#611 How does an ion thruster work?

How does an ion thruster work? It creates thrust by accelerating ions using electricity.

An ion is a charged atom or molecule. Atoms always have an equal number of electrons and protons. Electrons have a negative charge and protons have a positive charge. Because there is an equal number of each, the charges balance themselves out and the overall charge is zero, making the atom neutral. An ion is an atom with fewer electrons than protons. If an electron is stripped from an atom, it will have more protons and will become an ion, and will have a positive charge. If an electron is added to an atom it will become a negative ion and will have a negative charge. Atoms don’t like having a negative or positive charge and they want to regain their neutral charge. They do this by attracting an electron or losing an electron.

In an ion thruster, a propellant is injected into the thruster. A good fuel would be xenon because it has a high atomic mass and is easily ionized. Once the xenon is inside the thruster, it is bombarded with high energy electrons that are produced by a cathode. These electrons go whizzing around the chamber inside the thruster and collide with the xenon atoms. Each time there is a collision, the highly charged electron strips an electron from an atom making an ion. The electrons head towards an anode, but there are magnets inside the chamber that attract them back, forcing them to whizz around again, The longer the electrons are flying around, the more likely it is that they will hit a xenon atom.

The positively charged ions are directed towards a grid at the bottom of the thruster. The grid has two layers and thousands of tiny holes. The first layer of the grid is positively charged and the second layer is negatively charged. The ions enter through the first grid and are attracted to the second grid, the negatively charged one. This attraction and the differences in charges between the two grids accelerates the ions to roughly 90,000 mph and they are sent flying out of the back of the thruster in an ion stream. The electrons that were collected by the anode in the thruster chamber are ejected out of the thruster and into the ion stream after it has left the thruster, making the xenon atoms neutral again. This is to stop the ions being attracted back to the thruster.

Why is there thrust? Newton’s third law states that “for every force in nature there is an equal and opposite reaction.” That is why a gun recoils when you fire a bullet and why a rocket with regular fuel is able to fly. The rocket burns fuel that releases energy out of the back of the rocket. There is an equal and opposite reaction in the opposite direction equal to the amount of energy expelled out of the back. The ions accelerating out of the back of the thruster produce a force and that force has an equal and opposite reaction in the opposite direction for the thruster and the rocket.

So, why aren’t we using ion thrusters yet? They have several advantages, but they also have some disadvantages. They are able to produce far more energy from a kilogram of fuel to a regular rocket. They can produce ten times more thrust per kilogram of propellant, making them very fuel efficient. That means they wouldn’t need to carry as much fuel, or they could go ten times further on the fuel that they have. When a rocket is launched, 90% of it is fuel. If that amount could be reduced, it could make space flight cheaper.  

However, the biggest disadvantage of an ion thruster is they produce almost no thrust. That sounds very counterintuitive. If you held a piece of bread in your hand and felt how much gravity was pulling it towards the ground, that is the amount of thrust an ion thruster can produce. There is no way an ion thruster would get a rocket into space. So, what is the point of them? They may not work on Earth, but they would work in the vacuum of space. On Earth, they could not overcome the air resistance, but in space, there is no air resistance. That thrust that is equal to a piece of bread in your hand, if used continuously, for a very long time, will keep building up. If they continue to accelerate for years or decades, the rocket could reach speeds of 320,000 km/h, or even higher. And this is why they would be useful. And this is what I learned today.

Image By Jet Propulsion Laboratory – Electric Propulsion Technology Development for the Jupiter Icy Moons Orbiter Project, Public Domain, https://commons.wikimedia.org/w/index.php?curid=4213968

Sources

https://phys.org/news/2015-11-ion-propulsionthe-key-deep-space.html

https://insights.globalspec.com/article/10145/electric-spacecraft-propulsion-how-do-ion-thrusters-work

https://www.nasa.gov/centers/glenn/about/fs21grc.html

https://en.wikipedia.org/wiki/Ion_thruster

https://www.qrg.northwestern.edu/projects/vss/docs/propulsion/1-what-is-an-ion.html

https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Introductory_Chemistry/04%3A_Atoms_and_Elements/4.04%3A_The_Properties_of_Protons_Neutrons_and_Electrons

https://www.esa.int/Science_Exploration/Space_Science/SMART-1/Ion_drives_Science_fiction_or_science_fact

https://phys.org/news/2015-11-ion-propulsionthe-key-deep-space.html

https://space.stackexchange.com/questions/22140/what-is-the-maximum-speed-an-ion-engine-can-propel-a-spacecraft-at