A rocket fuel that is in a solid state. It is mixed with an oxidizer to create a solid propellant like a plastic cake. Solid-fuel rockets are more reliable because they have fewer working parts than liquid-propellant rockets. However, they can not be turned on and off whenever desired. The fuel, known as the grain, is shaped around the inside of the rocket.

The space shuttle uses solid rocket boosters (SRBs) during its launch, the largest solid-fuel motors ever flown. Their aluminium fuel and ammonium perchlorate oxidizer are bound together by a polymer. Each SRB weighs approximately 57 tonnes on the launch pad, with 85% of that being the solid fuel.