Command and Control

Early versions of the Atlas and Titan missiles had a radio-controlled guidance system. After liftoff, ground stations received data on the flight path and transmitted commands to the missile. The system eventually proved to be quite accurate, landing about 80 percent of the warheads within roughly a mile of their targets. But radio interference, deliberate jamming, and the destruction of the ground stations would send the missiles off course.

The Titan II was the first American long-range missile designed, from the outset, to have an inertial guidance system. It didn’t require any external signals or data to find a target. It was a completely self-contained system that couldn’t be jammed, spoofed, or hacked midflight. The thinking behind it drew upon ancient navigational rules: if you know exactly where you started, how long you’ve been traveling, the direction you’ve been heading, and the speed you’ve been going the whole time, then you can calculate exactly where you are — and how to reach your destination.

“Dead reckoning,” in one form or another, had been used for millennia, especially by captains at sea, and the key to its success was the precision of each measurement. A poor grasp of dead reckoning may have led Christopher Columbus to North America instead of India, a navigational error of about eight thousand miles. On a ship, the essential tools for dead reckoning were a compass, a clock, and a map. On a missile, accelerometers measured speed in three directions. Spinning gyroscopes kept the system aligned with true north, the North Star, as a constant reference point. And a small computer counted the time elapsed since launch, calculated the trajectory, and issued a series of instructions.

The size of the guidance computer had been unimportant in radio-controlled systems, because it was located at the ground station. But size mattered a great deal once the computer was going to be carried by the missile. The Air Force’s demand for self-contained, inertial guidance systems played a leading role in the miniaturization of computers and the development of integrated circuits, the building blocks of the modern electronics industry. By 1962 all of the integrated circuits in the United States were being purchased by the Department of Defense, mainly for use in missile guidance systems. Although the Titan II’s onboard computer didn’t rely on integrated circuits, at only eight pounds, it was still considered a technological marvel, one of the most powerful small computers ever built. It had about 12.5 kilobytes of memory; many smart phones now have more than five million times that amount.

The short-range V-2 had been the first missile to employ an inertial guidance system, and the Nazi scientists who invented it were recruited by the Army’s Redstone Arsenal after the Second World War. They later helped to give the Jupiter missile an impressive Circular Error Probable — the radius of the circle around a target, in which half the missiles aimed at it would land — of less than a mile. But the longer a missile flew, the more precise its inertial guidance system had to be. Small errors would be magnified with each passing minute. The guidance system had to take into account factors like the eastward rotation of the earth. Not only would the target be moving toward the east as the world turned, but so would the point from which the missile was launched. And at different latitudes, the earth rotates at slightly different speeds. All these factors had to be measured precisely. If the missile’s velocity were miscalculated by just 0.05 percent, the warhead could miss its target by about twenty miles.

The accuracy of a Titan II launch would be determined early in the flight. The sequence of events left no room for error. Fifty-nine seconds after the commander and the deputy commander turned their keys, the Titan II would rise from the silo, slowly at first, almost pausing for a moment above the open door, before shooting upward, trailed by flames. About two and a half minutes after liftoff, at an altitude of roughly 47 miles, the thrust chamber pressure switch would sense that most of the oxidizer in the stage 1 tank had been used. It would shut off the main engine, fire the staging nuts, send stage 1 of the missile plummeting to earth, and ignite the stage 2 engine. About three minutes later, at an altitude of roughly 217 miles, the guidance system would detect that the missile had reached the correct velocity. The computer would shut off the stage 2 engine and fire small vernier engines to make any last-minute changes in speed or direction. The vernier engines would fire for about fifteen seconds. And then the computer would blow the nozzles off them and detonate an explosive squib to free the nose cone from stage 2. The nose cone, holding the warhead, would continue to rise into the sky, as the rest of the missile drifted away.