# An Overview of Cube-Satellite Propulsion Technologies and Trends
## 8 Electrospray Propulsion System/Electrospray Thrusters  
### Operating Principle  
Electrospray thruster is a plasma-free electric propulsion system [93] that works on the principle of electrostatic extraction and acceleration of charged particles (ions) from a liquid (propellant) surface to produce thrust. Their fundamental working mechanism is based on a process by which the conductive liquid surface of the propellant is deformed into a sharp cone-shaped meniscus called _Taylor Cone_; when a certain threshold of the electric potential is surpassed, ions are extracted from the cone's apex [94; 95; 96]. Electrospray thrusters accelerate positive or negative ions, respectively generating either a positive or negative ion beams thereby eliminating the need for an external cathode to neutralize the ejected ions unlike in plasma propulsion devices (ion and Hall thrusters) where an external cathode is essential [97]. The propellants used for electrospray thrusters are usually ionic liquids, and their  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Company/Institution with Location** & **Engine** & **Thrust (mN)** & **I\({}_{up}\) (s)** & **Power (W)** & **Propellant** & **Remarks** & **Ref.** \\ \hline \hline Bussek, Natick, MA, USA & BHT-200 & 12.8 & 1390 & 200 & Xe, I Kr &
\begin{tabular}{c} flown TacSat-2 (370 kg) and \\ FalconSat-5 (180 kg) \\ \end{tabular} & [46; 85; 89] \\ \hline Bussek, Natick, MA, USA & BHT-600 & 39.1 & 1530 & 600 & Xe, I Kr & & [85] \\ \hline Sitzel Aerospace, Mola di Bari, Italy & HT 100 & 10 & 1100 & 100 & Xe, Kr & - & [00; 91] \\ \hline Sitzel Aerospace, Mola di Bari, Italy & HT 400 & 50 & 1750 & 100 & Xe & - & [92] \\ \hline MIT, Cambridge, MA, USA & MIT-9 & 20–50 & 300–1500 & 30–200 & - & - & [46] \\ \hline UTIAS-SFR, Toronto, ON, Canada & CHT & 1–10 & 1139 & \textless{}200 & Xe, Ar & - & [79] \\ \hline \hline \end{tabular}
\end{table}
Table 6: Summary of Hall Thrusters.  
negligible vapor pressure serves as an advantage by resolving the need for propellant pressurization and helps with system miniaturization [96]. The schematic of an electrospray propulsion system is shown in Figure 7, and the major components comprise of propellant storage, emitter and extractor electrode. The performance of an electrospray thruster can be varied by changing the voltage passed through th emitter and the extractor electrodes [93].  
In an electrospray thruster, the mass-to-charge ratio plays an important role in determining the exit velocity and thrust. The average mass-to-charge-ratio \(\left\langle\frac{q}{m_{ion}}\right\rangle\) (shown in Equation (15)) is inversely proportional to the density (\(\rho\)) of the propellant ion/droplet and the volume flow rate (\(Q\)) of ion/droplet. The exit velocity (described in Equation (16)) is a function of the square root of ion accelerating voltage (\(V_{i}\)), ion beam current (\(I_{i}\)) and average mass-to-charge-ratio. The thrust (shown in Equation (17)) is a function of ion accelerating voltage, ion beam current and mass flow rate of ions (\(\dot{n}_{ion}\)) [98]. The underlying mathematical relations summarizing the relationships are given below:  
\[\left\langle\frac{q}{m_{ion}}\right\rangle=\frac{1}{\rho Q} \tag{15}\]  
\[v_{e_{i}}= \sqrt{2V_{i}I_{i}\Big{\langle}\frac{q}{m_{ion}}\Big{\rangle}}= \sqrt{\frac{2V_{i}I_{i}}{\rho Q}} \tag{16}\] \[\tau=\dot{m}_{ion}v_{e_{i}}= \sqrt{2V_{i}I_{i}\dot{n}_{ion}} \tag{17}\]