Electric Propulsion Systems
October 4, 2024
Imagine powering spacecraft without a chemical reaction of fuel and an oxidizer. Little weird, but a completely viable option. But what about a conductive wire? Now it’s definitely getting weird. This article is the second of three that will explore non-chemical propulsion methods.
Another form of rocket engine propulsion is electric propulsion, which does not use chemical reactions to produce thrust. Instead, they utilize the energy of positively-charged ions or plasma, which is a state of matter involving a “gas-like state” of positively charged ions and free electrons. Electric propulsion is meant for spacecraft and satellites in space rather than rocket launches because it cannot produce as much thrust as a chemical combustion. This article will explain the design of four different types of electric propulsion: ion thrusters, hall effect thrusters, magnetoplasmadynamic thrusters, and electrodynamic tethers.
Ion thrusters are made of a few main parts, such as the ionization chamber, grid system, and a gas input. Ion thrusters typically use inert gases because they aren’t reactive (typically noble gases which already have a full shell of valence electrons, stable compared to other elements). Xenon is the most common choice for ion thrusters for several reasons. Xenon has a lower ionization energy than other noble gas elements due to its plentiful core electrons, which induce shielding, as well as a lower effective nuclear charge. Additionally, Xenon has a higher molar mass, giving it a low ionization energy per unit mass. Essentially, it will take less energy to ionize a Xenon atom, and there will be greater thrust output due to higher momentum of the Xenon ions (high atomic mass).
Image Courtesy of Wikipedia
Electrons are then released into the ionization chamber with the gas atoms in a process called electron bombardment. For now, we will assume the atoms are Xenon so it will be easier to follow along.
The velocity of the electrons being ejected into the chamber generates enough energy to match and overcome the ionization energies of Xenon’s electrons (this basically means that the electrons have enough energy to free Xenon’s outermost electrons from the binding energy of its nucleus), causing Xenon atoms to lose one of their electrons and become positively charged ions. As this process occurs, there becomes a growing, yet equal amount of positively-charged Xenon ions, and free negatively-charged electrons–a state of matter called plasma.
At the end of the thrusters sit two electric fields called “grid systems.” Here, the positively-charged Xenon ions are accelerated through the grids and exit the engine, producing thrust (grid is negatively charged to attract ions). A cathode sits near the end of the thruster next to the grids, ejecting more electrons to neutralize the Xenon ions being expelled. This neutralization is extremely important, as a larger net negative force will develop since positive ions were released but the number of negatively-charged electrons remained constant. To prevent the Xenon ions from being attracted back into the thruster, electrons are released with the Xenon ions.
Another type of electric propulsion is the Hall Effect thruster and it works in a similar way to ion thrusters. Instead of electrically charged grids used in ion thrusters, hall effect thrusters have a cathode that not only neutralizes ejected ions but also creates an electric field. In the diagram, the cathode streams out two pathways of electrons, with one being bound in a circular motion inside the thruster by a magnetic field (aided by electromagnets). The same type of stable propellant (typically Xenon) is released into the thruster chamber where its atoms interact with the electrons and produces positively-charged ions, similar to ion thrusters.
Image Courtesy of BeyondNERVA
Unlike ion thrusters, the spare electron that is removed from the noble gas atoms ends up joining the electric field of electrons that are going through a circular motion on the inside of the thruster. This process is called avalanche ionization, where the addition of more electrons increases the probability of collisions between noble gas atoms and electrons, creating more spare electrons that continue this process.
However, Hall effect thrusters have a lower specific impulse than ion thrusters. Specific impulse is a value that corresponds to the efficiency of a propulsion system; in other words, if you have 2 rockets with equal propellants, the rocket with the higher specific impulse is capable of producing greater thrust after both rockets use the same amount of propellant. Both thrusters, however, have higher specific impulses than chemical rockets. But while their efficiencies are super high compared to chemical engines, the amount of thrust capable of being produced is drastically different, as chemical rockets can do much better at that.
The final type of electric thruster I will mention in this article is the Magnetoplasmadynamic (MPD) thruster, which uses magnetic fields to accelerate plasma. The design is made of an exterior anode (positively charged) and an interior cylindrical cathode (negatively charged). Electrons are ejected from the interior cathode tube to ionize the propellant atoms (most likely Xenon, but others work too).
When a voltage is introduced, electrons become attracted to the anode and create an electric field, and as the electric current interacts with the cathode, a magnetic field is created around the cathode. The plasma (created in the same process as the other two thrusters) is accelerated out of the thruster via Lorentz Force, which is where the positively-charged ions from the propellant are electrically charged due to the electric field. These particles exist and interact with the perpendicularly-positioned magnetic field, causing a force to be exerted (force that ejects the plasma out of the thruster).
Image Courtesy of NASA
Now imagine a propellant-free propulsion system. You might think solar sails…but have you heard of electrodynamic tethers? The term “tethers” is just what it sounds like: a conductive wire. This type of propulsion can work with two seperate super small vehicles by using the Lorentz Force, where an electric current is induced through the conductive wire which causes mobile electrons to flow up and down (this occurs due to electromagnetic induction, where the conductive tether moves through Earth’s magnetic field, creating voltage and movement of electrons). As the electrical current interacts with Earth’s magnetic field, the Lorentz force is produced, allowing a small amount of propulsion. While these electrodynamic tethers are unhelpful for orbital launches, they can be useful for slowing down descending spacecraft. This form of propulsion has been tested before on a CubeSat near Earth during the Tether Electrodynamic Propulsion CubeSat Experiment (TEPCE).