# MEMS Vaporizing Liquid Microthruster: A Comprehensive Review
## 1 Introduction  
The reduction in the launch and space mission costs is one of the main objectives of the space industry. To this purpose, the development of small satellites (total mass less than 20 kg) has increased its relevance and, recently, the efforts of academic, scientific, and industrial communities have focused on the development of nano- and pico-satellites. These last ones, consisting of multiples of 10 \(\times\) 10 \(\times\) 10 cm\({}^{3}\) cubic units with a total mass from 0.1 kg to 10 kg, need to be equipped with secondary propulsion systems to satisfy the requirements for a multitude of maneuvers, such as orbit altitude modification, orbit inclination correction, drag compensation, constellation deployment from a shared launch and formation flight, and de-orbiting and the control of the re-entry process [1]. Typically, the secondary propulsion system on-board small satellites should provide thrust forces between a few micro-newtons up to some milli-newtons, a high specific impulse (from some tens of seconds to some thousands of seconds), and an impulse bit ranging from the mNs range for relatively coarse attitude requirements down to nNs for very tight pointing requirements and very small spacecrafts. At the same time, it should satisfy stringent constraints of mass, volume, and power consumption, ranging from 0.25 W to 20 W, depending on the spacecraft's total mass and, hence, the available nominal power [2]. Furthermore, the higher the spacecraft weight is, the more the impulse bit is required, especially when attitude control and pointing must be performed in short firing intervals. Furthermore, the need for microthruster miniaturization has met the continuous and rapid advancements in MEMS technologies experienced in the last twenty years, allowingfor the development of different demonstrators of MEMS-based microthrusters [3]. Among them, micro-resisttojets represent an interesting choice thanks to the simplicity of their concept. When fueled by a liquid propellant, micro-resisttojets are better known as VLMs. In this case, an electrical resistor provides the heat to vaporize and superheat the propellant into a properly designed micro-chamber. Therefore, the high enthalpy vapor is expanded through a micronozzle. Nowadays, they represent one of the most promising solutions for secondary propulsion able to provide nominal thrust levels in the range (0.5-2) mN [4] and specific impulse above 100 s (see Table 1), as required for the attitude control and the pointing systems of miniaturized spacecrafts [5]. This combines with additional advantages, such as a high compactness and low mass, high integration capability, high reliability, and fast response, as underlined in [4]. Compared to cold/hot gas microthrusters, they store the propellant in low-pressure and lightweight fuel tanks, eliminating the need for safe confinement and high-pressure storage. Consequently, weakly pressurized tanks storing propellant are coupled with a micro-valving system placed downstream to control the feeding pressure. Nowadays, the last one represents the easiest, cheapest, and most reliable propellant management system for application to micro-propulsion systems. An advantage of such a system is that the valve can be turned on and shut off for different run times. This allows matching requirements for attitude control of microsatellites in terms of a minimum impulse bit. Since the impulse bit is obtained by integrating the thrust over the maneuver run time, the minimum valve cycle time directly affects the minimum impulse bit achieved. Consequently, to ensure impulse bits ranging between nNs and mNs, valve-driven feeding systems must have open-close times less than 10 ms, in combination with thrust forces from \(\upmu\)N to mN, as specified in [6; 7]. In this regard, today, electrostatic, electromagnetic, and piezoelectric MEMS microvalves represent the most promising solutions [8]. In the present paper, first, we propose a review focused on MEMS VLMs geometries and heating solutions proposed in the literature.