## 3 Background
### Propulsion System Technologies Overview
#### 3.3.2 Electrical Propulsion Systems  
Although CubeSat technology is still developing as interest and investment rises significant improvement in the field of Electric propulsion has occurred. Small CubeSat craft require high values of specific impulse which electrical systems can provide; however, this comes at the cost of low thrust output meaning maneuvers take an extended period of time. Given that the system's main requirement is to maintain the altitude of the craft, and it is not required to complete complex maneuvers--for example orbital positioning for imaging--it would be suited to an electrical propulsion system which can provide a long lifespan with low specific fuel consumption. The thruster is required to perform small correction and altitude control maneuvers rather than interplanetary spiral trajectories [23]. A well-suited system would be a Pulsed Plasma Thruster/Electrospray using polytetrafluoroethylene. The most basic form of electric propulsion system is a Resistojet. The system works by a heating mechanism, combusting the propellant electrically to create a gas which expands and is expelled at a high velocity propelling the craft in a desired direction [3]. As the heating process increasing the exit velocity compared to a purely pressurized release, it has a greater \"fuel efficiency\" as it can generate more thrust per unit fuel, as the heating is usually powered by energy generated from the solar cells [15]. Collaboration between CU aerospace and VACCO has produced a CubeSat compatible Resistojet propulsion system targeted at 2U to 6U buses. Busek [36] have development an Ammonia Micro-Resistojet system incorporating a rocket nozzle. Table 10 demonstrates Busek Ammonia Micro-Resistojet performance. An electrospray would serve as an extremely viable solution (see Table 11 and Figure 17) to this mission requirement, due to its low fuel consumption and strong performance. An electrospray system operates upon the phenomenon of electrostatic extraction and acceleration of ions from a given propellant from a vapor pressure conductive salt. No gas-phase ionization is required which serves a major advantage of this design [23]. There is also no need to store pressurized \begin{table}
\begin{tabular}{c c} \hline \hline
**Performance Category** & **Achieved Value** \\ \hline Dimensions & 100 \(\times\) 100 \(\times\) 113.5 mm (1U) \\ Power Consumption & 15 W \\ Weight & 1.25 kg \\ Thrusts & 10 mN \\ \(\Delta V\) (assuming 4 kg weight) & 60 m/s \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Busek Ammonia Micro-Resistojet Performance [36]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Product** & **Manufacturer** & **Total Mass** & **Average Thrust** & **Specific Impulse** & **Status** \\ \hline ISP 30 Sec Motor & Industrial Solid Propulsion & & & & \\ \multirow{2}{*}{STAR 4G} & \multirow{2}{*}{Orbital ATK, Dulles, VA, USA} & 0.95 kg & 37 N & 187 s & TRL7 \\  & & & & & \\ \multirow{2}{*}{CAPS-3} & \multirow{2}{*}{DSSP, Reno, NV, USA} & 1.5 kg & 258 N & 277 s & TRL6 \\  & & & & & \\ \multirow{2}{*}{} & \multirow{2}{*}{DSSP, Reno, NV, USA} & 2.33 kg & 0.3 N & 900 s & TRL8 \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Solid Rocket Motor Performance [23]. propellant which is advantageous due to limited space and the maximum pressure vessel regulation of 1.2 atm [1]. The most commonly used propellant is an ionic liquid (1-ethyl-3-methyl-imadazolium, tetra fluoroborate). Ion propulsion systems have been deemed outside of this projects scope, this is due to their significant power consumption of at least 10 W [15]. The BIT 1, 2 and 3 models supplied by Busek have respective power consumptions of 28 W, 75 W and 460 W [36]. A pulsed plasma thruster produces thrust by generating a high-voltage discharge between two separate electrodes, leading to an electric arc formation which ablates a solid material [23]. The thruster generates a magnetic field which accelerates the particle from the thruster nozzle.