Monopropellants

Monopropellants are rocket propellants that consist of a single chemical species capable of producing thrust without an external oxidizer. In a monopropellant thruster, the propellant is introduced into a catalyst bed or heated section where it decomposes or oxidizes exothermically, generating hot gases that expand through a nozzle to provide propulsion. Thrust and operating conditions are controlled by feed rate, catalyst activity, and temperature; some designs allow restart and throttling by regulating the decomposition rate.

Common monopropellants include hydrogen peroxide at high concentrations (high-test peroxide, HTP), which decomposes catalytically to steam and oxygen. Hydrazine and certain hydrazine derivatives have also been used as monopropellants in catalyst-driven thrusters, though these compounds are more widely associated with bipropellant systems when paired with oxidizers. The exact behavior of a monopropellant—its decomposition products, heat release, and stability—depends on chemical composition, concentration, catalyst, and operating temperature.

Applications are most prominent in spacecraft propulsion for reaction control and attitude control systems, where simplicity, restart capability, and reduced plumbing outweigh the lower performance relative to bipropellants. Safety and handling concerns are major considerations: many monopropellants are highly toxic or corrosive, with hydrogen peroxide capable of severe causticity and reactive hazards, and catalysts susceptible to poisoning. Storage and contamination control are essential in systems employing monopropellants.