What is chemical propulsion in spacecraft?
Chemical propulsion generates thrust by combusting or decomposing chemical propellants, releasing energy stored in molecular bonds to produce high-velocity exhaust gas. Unlike electric propulsion which uses external electrical energy, chemical propulsion is energy self-contained — the propellant carries its own energy source. This makes chemical propulsion capable of producing high thrust (kilonewtons) in a compact package, at the cost of lower specific impulse (200–450 seconds) compared to electric alternatives.
Types used in satellites
Bipropellant systems: Burn two reactants simultaneously — typically monomethyl hydrazine (MMH) as fuel and nitrogen tetroxide (NTO) as oxidiser. Isp of approximately 320 seconds. Used on GEO satellites for apogee engine burns (orbit insertion) and station-keeping in hybrid propulsion systems. Monopropellant hydrazine: Decomposes catalytically over a heated catalyst bed (Shell 405 catalyst) to produce hot nitrogen, hydrogen, and ammonia. Isp of approximately 220 seconds. Used for attitude control thrusters and station-keeping on smaller satellites. Cold gas thrusters: Expel compressed nitrogen or helium — extremely simple and reliable but very low Isp (50–70 seconds); used only for fine attitude control. Green propellants: Emerging alternatives to toxic hydrazine — AF-M315E (ammonium dinitramide based, ECAPS HPGP thrusters), LMP-103S — offer Isp of 250–260 seconds with lower toxicity and handling costs.
Role in modern satellite architecture
Most modern GEO satellites use a hybrid chemical+electric architecture: chemical bipropellant for the apogee engine (inserting from GTO to GEO in 1–2 days) and electric thrusters for station-keeping over the 15-year operational life. Fully-electric GEO satellites eliminate the chemical apogee engine, saving 40–50% of launch mass at the cost of a 6–12 month all-electric orbit-raising sequence.