Space propulsion encompasses all technologies that accelerate a spacecraft by expelling mass (reaction principle). The central trade-off is between specific impulse (Isp, a measure of propellant efficiency, units: seconds) and thrust level, governed by the rocket equation: higher Isp means less propellant mass required for a given delta-v but is typically accompanied by lower thrust.
Chemical propulsion — combustion of propellant + oxidiser; highest thrust, modest Isp (250–460 s). Liquid bipropellants (e.g. LOX/LH2, Isp ~450 s) dominate launch vehicles. Monopropellants (hydrazine, Isp ~220 s; green alternatives 240–260 s) are the workhorse for small-sat attitude control.
Electric propulsion — converts electrical power to kinetic energy of ions. See Electric Propulsion (Hall, ion) and In-Space Propulsion (transfer stages, OTVs) for Hall-effect thrusters (Isp 1,000–3,000 s), gridded ion engines (Isp 2,000–10,000 s), and electrospray thrusters. Very high Isp but very low thrust; suited to station-keeping, orbit-raising over months, and deep-space cruise.
Nuclear — Nuclear Thermal Propulsion (NTP) heats propellant via a nuclear reactor (Isp ~800–900 s); nuclear electric propulsion (NEP) uses reactor power for electric thrusters. Both are mission-enabling for Mars cargo and deep-space missions but face regulatory and political friction on launch licensing.
Investment relevance: the small-sat boom has created demand for compact, low-cost propulsion at cubesat scale (1–12U). Green propellant systems and miniaturised Hall thrusters are the high-velocity segments. Launch propulsion (reusable rockets) is heavily consolidated; the fundable space is in-space and last-mile manoeuvring.
Frontier
- Can green monopropellants fully displace hydrazine in LEO small-sat propulsion?
- What is the practical Isp ceiling for Hall-effect thrusters at sub-100W power levels?
- When does nuclear electric propulsion become mission-enabling for outer-planet science missions?