Solid Fuel Rockets
Authors: Bennet Robert R. , Berkley George A. , Cook Douglass B. , Allen Brian , Graham Robert P. , Neunzert Arleigh P. , Lyman Ronald W. , Prescott Billy H. , Paul Pulver C. , Wassom Steven R. , Whitehead Nicholas J. , Kliger Joseph J. , Sauvageau Donald R. , Crippen Robert L. 
Published: October 2003

A solid fuel rocket is distinguished from a liquid fuel rocket by the type of fuel that it uses. It is more accurate to refer to the two basic types of rockets as solid propellant and liquid propellant rockets. Both types of rockets generate thrust by exhausting combustion products through a supersonic nozzle. Rocket propellants contain both a fuel and an oxidizer to produce energy. Both carry their own oxygen, so they can operate above the atmosphere and can be used in space.

A solid fuel rocket, commonly called a solid rocket motor, is a completely self‐contained device that converts chemical energy into kinetic energy in a controlled way. Although there is much to know about the science, engineering, and manufacture of solid rockets, they are simple devices. Solid rockets consist of four main components: (1) propellant grain, (2) a case that is the thrust chamber that contains the pressurized combustion gases, (3) a nozzle for directing and accelerating the gases away from the motor, and (4) an igniter.

The propellant grain consists of the propellant charge shaped to deliver the desired thrust profile. The case contains the pressure of propellant combustion and is frequently a major portion of the vehicle airframe. Insulation is necessary to protect the case from the high‐temperature combustion products. The igniter provides the heat necessary to initiate combustion at the propellant surface. The nozzle directs and accelerates the propellant exhaust gases.

The propellant is typically a solid rubberlike material, similar to a pencil eraser, that contains fuel and oxidizer particles. The propellant grain can have a wide variety of shapes, but it is generally a hollow cylinder designed to burn from the inside out, thereby not exposing the case to the extreme temperatures until near the end of the burn. The propellant is bonded to the case.

The rate at which the propellant burns and thereby generates thrust is designed into each propellant formulation. For instance, propellants that have fine oxidizer particles burn at higher rates than formulations that contain coarse oxidizer particles. Burn rate also varies with combustion chamber pressure, initial temperature of the propellant grain, and other factors.

The internal shape of the grain, which can vary the amount of exposed surface area, hence the burn rate, is typically established when it is cast (poured) and cured inside the case. An igniter, generally located in the head end and fires down the bore or grain center perforation, initiates the motor.

Hot combustion gases are ducted from the motor through a supersonic nozzle, which accelerates the gas and converts the pressure and temperature of propellant combustion to kinetic energy. Directional control is attained through a number of different means, but commonly the nozzle is jointed, allowing it to be vectored by mechanical actuators.