Solid-
Propellant Rocket MotorsThis kind of system is commonly called a rocket motor. All the solid propellant to be burned
is contained in a pressure vessel called a case. The
propellant body, called a grain, contains all the chemical species for complete burning. A nozzle is mounted to the case on one end, and there is a pyrotechnic igniter to start combustion of the propellant. Once ignited, a motor burns smoothly on all its exposed propellant
internal surfaces.By changing the design of the internal exposed grain surfaces, it is possible to vary the amount of propellant exposed and thus the amount of propellant that will burn per unit time. The burning rate of the solid propellantÑusually between 0.3 and 3.3 cm/sec (0.1 and 1.3 in/sec) in a direction perpendicular to the burning surfaceÑdepends on the propellant ingredients. The rate increases with chamber pressure (which in turn is determined by nozzle design and grain configuration) and the ambient temperature of the grain prior to ignition.The internal grain geometry or initial burning surface may feature a hollow cylinder and several radial slots. This surface is designed to have about the same burning area as the peripheral grain surface near the case wall just before running out of propellant. This kind of internal grain configuration results in an approximately constant burning surface for the duration of operation, which means approximately constant propellant mass flow and thrust. A different internal grain surface (that is, more slots) would alter the thrust-time profile. In most motors today, the grain is bonded or glued to the inside wall of the case. In locations where hot gases could reach the case wall directly, it is necessary to provide some thermal insulation to prevent the case material from becoming too hot and thus too weak.The number of solid-propellant motors being built is much larger than the number of liquid-propellant engines. Almost all of the newer military missiles and some small-payload space launch vehicles use solid-propellant rocket motors because of their inherent design simplicity, almost instant operating readiness, relatively high propellant density (which allows compact small missiles with low drag), and lack of potentially hazardous spills. Exhaust gases of most rocket motors are toxic, and the storage life is limited to 8 to 25 years because of propellant deterioration or aging. The propellant will burn stably under normal conditions, but many solid grains can become explosive in some unusual circumstances.The most common solid propellant, called composite propellant, is a mixture of many small (5- to 200-micrometer) crystals of ammonium perchlorate; a powerful oxidizer (65 to 70 %); many small particles of aluminum; the principal fuel (15 to 20 %); and a rubberlike organic polymer used as a glue or binderÑand also as a fuelÑto hold the grain together and give it some physical strength (10 to 15%). The largest such motor, the Space Shuttle solid booster rocket motor, contains more than 1.1 million pounds of composite propellant, weighs 1.256 million pounds at takeoff, burns for a nominal duration of 123 seconds, and has an average thrust, at altitude, of about 2,372,000 pounds.