## 3 Background
### Propulsion System Technologies Overview
#### 3.3.1 Chemical Propulsion Systems
The cold gas system (Figure 14) uses an inert gas stored at pressure to produce thrust. Nitrogen is the most commonly used inert gas. As space is a vacuum there is a significant pressure difference between the stored gas and the release environment. A valve is opened releasing the gas, this is directed through a nozzle to provide directional control [3]. The pressure difference generates a reaction force upon the CubeSat propelling it through space. As the system is controlled by a single valve it is an extremely simple system with low mechanical complexity. The inert gases such as nitrogen are not intrinsically corrosive or dangerous which meets the safety requirements set by the CubeSat design guide [1]. It also reduces risk to the launch vehicle and orbital deployer. The main disadvantage of cold gas thrusters is that they provide a very low level of performance compared to other propulsion systems [19]. Even at high pressures, the release of the inert gas only produces a very small amount of impulse upon the CubeSat, meaning low maneuverability and acceleration even for low-mass nanosatellites. This factor alone means cold gas thrusters are commonly used for CubeSats [2]. The maximum theoretical specific impulse for a nitrogen cold gas thruster in a vacuum is 76 s. Another issue with cold gas thrusters is that commonly used gases such as nitrogen have low densities. Due to the fact volume is limited; this is an important issue as gas supplies are \begin{table}
\begin{tabular}{c c} \hline \hline
**Performance Criteria** & **CHAMPS Performance Per Thruster** \\ \hline Dimensions & 100 \(\times\) 100 \(\times\) 113.5 mm (1U) \\ Mass & 1.3 kg dry, 1.6 kg wet \\ Operating Temperature Range & 5–50 degrees Celsius \\ Power Consumption & 4 W Start-up, 1 W operation \\ Operational Voltage & 5 V Nominal \\ Thrust & 0.26–2.79 N \\ Minimum Impulse & 0.00484 s \\ \hline \hline \end{tabular}
\end{table} 
Table 6: CHAMPS Performance Specification. Figure 14: Cold Gas Thruster [33]. rapidly consumed vastly limiting the operational time of the satellite. Technology is currently under development by NASA to allow solid fuel to be the primary source, which is then combusted in a plenum to produce the desired gaseous output. This allows a much larger quantity of gas to be carried within the same volume extending the operational life of the satellite [23]. Cold gas thrusters represent the most mature propulsion technology for CubeSats but however provide limited spacecraft propulsion capabilities, advanced system use highly pressurized gas, with a vessel pressure greater than that allowed by the CubeSat design guide [1]. As the technology has developed over time, warm gas thrusters have been selected by developers and integrated to orbiting designs [23]. The basic operation of a warm gas thruster is identical to that of a cold, providing a higher value of specific impulse [33]. Table 7 illustrates cold and warm gas propulsion systems performance and the stage of the development status from various manufacturers. The CPOD system manufactured by VACCO represents a plausible propulsion system solution for the mission. It has 1U dimensions, is center mounted, has a nominal thrust of 25 mN and a total impulse of 186 N-s [34]. Non-toxic propellants (Table 8) represent are classed as \"green fuels\" due to their reduced toxicity which help meets the CubeSat requirements [1]. Compared to Hydrazine they have a reduced vapor pressure, making them a safer alternative. Non-toxics are still in the development phase, but are expected to provide higher performance than current market-leading state-of-the-art fuels. The HYDROS system shown in Figure 15 represents a water electrolysis propulsion system, electrolyzing water into hydrogen and oxygen to produce a bi-propellant thruster. It has a modular nozzle and injector design allow it to be manufactured to any CubeSat size specification [35].