What was SAGE?

Following me watching and commenting on an Air Force video detailing their 70th anniversary, I realized that I’ve never mentioned the infamous SAGE computer.

The year is 1956. The Cold War is in full swing between the United States and the Soviet Union. Threats of an all-out nuclear war hovers above everyone’s heads like a stormy cloud. Duck-and-cover, fallout shelters, and other precautions were just a part of the culture. But what about a computer system that can detect hostile air planes and summon a missile to eliminate hostile planes? Introducing SAGE.

SAGE, or Semi-Automatic Ground Environment, was a computer constructed by MIT with subcontractors (IBM and Western Electric) for the US Air Force. It served as our first computerized early-detection warning system, allowing us to see hostile planes before they posed an immediate threat.

Almost like a video straight from the Fallout series of video games, the SAGE was a real computer with real capabilities. The video above details the system in basic terms including exactly what it did. (Remember: the 1950s was a time where computer technology was new and the American people wasn’t entirely approving of it.)

source: United States Air Force

SAGE was operated entirely using vacuum tubes. Most direction centers, or buildings located on Air Force bases around the country which housed the SAGE systems, had two computers. One was the normal system, and the other was a fail-safe or backup. If one failed, the other would take over. In normal operation, however, the second computer would check the results of the first computer before turning it over to the output system.

A SAGE control/command center (front) with a portion of the computer system (background) located at the Computer History Museum in Mountain View, California. [source: Steve Jurvetson]
Unlike other computers of the era, the SAGE had something that would change computing: a display. Computers printed their results onto long sheets of paper, but this proved to be too slow and inefficient for the Air Force where immediate, easy-to-read results were required. Using a CRT similar to what would be found in an ordinary RADAR system of the time, the computer would display objects in the airspace at a given time.

Input was also ground-breaking. Data continuously poured in from local radar systems mounted on planes, naval ships, the ill-fated Texas Towers, and over-the-horizon systems. Each control station not only had a CRT, but also a light-wand to highlight planes to perform an action on. The computer was also fed by punch-card reports of scheduled flights, so that the local PanAm flight wasn’t marked as hostile.

The machine was more advanced than an over-glorified RADAR, though. The computer had memory (256KB to be exact) that could track the movement of a plane so that trajectory and speed could be determined. If a hostile plane was discovered, the computer would “keep an eye” on the plane. If a missile needed to be fired, the computer would automatically determine an intercept point where a BOMARC missile would hit the plane.

The machine was important to computing history as it introduced many new innovations that we take for granted everyday. For instance, a display. While it was slow (about 1 frame/second) and archaic, it was the first computer able to draw vector graphics onto a CRT. The light wand used to highlight planes could be considered the first “pointing device.” The SAGE system was also one of the first – if not THE first – computer system to be networked across the nation, with various systems installed across the country being able to communicate with each other using early telephone modems.

Even more remarkable was the reliability of the system. Despite being a vacuum-tube based machine that required several tons of cooling, the downtime was incredible.

The machine was completed in 1963, although development stretched back a decade prior. Unfortunately, the system was obsolete as soon as it was completed. Transistors had long replaced their unreliable vacuum tube counterparts, and the Soviet Union had developed long-range missiles and ICBMs that made SAGE practically useless as bombs would no longer be carried by plane. However, it wasn’t the end.

SABRE used SAGE (AN/FSQ-7) technology. SABRE was used for many decades as a flight reservation system. The SAGE system also got used as the backdrop of many science-fiction movies of years past, especially thanks to its blinking lights and vacuum tubes.

The advantages from SAGE are found in everyday computing today, although the remnants of the system has dwindled to pieces and parts scattered about, including a control station featured in the Computer History Museum. SAGE still holds the record to this day, however, for being the largest computer ever built.


SAGE SPECS (at-a-glance):

  • Manufacturer: MIT for USAF (subcontractor for the computer was IBM)
  • Introduced: 1963 (tested in 1956)
  • Installations: 24, at various Air Force bases
  • Memory: 256k magnetic core memory
  • Processing Speed: 75KIPS
  • Vacuum Tubes: 49,000 per computer
  • Power Consumption: 3 MW

Why was it important?

  • First world example of real-time computing
  • First graphical display (both vector graphics and binary images)
  • Nationwide network (through telephone modems)
  • Light-wand could be argued as first “pointing device”
  • Could compute intercept points and direct missiles to that point (via on-board guidance computer)
  • Memory could track previous locations of a plane (used to compute speed, direction of movement, and an estimated time for arrival)
  • Gave way to SABRE flight reservation system

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