Rocket propulsion systems are devices that generate thrust to propel rockets into space. They work by expelling gas at high speed in the opposite direction, following Newton's third law of motion: for every action, there is an equal and opposite reaction. These systems can be powered by various fuels, including liquid or solid propellants, which burn to produce hot gases. There are two main types of rocket propulsion: liquid propulsion and solid propulsion. Liquid propulsion systems use a combination of liquid fuel and oxidizer, allowing for adjustable thrust. In contrast, solid propulsion systems contain a pre-loaded mixture of fuel and oxidizer, providing a simpler design but less control over thrust once ignited.