With so much focus on rocket fuels like hydrogen and new alternatives like liquid methane, it is hard to believe that an advanced form of propulsion known as ion thrusters or drives has been around for over half a century.
In spaceflight, an ion thruster or drive is a form of electric propulsion used by spacecraft in the vacuum of Space. It works by creating positively charged atoms or ions, which are accelerated at very high velocities through electrically charged grids at the back of the engine to produce thrust.
As government and private space agencies are refocusing their attention on returning humans to the lunar surface and extending missions to Mars with the goal of eventually establishing a colony on the planet, several major obstacles need to be overcome.
One of the biggest obstacles is the availability or, rather, the lack of sufficient fuel to successfully complete these missions, especially to planets like Mars, at a distance of 81.5 million kilometers (50.6 million miles) from Earth at its closest approach.
NASA is currently persisting with and trying to optimize engines using liquid hydrogen, the most fuel-efficient liquid propellant currently in use, while SpaceX is looking at refueling their methane-powered Starship rocket in Space and even producing liquid methane on Mars.
While most of these technologies are not yet capable of covering these vast distances in an acceptable time frame or are still in development, a type of electric propulsion known as an ion thruster or drive may just provide the solution for interplanetary travel.
And although electric propulsion may sound like something out of a science fiction movie, ion thrusters have existed for more than 50 years and have already been successfully implemented in the past, as demonstrated by NASA’s Deep Space 1 and Dawn Spacecraft.
As illustrated in the upcoming sections, the technology has significant drawbacks and will never be able to produce the amount of thrust needed to propel an orbital rocket into orbit, but its ability to propel a spacecraft in Space for sustained periods is worth exploring.
What Is An Ion Thruster/Drive?
As mentioned in the introduction, an ion thruster or drive is a form of electric propulsion used by spacecraft in the vacuum of Space. It works by creating positively charged atoms or ions, which are accelerated at very high velocities through electrically charged grids at the back of the engine to produce thrust.
Electrostatic and electromagnetic thrusters are the two primary forms of ion propulsion, of which electrostatic thrusters are most commonly used. (The upcoming section provides a detailed explanation of exactly how an electrostatic thruster works.)
The electricity needed for the ionizing process is provided by a spacecraft’s solar panels, which convert the sun’s radiation to usable electricity. Xenon is the chosen gas predominantly used for ionization since it provides a number of benefits over other gases.
Xenon is a noble gas, which means it does not react with any other element under almost any circumstance, making it very stable as it will not interfere with other components in the engine. It also remains a gas at room temperature, which makes it easy to handle and store.
Its high molecular mass of 131.293 u makes Xenon easier to ionize since removing an electron from an atom with a higher molecular mass requires less energy. This is crucial for a spacecraft that relies on its solar panels for all of its electrical energy requirements.
As ions are being accelerated through an electric field, the higher mass of Xenon atoms also allows them to have more momentum, providing more thrust to a spacecraft. This is part reason why Xenon is chosen over other noble gases like Helium, Neon, Argon, and Radon.
To better understand how an ion thruster works, one needs to take a more in-depth look at how the most widely used type of ion thruster, the electrostatic thruster, functions:
How An Ion Thruster Works
Like liquid and solid-fueled rocket engines, ion propulsion work on the same basic principle, which is Newton’s Third Law Of Motion. It states that “For every action, there is an equal and opposite reaction.”
It means that when two forces interact with each other, one force’s reaction is equal but in the opposite direction of the force applied upon it. The way a spacecraft’s ion thruster works is the perfect example of this law in practice.
The first force that comes into play is the thrust created by the rocket’s engine as it pushes the charged particles out the back of the nozzle. The force created by this thrust creates the second force, which pushes the vehicle forward as a reaction.
However, unlike liquid or solid rocket propellant engines, which use fuel and oxidizers to produce a chemical reaction to provide the necessary thrust to propel a launch vehicle forward, ion drives use positively charged atoms or ions to produce thrust.
The best way to describe how an ion drive works is by breaking down the above illustration of a fully assembled ion thruster into its separate components, explaining the role each part plays in allowing the drive to produce thrust.
As illustrated below, it all starts with a cylindrical vessel with an open end on one side and a closed end on the opposite side.
On the closed side, a noble gas, typically Xenon, is released into the vessel’s chamber. Noble gases are chosen since they comprise of separate atoms with a neutral charge that do not form any chemical reaction with other substances under normal conditions.
(These properties make noble gases easy to handle and store, but Xenon, in particular, has the added advantage of having a high atomic mass of 131.293 u and remaining a gas at room temperature.)
At this point, no reaction takes place, and the neutrally charged Xenon only fills and is distributed inside the chamber area.
As illustrated above, a device called an electron gun, also known as a hollow cathode, is also located on the closed side of the vessel. This cylindrical device typically contains a material like tungsten covered by barium oxide at the front, which is surrounded by a resistor.
As the resistor is heated, the material inside warms up, and through a process called thermionic emission, it will start to emit electrons. At this point, a certain amount of Xexon is introduced from the back of the tube.
The high-temperature environment with the continuous emission of electrons allows for the creation of plasma since the Xenon atoms collide with the electrons, which causes the atoms to lose an electron, creating positively charged Xenon atoms known as ions.
(Plasma is essentially a high-temperature electrically charged gas or medium where electrons have been stripped away from their atoms, causing the resulting positively charged ions and negatively charged electrons to remain separate.)
This space is now filled with plasma consisting predominantly of negatively charged electrons, which are forced through a small hole at the front of the cylinder as more Xenon atoms are introduced from the rear.
A positively charged cover at the front of the electron gun helps to relay the electrons into the larger chamber area while preventing unwanted materials like the positively charged Xenon atoms/ions from entering.
At this point, as illustrated above, the electrons enter the larger chamber area filled with Xenon. The same reaction that occurred in the electron gun now takes place on a larger scale as the electrons in the chamber collide and remove electrons off the Xenon atoms.
To make the ionizing process in the larger chamber more efficient, a series of magnetic rings (as illustrated in the first image) is added around the vessel, which not only helps the process along but also helps to contain the plasma.
Two metal grids are located side-by-side on the open side of the vessel (as illustrated above), which serves a crucial purpose. As the volume of generated plasma continues to grow, so does the pressure inside the vessel.
The only way for the plasma to escape is through both grids, and both electrons and the positively charged Xenon ions, will try and escape through the small holes in the grid.
However, both metal grids are electrically charged, with the inner grid positively and the outer grid negatively charged. The negatively charged electrons will be pulled toward the positively charged grid but, once through, will be pushed back by the negatively charged grid.
Initially, the positively charged Xenon ions will be pushed away by the positively charged grid, but as the pressure builds up, they will force their way through. Once through, they will be attracted by and accelerated through the negatively charged grid.
This process allows the Xenon ions to be accelerated and pushed out the back of the second grid at very high velocities, much higher than the speeds achieved by the gases exiting the nozzles of traditional liquid-fueled chemical rocket engines.
For example, the ion thruster used on the Deep Space 1 spacecraft propelled ions at speeds of up to 146 000 km/h (90 700 mph) into Space.
However, the process is not complete at his point, as the illustration above demonstrates. The positively charged ions will normally be attracted back to the negatively charged grid after exiting through its holes.
To prevent this from happening, a second electron gun, placed on the outside of the second grid, fires electrons at the positively charged ions, returning them back to a neutral charge. As a result, the Xenon atoms can continue to exit the ion engine at a high velocity.
Advantages & Drawbacks Of Ion Thrusters
As the previous section clearly illustrated, ion thrusters or drives work by using processes that differ dramatically from that of chemically propelled spacecraft. This comes with several significant advantages but also a few notable drawbacks.
The vast majority of orbital rockets and spacecraft use a chemical reaction to produce thrust. Whether they use solid or liquid propellant, fuel typically reacts with an oxidizer to create combustion that results in the hot gases that propel the vehicle forward.
(Learn more about the different types of fuel orbital rockets use, their characteristics, as well as the various advantages and drawbacks of each fuel type in this article.)
Unlike a chemically fueled rocket, ion propulsion uses electrical energy to ionize and accelerate charged particles at high velocities from the thruster. This type of propulsion provides some unique advantages, of which the most important are:
- High Specific Impulse
- High Exhaust Velocities
- Relatively Inexpensive
To get a better understanding of how these features give ion thrusters specific advantages over other forms of propulsion, one needs to take a closer look at each one individually:
1) High Specific Impulse
Orbital rockets and spacecraft need to carry all the fuel they will require in their lifespan onboard, and there is currently no means of refueling in Space. As a result, fuel efficiency is a sought-after and critical consideration for any vehicle operating in Space.
The means by which fuel efficiency is expressed in spaceflight is known as Specific Impulse.
What Is Specific Impulse?
Specific Impulse is the Holy Grail for many rocket engineers and essentially refers to how efficiently a rocket engine can burn its fuel. Simply put, it refers to how long a certain amount of propellant can produce a certain amount of thrust.
To better understand Specific Impulse, one can compare it to how many miles per gallon a car can achieve. A more fuel-efficient vehicle will be able to travel much further than a vehicle with a much higher fuel consumption rate.
In the same way, a rocket with a higher Specific Impulse can burn its thrusters longer, accelerate for extended periods, and as a result, cover much longer distances in a significantly shorter time.
Specific impulse is generally calculated by measuring the number of seconds a kilogram (or pound) of fuel lasts while producing a kilogram (or pound) of thrust. It is typically measured in seconds. (A very unscientific but practical explanation.)
As a result, a spacecraft with a Specific Impulse of 900 seconds is much more efficient than a spacecraft with a Specific Impulse of 450 seconds, allowing it to travel longer and further on the same amount of fuel.
However, Specific Impulse specifically refers to energy efficiency and not power or thrust. A launch vehicle with a high Specific Impulse may not necessarily have the mass flow rate (or thrust) to propel a rocket out of Earth’s atmosphere during launch.
Compared to conventional chemical rockets, ion thrusters have a very high Specific Impulse. It can operate continuously for days, months, or even years. A liquid-fueled rocket engine, in comparison, can only fire its thrusters periodically for a few seconds or minutes at a time.
(For example, the electrostatic ion thruster of the Deep Space 1 spacecraft had a Specific Impulse of 3 120 seconds, compared to the 450 seconds achieved by spacecraft powered by liquid hydrogen, the most fuel-efficient liquid propellant currently in use.)
This means that, even though they are not capable of generating the high amount of thrust chemical rockets do, ion-thruster-powered spacecraft can achieve very high velocities by continuously firing for extended periods over vast distances within our solar system.
2) High Exhaust Velocities
The high velocities at which atoms are ejected into Space from an ion thruster also contribute to its overall performance. If one considers Newton’s Third Law of Motion, the faster matter is expelled through the back of a rocket, the faster it is propelled forward.
(These extreme high velocities are also one of the main contributors to the high Specific Impulse that ion engines are able to achieve.)
Ion thrusters have an average exhaust velocity of 20 – 50 km/s (12 – 31 miles/second), while chemical rockets like the Space Shuttle’s main engines are only capable of producing an exhaust velocity of approximately 3 km/s (1.86 miles/second) at sea level.
For example, as mentioned earlier, the ion thruster used on the Deep Space 1 spacecraft propelled ions at speeds of up to 146 000 km/h (90 700 mph) into Space.
3) Relatively Inexpensive
Ion drives not only have a high Specific Impulse that allows them to travel vast distances for extended periods of time, but they also use a surprisingly small amount of fuel to produce the energy for this type of propulsion.
This means the size of the spacecraft and the amount of fuel they need can be much smaller, which reduces weight, saves cost on manufacturing materials, and, as a result, the cost of launching a spacecraft into Space.
Currently, chemically propelled rockets can achieve a fuel efficiency of up to 35%, while ion thrusters are capable of achieving a fuel efficiency of over 90%.
The following may be a slightly unfair comparison since the first example had to operate in Earth’s atmosphere, where the thick atmospheric air and the planet’s gravity made it difficult to reach Space, while the second operates in the vacuum of Space with few external forces.
However, the Deep Space 1 spacecraft used less than 72 kg (159 pounds) of fuel to provide a total of 16 000 hours of thrust. In contrast, only one of the five F1 rocket engines used to power the Saturn V rocket consumed 2 578 kg (5 683 pounds) of propellant per second.
These vast differences in fuel requirements and the resulting cost savings become very apparent when comparing these two different types of propulsion.
These advantages are very significant. But as attractive as they may seem, ion propulsion comes with serious drawbacks, which currently makes it impossible for them to replace chemically propelled spacecraft. The most notable drawbacks are:
- Very Low Propulsive Force
- Unable To Operate In Earth’s Atmosphere
- Requires External Energy Source
Like its advantages, one needs to take a closer look at each drawback individually to gain a better understanding of why it is considered a disadvantage:
1) Very Low Propulsive Force
One of the big advantages of ion propulsion is the very high velocities at which the small ionized atoms are accelerated and propelled into Space.
However, the small size & mass of the atoms mean that ion thrusters cannot achieve the same amount of thrust as chemical rockets, which use propellants producing exhaust gases where the sheer volume & size of the particles in them produce large amounts of thrust.
In other words, the small charged atoms pushed out the back of an ion thruster at high velocities produce a lot less thrust or “raw power” than a chemical rocket that pushes a much larger volume of bigger particles at lower velocities out of its engine nozzle.
For example, the ion drive that was used on the Deep Space 1 spacecraft only produced 92 millinewtons or 0.33 pounds of thrust. This a about the same force as a sheet of paper resting on your hand.
To put this into perspective, the first stage of a modern Falcon 9 rocket produces 7 607 kN or 1.7 million pounds of thrust that allows it to push through Earth’s thick atmosphere while battling immense gravitational forces to reach Space and put a spacecraft in orbit.
As a result, existing ion thrusters will never be able to produce enough thrust to propel an orbital rocket out of Earth’s atmosphere and pull away from the planet’s large gravitational forces to reach Space.
As a result, existing ion thrusters will never be able to propel an orbital rocket out of Earth’s atmosphere and pull away from the planet’s large gravitational forces to reach Space.
However, there is another reason why ion propulsion is unsuitable for operation in our atmosphere, which is addressed in the following section.
2) Unable To Operate In Earth’s Atmosphere
Another drawback of ion propulsion, which adds to its inability to function within Earth’s atmosphere, has everything to do with the way in which the ion drives produce thrust.
Part of the reason ion thrusters are so effective is that they are able to propel ionized atoms at high velocities into the vacuum of Space where there are no other matter, specifically charged particles, which can react with them and interfere with their function.
However, our atmosphere is filled with gases and other small particles, specifically charged particles or ions. The atmospheric air contains positive and negative ions with an average concentration of 200–2500cm−3 (although concentrations of up to 5000cm−3 can occur).
These charged particles in the air will immediately react with and disrupt the positively charged Xenon atoms exiting an ion drive, which will essentially nullify the thrust produced by the engine.
In a nutshell, ion propulsion cannot be used within the planet’s atmosphere primarily since ion thrusters can not operate in an environment where ions are already present in the air outside the engine.
Additionally, the amount of particle present in the atmosphere also causes too much air resistance for the tiny Xenon atoms to continue accelerating and produce the necessary thrust to propel a spacecraft.
3) Requires External Energy Source
Chemical rockets need to carry all their propellant onboard, including the oxidizer, since no air is available in the vacuum of Space to draw oxygen from to achieve combustion. There is also currently no means of refueling in Space, which limits the spacecraft’s capabilities.
Spacecraft powered by ion propulsion do not have this problem since they use very little “fuel” in the form of Xenon gas. As mentioned earlier, the Deep Space 1 spacecraft used less than 72 kg (159 pounds) of fuel to provide a total of 16 000 hours of thrust.
However, the electrical energy required to create the reaction that ionizes the Xenon atoms to produce thrust comes from the vehicle’s solar panels, which rely on the Sun’s radiation, an external source of energy.
This means the craft’s solar panels need to be directly exposed to the Sun and orientated in such a manner that it can make full use of the solar energy. If a celestial body or any other object blocks its exposure, no electrical energy can be created.
A spacecraft’s onboard batteries can keep its electrical systems going for short periods when no direct exposure to the Sun’s rays is possible, but care needs to be taken when planning the spacecraft’s trajectory so that it receives an adequate amount of solar radiation.
Although they are not as well-known as other forms of rocket propulsion, ion thrusters or drives have been in use for more than half a century and have some clear advantages (and disadvantages) over their chemically fueled counterparts.
They are increasingly used in spacecraft exploring distant planets and other celestial bodies in our solar system and also in the Reaction Control Systems (RCS) of satellites that help them to maintain their altitude and correct orientation.
(Learn more about Reaction Control Systems, what they are, and how they allow spacecraft to maneuver in the vacuum of Space in this article.)
Although there is currently no form of ion propulsion available powerful enough to replace chemically fueled rockets, they will continue to take over smaller or different tasks requiring less energy as the technology evolves and grows.
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