Inertial Brains

The latest Atlas ICBM to rise from Cape Canaveral flew, to the naked eye, like many a previous successful Atlas. But it was very different. For the first time no umbilical cord of guiding radio signals connected it with the ground. As soon as it left the pad, it was on its own, depending on the guidance of its built-in brain and senses. The test was a first-try marvel: the Atlas hit within two miles of a target 5,000 miles down range.

What was different about the latest Atlas was its "full inertial" guidance system built by American Bosch Arma Corp. of Long Island, and founded on techniques worked out at M.I.T.'s famed Instrumentation Laboratory whose director, Professor Charles Stark Draper, is the Grand Panjandrum of inertial guidance. Early in World War II, Draper became convinced that bombsights could be made enormously more accurate by stabilizing them with improved gyroscopes. When long-range missiles came into the picture after the war, Draper and his M.I.T. group began developing gyroscopic instruments to steer the rockets through the sky.

Sagging Weight. Inertial guidance works on the childishly simple fact that a weight suspended on springs lags behind when the vehicle on which it is mounted starts to move. If this lag is measured carefully, the speed of the vehicle can be determined, as well as the distance covered. But to do the measuring properly, the motions of the suspended weight must be compared with some fixed system of reference. If a missile curves, for example, the guidance system must know it.

That is where the gyroscopes come in. A spinning gyroscope keeps its axis pointing steadfastly in a single direction in "inertial space," i.e., the space that is thinly filled by the distant stars. The puny motions of a vehicle on the earth, or the motions of the earth itself, have no effect on a gyroscope. If its axis is pointing at the Pleiades, it will continue to point in that direction, no matter how objects near it may twist and turn.

Stable Platform. In actual operation, gyroscopes fall short of the ideal. They have trouble with friction and are thus inclined to misbehave. But as developed by Draper and his Instrumentation Laboratory, gyroscopes can keep a "stabilized platform" about as level as if it had no connection with the roaring missile that is carrying it aloft.

On the serenely stabilized platform are mounted three spring-suspended weights, each to keep track of motion in a different direction. The lagging behind (or pushing ahead) of each weight is reported to a computer that works out the missile's speed and direction. The computer has been told in advance what course the missile should follow to hit a selected target. If the actual course and speed deviate from this course, the computer makes corrections. When the missile has reached the correct top speed, the computer cuts off the rocket fuel. An error of one foot per second at this point means a miss one mile from the target.*

When the U.S., in 1954, started its crash program to build long-range missiles, not everyone was as sure as Draper that full inertial guidance would prove accurate. Radio guidance systems were therefore developed simultaneously. They are very accurate, but they require elaborate ground equipment that is so expensive that separate guidance cannot be provided for each missile. This being the case, the missiles at a base cannot be fired in salvo. Each must wait its turn--and during the wait an enemy hit may wipe out the base itself. All future U.S. missiles will be inertially guided. Since they will be self-contained, an unlimited number of them can climb into space at the same instant, each carrying instructions to fly to a different target, and each bearing the self-containing wherewithal for devastating accuracy.

*Provisions for destroying a malfunctioning missile and for keeping its warhead safe until near the target can be the same for both radio and inertial systems.

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