bipropellants

Bipropellants are rocket propulsion systems that use two separate chemical propellants—a fuel and an oxidizer—that are stored independently and combusted together in a dedicated engine chamber to produce thrust. They are a subset of liquid propulsion and are distinct from monopropellants, which decompose or react in a single substance, and from solid propellants.

Common bipropellant combinations include storable hypergolic propellants, such as fuels based on hydrazine derivatives (for example, monomethylhydrazine or unsymmetrical dimethylhydrazine) paired with nitrogen tetroxide or inhibited red fuming nitric acid. These mixtures ignite on contact and allow reliable restart, but are highly toxic and corrosive. Cryogenic bipropellants use liquid oxygen with a light fuel such as liquid hydrogen, or with kerosene (RP-1); LOX/LH2 offers high specific impulse, while LOX/RP-1 provides good performance with simpler storage requirements.

In operation, each propellant is stored in its own tank and fed to the engine through separate lines and pumps. The mixture ratio is carefully controlled to optimize efficiency, thrust, and stability. Many bipropellant engines support throttle and restart, features valuable for upper stages and spacecraft maneuvering.

Advantages of bipropellants include relatively high performance and the ability to restart and throttle, along with the option to store propellants for extended periods (in the case of hypergolic and some cryogenic systems). Disadvantages include mechanical complexity, handling hazards due to toxicity or cryogenic risks, and the need for reliable ignition systems to ensure safe operation.

Bipropellants are widely used in launch vehicle upper stages and spacecraft propulsion, including maneuvering thrusters and orbital transfer vehicles, where controllability and restart capability are essential.