# MEMS Vaporizing Liquid Microthruster: A Comprehensive Review
## 2 VLM Design and Manufacturing: State of the Art
[20] designed and manufactured a VLM using low-temperature co-fired ceramic (LTCC) as the material (see Figure 5a), demonstrating that the LTCC is an alternative solution to silicon  
Figure 3: Manufactured VLMs in single channel configuration: (**a**) [PERSON] et al. [14]; (**b**) [PERSON] et al. [15]; (**c**) [PERSON] et al. [16] (units are in \(\upmu\)m). thanks to the good electrical conductivity of printed metallization and a relatively low production cost. Later, [PERSON] and [PERSON] [21] successfully tested a high-temperature co-fired ceramic (HTCC) microthruster characterized by a platinum-based microheater integrated on a three layers structure (see Figure 5b). Concerning the heating system, [PERSON] et al. [22] developed a tubular concept of a VLM equipped with a micro-heater core, an excitation coil, a vaporizing chamber, and the micronozzle, all integrated into a glass tube with a dimension of 3 mm (outer diameter) \(\times\) 18 mm (length). They conducted experiments by applying an optimal AC frequency obtaining a maximum thrust force of 680 uN at 5 mg/s. Later, [PERSON] et al. [23] developed a new concept of planar VLM in which the vaporization process was driven by planar induction heating (see Figure 5d). Their microthruster consisted of a single microchannel, a micronozzle, a micro heating plate, and an external excitation coil fabricated on a PCB. Experiments showed that the device could provide a maximum thrust of 970 uN at a 5.30 mg/s mass flow rate and about 9.31 W apparent power. Instead, [PERSON] et al. [24] designed an 8 W powered water-fed vaporizing liquid microthruster operating in the Leidenfrost boiling regime. As shown in Figure 5d, the device was equipped with miniature molybdenum heating elements located inside the vaporization chamber, producing a heat flux output of 370 kW/m\({}^{2}\). Direct thrust measurements estimated the specific impulse and thrust level ranging between 25.9-51.1 s and 1.00-2.00 mN, respectively. Despite the advancements described above, all VLMs have shown an intrinsically unstable behavior due to the establishment of flow boiling instabilities occurring in the inlet and the heating chambers. In this regard, [PERSON] et al. [25; 26] designed and fabricated a silicon-based water-propellant VLM equipped with embedded microsensors for real-time monitoring of the in-channel vapor/liquid fraction and fluid temperature during its operation (see Figure 4c). Further, a secondary low-power platinum thin-film resistive heater was placed inside each of the eight channels, allowing for localized precision fluid heating and flow control. A preliminary characterization of the embedded sensors demonstrated the operational feasibility of the fabricated microthruster, enabling a fine heating effect localization and vaporization control. Table 2 compares the actual propulsive performance measured during several experimental tests. It is worth observing that, from the propulsive perspective, the performance optimization of a VLM is driven by three key steps, i.e., the maximization of the thrust force \(T\), the maximization of the specific impulse \(I_{sp}\), and the minimization of the power consumption \(P_{d}\) for propellant vaporization and overheating. They were collected into the overall efficiency parameter \(\eta_{O}=g_{0}I_{sp}\left(\frac{T}{P_{d}}\right)\). Figure 4: _Cont._  
Figure 4: Manufactured VLMs in parallel microchannels configuration: (**a**) [PERSON] and [PERSON] [17]; (**b**) [PERSON] et al. [18]; (**c**) [PERSON] et al. [25]. Figure 5: \(Cont\).