Satellites utilize different propulsion systems to maneuver in space after launch. The three main categories of in-space propulsion are chemical, electric, and nuclear thermal propulsion. There are also some more speculative technologies that may become viable options in the future. This article provides an overview and comparison of these different propulsion methods in the context of their use on satellites specifically.
Chemical Propulsion
Chemical propulsion has been the most commonly used technology for satellite maneuvering over the past several decades. Chemical systems work by combusting a fuel and oxidizer to produce hot gases that are expanded through a nozzle to generate thrust.
There are monopropellant and bipropellant variants. Monopropellant systems use a single propellant, typically hydrazine, which is decomposed over a catalyst. These systems are simple, lightweight, and reliable but provide relatively low thrust and specific impulse. Major applications include reaction control systems for attitude control.
Bipropellant systems use separate fuel and oxidizer propellants that combust upon contact. Common propellant combinations include MMH/NTO and hydrazine/NTO. Bipropellants can achieve higher specific impulses than monopropellants, allowing more total impulse for a given propellant tank size. These systems are more complex however. Bipropellants have been used for primary propulsion on many satellites to perform orbit raising, station keeping, and end-of-life deorbit burns.
In general, advantages of chemical propulsion include technical maturity, high thrust, and simplicity. Limitations center around their low specific impulse which requires carrying considerable propellant mass. This reduces payload capacity for the launch vehicle.
Electric Propulsion
Electric propulsion (EP) utilizes electrical energy to accelerate propellant to high exhaust velocities. This results in much higher specific impulses than chemical systems, so less propellant is needed for a given mission. EP only produces low levels of thrust however. There are several types of electric thrusters used on satellites:
- Gridded ion engines accelerate and eject charged ions using electrostatic grids. Performance exceeds chemical systems but grid erosion can limit lifetime.
- Hall effect thrusters also accelerate ions to high exhaust velocities using a magnetic field and electric potential between anodes and cathodes. No grids are required.
- Pulsed plasma thrusters utilize electromagnetic forces to accelerate solid propellant. They tend to have lower specific impulse than other electric options.
Electric propulsion provides major propellant and launch mass savings benefits. EP systems have very high power requirements however, necessitating large solar arrays. They also cannot provide enough thrust for launching payloads from Earth, so chemical stages are still required.
Common electric propulsion applications on satellites include station keeping, orbit raising, inclination changes, and attitude control. As satellite electrical power systems continue advancing, electric propulsion usage is expected to expand.
Nuclear Thermal Propulsion
Nuclear thermal propulsion (NTP) utilizes a fission reactor to heat hydrogen propellant which is then expanded through a nozzle to produce thrust. In contrast with electric systems, NTP provides both high specific impulse along with substantial thrust levels similar to chemical engines.
NASA examined NTP technology under the NERVA program in the 1960s but development was discontinued due to budget cuts and shifting priorities. There has been renewed government and industry interest in NTP for future crewed Mars missions over the past decade.
NTP remains a relatively immature technology, with additional work needed to retire development risks and qualify flight systems. Safety reviews would also be required given the nuclear reactor. If realized however, NTP’s combination of high thrust and Isp could benefit satellites needing to perform large orbital maneuvers. The high power could also enable high-bandwidth communications.
Advanced Concepts
There are a range of more speculative propulsion methods at earlier stages of conceptual development and testing. Examples include solar and laser sail systems using radiation pressure for propulsion. Electromagnetic tethers also offer intriguing possibilities.
These alternatives tend to have very limited thrust capacity unsuitable for most satellite maneuvering applications. But they merit further research given their potential to provide propellantless propulsion, opening up new mission capabilities.
Current Status
Chemical propulsion remains the standard, go-to technology for the majority of satellite propulsion needs given its technical maturity and ability to provide substantial thrust. Electric propulsion delivers major propellant savings benefits thanks to its very high Isp and has carved out a niche for satellites with sufficient electrical power budgets.
Nuclear thermal propulsion offers notable performance gains but still needs to complete development and flight qualification efforts. More advanced concepts are still early stage and not ready to displace the other options. An ideal solution may be developing hybrid chemical-electric systems tailored to specific mission requirements.
Summary
Satellites rely on capable, reliable propulsion systems to maneuver on orbit and complete their missions. Chemical propulsion is a proven workhorse technology that generates substantial thrust using bipropellant or monopropellant engines. Electric propulsion greatly reduces propellant needs but is power constrained and provides low thrust levels. Nuclear thermal propulsion can offer the best of both worlds with high Isp and thrust, but technical challenges remain. Continued investments into these propulsion technologies is warranted to bolster satellite capabilities and enable more ambitious space missions.
