Compare monopropellant and bipropellant propulsion systems for spacecraft.

Monopropellant systems: Single propellant decomposed catalytically (hydrazine over iridium catalyst), Isp ~220-230 s, Simpler system (one tank, one feedline), Used for attitude control and small maneuvers, Lower performance but higher reliability. Bipropellant systems: Fuel and oxidizer (MMH/NTO typical), Isp ~310-320 s, More complex (two tanks, two feedlines, more valves), Used for main propulsion, orbit insertion, Higher performance, more propellant efficient. Selection factors: Delta-v requirement (high favors bipropellant efficiency), Simplicity needs (monoprop for attitude control), Heritage and reliability requirements, and Mass and volume constraints. Many spacecraft use both: bipropellant main engine for large maneuvers, monopropellant thrusters for attitude control. Green propellants (LMP-103S, AF-M315E) offer improved safety with similar performance.