Space Propulsion Technologies: From Chemical Rockets to Electric Thrusters

Propulsion is the fundamental technology enabling space exploration. The choice of propulsion system determines what missions are possible, how long they take, and how much payload can be delivered. This guide examines the propulsion technologies in use today and the advanced systems under development.

Chemical Propulsion

Chemical rockets remain the workhorses of spaceflight. They produce thrust by burning propellants and expelling the hot exhaust gases. High thrust makes chemical rockets essential for launch and other maneuvers requiring rapid acceleration.

Liquid Propellants

Liquid rocket engines use separately stored fuel and oxidizer, pumped into a combustion chamber. Common propellant combinations include:

• RP-1/LOX: Kerosene and liquid oxygen. Used by SpaceX Merlin, Rocket Lab Rutherford, and many first stages. Good performance with dense, storable fuel.
• LH2/LOX: Liquid hydrogen and oxygen. Highest efficiency (specific impulse) but low density requires large tanks. Used by upper stages like Centaur and the Space Shuttle main engines.
• Methane/LOX: Emerging as the propellant of choice for new vehicles. SpaceX Raptor, Blue Origin BE-4, and Relativity Aeon use methane. Good performance, reusability-friendly, and potentially producible on Mars.

Solid Propellants

Solid rocket motors combine fuel and oxidizer in a single solid grain. They're simpler and storable for long periods, but cannot be throttled or shut down once ignited. Used for strap-on boosters (Space Shuttle SRBs, Ariane 6 boosters) and missile systems.

Key Engine Manufacturers

SpaceX develops Merlin (kerosene) and Raptor (methane) engines. Blue Origin builds the BE-4 methane engine. Aerojet Rocketdyne manufactures the RS-25 (former SSME) and RL10 engines. ArianeGroup produces Vulcain and Vinci engines for Ariane rockets.

Electric Propulsion

Electric propulsion uses electrical energy to accelerate propellant, achieving far higher exhaust velocities than chemical rockets. This efficiency comes at the cost of low thrust—electric engines provide tiny forces sustained over long periods.

Hall Effect Thrusters

Hall thrusters use magnetic fields to trap electrons, creating an electric field that accelerates ionized propellant (typically xenon or krypton). Widely used for satellite station-keeping and increasingly for orbit raising. Specific impulse around 1,500-3,000 seconds (vs. ~450s for the best chemical engines).

SpaceX uses Hall thrusters on Starlink satellites. Major suppliers include Busek, Accion Systems, Exotrail, and Orbion Space Technology.

Ion Engines

Ion engines (gridded ion thrusters) use electrostatic grids to accelerate ions. Even higher specific impulse than Hall thrusters (2,000-10,000 seconds) but lower thrust. NASA's Dawn mission used ion propulsion to visit two asteroids.

Electrospray Thrusters

Electrospray systems extract and accelerate ions from ionic liquid propellants, offering very small form factors suitable for CubeSats. Accion Systems and Enpulsion are leading developers.

Pulsed Plasma Thrusters

PPTs create plasma pulses from solid propellant, offering simple, compact propulsion for small satellites. Limited efficiency but mechanically simple with no liquid propellant handling.

Green Propellants

Traditional satellite propulsion often uses hydrazine, a toxic propellant requiring extensive safety measures. "Green" propellants offer similar performance with lower toxicity:

• AF-M315E/LMP-103S: High-performance monopropellants with reduced toxicity
• Hydrogen peroxide: Non-toxic oxidizer, lower performance than hydrazine
• Water: Steam propulsion systems for very small satellites

Benchmark Space Systems and Bradford Space develop green propulsion systems for commercial satellites.

Nuclear Propulsion

Nuclear Thermal

Nuclear thermal rockets use a reactor to heat propellant (typically hydrogen), achieving approximately twice the specific impulse of chemical rockets. NASA tested nuclear thermal engines in the 1960s-70s (NERVA program) and has renewed development for Mars missions.

BWXT is developing nuclear thermal propulsion for NASA's DRACO program. Nuclear thermal could cut Mars transit times significantly compared to chemical propulsion.

Nuclear Electric

Nuclear reactors generating electricity for ion engines could enable missions requiring both high power and high efficiency. Kilopower-class reactors developed by NASA could power electric propulsion for outer planet exploration.

Advanced Concepts

Solar Sails

Solar sails use radiation pressure from sunlight for propulsion—no propellant required. While thrust is tiny, continuous acceleration over months can achieve high velocities. LightSail 2 demonstrated controlled solar sailing in Earth orbit.

Fusion Propulsion

Fusion rockets could provide both high thrust and high specific impulse, potentially enabling rapid interplanetary travel. No practical fusion propulsion exists yet, but companies like Helicity Space are developing concepts.

Beamed Power

Ground-based lasers or microwaves could power spacecraft remotely, eliminating the need to carry energy sources. Useful for keeping satellites aloft or accelerating interstellar probes. Breakthrough Starshot envisions laser-propelled probes reaching nearby stars.

Market Trends

The propulsion market is evolving rapidly:

• Electric propulsion adoption: Most new commercial satellites use electric propulsion for station-keeping, with many using all-electric orbit raising
• Methane engines: New launch vehicles almost universally choose methane over kerosene or hydrogen
• Small satellite propulsion: Growing demand for compact, affordable propulsion for CubeSats and smallsats
• Reusability requirements: Propulsion systems designed for multiple flights, rapid turnaround

Conclusion

Propulsion technology continues to advance across all fronts. Chemical rockets remain essential for launch, while electric propulsion dominates in-space applications. Advanced concepts from nuclear thermal to fusion may eventually enable missions impossible with today's technology. The propulsion choices of today determine which destinations we can reach tomorrow.