Once a satellite has separated from its rocket, its own propulsion system takes over. It raises or lowers the orbit, keeps the satellite in position and, at the end of the mission, removes it from its orbit. The two main families are chemical and electric propulsion, and the choice between them affects mass, mission timeline and cost.
Chemical propulsion
Chemical thrusters release energy stored in propellants. Monopropellant systems, often using hydrazine, are simple and reliable. Bipropellant systems mix a fuel and an oxidiser and deliver more performance. Chemical propulsion provides high thrust, which means manoeuvres are completed quickly, but it needs a lot of propellant mass for large velocity changes.
Electric propulsion
Electric thrusters, such as gridded ion engines and Hall effect thrusters, use electrical power from the solar arrays to accelerate ionised propellant, often xenon or krypton, to very high exhaust velocities. They are extremely efficient in their use of propellant, but their thrust is very low. Manoeuvres therefore take weeks or months instead of minutes. NASA has researched ion propulsion since the 1960s, and it is now common on commercial and scientific spacecraft.
Comparing the two
- Efficiency: electric propulsion needs far less propellant for the same velocity change.
- Speed: chemical propulsion completes manoeuvres much faster, which matters for orbit raising and collision avoidance.
- Power: electric propulsion requires large solar arrays and power processing units.
- Complexity and cost: simple chemical systems are well proven, while electric systems need more electronics but save launch mass.
Why this matters for the launch choice
Propulsion and launch are linked. A satellite with electric propulsion can accept a lower drop off orbit and raise itself, which can open up cheaper rideshare options, at the cost of a longer commissioning period. A satellite without propulsion depends fully on the orbit the rocket provides and must still meet deorbit rules, which increasingly favours carrying at least a small thruster.
Related reading
- Delta-v Explained: The Currency of Spaceflight
- Geostationary Transfer Orbit (GTO) Explained: How Satellites Reach GEO
- Satellite End of Life: Deorbiting, Graveyard Orbits and Passivation
Sources
- NASA Small Spacecraft Systems Virtual Institute: State of the Art of Small Spacecraft Technology
- NASA: Basics of Space Flight
Cover image: Electron bombardment ion thruster at NASA's Lewis Research Center, 1970. Credit: NASA (GRC-1970-C-02492), public domain.