# Advances in Remote Sensing and Propulsion Systems for Earth Observation Nanosatellites
## 4 Propulsion Systems in Nanosatellites
### Taxonomy Nanosatellites' Propulsion Systems
#### 4.2.2 Electric Propulsion Systems  
Resistojets  
Resistojets are a type of electric propulsion system that operates by electrically heating a propellant, which is then expelled through a nozzle to produce thrust. The propellant, usually a neutral gas like nitrogen or water, is passed over a resistive heating element, where it absorbs heat and expands. This process increases the exhaust velocity and efficiency compared to cold gas thrusters, making resisitojets suitable for fine-tuning spacecraft orbits and attitude [49; 55]. The schematic of a resisitojets propulsion system is shown in Figure 4. Resistojets require electrical power to heat the propellant. The power (\(P\)) required depends on the mass flow rate (\(\dot{m}\)), specific heat capacity (\(c_{p}\)), and the temperature increase (\(\Delta T\)) of the propellant, as shown in Equation (10). \[P=\dot{m}c_{p}\Delta T \tag{10}\]  
Electrospray propulsion systems use strong electric fields to spray charged particles (ions or droplets) from a liquid propellant [56]. The propellant, typically an ionic liquid, is fed through a porous emitter, and when a high voltage is applied, it forms a Taylor cone and emits charged particles. These particles are then accelerated by an electric field to produce thrust [57]. Electrospray systems are highly efficient and capable of precise thrust control, which is suitable for small satellites and precision missions. The schematic of an electrospray propulsion system is shown in Figure 5. Electrospray systems require power to generate high electric fields to ionize and accelerate the propellant. The power (\(P\)) is primarily used to create and maintain the electric field and to charge the liquid droplets and is calculated as shown in Equation (11), where the (\(\dot{m}\)) is the mass flow rate and (\(v_{e}\)) is the exhaust velocity. \[P=\frac{1}{2}\dot{m}v_{e}^{2} \tag{11}\]  
An example of electrospray propulsion is the ATHENA thruster developed by IENAI SPACE. It uses non-toxic liquid salts as a propellant and is designed for small satellites  
Figure 4: Resistojet propulsion system. Figure 5: Electrospray Propulsion System. like CubeSats. The ATHENA thruster was successfully flown aboard the Firefly Alpha 2 mission. It is highly efficient and low power-consuming, making it ideal for precise, low-thrust maneuvers. Electrospray technology is undergoing further development, with funding from the European Space Agency (ESA) to improve its performance for long-term missions [58; 59]. In addition, ENPULSION's Field Emission Electric Propulsion (FEEP) technology offers one of the highest specific impulse values (up to 6000 s) and has flown on more than 200 missions. Its indium-based propellant provides a reliable and scalable solution with no moving parts and a design that minimizes complexity [60]. FEEP systems, such as ENPULSION's NANO thrusters, have demonstrated high specific impulse and scalability, making them ideal for CubeSat missions [61; 62]. Ion propulsion systems, or ion thrusters, operate by ionizing a propellant gas, such as xenon, using an electron bombardment or a radio frequency (RF) ionization method [63]. The ions are then accelerated by electrostatic fields through grids or electrodes, creating a high-velocity ion beam that is expelled to generate thrust [64; 65]. Ion thrusters are highly efficient and provide continuous, low thrust over long periods, making them ideal for deep-space missions. The schematic of an ion thruster propulsion system is shown in Figure 6. Ion thrusters require power to ionize the propellant and to accelerate the ions through electric fields; the power (\(P\)) is usually supplied by solar panels and calculated as shown in Equation (11). Also, the power is required to maintain the electric field within the ionization and acceleration chambers. Ion thrusters are advantageous for nanosatellite missions due to their high efficiency and specific impulse. For example, CU Aerospace's Fiber-fed Pulsed Plasma Thruster (FPPT) is designed for CubeSats.