The proliferation of Rendezvous, Proximity Operations, and Docking (RPOD) and on-orbit servicing (OOS) missions have created a growing need for sophisticated propulsion systems on satellites.

According to Annelies Powell, Special Project Coordinator at Dawn Aerospace, “Satellites need propulsion systems primarily for orbital maneuvering, station-keeping, collision avoidance, and de-orbiting at the end of their lifecycle. Propulsion extends mission flexibility and lifetime, enabling satellites to maintain or change orbits as needed.”

Propulsion systems are critical for these missions, especially when satellites are subject to outside forces that can alter their placement in orbit and interfere with the precise conditions that are required for mission success. “Satellites in orbit can be perturbed by conditions from the Earth's gravity environment, the moon, solar radiation, pressure and magnetic fields,” explained Dr. Daniel Pérez Grande, CEO and Co-founder of IENAI SPACE. “If a satellite didn't have a propulsion system, it would be subjected to all these forces and remain in whichever orbit the launcher deployed them at and then be affected by these different perturbation sources.”

So, what are the different types of propulsion systems that are currently being leveraged for these RPOD and OOS missions? What are the limitations and challenges they face? And will the continued sophistication and evolution of satellite missions require propulsion innovation in the future?

Types of Propulsion Systems

Depending on the mission, satellites will use different propulsion systems that are tailored to the use case need. Due to the higher thrusts they produce, chemical propulsion systems are used for missions that require a greater amount of energy for maneuverability, while cold gas propulsion is leveraged for more precise maneuvers. “Chemical propulsion is typically used for launches and large maneuvers, where fuel and oxidizer are burned to create high-thrust exhaust gases,” explained Bohdan Yurkov, COO of SETS. “Cold gas propulsion is simpler and uses compressed gas, like nitrogen, to generate low thrust, mainly for small adjustments or altitude control.”

When missions require a constant level of propulsion, electric systems are best suited for such propulsion use cases. “Electric propulsion systems provide continuous, low-thrust propulsion, making them ideal for long-duration missions with efficient fuel usage,” said Powell. “They are often chosen for station-keeping and gradual orbit adjustments. Examples include ion and Hall-effect thrusters.”

Currently, electric propulsion is the most prevalent type of system being leveraged on satellites. “Electric propulsion is the most commonly used propulsion system today,” said Yurkov. “Because of its efficiency, we are seeing increasing demand from satellite developers.”

Though electric propulsion is the most common, chemical propulsion systems are the most ideal option for RPOD mission use cases, according to Grande. “For RPOD, we’ve been using chemical propulsion systems,” he explained. “There are, however, a number of companies over the last few years that have attempted to use electric propulsion systems for these kinds of operations, but that is a little bit harder to do. You need to get the satellites quite close to each other, and then you need to ensure that whatever perturbations are affecting your orbits are not bigger than the amount of thrust that you're able to generate with your propulsion system.

Limiting Factors and Challenges

Several challenges face satellite propulsion systems that industry must overcome as the demand for space operations continues to grow.

“Power consumption, dimensions and mass remain key issues,” said Yurkov. “In particular, chemical propulsion requires large amounts of fuel for long missions. Meanwhile, electric propulsion is much more efficient, but it relies on solar power, which is limited.”

Thrust is another challenge that creates a conundrum for satellite propulsion. “Chemical propulsion provides high thrust but is heavy, whole electric propulsion is efficient but generates low thrust,” Yurkov explained.

Grande echoed that thrust is a particular challenge between different propulsion systems and the mission sets they serve. “The minimum amount of thrust that you can generate can also be challenging, particularly for very close operations,” said Grande. “You need very little thrust to make sure that that you're able to dock that spacecraft with relative velocities that are not dangerous.”

Grande does see a path forward in finding a balance between what chemical and electric propulsion systems can offer satellite missions. “Potentially, I think finding a compromise between chemical and electric [can solve this challenge],” he said. “Having chemical propulsion systems for the rough proximity operations and then electric for the very close proximity operations could work out well, in order to bridge the gap between these two types of propulsion systems.”

To Powell, efficiency, endurance and cost pose challenges to propulsion development. “Many systems struggle to balance high performance with fuel efficiency, and increased efficiency in propulsion will reduce dependency on large fuel reserves, enabling smaller, lighter satellites,” explained Powell. “As satellite constellations grow, scalable and cost-effective propulsion solutions are essential to keep pace with deployment demands.”

But according to Yurkov, industry is hard at work on overcoming the limiting factors for chemical and electric propulsion. “In the last few decades, advancements in solar panel technology and battery storage have significantly improved power availability, making electric propulsion more viable,” said Yurkov. “Furthermore, one of the key issues in electric propulsion development is the cost of propellant (such as xenon). Therefore, currently, active work is underway to develop and test propulsion using alternative and cheaper propellants, such as krypton, argon, or even air.”

The Future of Satellite Propulsion

As industry continues to make advancements in current propulsion systems for RPOD and OOS missions, it is also looking forward to the long-term future of how propulsion systems will evolve to meet new mission requirements.

For OOS missions, propulsion innovation will be centered on orbital mobility. “The OOS missions are mostly considering new propulsion for enhancing their orbital mobility,” explained Xavier Roser, Product Line Manager for Exploration, Science and On-Orbit Servicing at Thales Alenia Space. “Depending on the type of services provided, different main propulsion strategies that are considered include electrical propulsion for main transfer, chemical propulsion for control and main transfer, and refueling capabilities.”

According to Powell, future RPOD missions will rely on propulsion systems that can be adaptable. “RPOD missions are driving the innovation of propulsion systems that are not only reliable but also highly adaptable,” said Powell. “For instance, in-orbit servicing and repair require precise, controllable propulsion systems that can operate safely in close-proximity environments.”

To meet these mission needs, Powell cites future developments and advancements in autonomous propulsion control, where AI-driven systems optimize maneuvers in real-time. Powell also echoed Grande’s outlook of hybrid propulsion systems, which will combine the benefits of electric and chemical propulsion.

For Yurkov, he sees a future in the development of a versatile electric propulsion system with the ability to provide a wide range of thrust. “This would allow satellites to adjust thrust levels, providing required parameters for precision tasks and higher thrust for quick maneuvers while maintaining high efficiency,” he said. “This will make it possible to handle more complex missions like satellite servicing and debris removal, without relying on heavy chemical propulsion.”

Grande points to smaller propulsion systems as part of the future of satellite maneuverability. “Miniaturization of propulsion systems is going to be something that we can use for proximity operations,” he said. “If you have a very small propulsion system, you can distribute that along the spacecraft in order to have more control over the position of your spacecraft - instead of having maybe one or two propulsion systems. If you have distributed propulsion systems, that gives you an additional control layer. I would definitely see an advantage to very small propulsion systems for these kinds of maneuvers or missions in the future.”

Satellite Network Requirements for Propulsion Systems

To leverage existing and future propulsion systems on satellites during RPOD and OOS missions, there are certain enabling satellite network capabilities that are required.

According to Powell, communication and control networks will be key for propulsion systems to enable mission success. “Robust communication systems are necessary to maintain control and adjust propulsion parameters remotely, especially for constellations and autonomous RPOD missions,” Powell said.

Positioning and navigation will also be critical to these propulsion systems. “For RPOD tasks, precise positioning and navigation systems are required, along with complex algorithms for performing docking procedures,” explained Yurkov.

Yurkov also echoed Powell’s point that communication and control will be paramount to mission success. “The spacecraft’s flight control system, based on data from onboard equipment and the RPOD phase, makes decisions regarding the need to activate the propulsion system and sends the corresponding command to it,” he said. “The role of the propulsion system is to execute this command and activate the thruster for the specified duration. Therefore, for the propulsion system itself to function, the spacecraft really only needs reliable power and data communication lines - for communication with the flight control system and spacecraft telemetry.”

For Grande, GPS and optical algorithms will also be critical during maneuver missions. “[For maneuvering] you need to know where the target satellite and the maneuvering satellite are,” he said. “GPS is a good option for force proximity operations of satellites. When you need to be more precise, you need to move into optical algorithms and leverage AI vision tracking to actually see the spacecraft in front of you or around you.”

Explore More:

Connectivity, TT&C and the Growing Space Servicing Needs

On Orbit Transport – Opening a New Space Economy

PODCAST: Morpheus Exec on Mobility, Propulsion and the Future of Smallsat

Falling Debris and Near-Miss Collisions: Mitigating Risk in LEO