Ion Propulsion: The Future of Space Exploration and Satellite Technology

Rajesh Uppal January 18, 2024 Energy & Propulsion, Space Comments Off on Ion Propulsion: The Future of Space Exploration and Satellite Technology 1,683 Views

Space propulsion systems are the heart and soul of any space mission, providing the thrust needed to navigate through the cosmos. These propulsion systems vary widely, including solid, liquid, and electric propulsion. Traditional chemical propulsion has served us well for decades, but it has its limitations, particularly for missions to distant planets like Mars and Mercury, as well as for military satellites and space planes.

Among them, electric propulsion (EP) has recently emerged as a groundbreaking technology with the potential to redefine space exploration, particularly for missions to Mars, and Mercury, military satellites, space planes, and anti-satellite weapons (ASATs). In this article, we will delve into the world of ion propulsion and explore how it is poised to transform the future of space travel and satellite technology.

Understanding Ion Propulsion or Ion Thrusters

Ion propulsion, often referred to as ion thrusters or electric propulsion, is a propulsion system that uses charged particles (usually ions) to create thrust. Unlike traditional chemical rockets, which burn fuel to produce high-speed exhaust gases, ion thrusters rely on electromagnetic principles to accelerate ions to high velocities and expel them at great speeds.

As these ionized particles are expelled from the spacecraft, they generate thrust in the opposite direction. The power sources for ion thrusters are typically electric solar panels, although nuclear power becomes necessary in regions distant from the sun.

Ion thrusters are categorized by how they accelerate the ions, using either electrostatic or electromagnetic force. Electrostatic thrusters use the Coulomb force and accelerate the ions in the direction of the electric field. Electromagnetic thrusters use the Lorentz force. The most common propellant used in ion propulsion is xenon, which is easily ionized and has a high atomic mass, thus generating a desirable level of thrust when ions are accelerated. It also is inert and has a high storage density; therefore, it is well suited for storing on spacecraft. In most ion thrusters, electrons are generated with the discharge hollow cathode by a process called thermionic emission.

Ion thrusters are adaptable for a wide range of missions, from station-keeping for communication satellites to interplanetary travel. NASA even considers ion propulsion mission-enabling in scenarios where traditional chemical propellants cannot provide the necessary thrust. This technology holds the promise of powering round trips to Mars without refueling, utilizing recycled space debris as fuel.

How Ion Thrusters Work

Ion thrusters function by ionizing propellant through a process called electron bombardment. High-energy electrons collide with propellant atoms, releasing electrons from the atoms and resulting in positively charged ions. This ionized gas, known as a plasma, has some properties of a gas but is influenced by electric and magnetic fields. In ion thrusters, positive ions are accelerated towards grids containing thousands of precisely aligned apertures. These ions pass between the grids and are accelerated to high speeds, eventually being expelled as an ion beam, generating thrust. To maintain spacecraft neutrality, a neutralizer, another hollow cathode, expels an equal number of electrons, ensuring that the exhaust beam has a neutral charge.

The Benefits of Ion Propulsion

Efficiency: Ion propulsion systems are incredibly efficient compared to chemical rockets. They can operate for extended periods while consuming very little propellant, making them ideal for long-duration missions.
Speed: Ion thrusters can achieve much higher speeds than chemical propulsion systems over time. This is especially crucial for missions to distant planets and interplanetary travel.
Precision: Ion thrusters can provide precise and controlled thrust, enabling spacecraft to make delicate maneuvers with precision.

The advantages of ion propulsion are numerous:

Highest Specific Impulse: Ion engines offer substantial mass savings with specific impulses exceeding 3000 seconds.
High Performance with Low Complexity: They are known for their high performance despite their relatively simple design.
Reduced Power Processing Unit Mass: Ion propulsion systems require smaller, lighter power processing units.
Narrow Beam Divergence: Ion thrusters have a narrow beam divergence, enhancing their precision.
Robust Design: They are robust, with a large operational stability domain.
Large Throttle Range: Ion propulsion systems can adapt to available electric power.
Excellent Thrust Stability: They provide excellent thrust stability and rapid thrust response.
High Growth Potential: Ion propulsion systems can scale up with increasing electric power.

Ion Propulsion vs. Chemical Propulsion

Despite their many advantages, ion thrusters have some limitations. They produce relatively low thrust levels, making them unsuitable for launching spacecraft through Earth’s atmosphere. They are also practical only in the vacuum of space since ion engines cannot function outside of space’s ion-free environment. Furthermore, the tiny thrust generated by ion thrusters is insignificant when faced with atmospheric resistance during launch.

Comparing ion propulsion to traditional chemical rockets is akin to the fable of the tortoise and the hare. While chemical propulsion delivers bursts of high thrust followed by coasting, ion propulsion provides consistent, albeit slower, thrust over long durations. Ion engines ionize propellant by adding or removing electrons to create positively charged ions. They offer high specific impulses, which means they are exceptionally fuel-efficient. In fact, ion thrusters can be more than ten times as fuel-efficient as traditional rocket engines. Additionally, ion propulsion operates at lower temperatures, eliminating the need for the extreme heat-resistant materials required by chemical rockets.

Components of an Ion Propulsion System

An ion propulsion system comprises several essential components:

Ion Thruster: The core component that generates thrust.
Power Processing Unit (PPU): Converts electrical power from sources such as solar panels or nuclear heat into the required voltages for the thruster’s operation.
Propellant Management System (PMS): Regulates the propellant flow from the storage tank to the thruster components.
Digital Control and Interface Unit (DCIU): Controls and monitors system performance and communicates with the spacecraft computer.

In an ion propulsion system, the generation of thrust involves a complex series of steps that rely on the behavior of electrons and ions. Here’s a summary of the process described:

Electron Generation: Electrons are produced by a discharge cathode within the ion thruster.
Electron Attraction: These electrons are attracted to the walls of the discharge chamber, which are positively charged due to the voltage applied by the thruster’s discharge power supply. This attraction occurs because opposite charges attract, and electrons are negatively charged.
Propellant Injection: Neutral propellant, often a gas like xenon, is injected into the discharge chamber. This propellant is not charged; it is in a neutral state.
Electron Bombardment: The electrons, drawn toward the positively charged walls, bombard the neutral propellant atoms. This collision between electrons and propellant atoms causes the atoms to lose electrons and become positively charged ions. Simultaneously, more electrons are released during these collisions.
Magnetic Field: High-strength magnets are used to prevent electrons from freely reaching the discharge chamber walls. This magnetic field confines the electrons and extends the time they remain within the discharge chamber.
Ion Acceleration: The positively charged ions resulting from the electron-propellant collisions migrate toward grids located at the aft end of the ion thruster. These grids have thousands of precisely aligned holes or apertures. The first grid, known as the screen grid, is positively charged. It is designed to maintain a high voltage, forcing the discharge plasma (containing ions and electrons) to also reside at a high voltage.
High-Speed Acceleration: As the ions pass through the grids, they experience a significant acceleration due to the high voltage gradient. This acceleration propels the ions to very high speeds, often reaching velocities of up to 90,000 miles per hour (mph).
Ion Beam Emission: The positively charged ions, now accelerated to high speeds, are expelled from the thruster as an ion beam. This ion beam produces the thrust necessary to propel the spacecraft.
Neutralizer: To ensure that the ion beam has a neutral overall charge, a neutralizer, which is another hollow cathode similar to the one used for electron generation, expels an equal amount of electrons. This balancing of charges ensures that the exhaust beam is electrically neutral.

Without a neutralizer, the spacecraft would accumulate a negative charge due to the expulsion of positively charged ions, which could lead to reduced thrust and potential erosion of the spacecraft over time.

In essence, the ion propulsion system takes advantage of the ionization of neutral propellant, controlled by electrons, to generate a high-speed ion beam that produces the thrust needed for space missions. The precise control of charges, magnetic fields, and voltage gradients is crucial to the efficient operation of this advanced propulsion technology.

Real-world Applications of Ion Thrusters

Ion thrusters are being designed for a wide variety of missions—from keeping communications satellites in the proper position (station-keeping) to propelling spacecraft throughout our solar system.

One notable mission featuring ion propulsion is the European Space Agency’s (ESA) BepiColombo mission to Mercury. Launched in 2018, it is currently on a seven-year journey to the closest terrestrial planet to the sun. BepiColombo will endure extreme temperatures exceeding 350°C while gathering valuable data during its one-year nominal mission, with a potential one-year extension.

Mars and Mercury Missions: The efficiency and long operational life of ion propulsion make it an excellent choice for missions to Mars and Mercury, where the travel times are considerable. Ion thrusters can significantly reduce mission durations and open up new possibilities for exploration and scientific research.
Military Satellites: Ion propulsion is becoming increasingly important in the development of military satellites. These satellites require precise positioning and maneuverability, which ion thrusters can provide. Additionally, the reduced need for refueling or replacement in orbit enhances their operational longevity.
Space Planes: Ion propulsion is a promising technology for space planes, enabling them to achieve higher speeds and travel to higher altitudes while using less propellant. This could lead to the development of more cost-effective and versatile spaceplanes for various purposes, including space tourism and cargo transport.
Anti-Satellite Weapons (ASATs): While the peaceful applications of ion propulsion are abundant, it’s important to note that the technology’s precision and maneuverability also have potential military applications, including ASATs. These systems could be used for satellite interception and defense in the future.

Moderate power level Gridded Ion Thrusters

Moderate power level Gridded Ion Thrusters, ranging from 1 to 20 kW, have a substantial history of development and utilization. The United States heavily invested in these thrusters during the 20th century, exemplified by the Xenon Ion Propulsion System family, initially developed by Hughes and later by Boeing. These thrusters found extensive use in various missions, including deep space explorations like Dawn, Hayabusa (at slightly lower power levels), and the current BepiColombo mission to Mercury. Ground-based testing has also played a pivotal role in advancing these thrusters, particularly in understanding and mitigating erosion-related issues.

However, despite their successful track record, challenges persist in enhancing the performance of moderate power Gridded Ion Thrusters. These challenges primarily revolve around making incremental improvements and addressing specific issues related to their testing and deployment. One critical consideration is the influence of facility effects, similar to those observed in Hall thrusters. For example, when assessing the NASA Evolutionary Xenon Thruster (NEXT), numerical modeling during life tests revealed that accelerator grid groove erosion could be reduced by up to 30% through the redeposition of sputtered material from the beam dump. Neglecting this effect could lead to overestimated thruster longevity, posing a significant risk to flight operations.

NASA and aerospace company, Aerojet Rocketdyne, have successfully completed qualification testing of the Advanced Electric Propulsion System (AEPS), which is a 12-kilowatt, solar electric propulsion (SEP) engine being built for use for long-term space missions to the Moon and beyond, and AEPS is being touted as the most powerful electric propulsion—also called ion propulsion—thruster currently being manufactured.

The goal is to use AEPS on NASA’s upcoming Gateway space station by mounting three AEPS thrusters on Gateway’s Power and Propulsion Element, which will be responsible for providing an assortment of duties, including maintaining Gateway’s desired orbit around the Moon, high-rate communications between Earth, and power for the entire orbiting outpost. Due for an expected launch in 2025, Gateway will be a collaboration between international and commercial partners as an essential piece for NASA’s upcoming Artemis missions to the lunar south pole in the next few years. While Gateway is the current goal for AEPS, the thrusters could potentially be used on deep space missions, as well.

Additionally, determining the lifespan of moderately-powered Gridded Ion Thrusters remains a persistent obstacle. While simulations can accurately replicate erosion patterns, understanding the multifaceted causes of cathode erosion and failure is an ongoing area of study. Furthermore, identifying and addressing long-duration failure mechanisms necessitates extensive and rigorous ground testing efforts.

In summary, moderate power level Gridded Ion Thrusters have a rich history of development and application, particularly in deep space missions. However, ongoing challenges involve fine-tuning these thrusters, accounting for facility effects, and comprehensively assessing their longevity and reliability, making them a subject of continuous research and improvement.

High-powered gridded ion thrusters

The development of high-powered gridded ion thrusters has the potential to bridge the gap between the applications of modern electric propulsion (EP) devices and those where chemical propulsion is traditionally favored. Chemical propulsion provides high thrust for rapid travel, crucial for crewed missions despite the payload mass trade-off. However, with increased power, EP systems could offer sufficient thrust while retaining their inherent propellant efficiency. Researchers are actively exploring the scalability of EP devices to higher power levels, aiming for 20 kW or more.

High-powered gridded ion thrusters have a historical foundation, with previous exploration, including a 130 kW mercury thruster, demonstrating their feasibility. This technology has progressed beyond its initial stages of development.

Apollo Fusion, a new company, is gaining attention for its propulsion system concept that employs mercury as a fuel. Mercury, although promising for its potential to generate more thrust due to its higher mass compared to xenon or krypton, poses environmental risks as a toxic substance if released into the atmosphere. NASA experimented with mercury for ion propulsion in the 1960s but discontinued its use due to safety concerns. Today, xenon and krypton are the common propellants for ion engines. Apollo Fusion’s pursuit of mercury-based ion engines could offer high-power, cost-effective solutions for satellites and spacecraft if successful, but it also carries the risk of mercury contamination if not properly managed.

Several factors have contributed to the relatively slow development of higher power Gridded Ion Thrusters (GITs). First, GIT beam current is limited by space charge, necessitating physically larger thrusters for higher power, making testing and deployment more challenging. For instance, the 130 kW mercury GIT tested in 1967 was notably large, at 1.5 meters in diameter, in contrast to modern flight EP systems. Second, past successful ground testing of large GITs relied on easily pumped propellants like mercury and cesium, which are now avoided due to health and spacecraft interaction concerns. Consequently, the high gas throughput of these GITs can overwhelm most ground vacuum facilities, posing practical limitations to their development and testing.

Sub-kilowatt Gridded Ion Thrusters (GITs)

The development of sub-kilowatt Gridded Ion Thrusters (GITs) has seen progress, with notable examples like the Radio-frequency Ion Thruster (RIT) series and JAXA’s Kiku ion thrusters contributing to an early shift in the technology’s development curve. Subsequently, there has been consistent advancement in low-power GITs, exemplified by the 350 W microwave discharge gridded ion engines used in JAXA’s Hayabusa mission, which have accumulated over 25,000 hours of in-flight operation. More recent additions to the field include Busek’s BIT series, Astrium’s µNRIT, and UCLA’s Miniature Xenon Ion (MiXI) thruster.

However, despite these recent developments, most sub-kilowatt GITs are not as mature as their higher power counterparts, primarily due to significant challenges. The foremost obstacle is the substantial power electronics required to support the multiple electrodes in these designs. Consequently, there is a pressing need for the development of more compact and efficient power processing systems to make sub-kilowatt GITs practical. Second, sub-kilowatt GITs face a fundamental limitation related to space charge, where the current extracted from the thruster is constrained by the grid area of the device. Unlike other technologies like Hall thrusters, where current density can theoretically compensate for smaller thruster sizes, this space-charge limitation restricts sub-kilowatt GITs to low beam currents.

Ion Propulsion Breakthroughs

Ion propulsion is experiencing significant innovation and expansion in various directions:

Higher Thrust Ion Engines: Recent advancements in ion propulsion technology have led to the development of higher thrust ion engines. These engines can produce significantly more thrust while maintaining the efficiency that ion propulsion is known for. This breakthrough opens up new possibilities for faster interplanetary travel.

Solar Electric Propulsion (SEP): SEP systems are designed to harness the power of solar panels to generate electricity for ion thrusters. The development of more efficient and lightweight solar panels has made SEP a practical and sustainable option for a wide range of space missions.

Miniaturization: Miniaturization of ion propulsion systems has made them suitable for a variety of spacecraft, including military satellites and space planes. Smaller, more lightweight ion thrusters are essential for reducing launch costs and increasing mission flexibility.

Advanced Propellants: Research into alternative propellants, such as mercury, holds promise for increasing thrust and efficiency.

Ion Propulsion for Deep Space Missions: Ongoing research aims to develop ion propulsion systems capable of powering deep space missions with higher thrust and efficiency.

3D Printing: Novel manufacturing techniques, like 3D printing, are being explored to create ion propulsion systems with optimized designs and materials.

In January 2021, researchers from the Massachusetts Institute of Technology (MIT) unveiled a groundbreaking 3D printed ion-emitting satellite propulsion system. This innovative thruster, believed to be the first fully 3D printed device of its kind, emits charged ionic particles from emitter cones on its outer shell, providing micronewtons of propulsion. In the frictionless environment of space, this level of thrust could make it a cost-effective and efficient alternative to traditional CubeSat engines.

The MIT research team emphasized the importance of optimizing shapes, materials, and all components in space hardware development. 3D printing offers advantages in achieving these optimizations. The researchers created two microelectromechanical system (MEMS) designs: one with a binder jetted SS 316L-based emitter array and another using an acrylic polymer. Both devices featured a fluidic connector, liquid reservoir, and an outer casing with an embedded array of external cone-shaped emitters.

During testing, both systems demonstrated consistent performance over several hours, producing only a thin removable layer of residue. The metallic MEMS exhibited higher thrust levels than the polymeric version, but the latter’s cost-effectiveness made it an attractive option for future applications.

The researchers envision that the affordability of the plastic electrode could pave the way for numerous college-led designs and orbital space missions in the future. Developing reliable, robust, and efficient electric propulsion systems while meeting specific mission requirements remains a key challenge, but innovative approaches like 3D printing show promise in addressing these challenges and expanding the possibilities of satellite propulsion technology.

Field-emission electric propulsion (FEEP)

Field-emission electric propulsion (FEEP) is an advanced form of electrostatic space propulsion, classified as an ion thruster, that employs liquid metal propellants like caesium, indium, or mercury. The FEEP propulsion system comprises an emitter and an accelerator electrode. A substantial potential difference, typically around 10 kV, is applied between these components, creating a powerful electric field at the tip of the liquid metal surface.

This intense electric field, combined with surface tension effects, triggers surface instabilities, leading to the formation of Taylor cones on the liquid’s surface. When the applied field reaches a sufficiently high level, ions are extracted from the tip of these cones through mechanisms like field evaporation. Subsequently, these ions are accelerated to high velocities, often exceeding 100 km/s or more.

To maintain the spacecraft’s electrical neutrality, a separate electron source is necessary. Due to its extremely low thrust, typically ranging from micronewtons to millinewtons, FEEP thrusters are primarily employed for precise spacecraft attitude control in applications requiring microradian-level accuracy and low thrust, such as in missions like the ESA/NASA LISA Pathfinder scientific spacecraft.

Austrian startup Enpulsion is focused on mass-producing Field Emission Electric Propulsion (FEEP) thrusters, initially targeting small satellites ranging from 3 to 100 kilograms. FEEP thrusters have been of interest to the European Space Agency (ESA) and the industry for over a decade due to their capability for extremely precise in-space movements and station-keeping. However, previous attempts to commercialize FEEP technology faced challenges transitioning from the laboratory to practical applications.

Enpulsion emerged from Fotec, a research division at the University of Applied Sciences Wiener Neustadt in Austria. They developed a breakthrough involving a “porous tungsten crown emitter,” which offers a stable and scalable technology suitable for mass production. This advancement has garnered significant interest from small satellite manufacturers worldwide.

Enpulsion’s CEO, Alexander Reissner, emphasized the appeal of their Indium-FEEP technology, which allows for customized propulsion solutions with a catalog price and a short lead time of less than two months. The company aims to produce 100 to 200 thrusters annually and has already received 150 pre-orders from customers in Europe and the United States. One notable customer is Iceye, a Finnish synthetic aperture radar startup that plans to use Enpulsion FEEP thrusters in their upcoming missions.

Paddy Neumann, a doctoral candidate in Physics at the University of Sydney, has created a groundbreaking ion space drive that surpasses NASA’s current fuel efficiency record.

This innovative drive can utilize various metals, including those present in space debris, according to the university’s student newspaper, Honi Soit.

NASA’s existing fuel efficiency champion is the High Power Electric Propulsion (HiPEP) system, boasting 9,600 (+/- 200) seconds of specific impulse. However, Neumann’s newly developed drive has achieved an impressive 14,690 (+/- 2,000) seconds of specific impulse, as reported by Honi Soit.

What sets the Neumann Drive apart is its adaptability to power sources. While NASA’s HiPEP relies on Xenon gas, this new drive can operate using a range of metals, with magnesium being the most efficient one tested so far. Significantly, since it can run on metals commonly found in space debris, there’s potential to recycle defunct satellites and transform them into fresh fuel for future space missions.

China Advances

China’s Tiangong space station’s inaugural module is set to feature an ion propulsion system that promises significantly enhanced energy efficiency, potentially reducing travel times to Mars, reports the South China Morning Post (SCMP). The core Tianhe module of the space station will be propelled by four ion thrusters.

According to the Chinese Academy of Sciences, the International Space Station (ISS) requires four tons of rocket fuel annually to maintain its orbit, while ion thrusters would only need 882 pounds (400 kg) to achieve the same effect.

One challenge with ion thrusters is their propensity to degrade engine components and provide relatively low thrust. However, Chinese scientists have addressed these issues. They developed a method involving a magnetic field over the engine’s inner wall to repel damaging particles, thus protecting the engine from erosion. Additionally, they created a unique ceramic material capable of withstanding high heat and radiation for extended periods. As a result, their ion thruster has operated continuously for more than 11 months without issues.

In 2016, reports emerged that China had developed the world’s most powerful ion thruster, surpassing existing technologies in terms of performance. This next-generation Hall-effect thruster unit was designed to improve the maneuverability and longevity of Chinese space assets. It was anticipated to outperform all existing ion thrusters, potentially achieving up to 30% better performance. Chinese researchers expressed confidence that they would become the first to test this technology on a high-altitude satellite, as other countries had not yet completed ground testing of similar ion thrusters.

NASA and Aerojet Rocketdyne have begun qualification testing on the most powerful solar electric propulsion (SEP) thrusters ever built.

SEP thrusters work by using solar power to accelerate xenon ions to very high speeds. This creates a small amount of thrust, but it is very efficient. SEP thrusters can operate for months or even years on a single fuel tank, making them ideal for long-duration missions.

AEPS thrusters are more than twice as powerful as any existing SEP thrusters, with a maximum power output of 12.5 kW. This allows them to generate a significant amount of thrust, even though they are very efficient.

AEPS thrusters are also very reliable, with a predicted lifetime of over 100,000 hours. This makes them ideal for long-duration missions, such as the Gateway lunar space station.

The AEPS thrusters will be used on the Power and Propulsion Element (PPE) of NASA’s Gateway lunar space station. The PPE will provide the Gateway with the power and propulsion it needs to maintain its orbit around the Moon and to support missions to the lunar surface.

The qualification testing of the AEPS thrusters is expected to last for several years. During this time, the thrusters will be subjected to a variety of tests to ensure that they can meet the demands of space travel.

Once the AEPS thrusters are qualified, they will be integrated into the PPE and launched to the Moon. The Gateway is expected to be operational in 2025, and it will play a vital role in NASA’s Artemis program to land the first woman and first person of color on the Moon.

Challenges and Testing Facilities

As ion propulsion continues to evolve, several challenges must be addressed. These include reliability, robustness, electromagnetic compatibility, radiation hardness, and energy efficiency. Furthermore, the speed of development and adaptation of propulsion systems to specific mission conditions must increase to remain competitive. This requires suitable test facilities with standardized measurement procedures and validated computer-aided modeling.

Conclusion

In conclusion, ion propulsion represents a transformative technology in space exploration and satellite technology. Its efficiency, versatility, and fuel economy make it a compelling choice for a wide range of missions.

Its efficiency, precision, and versatility make it a promising choice for future missions to Mars and Mercury, the development of military satellites, the creation of space planes, and even the potential use in ASAT systems. As we continue to push the boundaries of space exploration and satellite technology, ion propulsion will undoubtedly play a pivotal role in reshaping our understanding of the cosmos and how we interact with it. The future of space travel and satellite technology is electric, and ion propulsion is leading the way.