From micro-nano satellites as small as a few kilograms to high-orbit communication satellites as large as more than ten tons, propulsion is one of the core subsystems. In recent years, with the continuous increase in the number of micro-nano satellites launched, if the satellites cannot actively deorbit after reaching the end of their service life, they will continue to float in low-Earth orbit and become space debris. As space debris continues to increase, it will cause space traffic congestion and cause space safety issues. Therefore, countries around the world have introduced regulatory measures requiring microsatellites to be equipped with active deorbit devices before they are allowed to be launched. In May 2021, my country issued the “Notice on Promoting the Orderly Development of Microsatellites and Strengthening Safety Management”, which requires that “microsatellites should have the necessary capabilities to facilitate deorbiting and avoid long-term occupation of commonly used orbits”, which has further promoted the development of propulsion systems.
The real tasks of the satellite propulsion system in orbit are mainly four things. The first is apogee/perigee orbit change (such as moving from transfer orbit to geosynchronous orbit), the second is position maintenance (high-orbit satellites have to fight against the gravity of the sun and the moon, and solar light pressure every day), the third is attitude control and angular momentum unloading (cooperating with the flywheel to eliminate accumulated angular momentum and achieve large-angle attitude maneuvers), and the fourth is deorbit disposal (igniting at the end of its life, allowing the satellite to fall into the atmosphere or enter a “cemetery orbit”).
According to the working principle, there are many types of propulsion. Currently, it can generally be divided into two categories: chemical propulsion and electric propulsion. Although electric pushers are developing rapidly, chemical pushers are still irreplaceable for the time being.
Chemical propulsion uses chemical substances to burn or decompose to produce high-temperature and high-pressure gas, which is ejected through a nozzle and uses reaction force to generate thrust. It is mainly used for initial orbit capture, emergency obstacle avoidance, and large-angle attitude maneuvers after the satellite enters orbit. Chemical propulsion is mainly divided into single-component propulsion and dual-component liquid propulsion. Hydrazine is commonly used for single-component propulsion, which is mostly used for small satellites, attitude control, orbit fine-tuning, etc. The advantages are simple structure and low cost. The disadvantage is that hydrazine is highly toxic and extremely troublesome to handle. Now new models are basically eliminating it. The mainstream of bicomponent liquid propulsion is dinitrogen tetroxide + dimethylhydrazine, which has the advantages of instant ignition, high thrust, mature and reliable, and is mainly used for apogee engines and high-thrust orbit changes. For example, my country’s 7500 N bicomponent thruster is used for the descent, hovering and slow landing missions of the “Chang’e-3”, “Chang’e-4” and “Chang’e-5” aircraft.
The basic principle of electric propulsion is to first ionize the gaseous propellant into a plasma composed of ions and electrons, and then use an electric field or magnetic field to accelerate these ions to extremely high speeds and then eject them, thereby generating a reaction force. The biggest advantage of electric propulsion is its ultra-high specific impulse, which allows the spacecraft to carry less fuel, thereby significantly reducing launch weight, or performing more complex tasks at the same weight. However, electric thrust also has its inherent limitations. Since the thrust generated by electric thrust is very small, this means that it cannot help the spacecraft change its orbit quickly. It must work continuously for a long time to change the speed slowly. Hall thrusters are the absolute main force in current electric propulsion, and they are also a must-have for any propulsion startup company. When the power and efficiency are constant, the greater the specific impulse, the smaller the thrust of electric propulsion. The smaller the thrust, the longer it will take for the satellite to complete a certain speed increment. Therefore, specific impulse and thrust indicators need to be weighed for the same satellite, especially in missions that hope to climb orbit quickly. A very important reason why Hall electric propulsion is popular is that compared with other types of electric propulsion, its specific impulse and thrust power ratio are within a relatively suitable range. Hall electric propulsion mainly uses inert gas as the working fluid. The most commonly used working fluid is xenon. Other available working fluids include krypton, argon, bismuth and iodine.
Take Starlink as an example. In order to control costs, Musk abandoned the traditional xenon route. On the early Starlink (V0.9, V1.0, V1.5), Musk used krypton gas as the working fluid. The move from xenon to krypton has significantly reduced costs for Musk. Although this step seemed a little big at the time, it also caused certain losses to Musk. After entering orbit, some of the Starlink V0.9 satellites failed to reliably rise to orbit, and eventually fell into the dense atmosphere and burned up. Starting in 2021, the price of krypton gas has continued to increase. After February 2022, affected by the Russo-Ukrainian war and Russia’s restrictions on the export of rare gases, its price has soared. In order to further reduce the networking costs of the second-generation “Starlink”, Musk began to use cheaper argon gas (even at the expense of some system comprehensive performance).
In the long run, electric propulsion technology will continue to develop in a direction with more “commercial” characteristics, because according to the current plan for large-scale Internet constellations, Starlink plans to have 42,000 satellites, the “GW” constellation plans to have 12,992 satellites, and the “G60” constellation has 15,000 satellites. Therefore, it costs less on one satellite. Cost-saving efforts will be rewarded 4.2/1.3/15,000 times. Or, conversely, if we do not continue to reduce costs and develop products with more “commercial” characteristics, the rising costs will eventually be magnified by 4.2/1.3/15,000 times, which may ultimately lead to the failure of the entire constellation plan.
