SAGE: Air Defense

An airman works on the world’s biggest computer.

Scott Sitterly sat in a classroom eagerly awaiting his new assignment. The year was 1967, and he was one of more than a hundred thousand enlisted airmen to enter the United States Air Force. His father and father-in-law had served at sea in the Second World War, but Scott wanted to specialize in computers. He had just completed a six-week Air Force program where recruits were thoroughly tested on mathematics and electronics. He had scored well and hoped to get some positive results.

At long last, someone entered the classroom with marching orders. The guys in the second row, Scott’s row, were going into computers. The men behind him were selected for training as combat controllers, the specialists known for parachuting into a combat zone to prepare an airfield and direct aircraft. “I got this close to jumping out of an airplane,” he joked about that pivotal moment. As fate would have it, Sitterly would become ensconced in the cutting-edge technology behind the North American air defense system. Not long after, he shipped off to Biloxi, Mississippi for forty-seven weeks of specialized training.

In the new decade, the Air Force called for a new breed of intelligent, highly-trained airmen to study, operate, and maintain a variety of systems. Strategic missile squadrons in their hardened silo complexes were upgrading to the Minuteman II intercontinental ballistic missile (ICBM). Pratt & Whitney was working on the new F100-PW-100, a turbofan engine that would win a design contest in 1970 and soon propel the next generation of fighter jets. Meanwhile, electronic countermeasures (ECM) were becoming an essential part of operations, especially over North Vietnam where missiles and radar added a deadly new element to combat. To lose the cutting edge of all this technology was to lose a war.

Scott Sitterly wasn’t going to climb into a missile silo or fidget with a jamming pod. He was going to work on the world’s largest computer, what the Air Force called the Semi-Automatic Ground Environment (SAGE). After the surprise attack on Pearl Harbor less than thirty years before, the military could not ignore the benefits, and dangers, of modern air power. Combat aviation had dramatically improved in the post-war years. One of the greatest threats to the United States came from the Soviet Union, whose Tu-16 Badger and Tu-20 Bear strategic bombers could potentially strike industrial, transportation, supply, and nuclear launch targets. With realtime analysis capability, SAGE was the computerized counter-punch to prevent that kind of Soviet air attack.

The nucleus of SAGE was the massive AN/FSQ-7 computer that collected data from six radar sources: long-range monitoring stations, surveillance aircraft like the EC-121 Warning Star, vessels at sea, “Texas Tower” platforms off the coast, intermediate radar, and civilian and commercial air traffic authorities. Absorbing their signal information, the AN/FSQ-7 built a composite image of every air contact over the United States and parts of Canada. Seated at a special console that resembled a radar scope, an Air Force operator could pick up a light gun, use it to touch a track on his circular display, and receive an instant report on a bogey’s course, speed, and other characteristics. SAGE could also steer an interceptor, such as the F-101 or F-106, or even a surface-to-air missile (SAM) to the inbound contact via datalink.

The birth of an air defense computer

Pictured above is the “blue room”, the command post in a SAGE direction center. The regional radar plot is projected before the controllers. (Photo: Wiki Commons)

“Most people felt that this wasn’t going to work,” Sitterly recalled of the SAGE system. Skeptics just couldn’t believe in its capabilities, although the computer did have a noteworthy pedigree. The progenitor of the AN-FSQ-7 was the Whirlwind, a marvel developed at the Massachusetts Institute of Technology (MIT) by project leader Jay Forrester in the 1940s and 1950s. The machine’s selling point was its magnetic core memory, also known as coincident-current magnetic memory, which performed better than electrostatic storage. Whirlwind was designed at the behest of the Office of Naval Research to assist in aircraft flight simulation. In its infancy, the computer offered limited read-only memory (ROM) and random-access memory (RAM), but was relatively fast for the period. A 1950 experiment, in which the radar image of an aircraft over Hanscom Field in Bedford was sent via phone line to Whirlwind on the MIT campus in Cambridge, proved successful.

By 1951, Whirlwind had become popular for academic and government research, and this prompted some important changes in management. Non-classified projects with Whirlwind were conducted from MIT’s Digital Computers Laboratory. Classified work was done in a new campus at Hanscom roughly half an hour from MIT in Cambridge. The latter site was the start of Project Lincoln, which is known today as Lincoln Laboratory. The new facilities would support 1,800 workers with a twenty-million-dollar budget, an astronomical bump from the 100-person staff and $2 million budget first proposed by Air Force chief scientist Louis Ridenour.

If the first Whirlwind set the stage for modern processing power, Whirlwind II tested the feasibility of a magnetic core system used for air defense. In October 1953, Whirlwind II was tested as the centerpiece of an ambitious prototype network called the Cape Cod System. Twice a week, F-89C Scorpions out of Hanscom Field and F-3D Skynights out of Naval Air Station South Weymouth were tracked by an AN/FPS-3 radar at South Truro, Massachusetts and identified by Whirlwind at Lincoln Laboratory. The Air Force would eventually send B-47 bombers and more F-89s into the detection area. Radar coverage was augmented by “gapfiller” SCR-584 stations in Scituate and Rockport and other AN/FPS-3s in Maine and Long Island. The other critical role of the air defense system, intercept vectoring, was successfully performed via datalink with a B-29.

Events moved rapidly from this milestone. Whirlwind II, having demonstrated its value to the Air Force Air Defense Command, was renamed the AN/FSQ-7, and an operational plan for the Semi-Automatic Ground Environment (SAGE) was established. IBM built an experimental version, the XD-1, for Lincoln Laboratory and another, the XD-2, in Poughkeepsie, New York for hardware and software testing. An Experimental SAGE Subsector was set up with pieces of the Cape Cod System. By the mid-1950s, Division 6 of Lincoln Lab was divided up into multiple groups that worked on everything from SAGE testing and planning to storage and vacuum tubes. There would be other tests and integration with radar, interceptor squadrons, and SAM batteries. The first SAGE air defense sector, a span of territory from Philadelphia to New York, went active in June 1958. By 1961, there were twenty-three SAGE centers in the United States (including one in Canada).

In the block house

SAGE computers were installed in direction centers (DC’s) around the country. The four-story DC buildings were enormous concrete boxes, or block houses, with approximately 22,000 square feet of space for staff, equipment, and two redundant AN/FSQ-7s. Operators seated at cathode-ray tube (CRT) interactive display consoles—each unit with data-input keyboards and light guns for inputting track data to the computer—were stationed on the fourth floor. A large plenum and electronic frame equipment were on the third floor. The twin AN/FSQ-7s were installed on the second floor. Everything required to sustain the block house—heating, air conditioning, ventilation, telephones, and backup diesel generators—were built into the first floor. A large cooling tower was built to the side of the block house.

Screen Shot 2018-08-24 at 1.02.04 PM
Above, Airman Scott Sitterly (in white lab coat) works on computer equipment in the SAGE air defense system.

Sitterly described the computer level as an arrangement of big banks with blinking lights for every flip flop in the machine. Data went through an accumulator. The technicians could actually listen to the bits of data as they passed through a sound synthesizer—all those ones and zeroes composed a quaint musical tone that filtered out into the background, but when the tone drooped or diminished, Sitterly and his fellow techs responded immediately to any problem with the equipment.

A maintenance and programming area was positioned in the middle of the second floor. The left side of this area contained a console full of lights and toggle switches where IBM cards could be typed and printed. The right side contained another bank of lights and switches. The front console allowed a technician to switch between AN/FSQ-7s in the event of a problem and maintain constant coverage of the air defense system.

Hardware in an adjacent room joined SAGE to distant radar stations. If any of the modules malfunctioned, they could be pulled from their racks and examined. Sitterly and the other specialists were not allowed to wear dog tags in the building; if their metallic identification ever touched the thick copper conduit under the floor that served as the main electrical bus, they’d be fried to a crisp.

The airmen worked in rotating shifts—six days on, two days off. There wasn’t much time for relaxation, although “the meals were fantastic,” and Sitterly was able to share an apartment off base with a few other airmen.

SAGE was the world’s biggest computer at the time and a revolution in design and function. For the Air Force, SAGE evolved with the weapons and sensors of the period until smaller and faster computers replaced its giant systems. The air defense system worked, as Scott once witnessed in a direction center when an evening commercial flight from Hawaii passed dangerously close to an ICBM base. The bogey was tracked by radar, identified and categorized by a SAGE controller, and intercepted by fighter jets. The commercial aircraft had lost its radio and was unable to identify itself. The Air Force jets escorted the airliner to safety.

Sitterly became part of a select group of highly trained airmen with a specialty in air defense systems. He was sent to work in other SAGE facilities in Oregon and Virginia. The Air Force offered him a higher salary and an assignment in Europe, but he passed on the opportunity. After his enlistment in 1971, Scott applied for a job at Online Systems in Pennsylvania. He was confused when the interviewer “was trying to sell the job to me,” going on about the benefits package without asking Scott about his technical background. As it turned out, the company had already hired several of his fellow technicians.

Additional Sources

The Soviet War Machine: An Encyclopedia of Russian Military Equipment and Strategy. Chartwell Books, 1976.

President’s Report Issue. Massachusetts Institute of Technology Bulletin. Vo. 87, No. 1. October 1951.

President’s Report Issue. Massachusetts Institute of Technology Bulletin. Vol. 94, No. 2. November 1958.

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