Propellantfree

Propellantfree propulsion refers to propulsion methods that generate thrust without carrying propellant for reaction mass. These approaches rely on exchanging momentum with external agents, such as photons, charged particles in space plasmas, or external energy beams, rather than expelling onboard fuel. The term is commonly used in discussions of space propulsion, early satellite technology, and theoretical concepts seeking to reduce on-board propellant mass.

Common mechanisms include photon propulsion, where thrust is produced by radiation pressure from sunlight or directed energy. Solar sails use large, lightweight reflective surfaces to push a spacecraft from solar photons, while laser or microwave beaming systems can impart greater momentum by delivering energy from Earth or another platform to a sail or tethered spacecraft. Electric and magnetic sail concepts use interactions with the solar wind or planetary magnetospheres: an electric sail (E-sail) employs long charged tethers to repel solar wind protons, and a magnetic sail uses magnetic fields to deflect charged particles. Beamed-energy propulsion encompasses systems that provide energy from an external source to enable thrust, potentially using sails, drag devices, or other interaction mechanisms, without propellant on board. Electrodynamic tethers, which generate thrust through interaction with a planet’s magnetic field, are another example of propellantless propulsion under certain operating conditions.

Advantages of propellantfree methods include reduced launch mass and the potential for long-duration, high-velocity capabilities. Challenges involve very low thrust, efficiency limits, dependence on external energy sources or environments, material and deployment complexities, and precise attitude control requirements. While practical demonstrations exist for solar sails and related concepts, many propellantfree technologies remain at the research or demonstration stage and are the subject of ongoing aerospace study.