## 3 Background
### Propulsion System Technologies Overview
#### 3.3.2 Electrical Propulsion Systems
The magnetic field is often shielded using a Faraday guard to prevent magnetic interference to components such as the altitude determination and control system for example. The propellant is usually fed into the arc by an actuating spring; Figure 18 illustrates a typical Phase Four thruster [38]. Pulsed Plasma thrusters were derived from larger scale space craft applications. Pulsed Plasma Thrusters are suited to altitude control and fine pointing which meets the mission requirements; this is because the pulse impulse magnitude is adjustable [38]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Product** & **Manufacturer** & **Thrust** & **Power** & **Specific Impulse** & **Status** \\ \hline S-iEPS & MIT, Cambridge, MA, USA & 74 \(\mu\)N & 1.5 W & 1160 s & TRL6 \\ IMPACT & Accion Systems Inc., Boston, MA, USA & 60 \(\mu\)N & 0.75 W & 1200 s & TRL5 \\ MAX-1 & Accion Systems Inc., Boston, MA, USA & 120 \(\mu\)N & 1.6 W & 2000 s & TRL5 \\
1 mN Electrospray & Busek, Natick, MA, USA & 0.7 mN & 15 W & 800 s & TRL5 \\
100 \(\mu\)N Electrospray & Busek, Natick, MA, USA & 0.1 mN & 5 W & 2300 s & TRL5 \\ \hline \hline \end{tabular}
\end{table} 
Table 11: Electrospray Thruster Performance data [23]. Figure 17: MIT Electrospray Thruster Compared to a U.S. Quarter Coin [37]. Pulse control is usually achieved by electronically controlling the discharge rate of the capacitor operating the thruster using a PPU. The Lorentz force created due to the magnetic field and electric current is quantified using:  
\[\overset{\rightarrow}{F}=q*\overset{\rightarrow}{v}*\overset{\rightarrow}{B} \tag{1}\]  
\(F\) is equal to the force, \(q\) is the electric charge, \(v\) is the velocity of the charge and \(B\) is the strength of the magnetic field (Tesla's) [39]. A Pulsed plasma thruster design is illustrated in Figure 19. Failure in this thruster often occurs due to electrode surface charring, limited total impulse due to fuel consumption and pulsed thruster instead of continuous discharge [4]. Below (Table 12) are typical Pulsed Plasma and Vacuum Arc propulsion system performance values:  
Figure 19: Pulsed Plasma Thruster Design [4]. Figure 18: Phase Four RF Thruster [38]. The P4 RFT (Phase Four Company) shown in Figure 20 is of particular interest to this project. It is a \"CubeSat Ampibolar Thruster\" (CAT) developed by the university of Michigan and exclusively licensed for production [37]. It utilizes a magnetic helicon discharge when ionizing the chosen propellant [23]. This system eliminates the need for a separate electron source and produces no magnetic dipole. The device generates plasma using helicon radio frequency source. As the system is electrodeless, a vast range of propellants may be employed, including water [4]; however, iodine is emerging as the most popular selection due to low cost and high energy density [23]. Iodine operating P4 RFT systems have achieved specific impulse values of 1010 s.  
Hall effect thrusters (Figure 21) are another example of mature technology which has undergone miniaturization to make it a viable nanosatellite technology. The miniaturization of components such as neutralizers is still ongoing; therefore, overall power consumption is very high [23] compared to viable alternatives such as pulsed plasma thrusters and electrojets.