A propellant is a substance that produces thrust to propel an object, typically in the context of rockets or engines. It can be in the form of a gas, liquid, or solid, and is often used in rocketry and aerospace applications. When ignited or expelled, propellants create a high-pressure gas that pushes against the walls of a combustion chamber, generating force that moves the vehicle forward. There are two main types of propellants: liquid propellants, which are stored in separate tanks and mixed before combustion, and solid propellants, which are pre-mixed and ignited as a single unit. Each type has its advantages and disadvantages, influencing their use in various space missions and military applications.