A Solid Rocket Booster (SRB) is a type of rocket engine that uses solid propellant to produce thrust. Unlike liquid rocket engines, which require fuel and oxidizer to be stored separately, SRBs contain a mixture of fuel and oxidizer in a solid form. This design allows for simpler construction and reliable performance, making them ideal for launching spacecraft like the Space Shuttle. SRBs are typically used during the initial phase of a rocket's flight to provide the necessary lift to escape Earth's gravity. Once their fuel is expended, they are jettisoned and fall back to Earth, where they can be recovered and refurbished for future missions.