## 4 Propulsion System Design and Selection
### Propulsion System Selection
The maneuvers specified would require short, high-impulse bursts of energy [4] which tends the selection towards traditional flight-tested propulsion systems of micro-pulsed plasma thrusters and cold gas thrusters. Either selected system should ensure the pressure of stored propellant is in line with the CubeSat Specification [1]. A direct comparison between the performance of a micro-pulsed plasma thruster and cold gas thruster was considered (Table 24) to ensure the optimum technology is selected. The comparison above utilizes the Cold Gas MOOG 58E135 thruster. A limitation of cold gas thrusters is that advanced technologies require very high propellant storage pressure, exceeding the CubeSat specification [3]. Therefore, only primitive cold gas systems may be employed into the design. This low-pressure storage leads to low specific impulse generation at the exit nozzle. This makes the technology useful for small attitude alterations and low \(\Delta V\) maneuvers [3]. A Pulsed plasma thruster has been selected as the propulsion system technology. This is because the thrusters Teflon propellant can be stored at a low pressure relative to that of advanced cold gas systems [4] as thruster's input electrical energy to raise the temperature of the propellant to a pragmatic state, whereas cold gas thrusters rely solely upon the enthalpy of the stored gas [3]. Micro-pulsed plasma thrusters are typically lower in mass than cold gas systems, ensuring the CubeSat specification guidance of a maximum weight of 4 kg is maintained [1]. For orbital maneuvers, a pulsed plasma thruster can offer a much higher value of specific impulse [3], making it the optimum technology for performing altitude adjustment maneuvers. The pulsed plasma thruster also consumes a much lower mass of propellant compared to the cold gas thruster--approximately 800% less according to Table 24 above. As the amount of propellant is finite minimizing consumption is vital to ensure maximum mission length, as the propellant cannot be refilled in orbit. The power draw of the PPT is higher than the cold gas alternative; however, the power system verification has shown ample additional stored power to accommodate this added power consumption. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Propulsion** & **Total** & **Specific** & **Impulse** & **Thrust** & **Propellant** & **AV time** & **Energy Per** & **Peak** \\
**System** & **Mass** & **Impulse** & **Bit (\(\mu\)N s)** & **(mN)** & **Mass Per AV** & **Duration** & **\(\Delta V\) (J s/m)** & **Power** \\
**Type** & **(kg)** & **(s)** & & & **(g s/m)** & **(g s/m)** & **(g\({}^{2}\)/m)** & **(W)** \\ \hline \(\mu\)PPT & 3.8 & 500 & 70 & 0.14 & 2 & \(1.43\times 10^{5}\) & \(17.9\times 10^{6}\) & 12.5 \\ Cold Gas & 4.58 & 65 & 100 & 4.5 & 16 & \(2.22\times 10^{3}\) & \(1\)–\(5\times 10^{4}\) & 10.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 24: Comparison of typical Cold Gas Thruster and Pulsed Plasma Thrusters [3]. \begin{table}
\begin{tabular}{c c} \hline \hline
**Mode** & **Configuration, Purpose** \\ \hline Normal & Nadir (Earth Completely within FOV), VACNT Radiometers only, Normal air data collection \\ Solar & Point at Sun, absolute calibration \\ Deep Space & Point at Deep Space, offset calibration \\ Internal & Doors closed, calibration with gallium blackbody \\ Inter-Calibration & Both doors open, intercompare VACNT and cavity radiometers \\ \hline \hline \end{tabular}
\end{table} 
Table 23: Orbital Maneuvers Required for the RAVAN Payload [59].