# Advances in Remote Sensing and Propulsion Systems for Earth Observation Nanosatellites
## 4 Propulsion Systems in Nanosatellites
### Taxonomy Nanosatellites' Propulsion Systems
#### 4.2.1 Non-Electric Propulsion Systems  
Liquid rocket systems, known for their remarkable efficiency, operate by storing propellants in liquid form, typically an oxidizer and a fuel, in separate tanks. These propellants are pumped into a combustion chamber, mixed, and ignited, creating high-pressure and high-temperature gases [49; 50]. The rapid expansion of these gases is then expelled through a nozzle to produce thrust. Liquid rockets offer high efficiency and controllability, making them suitable for various space missions. The schematic of a bipropellant liquid propulsion system is shown in Figure 1. Furthermore, the primary power consideration for liquid rockets is the energy required to operate the pumps that deliver the propellant to the combustion chamber, as shown in Equation (7).  
\[P_{pumps}\ =\ \dot{m}\Delta F \tag{7}\]  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Propulsion** & **Type** & **Thrust (mN)** & **Specific** & **Power** & **Applications** \\
**System** & **Type** & **Thrust (mN)** & **Impulse (s)** & **Requirement** & **Applications** \\ \hline Liquid Thrusters & Non-electric & 5–50 & 20–300 & - & Short burns, orbital insertion \\ Cold Gas & & & & & Attitude control, short \\ Thrusters & Non-electric & 10 & 50–70 & \textless{}1 & maneuvers \\ Solid Rocket & & & & & Orbit transfer, deorbiting \\ Thrusters & Non-electric & 100–500 & 250–300 & - & maneuvers \\ Resistojets & Electric & 1–10 & 200–400 & 10–50 & Low thrust maneuvers, \\ Electrospray & Electric & 0.04 & 500–1500 & \textasci{}2 & station-keeping \\ Thrusters & Electric & 0.5 & 1500–3200 & 50–100 & Deep-space missions, efficient \\  & & & & & adjustments \\ Hall Thrusters & Electric. & 10–100 & 1500–2500 & 100–500 & High-efficiency orbital \\ Pulsed Plasma & Electric & 0.04 & 800–1200 & \textasci{}2 & Adjustments \\ Thrusters & & & & & Attitude control, precise \\ \hline \hline \end{tabular}
\end{table}
Table 8: Propulsion System Comparison.  
Cold gas propulsion systems use a pressurized inert gas, such as nitrogen or helium, stored in a tank. Upon release, the gas expands and is expelled through a nozzle to generate thrust [51]. This type of propulsion system is simple, reliable, and safe, though it provides relatively low specific impulse compared to other propulsion methods. Cold gas systems are commonly used for attitude control and small maneuvers [52]. The schematic of a cold gas propulsion system is shown in Figure 2. Also, cold gas systems require minimal power, primarily for operating valves that release the pressurized gas, as shown in Equation (8). The thrust is generated by the expansion of the gas through a nozzle.  
\[P_{valves}\approx Minimal \tag{8}\]  
Solid rocket propulsion systems utilize a solid propellant consisting of a mixture of fuel and oxidizer. The propellant burns rapidly when ignited, producing hot gases expelled through a nozzle to generate thrust. Solid rockets are known for their simplicity, high thrust, and reliability [53; 54]. However, they cannot be throttled or shut down once ignited, limiting their flexibility [49]. The schematic of a solid rocket propulsion system is shown in Figure 3. Solid rockets do not require external power to operate during burn. The combustion of the solid propellant provides all the energy needed for thrust, as shown in Equation (9).  
\[P_{combustion}\approx Propellant_{Provided} \tag{9}\]  
Figure 1: Bipropellant liquid propulsion system.  
Figure 3: Solid rocket propulsion system.  
Figure 2: Cold gas propulsion system.