The LM User Interface

The layout of the LM reflected the design of the landing (as well as the ascent and rendezvous), and the user inputs to the digital autopilot. Each operator looked through a window on either side of an instrument panel, which had a number of common indicators and other switches and controls unique to each side. The eight ball indicator, similar to that in the CSM, resembled the artificial horizon in an aircraft and indicated the LM's attitude relative to a reference. Though an analog indicator, it was driven by the digital autopilot. During landing, the eight ball functioned like an artificial horizon, showing the LM's relationship to the local vertical of the moon. A velocity display (also driven by the computer) with x and y ''crosspointers'' (needles) indicated the for-

Figure 8.5

Arrangement of crew and controls inside the LM. Note that the LMP is designated ''systems engineer.'' (Klumpp, ''A Manually Retargeted Automatic Descent and Landing System for the LEM,'' 130.)

Figure 8.5

Arrangement of crew and controls inside the LM. Note that the LMP is designated ''systems engineer.'' (Klumpp, ''A Manually Retargeted Automatic Descent and Landing System for the LEM,'' 130.)

ward and lateral velocities. A dual tape-like indicator showed the altitude and descent rate from the digital autopilot's state vector in the computer. Each operator also had two hand controllers—one to control attitude, and one to control translations and vertical velocity (figures 8.5 and 8.6).

Under normal conditions, the commander did the hands-on flying while the LMP monitored the systems. Each time the commander moved his stick from its center position, it sent two signals to the computer, an analog voltage proportional to the amount of deflection, and a switch closure indicating that the stick had been moved.

The digital autopilot offered the astronaut several different modes to control the vehicle. These ranged from fully computerized controls (when the computer determined attitude and thruster firings), to complete manual takeover. Between them was a host of servos, mode switches, feedback loops, and software. An impulse mode, for example, enabled short, timed bursts of thrust in response to each joystick command, useful for precision maneuvers such as docking. An extreme ''hard over mode'' (when the operator pushed the stick all the way to its stops), bypassed the computer

Figure 8.6

Partial view of LEM instrument panel showing indicators and flight controls. Commander stands on the left, LMP at right, with DSKY between them. Commander has additional hand controller and rate-of-descent switch in left hand. (Cradle of Aviation Museum archives, Bethpage, N.Y., Courtesy of Paul Fjeld.)

Figure 8.6

Partial view of LEM instrument panel showing indicators and flight controls. Commander stands on the left, LMP at right, with DSKY between them. Commander has additional hand controller and rate-of-descent switch in left hand. (Cradle of Aviation Museum archives, Bethpage, N.Y., Courtesy of Paul Fjeld.)

altogether and enabled direct control of the valves on the RCS thrusters. This would be an extremely inefficient use of fuel, but might work in an emergency if the computer failed and stopped accepting commands. As Jim Nevins wrote of the interface, ''there is great flexibility and redundancy, but heavy burden on the crew.''21

The most important LM digital autopilot mode was ''rate command/attitude hold,'' entered by Verb 77 in the DSKY, or by flipping a switch from PNGS AUTO to ATT HOLD. In this semiautomated mode (similar to the ''rate command'' mode in the X-15), the astronaut could deflect the stick and change the vehicle's attitude. When he released the stick, the digital autopilot would automatically hold that new attitude. A similar mode (used simultaneously) for vertical control enabled the computer to maintain a precise rate of descent, allowing the astronaut to increase or decrease the rate with a switch.

The manual control modes in the LGC started out as digital reproductions of the analog loops found in the lunar landing research vehicle, or LLRV (discussion to follow), but over time they evolved to be ''quintessentially digital, making freer use of the logical branches, counters, and nonlinearities which are so readily, and reliably programmed in the digital computer.''22 In this rich scheme, the computer did not replace the pilot's skill but rather coalesced the complex craft into an interface that provided both simplicity and variety.

The interface also included instructions for the crew. Figure 8.7 shows the checklist the astronauts used in the cockpit, in this case for Apollo 12. It is part of a larger timeline used for the overall mission, and one of about five pages used for the LM from undocking to landing. This piece of paper lay out on a notebook between the two astronauts just below the DSKY. The procedure started at the left one minute before the engine began to fire to slow the LM out of orbit. The first instruction was for the astronauts: ''reset watch.'' They next read down the column and then up to the top of the next column and down to the conclusion, the landing. Areas in the dark boundaries indicated abort conditions or critical moments such as ''bingo fuel.'' The final instructions on the bottom right indicated procedures to be taken after touchdown to ''safe'' the vehicle, and can be heard in Aldrin and Armstrong's words over the radio just before ''the Eagle has landed.''23

Like the programs embedded in the computer that commanded spacecraft systems to perform certain duties, these coded instructions directed the humans to perform particular behaviors and to make decisions based on data they observed. Like the ones and zeroes in the core ropes, the paper timelines in the LM cockpit helped tie the system together, binding human and machine into a single, integrated mechanism.

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