Solid propellant engines are a type of rocket engine that use a solid mixture of fuel and oxidizer to produce thrust. Unlike liquid engines, which store fuel and oxidizer separately, solid propellant is pre-loaded into the engine and ignited to create combustion. This design makes solid propellant engines simpler and more reliable, often used in military missiles and space launch vehicles. The combustion process in solid propellant engines generates hot gases that expand and are expelled through a nozzle, creating thrust. These engines are known for their high thrust-to-weight ratio and are commonly found in systems like the Space Shuttle boosters and NASA missions.