Rockets and Missiles

Like liquid-fuel rockets, solid-fuel rockets have both advantages and disadvantages. The rocket can be fueled a long time in advance of a launch without too much danger of the fuel's deteriorating or damaging the rocket body. The construction of the rocket body needed to accommodate the solid fuel is also much simpler than that which is needed for a liquid-fuel rocket. Finally, the fuels themselves in a solid-fuel rocket tend to be safer and easier to work with than those in a liquid fuel rocket.

Still, solid-fuel rockets have their own drawbacks. Once the fuel in a solid-fuel rocket begins to burn, there is no way to slow it down or turn it off. That means that some of the most serious accidents that can occur with a rocket are those that involve solid-fuel combustion that gets out of control.

The solid fuels used in rockets tend to have a clay-like texture. The material, called the grain, contains the oxidizer, the fuel, a binder, and other components all mixed with each other. Ignition occurs when a spark sets off a chemical reaction between the oxidizer and the fuel. The chemical reaction that results produces large volumes of hot gases that escape from the rear of the rocket engine.

Many combinations of materials have been used for the grain in a solid-fuel rocket. One common mixture consists of powdered aluminum metal as the fuel and ammonium perchlorate or ammonium nitrate as the oxidizer. The flame produced by the reaction between these two substances has a temperature of at least 5,400°F (2,982°C). Nitroglycerine in combination with easily oxidizable organic compounds is also widely used. Such combinations have flame temperatures of about 4,100°F (2,260°C).

The shape into which the grain is formed is especially important in the operation of the solid-fuel rocket. The larger the surface area of grain exposed, the more rapidly the fuel will burn. One could construct a solid-fuel rocket by simply packing the rocket body with the fuel. However, simply boring a hole through the center of the fuel will change the rate at which the fuel will burn. One of the most common patterns now used is a star shape. In this pattern, the solid fuel is actually put together in a machine that has a somewhat complex cookie-cutter shape in its interior. When the fuel has been cured and removed from the machine, it looks like a cylinder of cookie dough with its center cut out in the shape of a seven-pointed star.

In some cases, a rocket engineer might want to slow down the rate at which a solid fuel burns. In that case, the surface area of fuel can be decreased or a slow-burning chemical can be added to the fuel, reducing the fuel's tendency to undergo combustion. A grain that has been treated with an inhibitor of this kind is known as a restricted-burning grain.

First flight of the Ariane 4 rocket. European Space Agency/Photo Researchers, Inc. Reproduced by permission.