The Mercury Control System

In Mercury, as with the X-15, questions about the human role were built into the machine. McDonnell-Douglas built the Mercury capsule itself, but its control system was designed by the Minneapolis-Honeywell Regulator Company (then also building the MH-96 adaptive controller for the X-15). Company experts initially wanted a fully automatic spacecraft, with little human operation, but after consulting a ''human engineering'' expert who favored a more active pilot, McDonnell engineers believed more human control could improve the reliability of their spacecraft. Nevertheless, all agreed with Robert Chilton's view that unlike in an aircraft, the automatic modes would be primary, with the human-actuated controls reserved for off-nominal conditions or emergencies. Indeed, humans were not essential for flight: four Mercury capsules launched on Redstone rockets and three on Atlases under fully automatic control before being qualified for human flight.39 The conception gradually shifted from complete automation to ''monitored automatic.'' The human function was ''something more than secondary, if still less than primary.''40

Automatic

Figure 4.4

Automatic versus manual procedure for Mercury Retro Maneuver, showing the numerous pathways by which the pilot can switch between automatic and manual modes. (Redrawn by the author from Voas, ''A Description of the Astronaut's Task in Project Mercury.'')

Manual

Figure 4.4

Automatic versus manual procedure for Mercury Retro Maneuver, showing the numerous pathways by which the pilot can switch between automatic and manual modes. (Redrawn by the author from Voas, ''A Description of the Astronaut's Task in Project Mercury.'')

The Mercury spacecraft had two redundant sets of control systems, one automatic and one manual, each with their own separate reaction jets and fuel supplies. The pilot had no control over the spacecraft's position; in other words, he could not change its orbit. He could, however, control its attitude, that is, the orientation around the center of gravity. Prior to firing the retro rockets for reentry, the attitude was normally set automatically, but if the primary failed the pilot could point the spacecraft and fire the retro rockets under his own control. During reentry itself, Faget's capsule design meant that the vehicle would passively orient itself with the heatshield down, though it might oscillate a bit. The pilot could use either automatic or manual control to damp the oscillations.

Some pilots, particularly Deke Slayton, wanted rudder pedals in the cockpit, but in the end astronauts used one hand on a three-axis stick. The pilot could push fore and back for pitch, side to side for roll, and twist the stick (instead of pressing on pedals) for the yaw control. In automatic mode, the system would assume the proper attitudes and hold them via feedback control with an autopilot. On the other extreme, in fully manual, the pilot could control the attitude of the spacecraft by commanding the thrusters directly, opening valves for fuel in proportion to the deflection of the stick.

This fully manual mode took a great deal of thrusting to hold attitude, so it was hopelessly inefficient with fuel. A more efficient, hybrid solution allowed ''rate control,'' where the pilot commanded a servo that stabilized the vehicle's rate of motion about each axis. As with the automatic control on the X-15, when the pilot moved the stick, the spacecraft would move, and when he took his hand off, the spacecraft would stop and hold where it was. This simple idea required a finely tuned servo loop to be responsive and stable. In a mode known as ''fly by wire'' (although rather different from the modern usage of that term for aircraft), the pilot could issue ''on-off'' control commands to the solenoid valves in the automatic control system. The pilot could use more than one of these modes at one timeā€”for example, if the autopilot was holding in pitch and yaw, the pilot could still control roll with the fly-by-wire mode. He also had display of rate information in three axes, a periscope, and the window, added to the second capsule after a request by the astronauts.41 The newly prominent astronauts, of course, had opinions about these engineering decisions, and were still fighting the battle of whether they would fly the rocket off the pad.

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