Another kind of rocket has come into wide use as part of military weapon systems and spaceflight boosters. This is the solid-propellant rocket, such as the Air Force’s intercontinental ballistic missile, or ICBM, and the Navy’s submarine-launched fleet ballistic missile, or FBM. The space shuttle and other space boosters use both solid- and liquid-fueled rockets as a part of their stack.
There is a big difference between these solid rockets and the early solids such as the Chinese fire arrows and the British-made Congreves.
It is true those modified skyrockets had one characteristic necessary for spaceflight, even though it was not recognized then: their basic gunpowder propellant would ignite in a vacuum because the ingredients of gunpowder contain an oxidizer.
But the relatively small amount of thrust per pound produced by ignited gunpowder prevented its use as a propellant. Producing only a relatively small amount of thrust per pound—a range of two miles for rockets—was considered good through the 19th century.
As conventional guns gained greater range and accuracy, military rockets fell from favor until they were virtually forgotten by all, except for a few like Dr. Goddard, who realized that liquid fuels held much more promise than the solid propellants of his day.
New interest in solid propellants began during and after World War II from experimental work at the Jet Propulsion Laboratory using a liquid polymer developed for industrial use as a propellant. (A polymer is made by stringing identical molecules together, necklacelike, to create larger molecules without changing the basic chemical composition.) Polymers are the basis of synthetic rubber and plastics and include substances like nylon.
It was discovered that mixing a chemical oxidizer and fuel, often aluminum powder, with the polymer to provide the needed oxygen with the polymer resulted in a substance with a consistency similar to peanut butter. This substance could be poured into forms and baked into a rubbery solid material that burned furiously when ignited and created large volumes of gases, producing much greater thrust per pound than gunpowder.
In solid rockets the propellant mixture is called the grain, a word carried over from gunpowder days. The perforation, a carefully designed hole through the propellant, is formed when the fuel is cast to fit into the rocket’s cavity. Varying the size and shape of the perforation determines the rate and duration of combustion, which controls the thrust.
Liquid- and solid-fuel rockets each have special capabilities, advantages, and applications. For military purposes the “rifle-readiness” of solid rocket motors gives them an advantage over liquid rockets. There is no need to lose the precious minutes required for fueling liquid propellants with a solid motor.
Liquid rockets are often preferred for space missions because of their more efficient use of propellants. Because of their high thrust and simplicity, however, solid-fuel rockets are also used with space launch vehicles. Some launch vehicles, such as the space shuttle, combine both liquid engines and solid motors.