Analog Simulators at NACA

The techniques and technology of simulation were just beginning in aerospace in the 1950s, matured greatly in the course of the X-15 program, and played a central role in Apollo. Modeling flight was not a particularly radical idea for aeronautical engineers, for they had always worked with wind tunnels, which we might see as highly

Figure 3.4

Joe Walker in an X-15 simulator. Note the cathode-ray tube above the instrument panel for simulating a horizon view out the window in real time. (NASA Dryden photo E-10251.)

Figure 3.4

Joe Walker in an X-15 simulator. Note the cathode-ray tube above the instrument panel for simulating a horizon view out the window in real time. (NASA Dryden photo E-10251.)

specialized simulators, designed to replicate the conditions of flight inside a laboratory. In the early 1950s, NACA engineers began using simulators to model new aircraft as they were testing them, and eventually to model them before they were built. Engineers even sometimes spoke of the airplane itself as an analog computer, solving a host of differential equations in parallel and real-time (figure 3.4).16

In an analog computer, the idea basically was to connect a vast array of electronic building blocks into a large model of a system. Because of their complexity, and the parallel nature of their computations, each simulation seemed to develop a personality of its own, being ''frequently described as cantankerous, malicious, mulish, or other less friendly terms.''17 Yet this very physicality gave the engineers a deep understanding of the problems they were studying.

While they were designing the X-15, North American Aviation built an analog simulator of the aircraft. Eventually, it grew to six analog computers connected together, and about five hundred patch cords that ''mechanized'' the equations (the analog computer equivalent of programming).18

Simulators quickly became indispensable in the X-15 program. Flight planners ''spent their entire working life in the simulator,'' and hence were experts on the handling of the aircraft. Pilots would spend fifteen to twenty hours in the simulator practicing for an eight- to ten-minute flight.19 Older NACA pilots tended to be reluctant to accept the devices, and they avoided practicing maneuvers, or even entire flights, in the simulators before flying them in the air. Younger pilots, by contrast, like Armstrong, embraced the simulators more quickly, and became active participants in their design and improvement.20

Pilot Milt Thompson believed he could not have flown the aircraft successfully without practicing in the simulator, and he was not alone.21 Pilots understood the benefits of the simulators—it was their lives, after all, that were at stake—but were also keenly aware of their limitations. Most of the simulators used for research were ''fixed base,'' which meant they did not move, depriving the pilots of a significant source of sensation. Early simulators also lacked the ''feel'' of the control stick that, in a real airplane, allowed a pilot to sense the airflow and resistance forces around a plane. Until these features were added in the 1960s, one pilot commented, ''it was just a pinball game,'' and pilots would sometimes correct the engineers when they found the simulators did not ''feel'' right.22

Thompson noted how the X-15 simulator never quite prepared him for the experience of flight. ''In all my simulation practice for the flight, I had been very relaxed, smoking cigarettes, drinking coffee, and sitting in a slumped, head forward position,'' he recalled. On his first real mission, the drop from the B-52 jolted him to the top of the cockpit. Said Thompson: ''Now all of a sudden I was viewing the instrument panel from a completely different perspective and suffering from tunnel vision.''23 In addition to their lack of acceleration or motion cues, the simulations did not include all physical phenomena, but rather only those the engineers already understood, such as the basic aerodynamics, the aircraft's major structural features and their bending modes. During one flight, feedback through the control system excited a high-frequency oscillation in the X-15's structure. Because the simulator model did not include subtleties of the structure, it did not predict the oscillation. ''The success of a mission,'' Thompson concluded, ''is still up to a pilot.''24

The most glaring, and the most troubling, limitation of the simulators was their lack of visuals. During the 1930s, pilots had learned how to fly on instruments alone, especially through fog or bad weather, and for much of the high-altitude research flying of the 1950s and 1960s, the view outside the window was less important than the dials and gauges in the cockpit. But at low altitudes, especially during the critical operations of landing, the scene outside the window was key for the pilot. ''The biggest problem I had was with the displays,'' wrote Thompson. ''They didn't seem real. It wasn't a true-life thing. It was hard to correlate between real-life and looking at a meter.''25 Problems in simulating the visual environment of the moon would be endemic to Apollo, and an erroneous model of the lunar environment would in fact cause the Apollo 15 astronauts to lose their bearings in critical moments.

The other critical effect missing from a simulator experience was anxiety. As Thompson pointed out, the pilots could work in simulators with coffee and cigarettes, and could easily take bathroom breaks. They could start or stop the simulation with a switch installed in the cockpit. Yet when they flew the real airplane, particularly for the first time, the drama and danger of the experience changed the pilots' characteristics as black boxes in the control systems—frequently operating with higher ''gains'' than they did in the laboratory. ''It is simple to evaluate a flight condition on a simulator, rate it subjectively, and reset when you lose control.'' But Thompson concluded, ''Until a reset capability is provided in the airplanes, the success of a mission is still up to the pilot.''26 No matter how good the virtual world, no computer was going to keep pilots from flying. As one contemporary account dryly put it, the X-15 simulator ''lacked the in-flight realism afforded by the rapid approach of the ground.''27

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