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Guidance Problem. For advanced missiles, guidance is a more serious problem than propulsion. Two guiding systems are of obvious value for an ICBM, and both are being developed. One, under contracts with American Bosch Arma, AC Spark Plug and M.I.T., is "inertial guidance." Its heart is a subtle instrument that senses every force that acts on the flying missile, the enormous force of the rocket thrust and the delicate forces of crosswinds and yawing motions. This information goes to a computer (contracts with Burroughs and Sperry Rand) that figures out the missile's position, speed and direction. If any one of these is not right for the programmed trajectory, the computer makes corrections, moving the missile's fins or regulating its fuel to put it back on its proper course.
The alternative system ("radio inertial") uses a similar instrument in the missile, but readings that show the missile's behavior are sent back to the launching site by radio waves. Then a computer on the ground tells the missile, also by radio, what to do. Each system has its advantages. Radio inertial guidance, for instance, keeps the computer on the ground, where it can be as big and heavy as necessary. Pure inertial guidance, on the other hand, is self-contained and unaffected by radio interference or enemy jamming.
Both systems must exert their influence while the missile is still in the atmosphere or the motor is still thrusting. In space, with the rocket cold, a ballistic missile is as independent as an asteroid. But another guidance problem remains. The missile ascends toward space nose up and cruises toward its target around the curve of the earth. Thus, in natural flight it will re-enter the atmosphere more or less broadside on. This is undesirable; so a "positioning device" must be provided to turn its nose toward its target. There are several possible ways of doing this, such as gyroscopes, flywheels and gas-jets.
Re-Entry Crisis. Somewhere during the passage through space, which will last only 30 minutes over a 5,000-mile range, the bulk of the missile separates from the "reentry body," i.e., the nose cone and warhead. Now comes the crisis of the missile's life. As it drops down into the fringe of the atmosphere 60 to 80 miles up, it is moving at about 16,000 m.p.h. At this enormous speed, even the thin upper air generates temperatures that will vaporize any known substance. The dense lower air is even worse, and it smacks the re-entry body with jarring deceleration forces 20 times gravity. The situation is complicated by the fact that the air sweeping past the missile is ionized by high heat. This absorbs some energy, but creates corrosive particles. It is also responsible for the meteor-like trail.
The designers of the ICBM believe that re-entry is their worst problem. The missile must not burn up, as most natural meteors do, and it must not lose its shape. Its thermonuclear warhead must not be exploded prematurely, and it must not be so damaged that it will not explode at all.