solid fuel rocket motors have the fuel (called propellant) as a solid mixture of an oxidizer and fuel. an oxidizer supports the burning of fuel much like oxygen supports burning. the common oxidizer is powdered ammonium perchlorate, while the common fuel is powdered aluminum metal. the two powders are stuck together with a third component known as the binder. the binder is a rubbery solid that also burns as a fuel. the simple idea makes solid rocket engines cheaper, but they cannot be turned off or controlled, and are more likely to explode than liquid rocket engines. solid rockets also provide a smaller specific impulse, hence must be heavier to launch the same payload.