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The rocket motor takes an appreciable time to get the missile moving fast enough for the tail fins to grip the air. So most surface-to-air missiles are launched by boosters attached behind the missile's tail. These contain a solid propellant (a slow-burning explosive) that gives an enormous push for a second or so, and starts the missile fast enough to fly straight and true. When the booster bums out, it falls to earth with a whickering scream. The liquid-fueled motor takes over and brings the missile up to speed—several thousand m.p.h.
Ram-Jets & Planes. Not all missiles use rocket motors exclusively. Some have ram-jets, powerful jet engines that burn fuel (gasoline or kerosene) with the air that is crushed into their open noses by the speed of their flight. The great advantage of ram-jets is that they need no oxidizer (e.g., fuming nitric acid), which makes up two-thirds of a rocket motor's fuel load. Their chief weakness is that they have no starting thrust, and are not very efficient until they reach supersonic speed.
Ramjet enthusiasts are sure that these faults can be overcome by rocket boosters to get the ram-jets started, or by launching them from fast airplanes. Many ramjet missiles have been tested already, and some have vigorous admirers in the armed services. They will have to stay in the atmosphere, say at 70,000 ft., but they will gather oxygen as they fly, and their controlling fins will always have air to act upon.
It is the vast power of rockets and ramjets, realized in speed, that makes guided missiles so important for war. The German V-2 was as brainless as an artillery shell, but it plunged toward the ground at 3,000 m.p.h. Not a single V-2 was ever shot down and most were not even seen.
Since the V-2 days, the missiles have taken on even more range and speed. Just as important, they have acquired brains (computers) and senses (guidance systerns) to put them on their targets. These new devices, which bring weapon and target together, give the new missiles their devastating power to destroy.
Riders & Seekers. The simplest guidance system for surface-to-air missiles is radio "command guidance." The invading bomber will be tracked by a radar on the ground. When the missile is fired, its image will appear on the radar screen. Then the missile's radio pilot (safe on the ground) will steer his destroying bird close to the bomber, where a proximity fuse will explode its powerful warhead.
Another possibility is beam riding. As soon as the bomber is discovered by radar, a narrow radio beam will be trained upon it. The missile will contain electronic apparatus to pick up the beam and steer the missile toward its center. As the beam swings with the motion of the bomber, the missile will swing, too. Coached by an automatic computer, the beam operator can lead the target (like a hunter leading a duck) and set the missile on a true collision course.
These systems have a common weakness. Their guidance is fuzziest just where it should be clearest: close to the target. Seeking or homing missiles will be free of this disability. Ground guidance will bring them to the general vicinity of the target. Then they will be on their own, to search for the target with their own senses and brains.