \(\eta_{0}\) lower than 0.2. The significant discrepancy between the theoretical performance and the real one is due to the establishment of explosive flow boiling instabilities, which reduce the heating efficiency and induce flow oscillations affecting the stability of the supersonic expansion with the occurrence of thrust instabilities and low specific impulse efficiency. The thrust instability due to boiling instabilities is harmful for high-precision attitude control of micro-satellites. Furthermore, flow boiling instability couples with the establishment of the critical heat flux condition inducing thermal stresses, which deteriorate the VLM lifetime. Consequently, the combination of passive and active strategies for flow boiling instability control must be considered to improve the overall performance of VLMs. To this purpose, the definition of the heating chamber geometry, composed of the inlet manifold and the microchannels, is a key step in the VLM design's optimization process. The existing literature dealing with the optimal design of micro heat exchangers has suggested different promising solutions which successfully demonstrate to mitigate flow boiling instabilities, for instance, through the use of a divergent microchannel geometry or the use of a saw-tooth and triangular microchannel, in combination with the use inlet restrictors at the microchannels entrance and/or artificial nucleation sites. Concerning the active control of flow boiling instability, the dynamic behavior of VLMs is driven by the fast phase change phenomena in an open-loop system at high wall temperature and full vaporization conditions. The classical active control approach uses the modification of the feeding pressure or the mass flow rate using a micro-valving system placed upstream of the inlet section, bubble seeding, or synthetic jet in the crossflow. An alternative control strategy is based on the active control of the heat fluxes.