# An Overview of Cube-Satellite Propulsion Technologies and Trends
## 9 Pulse Plasma Thruster (PPT)  
### Operating Principle  
Pulsed Plasma Thrusters (PPTs) operate by creating a pulsed, high-current discharge across the exposed surface of a solid insulator (for instance, Teflon) that serves as a propellant. The arc discharge ablates (sublimates/vaporizes) the propellant material from its surface, thereby ionizing and accelerating the propellant to high speeds. A current pulse lasting few micro-seconds is generally driven by a capacitor that is charged and discharged approximately once every second [110]. The schematic of a PPT is shown in Figure 8 containing a spring loaded mechanism, propellant, capacitor, anode, cathode, acceleration chamber and a spark plug. During the process of propulsion, the spring feeds the propellant (usually solid) between the two electrodes (anode and cathode) and the spark plug is simultaneously fired (through a small discharge) to raise the electrical conductivity of the acceleration chamber. Now, the electric current from Power Processing Unit (PPU) flows to the electrodes through the capacitor and then into the arc, thereby completing a current loop and simultaneously generating a magnetic field. The electric arc formed ablates the propellant and ionized plasma is formed. The plasma is then accelerated due to _Lorentz Force_ generated by electric arc and the induced magnetic field [111]. The advantages of a PPT are its ability to provide small impulse bits for precision maneuvering, robustness by programming impulse bits to cater to mission needs, design simplicity owing to the ability of using wide variety of propellants (solid/liquid), and its ability to maintain constant specific impulse and efficiency over a wide range of input power levels. However, these advantages come at the cost of issues that result due to electrode erosion, presence of macro-particles in the plume due to non-uniform ablation and very low thruster efficiency [111; 112; 113; 114; 115]. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Company/Transition with Location** & **Engine** & **Thrust (nm)** & **Ly (s)** & **Power (W)** & **Propellant** & **Heritage** & **Remarks** & **Ref.** \\ \hline MIT, Cambridge, MA, USA & 5-EIFS & 0.1 & 1200 & 1.5 & ionic liquid & Aero-Cube-8 (1.5U) & also called IMPACT & [9; 94; 106] \\ Action Systems, Boston, MA, USA & TIFS & 5000 & 1.5 & 1800 & 30 & ionic liquid & - & [30] \\ Busak, NaIck, MA, USA & BET-ILNN & 0.7 & 800 & -9 & ionic liquid & - & flown on LISA Pathfinder (EIFS 3 kg) & [70; 71; 72; 73; 74; 75; 76; 77; 78; 79; 80; 81; 82; 83; 84; 85; 86; 87; 88; 89; 90; 91; 92; 93; 94; 95; 96; 97; 98; 99; 100; 99; 101; 98; 102; 99; 103; 104; 105; 106; 107; 108; 109; 103; 105; 107; 109; 104; 106; 107; 108; 109; 111; 112; 113; 114; 115] & - & [9] \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Summary of Electrospray Propulsion Systems. The thrust (shown in Equation (18)) produced is calculated from the law of conservation of momentum. For any electric thrusters, _Lorentz Force_ describes the relationship between the force (thrust) produced due to charged particles moving through a self induced magnetic field. Thrust produced also depends on the charge of ions (\(q\)), sum of all collision forces per particle (propellant) over all particles (\((P_{i})_{k}\)) and particle velocity (\(\mathbf{\tilde{u}}_{i}\)). The effective exit velocity (shown in Equation (19)) of the ionized propellant is a function of the thrust generated and the mass flow rate of propellant (\(\dot{m}\)) and efficiency (\(\eta\)). Effective exit velocity can also be calculated in terms of the radius of anode (\(R_{a}\)) and radius of cathode (\(R_{c}\)). The specific impulse of a PPT can be obtained from the Equation (2) [116]. \[\tau=m_{ion}\frac{d\mathbf{\tilde{v}}_{i}}{dt}=q\Big{(}\mathbf{\tilde{E}}+ \mathbf{\tilde{u}}_{i}\times\mathbf{\tilde{B}}\Big{)}+\Sigma(P_{i})_{k} \tag{18}\]  
\[v_{e}=\frac{\tau}{\dot{m}\eta}=\frac{1}{\eta}\frac{\mu_{0}}{4\tau}ln\left( \frac{R_{a}}{R_{c}}\right), \tag{19}\]  
where \(\mu_{0}\) is the permeability of free space.