Rocket propellants are substances used to produce thrust in rockets. They work by burning fuel and creating hot gases that are expelled at high speed from the rocket's engine. There are two main types of propellants: liquid and solid. Liquid propellants, like liquid oxygen and kerosene, can be mixed and controlled during combustion, while solid propellants are pre-loaded and ignite all at once. The choice of propellant affects a rocket's performance and efficiency. For example, NASA often uses liquid propellants for their flexibility, while military missiles may use solid propellants for their simplicity and reliability. Understanding these propellants is crucial for successful space missions.