The X15 Control Systems

The X-15 was an unusual aircraft to fly. The pilot could not see the nose, and he could not see the wings. His full-pressure suit wrapped him up tight and isolated him from the outside world. He could not feel or touch anything directly other than the suit and gloves. He could smell nothing other than the pure oxygen he was breathing. In Thompson's words, ''I was in my own little world. I was comfortable and secure and protected from harm. I had complete control over the environment in the cockpit.'' He remembered the wonderful feeling of being able to breathe and smell fresh air when the canopy was raised after the flight, but also a sense of intrusion, ''that someone was invading my privacy'' when he had to step out of the womb-like vehi-cle.28 Many of these characteristics, of course, would reappear in subsequent manned spaceflights.

As if symbolizing the air force's preference for bombers over missiles, the X-15 didn't take off on its own, nor was it boosted from a rocket, but was dropped from an aircraft like the B-52's other, more deadly payloads. The mother ship, in effect, served as the first stage of rocket flight;then the pilot took over and flew the vehicle out of the atmosphere into space. In fact, the X-15 was intended to investigate the other end of the flight—what was then called ''entry'' but was soon rephrased ''reentry'' into the atmosphere. How would a vehicle negotiate the dynamic and uncertain transition from the vacuum of space to the flow and friction of the atmosphere? From spacecraft to airplane: this problem called for the latest in control systems, and, of course, for the pilots' skill.

The X-15 actually had two control systems, one with small rocket thrusters to control its attitude in space (''reaction controls''), and another with the traditional aerodynamic control surfaces (i.e., ailerons and elevators) for use in the atmosphere (''aerodynamic controls''). Spacecraft, of course, require reaction controls because the usual flapping control surfaces on an aircraft require air flowing over them to be effective. The real trick, when flying reentry, would be to make a smooth transition from reaction controls to aerodynamic controls. Could a pilot handle this complex task, especially the difficult period when both sets might be necessary (figure 3.5)?

To investigate this question, the X-15 actually had three control sticks. It had a traditional stick between the pilot's legs for aerodynamic controls. A smaller stick on the left side of the cockpit controlled the reaction thrusters. A third stick on the right-hand side also controlled the aeronautical control surfaces, but it was intended for high-acceleration flight when high g-force loads might preclude the pilot from handling the center stick. Thompson remembered that it became ''a macho thing'' not to use the center stick, but to fly the whole mission in the atmosphere with the side stick. Thompson felt he could fly better with the traditional center stick, but ''my ego would not let me use the center stick, even in an emergency.''29

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