# An Overview of Cube-Satellite Propulsion Technologies and Trends
## 12 Conclusions  
This paper provides an overview of micro-propulsion systems that have flown on CubeSats or are being developed for future CubeSat missions. A subset of the latter have flown on larger spacecraft as secondary propulsion systems. The operating principles of the surveyed systems, their key design considerations and performance characteristics have also been discussed. Figure 9 summarizes the comparison between thrust and specific impulse for the surveyed propulsion systems. Liquid and solid rocket propulsion systems generate the highest thrust among all propulsion systems owing to the expansion of the burnt propellants (liquid and solid respectively) in the nozzle. On the other hand, the specific impulse of liquid and solid rocket systems is low (relative to most electric propulsion systems) because the exit velocity of the propellants is lower than that of electric  
Figure 9: Thrust vs. Specific Impulse for Propulsion Systems from Survey. population systems. The thrust generated by solid rocket systems is greater than than liquid propulsion systems mainly due to the greater propellant density (more propellant packed within the same volume). Cold gas propulsion systems and resistojets have the lowest specific impulse among all propulsion systems due to their simple operational physics devoid of chemical transformations, only involving the expansion of propellants. Resistojets possess the ability to super-heat the propellant prior to the expansion in the nozzle; hence, additional kinetic energy is imparted to the propellant resulting in higher specific impulse and thrust compared to cold gas systems. For electric propulsion systems, two comparisons are summarized: first, power and specific impulse (shown in Figure 10); second, thrust-to-power ratio and specific impulse (shown in Figure 11). Inspite of their average power consumption being slightly higher than that of electrospray and pulsed plasma systems, resistojets provide a specific impulse an order of magnitude lesser. Resistojets also have the highest average thrust-to-power ratio amongst all the surveyed electric propulsion systems. Figure 11: Thrust to Power Ratio vs. Specific Impulse for Electric Propulsion Systems from Survey. Figure 10: Power vs. Specific Impulse for Electric Propulsion Systems from Survey. From Figures 9-11 it can be observed that electrospray, radio frequency ion, Hall and pulsed plasma thrusters provide similar performance. RF ion thrusters feature the highest specific impulse due to their high operating efficiency (approximately 70% [61]) but offer a lower average thrust-to-power ratio. Hall thrusters on the other hand have a lower I\({}_{sp}\) than RF ion thrusters because of their lower thruster efficiency (approximately 50% [61]), however they possess higher average thrust-to-power ratio. Though RF ion and Hall thrusters have the highest specific impulse amongst electric engines, they consume a considerably larger amount of power mainly owing to complex design systems that include the series of grids, external cathode and RF ion power for RF ion thrusters; external cathode, induced magnetic and accelerating electric fields for Hall thrusters. Electrospray thrusters have a specific impulse higher than PPTs and lower than both RF ion and Hall thrusters. Their efficiency of electrical-to-jet power conversion is \(>\)70% at 1 mW power because their propulsion process involves atomization and charging of the propellant unlike the plasma discharge engines [93]. Inspite of having high efficiency, electrospray thrusters have a relatively lower specific impulse than Hall and RF ion thrusters because of the formation of larger clusters of droplets during the extraction of ions [94]. The average thrust-to-power of the surveyed electrospray thrusters is found to be similar to that of RF ion thrusters. PPTs have a relatively lower thrust and specific impulse amongst the electric propulsion systems due to their very low thruster efficiency (10-20% [116] ).