A propulsion system that produces thrust to push the spacecraft forward into space is essentially the core of space exploration missions. These systems are becoming increasingly crucial in the space industry as corporations and governments put more low Earth orbit (LEO) satellite constellations and commercial space exploration into operation.

According to the BIS Research data, more than 45,000 satellites are anticipated to be launched between 2022 and 2032, with 90% to 92% of them projected to be small satellites operating in LEO.

In earlier space missions, such as the Apollo mission, the bulk of the massive rocket's fuel was used to propel a small spacecraft carrying a crew into orbit. In the propulsion process, a small thruster was used to burst beyond the pull of Earth's gravity and propel the Apollo spacecraft to the moon and back.

Since then, various alternative thruster technologies without the use of heavy fuels have been created by space scientists. For instance, the rockets ionize stable gases such as xenon and krypton by removing the electrons from the gas atoms using power from solar cells to produce plasma or a stream of positively charged ions. This plasma is forced out of the spacecraft's exhaust as it travels through the empty, weightless vacuum. This system is known as plasma propulsion.

Plasma propulsion is a type of electric propulsion system (EPS) that creates thrust from a quasi-neutral plasma. Electric propulsion technology uses electrical power to accelerate a propellant using a variety of electrical and/or magnetic techniques.

Figure 1 Satellite electric propulsion systems features

When compared to conventional chemical thrusters, the electric propulsion (EP) thrusters' propulsive performances are improved by using electrical power. EPS also takes comparatively less energy than chemical propulsion to accelerate a spaceship.

Due to the propellant being expelled up to twenty times quicker than from a conventional chemical thruster, the system as a whole has a significantly higher mass efficiency.

Therefore, the development of high-thrust variants of electric propulsion systems is resulting in their higher uptake for various applications in space missions.

According to the BIS Research report, the global satellite electric propulsion market is estimated to reach $1.3 billion by 2032 from $522.3 million in 2021, growing at a CAGR of 4.10% during the forecast period 2021-2032.

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Currently, hundreds of global positioning systems (GPSs) and military and communications satellites are equipped with such advanced thrusters, also referred to as electric propulsion engines or plasma thrusters, to enable minute course changes and maintain stable orbits.

Today, researchers are attempting to create a variety of new advanced electric propulsion systems for different satellite types and technologies that will eventually enable spacecraft to travel long distances around the solar system. A few of these emerging technologies are discussed further in the article.

Emerging Technological Trends in Electric Propulsion Systems

1. Solar Electric Propulsion System: Solar sail or solar electric propulsion is one of the most popular alternatives for chemical propulsion. The system is powered by the Sun. It uses sunlight to propel vehicles through space by reflecting solar photons from a sizable sail made of a light and highly reflective material.

The system usually requires a surface on the exterior of the satellite or the spacecraft because the combined effect of numerous photons is required to produce a significant momentum transfer.

However, the mass of the sailcraft should be kept to a minimum since, for a given thrust force, acceleration is inversely related to mass. Sunlight rays that happen to hit the solar sail reflect off it at an angle of 0 degrees to the sail's usual direction.

The technology is proving to be highly sustainable and energy efficient. Many budding space technology companies are developing solar electric propulsion systems for their space missions.

For instance, recently, in June 2021, ExoTerra Resource (ExoTerra), a U.S.-based space technology company, received a National Aeronautics and Space Administration (NASA) Phase II Small Business Innovation Research (SBIR) award for producing solar electric propulsion upper stages for microsatellites and micro-landers to reach geostationary Earth orbit (GEO).

2. Electric Iodine Thrusters: Electric propulsion is ideal for space applications since it is particularly effective at producing thrust with little propellant. Xenon is used as a propellant in electric thrusters on spacecraft because it is easily ionized and produces thrust.

In this system, bulky pressurized tanks are needed to store the propellant, as well as a complex network of pipes, valves, and pumps to keep the xenon gas moving through the propulsion system. The propulsion system becomes hefty and expensive as a result. Iodine may be kept in its solid state before it transforms into a gas; hence there is no need for massive, high-pressure gas tanks in the case of iodine thrusters.

3. Field-Emission Electric Propulsion: Ion thrusters of the field-emission electric propulsion (FEEP) variety use liquid metal propellants such as indium. In most cases, they ionize the liquid metal before accelerating ionized particles through the ion emitter with a strong electric field. The charged ions are then neutralized by the neutralizer’s electrons, which stabilize the satellite’s electrical neutrality.

This propulsion technology helps small spacecraft maintain their orbits and manage their altitudes. It also offers incredibly precise thrust maneuvers so that small satellites can precisely focus on any location on the surface of the Earth. Due to its high specific impulse and thrust-to-power ratio, the FEEP technology is widely used.

4. Air-Scooping Electric Propulsion: A spacecraft powered by solar panels and supplemented with electric propulsion that uses the surrounding air in space as a propellant is known as air-scooping electric propulsion (ASEP).

Very low Earth orbit (VLEO) satellites can have their lives extended using ASEP. Regular re-boosts are provided by this propulsion to maintain orbital heights. The goal of the ASEP technology is to preserve lower orbital altitudes while boosting resolution for remote sensing satellites or reducing latency for communication satellites.

Additionally, a reusable space tug might be included in an ASEP spacecraft that stores extra fuel in its fuel tank, eliminating the need for powerful chemical rocket boosters to launch satellites into their final orbit.

5. Lightweight Amplifiers: Power amplifiers are a crucial component for many satellite applications because they facilitate the transmission of high-frequency signals.

Gallium arsenide is the material now utilized in power amplifiers because it offers a lighter alternative to silicon-based circuits. Although complementary metal-oxide-semiconductor (CMOS) is frequently utilized for chip production, designing the chip using alternative materials may be more practical.

As compared to gallium arsenide, silicon has a longer lifespan and is more cost-effective. The development of the market for electric satellite propulsion is facilitated by this technology trend.

Conclusion

The electric propulsion system has garnered significant interest from several key stakeholders in the satellite industry. The technology has been rapidly growing owing to the increasing requirement of launching satellite constellations for communication and Earth observation.

Moving forward, more innovation in existing propulsion systems is expected as the ambition for deep exploration of space is getting stronger.