Ion thrusters

This technology sounds and looks like it is more fiction than it is science and “Ion engines” are common in the Star Wars universe on TIE-fighters (Twin Ion Engine), but are they real?

Short answer: Yes!
Long answer: Yeeeeeeeeeeees!

The first time ion propulsion has been mentioned is as early as 1906. From then it took a long path to the 1970’s to actually employ this technology as was done on American and on Soviet satellites. Deep Space 1 was NASA’s first interplanetary mission using ion thrusters in 1998.

So if this technology is real, then why aren’t we seeing anything of this when we watch rocket launches now and then?

To get a rocket off the ground and away from earth’s gravity it needs thrust which is the formal term for the force pushing you up. If a spacecraft has an effective thrust of 10 Newton it could accelerate an object with a mass of 1 kg into outer space (considering the gravitational constant g=10 m/s²). Ion thrusters simply have a very low thrust; we’re talking about a maximum of 250 mN! This could carry a rocket weighing about 25 grams. Or in other terms: It would take more than a day to accelerate your car to highway speed with an ion thruster, not taking any resistances into account (of course!).

1280px-Ion_engine.svg
photo credit: wikimedia.org

Before I get into why we even use this propulsion even though it is so weak, I want to take you through a simple explanation of how ion thrusters work. Generally, we can differentiate between electrostatic and electromagnetic thrusters, the former using the Coulomb force and the later using the Lorenz force as the main acceleration mechanism. There are, of course, several different variations but I will only explain one of the most common: The gridded electrostatic ion thruster.
As seen in the diagram, gas is vented into the ion chamber where it is ionized (kick out an electron) by fast electrons coming from the hot cathode on the left. Magnetic coils help hold the ions and electrons in place, as well as help ionize the gas. The ions then move to the positive grid on the right and are then accelerated to about 1 keV and shoot out of the chamber. A second cathode emits electrons into the ion beam in order to compensate for the positive charge leaving the spacecraft and stop the ions from being attracted back to the thruster.

For most thrusters, Xenon is used as a propellant gas. What you are looking for is an easy to ionize and heavy atom because the thrust is proportional to the mass of the accelerated ions. Xenon is also a noble gas and therefore doesn’t erode any parts of the thruster. However, Xenon is globally in short supply and expensive.

Now that we have a crude understanding, we can see how these thrusters are different from conventional rocket propellant. The biggest difference is the effective speed at which particles leave the spacecraft. For conventional rockets, e.g. the Space shuttle, the escaped velocity is around 4.400 m/s (16.000 km/h; 10.000 mph). For our standard ion thruster, the ions are accelerated to about 30.000 m/s! In space exploration the specific impulse of a propellant is very important, that is the total impulse delivered per unit of propellant consumed. If you accelerate your particles to high velocities, every little bit of mass you eject will have a relatively big impact in comparison to lower escape velocities. So ion thrusters have about ten times the specific impulse of conventional rockets which is really important because that simply means you need less propellant. Thus the overall mass of the spacecraft is smaller, reducing the amount of propellant again. This goes on until the spacecraft has no mass at all… just kidding.

Atlantis_taking_off_on_STS-27
Space Shuttle Atlantis; photo credit: wikimedia.org

There are two reasons conventional rockets don’t just use higher escape velocities as well. Firstly, conventional rockets are like big heat machines and a thing called Carnot’s law limits their exhaust velocity; secondly, the energy you need to accelerate stuff goes with the velocity squared and conventional rockets have limited energy reserves, ion thrusters though basically have unlimited electrical energy from their solar panels!


So in the end, ion thrusters are much more efficient, lighter and precise, but lack the ability to create strong thrust and can only be operated in outer space. A conventional rockets brings a spacecraft to outer space and then the ion thrusters take it from there in a well … slow manner.

This definitely isn’t the only promising technology out there but it is the most practical at the time and has been and will be a crucial part of many space mission.

And as always, stay curious!