Things You Should Know About Electric Propulsion Systems in the Spacecraft

The global shift towards decarbonizing energy systems has paved the way for an era of electric mobility. Electrically powered vehicles, including cars, heavy trucks, buses, drones, and trains, are becoming increasingly popular. There are also plans to introduce electric flying taxis in order to revolutionize on-demand and private transportation systems. Sweden and Denmark are leading the way in preparing to replace their domestic fleet of airplanes with fossil-fuel-free versions by 2030, with all-electric 186-seat passenger airplanes expected to enter into service. Smaller, 100-seat Wright Spirit electric airplanes will also be released in 2026.

Electric pump-feed rocket engines have also made their way into launch systems. Rocket Lab, for example, leveraged this technology 5 years ago to deliver a commercial payload of several satellites to orbit. Electric propulsion thrusters are also utilizing many small satellites in space; however, SpaceX's Starlink constellation remains the most outstanding example. Despite these advancements, many space assets - from small probes and satellites to large spacecraft - continue to rely on conventional chemical-based propulsion. 

As a result, in-space mobility systems' electrification needs to catch up to Earth systems. Nevertheless, this may soon change. The electric propulsion system in spacecraft could significantly extend the useful life of billion-dollar missions in outer space.

What Is Electric Propulsion System in Spacecraft?

Electric propulsion is a type of spacecraft propulsion that leverages electrical and magnetic fields to change the spacecraft’s velocity. These propulsion systems typically work by expelling propellant at high speed, using much less propellant compared to chemical rockets due to their higher exhaust speed. However, the thrust generated by electric thrusters is much weaker than that of chemical rockets due to limited electric power, though it can be provided longer.

In all-electric propulsion systems, the source of electric power - whether solar, nuclear radiation batteries, or receivers - is physically separated from the thrust mechanism. Heavy and inefficient power sources have limited this type of propulsion. The thrust generated is typically low, ranging from 0.005 to 1 N. Therefore, to achieve significant increases in velocity, downward thrust and small accelerations must be applied for extended periods (weeks or months).

Ion Thrusters

Ion thrusters are propulsion systems that shoot out a propellant gas at a much higher velocity than a chemical rocket, delivering about 10x more thrust per kilogram of propellant. These engines use charged ions, or atoms, which electric guns accelerate. When the power comes from a spacecraft's solar panels, the technique is called "solar-electric propulsion."

While ion engines may not match the explosive power of a chemical rocket due to the relatively low power output of typical spacecraft solar panels, they make up for this with their endurance. Chemical rockets typically burn out after just a few minutes, whereas an ion engine can provide gentle but steady thrust for months or even years, as long as there is a supply of propellant and the Sun continues to shine.

NASA's Deep Space 1, launched in 1998, was a demonstration spacecraft that flew by a near-Earth asteroid and later intercepted a comet, showcasing the potential of ion thrusters in deep space missions.

In 2017, a new milestone was achieved in ion thruster technology with the development of the X3, which set a new record for maximum thrust at 5.4 newtons. It's a powerful ion thruster that has the potential to propel humans beyond Earth, and recent successful tests suggest that it could be a crucial component of propulsion systems for future Mars missions.

The X3 is a Hall-effect thruster, an ion thruster that uses electric and magnetic fields to accelerate propellant, typically xenon gas. Compared to traditional chemical rockets, Hall-effect thrusters are safer and more fuel efficient. However, they still offer relatively low thrust and acceleration, and further engineering is needed to increase their power.

Xenon

Electric thrusters use xenon gas as the primary propellant, but its rarity and expense pose significant challenges. Xenon constitutes less than 1 part/10 million in Earth's atmosphere and costs around $3,000/1 kilogram. Furthermore, the gas necessitates large, pressurized tanks and complex networks of valves, pipes, and pumps for transportation within a propulsion system.

Researchers have been exploring iodine as a possible alternative to xenon for the past 20 years. Iodine is a more abundant and cheaper element than xenon, and it can be stored in an unpressurized solid form that directly transforms into a gas when heated. This unique property of iodine can enable significant miniaturization and simplification of the propulsion system. Moreover, ground-based tests have demonstrated that electric thrusters utilizing iodine can be more efficient than those using xenon.

However, using iodine as a propellant poses its own set of challenges. For instance, iodine is highly corrosive, which may endanger the electronics and other systems onboard the spacecraft. Additionally, solid iodine can break into pieces during launch vibrations and spacecraft motion, potentially damaging the propulsion system and causing other problems.

But, researchers have made significant progress in addressing these challenges. They have recently achieved a breakthrough by launching an iodine-based electric thruster into space for the first time and demonstrating its ability to propel a spacecraft in orbit.

Advantages of Electric Propulsion in Spacecraft

An electrical spacecraft propulsion system offers several advantages over traditional chemical rocket engines. These include:

• Greater Fuel Efficiency

Electrical propulsion systems can use a much smaller propellant to achieve the same thrust as a chemical rocket. It makes them ideal for long-term missions where fuel efficiency is critical.

• Longer Mission Duration

Since electrical propulsion systems are much more fuel efficient, they can continue to operate longer than chemical rockets, making them ideal for extended missions to far-off destinations in space.

• More Excellent Maneuverability 

Electrical propulsion systems offer precise control over thrust, allowing for greater maneuverability and complex flight paths.

• Reduced Launch Weight

Because electrical propulsion systems use much less propellant than chemical rockets, they can reduce the overall weight of a spacecraft, making launches easier and less expensive.

• Lower Cost

Electrical propulsion systems can be less expensive over the long term than chemical rockets since they require less propellant and can operate longer.

Electric Propulsion in Spacecraft: Applications

Electric Propulsion can be used in the following applications:

• EP can change a spacecraft's orbit more competently and with less propellant compared to chemical propulsion for orbit transfer. However, this process may take longer and require more power.
• For station-keeping, EP can maintain a spacecraft's orbit, such as a satellite or space station, with minimal propellant consumption and a long operational lifetime. However, this process requires continuous power and control.
• For drag compensation, EP can counteract the atmospheric drag that stalls a spacecraft in a low orbit of the Earth with downward thrust and high specific impulse. However, this process requires large solar arrays and may encounter plasma interactions.
• EP can provide high delta-v and a large payload mass fraction for interplanetary exploration. However, this process requires nuclear power and a long transit time.

Conclusion

The demand for electric propulsion systems in spacecraft is expected to grow in the coming years due to their advantages over traditional chemical propulsion systems. The increasing need for satellite launches, especially for communication and remote sensing purposes, is one of the main drivers of demand. The growing interest in space exploration and the need for more efficient and sustainable propulsion systems for long-duration missions also contribute to the market for EP. 

Also, with the growing demand for cost-effective and efficient propulsion systems for space exploration and satellite operations, employment opportunities in the electrical spacecraft propulsion field are expected to increase in the coming years.

If you're interested in delving deeper into electric propulsion, Skill-Lync offers courses to help you develop the skills and knowledge necessary to excel in this growing industry.

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