A chemical rocket is a type of spacecraft propulsion system that uses stored energy in the form of fuel and oxidizer to generate thrust. The term refers specifically to rockets that use chemical reactions for their motive power, as opposed to other methods such as electric or nuclear propulsion. In a typical chemical rocket, fuel and oxidizer are mixed together and combusted in a nozzle, producing hot gas which is expelled out the back of the vehicle at high speed. This generates an equal and opposite force on the rocket itself, causing it to accelerate upwards. Chemical rockets have been used for spaceflight since the 1950s, and remain the most common form of propulsion in use today.