What defines a "hybrid rocket"?

A hybrid rocket is specifically defined as a propulsion system that utilizes a combination of both solid and liquid propellants. In this type of rocket, the oxidizer is typically in a liquid state, while the fuel is in a solid form. This configuration takes advantage of the benefits offered by each type of propellant: the ease of handling and storage of solid fuels, along with the controllability and performance characteristics of liquid oxidizers.

This setup allows for flexibility in thrust modulation, as the flow of the liquid oxidizer can often be adjusted, improving control over the rocket's performance during flight. Consequently, hybrid rockets represent a unique approach to rocket design, blending elements that can enhance efficiency and operational capabilities.

Other options refer to types of rockets that do not fit this definition—for instance, a rocket using only solid propellant or relying solely on gas or compressed air does not incorporate both solid and liquid components in its propulsion system.