# MEMS Vaporizing Liquid Microthruster: A Comprehensive Review
## 2 VLM Design and Manufacturing: State of the Art  
Figure 1 shows the working principle of a VLM where the propellant management system (storage and feeding), the heating chamber, and the micronozzle represent the main sub-systems [1]. After propellant injection into an inlet manifold, the heating process undergoes into the heating chamber before the supersonic expansion of the vapor flow occurring through the micronozzle. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Micropropulsion System** & \(T\) & \(I_{sp}\) & \(P_{d}\) \\  & **(N)** & **(s)** & **(W)** \\ \hline Electrospray & \(2\times 10^{-5}\)–\(1.5\times 10^{-4}\) & 1500–6000 & 0.08–8 \\ Solid Propellant & 0.5–2 & 25–130 & 0.15–2 \\ Cold Gas & \(2\times 10^{-4}\)–0.002 & 45–70 & 1.5–2.5 \\ Low-Pressure Micro-resisttojet & \(2\times 10^{-4}\)–0.004 & 45–100 & 0.8–8 \\ Vaporizing Liquid Microthruster & \(5\times 10^{-4}\)–0.002 & 15–200 & 0.7–18 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of nominal power, thrust, and specific impulse between different types of micropropulsion systems [4]. The first concept of a silicon-based MEMS VLM was developed at the end of the 1990s, thanks to [PERSON]'s studies [9; 10; 11]. They manufactured the device shown in Figure 2a and performed a characterization of the heat losses in relation to its packaging and the assessment of the influence of the feeding pressure on the vaporization process. Later, different configurations, materials, and manufacturing technologies were investigated, as summarized in [PERSON] et al. [5]. In the field of silicon technology, [PERSON] et al. [12] developed a MEMS-based VLM using water as a propellant, which was equipped with an external heater (see Figure 2b). Their experiments showed thrust force magnitudes ranging between 0.15 mN and 0.46 mN. Similarly, [PERSON] et al. [13] first designed and fabricated a water-fed silicon-based VLM with an integrated micro-heater (see Figure 2b), measuring thrust forces in the range (5-120) \(\upmu\)N with a heating power between 1 W and 24 W. Later, [PERSON] et al. [14] proposed a new single-channel VLM configuration, including the use of two integrated heaters to ensure a more uniform  
Figure 1: Schematic of a typical VLM system, including the feeding system components (green) and the thruster components (blue) (reproduced from [1]). Figure 2: Manufactured VLMs in single channel configuration: (**a**) [PERSON] et al. [12]; (**b**) [PERSON] et al. [13]. heating (see Figure 3a). The measured thrust force was about 1 mN using a maximum heating power of 3.6 W. Instead, [PERSON] et al. [15] first applied pulsed heating to a silicon-based VLM (see Figure 3b), measuring a total impulse of 0.2\(\cdot\)10\({}^{-6}\) Ns produced in a second with a pulse power of 30 W. Instead, [PERSON] et al. [16] first performed a characterization of the two-phase heater in a single-channel VLM (see Figure 3c), distinguishing four different flow patterns, i.e., snake flow, vapor-droplet flow, vapor-droplet-jet flow, and vapor flow. Similarly, [PERSON] and [PERSON] [17] first fabricated a MEMS-based VLM created by parallel microchannels (see Figure 4a) and described the establishment of flow boiling instabilities in relation to the measured performance. More recently, at TU Delft (the Netherlands), [PERSON] et al. [18] developed a VLM with integrated molybdenum heaters (see Figure 4b) and temperature sensing using water as a propellant; later, [PERSON] et al. [19] described its integration on the first demonstrator of dual-mode water-propelled micro-resistojets operating in a wide range of feeding pressure for in-orbit pico-satellite applications. The launch into orbit of their PocketQube platform Delfi-PQ equipped with the VLM as a secondary propulsion system confirmed that the VLM technology reached the technology readiness level (TRL) of four. Concerning new materials and manufacturing technologies, [PERSON] et al.