Ion Thrusters — Paving the Way to Mars Using Electricity
By: Krupal Patel
Tags: Mars, Space Exploration, Innovation, Space Technology

The Past

First let’s discuss chemical rockets, the primary way of getting humans into space. Unfortunately they’re also one of the reasons we’re still stuck on Earth. While the history of jet propulsion can date back to c. 400 BCE, using chemical rockets became most popular in the 1900’s when they were used to launch missiles in war.

In the mid-1900’s, scientists started trying to find ways to develop launch vehicles, using chemical propulsion, to take humans and animals into space. I mean, why not, right?

Photo by Bill Jelen on Unsplash

Chemical rockets, depending on the type of propellant, burn a fuel (most likely a combination of different elements) and an oxidizer. The energy and force created by this combustion reaction is accelerated outside of the engine through the nozzles. This force (thrust) blasts the rocket into the air, following Newton’s 3rd Law.

As easy as this process may sound, it’s very expensive (rocket fuel is not cheap, trust me) and fuel is very heavy. For example, if you want to get to Mars, you need a lot of fuel to travel the ~73 million km separating Earth and the Red Planet. Now the rocket is heavier, so now you need more fuel to thrust such heavy craft all the way to Mars. So you add more fuel, and now the spacecraft is even heavier…

This cycle would go on and and it starts to sound pretty ridiculous. But this is how we’re getting off Earth currently.

Seems kind of inefficient, right?

I thought so too.

A More Efficient Way

Now that we’ve covered why chemical rockets are inefficient, a new question arises: is there a more efficient way to get to space? I’m happy to introduce you to the idea of ion thrusters. Instead of burning chemicals, ion thrusters use the thrust from ion collisions using electricity. We’ll get into how they do this exactly in a bit, first let’s just go over why they’re more efficient.

Right away, we know that since ion thruster systems use electricity instead of burning chemicals, that’s a clear indication that the energy sources for ion thrusters are more accessible. Above, we discussed that chemical rockets aren’t efficient for long term space travel because they’re costly since fuel is so heavy.

They’re also not as fast as we need them to be for future missions to venture outside our solar system. The traditional Space Shuttle could only go up to speeds around 28,968 km/h while spacecraft using an ion thruster system could travel with a velocity of up to 144,840 km/h. That sounds like a much better deal if you ask me.

Now, you must still be wondering: “Can I please know how this system even works?!” Well, you’ll be happy to know that that’s exactly what we’re going to be discussing next.

Electrical Collisions

As previously stated, ion thrusters are powered by ions being accelerated out of the thruster and I’m about to show you exactly how that happens.

Credit: Studzine

The thrusters work through an ionizing propellant using electron bombardment, which occurs in a discharge chamber. The chamber is injected with a neutral propellant to aid in the process. The most common propellant used is xenon since:

it’s easily ionized which optimizes the thrust created it has a high storage density which is suitable for the small storage available on spacecraft.

In the process of electron bombardment, high-energy electrons collide with neutral propellant atoms. From this collision, the results are electrons released from the propellant atoms and a positive ion.

There is also a plasma that is produced in the chamber that has properties of a gas but is affected by electric and magnetic fields. Magnets line the discharge chamber walls, preventing the electrons from reaching the channel walls. This lengthens the time electrons spend inside the chamber and also increases the chances of ionizing events happening.

The positive ions that are produced from the collisions move to a series of grids at the other end of the thruster. The very first grid is a positively-charged electrode ( a screen grid) which is configured to force the discharge plasma to reside at a high velocity. As the ions themselves pass through the grids, they are accelerated towards a negatively-charged electrode (an accelerator grid). As the positive ions pass through the accelerator grid, they reach very high speeds. As they are accelerated out of the thruster, they create… well thrust (Newton’s 3rd Law).

While this is happening, there is also a neutralizer (which is a hollow cathode) hard at work. It expels an equal amount of electrons as the positive ions that are being accelerated out, resulting in a neutral exhaust beam.

Now we know how this crazy system works but let’s go deeper.

The Anatomy

Just like our human bodies have many different parts that work together to help us perform all our bodily functions, ion thrusters are not different in that concept. In a complete ion thruster propulsion system, there are several different parts that help the thruster perform the proper functions to help propel the craft in space.

So let’s go over the details of the structure of the entire system. We know that there’s a thruster but obviously, there has to be more to the system than just that, right? A full ion propulsion system is composed of:

The Ion Thruster — I think we’ve covered what this is and what it does

— I think we’ve covered what this is and what it does Power Processing Unit — the PPU is responsible for converting electrical power from the power source (solar cells, nuclear heat source, etc) to the voltage needed for the hollow cathodes to operate, bias the grids, and to provide the currents needed to produce the ion beam.

— the PPU is responsible for converting electrical power from the power source (solar cells, nuclear heat source, etc) to the voltage needed for the hollow cathodes to operate, bias the grids, and to provide the currents needed to produce the ion beam. Propellant Management System — the PMS is divided into two separate systems: the high-pressure assembly (HPA) which reduces the xenon (most common propellant) from a high-storage pressure; the low-pressure assembly (LPA) is responsible for metering accuracy for the ion thruster components

— the PMS is divided into two separate systems: the high-pressure assembly (HPA) which reduces the xenon (most common propellant) from a high-storage pressure; the low-pressure assembly (LPA) is responsible for metering accuracy for the ion thruster components Digital Control + Interface Unit — the DCUI controls and monitors the system performance and is responsible for communication functions with the spacecraft computer

The Future

From what we’ve looked at, ion thruster systems seem amazing so how come we aren’t already using them? Good question. It’s because while they allow for great velocities, they lack the thrust that traditional chemical rockets have. This, in addition to the fact that ion thrusters need constant fuel, makes some engineers and scientists skeptical.

But technologies are advancing.

NASA’s most recent thruster, the X3, can generate up to 5.4 N of thrust. This is the highest record for ionic thrust to this date.

Ion thrusters are slowly paving the way to Mars as you read this. Further advancements mean that we can begin to leave behind the old chemical rockets for something more revolutionary.