Lpd

After pitch over the commander needed to find the predetermined landing spot and decide whether it would be safe. How did the commander select a new landing spot? The approach program P64 would display a new quantity, ''LPD angle,'' on the DSKY. This allowed the commander to identify the landing site out the window, or at least know where to look, using a clever device called the ''landing point designator'' (LPD), the brainchild of Donald Cheatham, implemented by Allan Klumpp. Klumpp called it a ''hybrid'' control system, because ''the LM commander can manually steer the LM to the selected landing site, yet the trajectory he flies is produced by an automatic system'' (figure 8.9).33 The LPD consisted of a few lines of code and a very simple piece of weightless hardware—a set of markings on the commander's window that labeled the angles of his sight. Two separate grids were printed on the inner and outer windows of the LM, slightly offset; when the commander positioned his eye so the two grids lined up, his vision too was aligned. The DSKY gave him a number and the commander could look through the corresponding grid point on the window, where he should see the landing spot.

The commander could also redesignate the landing site, moving the computer's aiming point. He would find a new site, sight it through the LPD grid on the window, and read out the number to the LM pilot, who would enter it into the computer with a few DSKY commands. Klumpp found that process clumsy and error-prone, so he added a feature: the commander could also nudge the joystick and the landing site would ''click'' a degree or two in the corresponding direction (figure 8.10). One nudge of the

jlUAM OultOjuo] Figure 8.9

Landing Point Designator (LPD) scribed into the LM commander's window. (Grumman Aerospace Corporation, ''Apollo Operations Handbook, Lunar Module, LM 10 and Subsequent, Volume I: Subsystems Data,'' April 1, 1971, 1-11.)

jlUAM OultOjuo] Figure 8.9

Landing Point Designator (LPD) scribed into the LM commander's window. (Grumman Aerospace Corporation, ''Apollo Operations Handbook, Lunar Module, LM 10 and Subsequent, Volume I: Subsystems Data,'' April 1, 1971, 1-11.)

stick left or right would change the landing site by two degrees in the commander's field of view;one nudge fore or aft would move the landing site ahead or behind by a half degree (translating these angles to the actual distance change of the landing site depended on altitude: from 6,000 feet high, one click corresponded to moving the landing site 600 feet;at 500 feet altitude, one click only moved it 79 feet). The computer then recalculated the new landing position, and a new trajectory, and flew the LM accordingly to the new spot. The LM pilot could read out the number again, and the commander could look to find the new landing area, and redesignate again if necessary, in an iterative loop that should converge on the right spot. Through this repetition, Klumpp wrote, ''the commander literally steers the current landing site into coincidence with the desired site,'' via a computer-mediated feedback loop.34

The pilot could continue to redesignate, evaluate landing sites, and make decisions for several minutes after pitch over. At some point, however, the LM would be so close to the surface that the LPD would be unusable. Just before the low gate of about 500 feet, the computer would not accept further redesignation and would switch into program P66 to guide the LM toward the site.35

Landing point designation (LPD)

4) Computer updates new trajectory to new landing point

2) Computer calculates \ trajectory to landing point \

Lunar surface

4) Computer updates new trajectory to new landing point

2) Computer calculates \ trajectory to landing point \

Lunar surface

1) Initial LPD designation

3) Commander or LMP updates LPD angle by keypad or control stick, which updates landing point

Figure 8.10

Landing point designation, showing how the commander or LMP can move the aiming point by updating the LPD angle, either by entering a new LPD angle into the DSKY or by nudging the control stick one ''click.'' The process could be repeated an indefinite number of times. (Redrawn by the author from Cheatham, ''Apollo Lunar Module Landing Strategy,'' fig. 40.)

1) Initial LPD designation

3) Commander or LMP updates LPD angle by keypad or control stick, which updates landing point

Figure 8.10

Landing point designation, showing how the commander or LMP can move the aiming point by updating the LPD angle, either by entering a new LPD angle into the DSKY or by nudging the control stick one ''click.'' The process could be repeated an indefinite number of times. (Redrawn by the author from Cheatham, ''Apollo Lunar Module Landing Strategy,'' fig. 40.)

The LPD formed the core of the human-machine system for landing. It allowed the commander a decision-making input and a high-level control of the vehicle's trajectory without burdening him with the actual control of the engines and thrusters, or of maintaining the velocity and attitude of the LM. It seemed an ideal allocation of tasks, using the human for what he was good at—(finding and assessing the landing site)—and the machine for its strengths (precision control and calculation). Apollo commanders did indeed use the LPD to steer them toward safer terrain. None, however, ever let the automatic feature actually land the LM on the spot designated by the LPD.

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