Bi propellant is a rocket propellant consisting of two unmixed or non-combined chemicals (fuel and oxidizer) fed separately into the combustion chamber.

Bipropellant engines produce thrust when the two propellant valves open and liquid fuel (typically monomethyl hydrazine, or hydrazine) and liquid oxidizer (nitrogen tetroxide) hypergolically ignite in the chamber (which means they ignite without external aid ) . The resulting hot gas exits the nozzle, creating thrust.

Hypergolically-fueled rocket engines are usually simple and reliable because they need no ignition system.