Jon Mendini, age 12, of Charlotte, N.C., for his question:
HOW DOES AN ICBM WORK?
As early as 1200, the Chinese were using unguided rockets as fireworks. Then during the early 1800s, a British army officer by the name of William Congreve developed an unguided rocket powered missile that could carry explosives. Rockets were used widely during the 1800s in wars fought in Europe, Asia and both North and South America. Guided missiles weren't far behind.
An ICBM is an intercontinental ballistic missile. It is a rocket that can fly from one continent to another. An ICBM can be programmed to strike a target from 3,400 to more than 8,000 miles away after it first soars to as high as 700 miles.
There are two main types of ballistic missiles. In addition to the ICBM, there is also an intermediate range ballistic missile. This IRBM is designed to fly shorter distances and usually travels from 1,700 to 3,400 miles.
Both an ICBM and an IRBM can be launched from a submarine. When this practice is followed, it is called a SLBM a submarine launched ballistic missile.
Equipped with a nuclear warhead, an ICBM would be capable of completely destroying a distant city. Because of its military importance, it is called a strategic missile.
Ballistic missiles get their name from the way they fly. They follow an arching, ballistic bath like that followed by a ball thrown through the air.
Missiles use engines that work by burning chemicals. The burning produces gases that have a very high pressure. The high pressure forces the gases backward out of the engine and at the same time pushes the engine and the missile ¬forward.
Chemicals used in missile rocket engines are called propellants. Most use a solid propellant that is shaped in the form of a hollow rod called a grain. Fuel provides the basic burning substance, while an oxidizer supplies the oxygen needed to make the fuel burn.
In most rockets, the fuel and the oxidizer are both in solid form and are combined in the grain. Once the grain has been set on fire, the two propellants unite and continue to burn.
A jet engine is used to power some guided missiles. When this is the case, the jet engine uses fuel in the same way a rocket does, but it gets its oxygen from the atmosphere, taking in air as it flies.
Guidance and control systems, which use computers and other special instruments, are built into many guided missiles.
The flight of a ballistic missile has two parts. At first the rocket engine blasts the missile into its planned course and gives it the necessary speed. Then the engine shuts off,and the missile coasts through the second part of the flight until it drops on its target.