In the 1940s, flight simulators were servo-operated, electro-mechanical devices that mimicked an airplane's attitudinal changes in response to movements of its controls. They allowed pilots in training to practice flying in a safe and relatively inexpensive environment. A sufficiently accurate simulation could also allow engineers to study alternative sets of characteristics before building a prototype of a new design. In 1943-44 Captain Luis de Florez, director of the Navy's Special Devices Division, realized that a general simulator, one that could be programmed to simulate any desired set of characteristics, could in theory vastly reduce the time and expense of both aircraft development and pilot training. In principle, the flight simulator was what in now know as a "dual-use" technology, equally applicable to training military and civilian pilots.

		In 1944 Jay Forrestor was an advanced graduate student at MIT. As one of Gordon S. Brown's two assistants, he had helped found the Servomechanisms Laboratory in 1940. He was present when Captain de Florez discussed the idea of a general simulator with Brown's group, and when Special Devices Division issued a contrat for the ASCA in December 1944, Forrester took charge of the project. The Servo Lab was then the most important enter of analog control research in the United States, and Forrester spend his first year working on an analog computer for the ASCA. The complexity of the calculations involved - requiring simultaneous solutions of a hundred or more differential equations - frustrated his efforts, but it is important to emphasize that this was not because analog techniques were unable, in principle, to solve the equations. Forrester needed to overcome two other problems.
In December 1944, Jay Forrester took charge of the ASCA simulator project.

Differential equation - "Newton's Laws allow one to relate the position, velocity, acceleration and various forces acting on the body and state this relation as a differential equation for the unknown position of the body as a function of time."

First, the speed of the electromechanical analog equipment in terms of which Forrester had been trained to think - the servomechanisms and differential analyzers of the Vannevar Bush era - was too slow. To make a simulator feel realistic, its controller would need to solve the necessary equations virtually instantaneously, that is, without a noticeable delay between the pilot's actions and the machine's response. Computational delays of even significant fractions of a second, as were typical of electromechanical devices, would be intolerable. This was the problem of "real-time" control. In principle, at least, this problem was not unsolvable; electronic analog computation could have achieved the requisite speeds.
		Whirlwind simulation figure from a 1951 sales booklet.

A second, more intractable difficulty was the limited accuracy of analog techniques. Because they employ measured physical quantities rather than counts of discrete units, analog devices unavoidably introduce increasingly large errors as their complexity rises. In 1945 Forrester and some associates paid a visit to MIT colleague Frank Verzuh. Verzuh had worked on Bush's Rapid Arithmetic Machine and the various differential analyzers before and during the war, but he was now helping to design the Rockefeller Electronic Calculator, a small digital computer. He "told Jay... he would have to use digital techniques," because the best MIT differential analyzer achieved only five significant figures, whereas the ASCA would require as many as ten. Though a research effect on Whirlwind's eventual scale would surly have led to major improvements in analog accuracy, in late 1945 Forrester began to explore digital techniques.

		Forrestor's interest in digital possibilities was piqued by three further encounters. First, his former fellow graduate student Perry Crawford, who had written a master's thesis on applying digital computation to the automatic control of antiaircraft guns, strongly suggested that Forrester look into digital methods. Then, in late 1945, Forrester attended the Conference on Advanced Computation Techniques, whose major theme was ENIAC research. Finally, he visited the Moore School to learn about the "Pennsylvania technique" and read the ENIAC designers widely circulated "First Draft of the Report in the EDVAC." Even together, however, these did not amount to some kind of digital conversion experience. The choice, at this point, was anything but clear-cut: Forrester spent the better part of the following year (1946) weighting analgo and digital methods against each other.
The operational ENIAC digital computer in 1945.

By mind 1946 Forrester had abandoned the analgo approach and recriented the ASAC project toward a general-purpose digital mahine, the Whirlwind, that would have the flight siulator as just one of its possible applications. This practical program in simulator design separated Whirlwind from almost all other digital computer projects of this era because it required a device that could be used as a real-time control mechanism. This was a far from obvious goal for digital computer, given the technology of the day. Most other computer projects of the 1940s saw digital machines as giant calculators for scientific computation. Many believed that only a few would ever be needed, and even Forrester at one time apparently though that the entire country would eventually be served by a single mammoth computer. By 1948, the ONR's interest in a supersophisticated and by then extremely expensive flight simulator was on the wane. Meanwhile Forrester, increasingly less interested in the simulator application and more determined to build a high-speed, highly reliable general-purpose digital computer, had openly abandoned work on the simulator cockpit in June 1948.

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During World War II, the U.S. Navy approached MIT about the possibility of creating a computer to drive a flight simulator for training bomber crews. They envisioned a fairly simple system in which the computer would continually update a simulated instrument panel based on control inputs from the pilots. Unlike older systems like the 'Link Trainer', the system they envisioned would have a considerably more realistic aerodynamics model that could be adapted to any type of plane.

A short study by the MIT Servomechanisms Laboratory concluded that such a system was certainly possible. The Navy decided to fund development under Project Whirlwind, and the lab placed Jay Forrester in charge of the project. They soon built a large analog computer for the task, but found that it was inaccurate and inflexible. Solving these problems would require a much larger system, perhaps one so large as to be impossible to construct. In 1945 Perry Crawford, another member of the MIT team, saw a demonstration of ENIAC and suggested that a digital computer was the solution. Such a machine would allow the accuracy of the simulation to be improved with the addition of more code in the computer program, as opposed to adding parts to the machine. As long as the machine was fast enough, there was no theoretical limit to the complexity of the simulation.

In March 1946, the navy agreed to fund development of a digital computer called Whirlwind to run a general-purpose flight simulator. Forrester and Robert Everett, led a team that got Whirlwind running at least intermittently in 1949. It used 4,500 vacuum tubes, occupying the space of five or six offices. But vacuum-tube memory proved wildly expensive and unreliable. Forrester recalls that Whirlwind "always worked when the admirals were there, but it quit pretty much when they walked out of the building." The development of Whirlwind itself ultimately eclipsed the flight-simulator project. Forrester's team had stopped working on the simulator in June 1948, and navy support for Whirlwind waned. "We probably would've been pretty well budgeted out of existence, except for Russia exploding the atomic bomb" in 1949, says Forrester. "That brought into sharp relief the weakness or almost nonexistence of the American air defense system."

The air force stepped in to fund Whirlwind, and Forrester led the division at MIT's Lincoln Lab that designed the Whirlwind-based SAGE air defense system, which operated until 1983. Whirlwind's greatest legacy, however, was magnetic-core memory, which Forrester says made the first two-plus decades of digital computing possible. "We spent about seven years trying to convince the industry that it was a good idea," he says. "Then we spent about seven years in patent courts trying to convince them that they hadn't all thought of it first."