High Gate 9000 Feet

At first glance, the lunar landing trajectory seems purely an engineering design, almost a high school physics problem employing the time-honored principles of Newtonian mechanics. Indeed, were the system totally automated, the LM would continue along a flat, fairly horizontal path, only orienting itself vertically as it neared the ground. But at about 9,000 feet the LM reached an imaginary point called the ''high gate,'' a term derived from aviation denoting the beginning of an approach to an airport (the term for the next point, ''low gate'' has a similar origin). Here the landing changed from an ideal trajectory to one that accommodated human judgment and decision. At this point the computer would select ''P64,'' the approach phase program.

The landing sites had been chosen with a variety of maps, made by robotic orbiters, previous Apollo missions, or ground telescopes. But the resolution of these maps was too low to depict features that might disturb a craft the size of the LM, so none could show for certain that the sites would be perfect and flat. Early plans called for a previous flight to drop a beacon or radar transponder for the LM to home in on, but the idea never made it off the drawing boards (other plans even called for clearing special landing fields for fully automated LMs carrying cargo).28 Lacking such fixed reference points, precise landings required some sort of terminal guidance for the last few seconds. Here NASA relied on human vision and decision. Was the predetermined landing area suitable? Was the ongoing trajectory going to lead to a safe landing? The commander made these judgments. His life was on the line, so despite the variety of instruments and computers, he would verify the landing site on his own.

To do that he had to see.

But until high gate he was coming in feet first on his back, looking out into space.

Thus the LM needed to ''pitch over'' as it began the approach phase, rotating the astronauts to a standing position, bringing their eyes and their windows to bear on the landing site as it loomed ahead. This simple motion raised a series of questions: How close to the surface must the pitch over occur, to allow the commander enough time, and enough detail to make the decision? How would he identify the proposed landing site? How much time would he need to identify it and make a decision about where and whether to land? How much flexibility would the commander need to ''redesignate'' the landing site if necessary to a more suitable location (assuming he could find one)? ''It soon becomes obvious,'' one flight plan explained, ''that a strategy is needed that will trade off the system capabilities of the spacecraft and the crew capabilities against the unknowns of the lunar environment.''29 More systems engineering: trading energy (fuel) for information (vision), buying safety by reducing uncertainty with the resource of human perception.

Apollo mission planners and designers sacrificed much to save on weight and fuel, eliminating useful equipment to make the vehicle and its trajectory highly efficient.

Variation of footprint capability with altitude

Variation of footprint capability with altitude

Figure 8.8

Analysis of LPD redesignation footprint from an 8,000-foot altitude. If the LM is at the point on the bottom of the graph and targeted to the center, the commander can redesignate the target and land anywhere within the top of the cone. As the LM descends, the cone becomes smaller, limiting the options for alternate landing sites. (Redrawn by the author from Cheatham, ''Apollo Lunar Module Landing Strategy,'' fig. 36).

Figure 8.8

Analysis of LPD redesignation footprint from an 8,000-foot altitude. If the LM is at the point on the bottom of the graph and targeted to the center, the commander can redesignate the target and land anywhere within the top of the cone. As the LM descends, the cone becomes smaller, limiting the options for alternate landing sites. (Redrawn by the author from Cheatham, ''Apollo Lunar Module Landing Strategy,'' fig. 36).

But saving fuel was in direct conflict with providing visibility to the commander at decision time.30 Human vision won: ''the trajectory is shaped at the cost of fuel, in order to provide the crew with visibility of the landing area.''31 They were devoting their scarcest resource, more than a hundred pounds of it, to a moment of human judgment. More fuel, more time to decide. For the first landing, the fuel budget included a hover time of about one minute;the commander began to see the landing area at 8,000 feet, and if necessary he could move the landing spot, or ''redesignate'' it, up to four miles away.

Figure 8.8 is typical of the many analyses done for these critical moments of assessing the landing site and ''redesignating'' to an alternate as needed.32 The trade-offs of fuel were carefully plotted against the astronaut's sighting angle, as was the trade-off for altitude of the commander's assessment. At 5,000 feet, the commander could still redesignate up to 3,000 feet;going that far would require about 45 feet per second of additional velocity (fuel was measured in the ''delta v budget,'' a precise accounting of possible changes in LM velocity). As the LM approached the surface and altitude decreased, the options for redesignations became smaller.

Thus the high gate altitude was chosen as 8,000 to 9,000 feet. At this point the computer automatically sequenced from P63 (for PDI) to P64, the approach program. At the high gate the vehicle would ''pitch over''—rise from its near-horizontal attitude to a vertical position before landing. Obviously, this move would need to occur under any circumstances, so the vehicle could land properly on its legs, but the presence of the pilot meant that it would occur significantly higher and earlier than otherwise.

The pitch over provided a dramatic moment of revelation. The long flight from earth culminated in a speedy descent with the astronauts laying feet first on their backs. At pitch over, as though waking from the dead, the astronauts suddenly rose to a standing position, seeing the approach and landing area for the first time. On several of the landings this moment was accompanied by exclamations of joy and recognition. During others, it generated confusion and uncertainty.

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