The Theory of Nothing

How does an ion thruster rocket work?

How does an ion thruster rocket work?

If you haven’t already, please read my post: How do rockets work? This explains how Newton’s third law of motion makes a rocket accelerate. It comes down to shooting something out the back at high velocity to cause the rocket to react by moving in the opposite direction.

An ion thruster rocket engine works on the same principle, except instead of using a chemical burning reaction to create high velocity exhaust gasses; an ion thruster engine uses a gas plasma of charged ions to push the rocket. The idea here is to form ions using an electron gun and then use a magnet to squeeze them into a beam and accelerate them out of the rocket’s nozzle by attracting them with an opposite charged grid. There are several clever ways to do this. The earliest method employed is Gridded electrostatics. One takes a gas like xenon and bombards it with energetic electrons from a hot cathode filament to ionize it into a plasma (change it into positively charged ions much like is done in a neon tube) and then attract it to oppositely charged grids at ever-increasing potential. This makes those ions go like the devil out of the rocket nozzle as a beam of charged xenon gas ions.

Remember that this is the main idea to make a rocket accelerate: shoot something out of the rocket’s nozzle at high velocity.

There is a major problem with this idea. One is that the charged ion beam will charge the entire spaceship. This is eliminated by placing a cathode near the engine’s nozzle to pump electrons into the ion beam to prevent the ions from returning to the ship and cancelling the thrust. This Cathode neutralizes the positive ions with negative electrons. There are inefficiencies in this process. One is that the specific thrust of an ion thruster is low. Another problem is that the electrodes are constantly bombarded by ions and erode, reducing their efficiency and operational life. However, ion thrusters can operate for an extended period of time and thus provide thrust to accelerate a spacecraft to speeds not obtainable with a conventional chemical rocket. This is sort of like the old tale of the tortoise and the hare. The rabbit is faster but tires too soon, whereas the turtle is slow but steady and ultimately wins the race.

As you might expect there are several enhancements to the ion thruster rocket idea. One is the Hall effect thruster, in which xenon gas is introduced near a anode that negatively ionizes it after which the ions are attracted towards a cathode where electrons are introduced to neutralize it before it shoots out of the engine nozzle. The secret here is that the central magnet doesn’t affect the plasma ions, instead the cathode electrons are trapped in the magnetic field and circulate around in a Hall current that smacks into the uncharged Xenon and cause it to be ionized back at the anode. This increased the efficiency of the system.

Keep in mind that anodes are negative and cathodes are positive in charge in this application.

Some ion thrusters use a liquid metal (cesium or iridium) reservoir that is easy to ionize and heavier than gas. This makes the system have more thrust, but it emits metal into space.

There are several electromagnetic thruster concepts. Pulsed inductive thrusters run in pulses of thrust instead of running steady. These engines use ammonia as a fuel often relying on Lorentz Force, which is a combination of electric and magnetic force. The equation for this is: Force F is equal to the electric charge q times the combination of Electric Field E plus instantaneous velocity v times magnetic field B. Again, this results in a greater efficacy.

There is an Electrode-less plasma thruster. It works just as the name suggests: instead of using cathodes and anodes the gas is ionized by electromagnetic waves and then transferred to another chamber where it is accelerated by an oscillating electric and magnetic field. This process allows the engine to be throttled.

My favorite is the Helicon double layer thruster, which uses a shaped antenna around the gas ionization chamber. A radio signal at precisely 13.56 MHz is broadcasted through the antenna to produce a helicon wave (an electromagnetic wave that can propagate through pure metal and induce heating) in the plasma to heat it, which is shot out of a magnetic nozzle. This is rather complicated but it makes a very effective rocket engine.

Needless to say, NASA has experimented with these ideas and has used them on several missions. There is no doubt that they are the rocket engines of the future and will perhaps get us to the stars.

Thanks for reading.