An Instrument Landing on the Moon

By the time the crew of Apollo 12 arrived at their PDI point 50,000 feet above the moon, ground controllers uploaded a new state vector from the ground into the LM's computer—temporarily seizing control of the DSKY from the astronauts in the LM. The new state vector told the computer it was five miles off course, north of the intended descent track. Before the burn, a KALCMANU maneuver automatically pointed the LM back toward the track, and the guidance drove the error to zero during the descent.

As on Apollo 11, Apollo 12 began firing its engine at 50,000 feet to initiate powered descent. Where Apollo 11 had reached this point a little bit long, Apollo 12 was about a mile short.

At this point Conrad joked with the ground, ''You can turn the mirror on;give me the fox-corpen, and we got the hook down.'' Terms from his days as a carrier pilot—the fox-corpen was the heading of the aircraft carrier's runway, the mirror referred to the ''meatball'' that showed his position above or beneath the glide slope, and the hook referred to the tailhook on a carrier aircraft.

Houston then called up ''Noun 69 + 4200.'' This new command allowed the crew to retarget the vehicle 4,200 feet down range, something Apollo 11 could not do. Bean keyed it in; the ground controllers reviewed his keystrokes to check for errors, then okayed him to hit enter. A mistake entering this correction could cause a serious problem.

Ground then reported: ''MSFN agrees with PGNS and AGS.'' The Manned Space Flight Network [MSFN] was tracking Apollo 12 from the ground, and came up with similar numbers.

Conrad replied: ''Looks good. We're smoking right down there.'' A few minutes later he pointed out, ''I have an altitude light out;and I have a velocity light out.'' The landing radar was sending good data, at about 40,000 feet. Conrad then instructed the computer to incorporate the radar data into the guidance equations. The radar showed a discrepancy (DELTAH) of about 1,700 feet. Within a couple of minutes, the computer had adjusted its solution so the two converged to within one hundred feet.7

When he incorporated the radar into the guidance solution, the RCS (reaction control system) thrusters began to fire to adjust the spacecraft to this new source of data. ''I'm getting a fair amount of RCS firing,'' Conrad reported, ''more than I think I should,'' and a few minutes later, ''Boy, it's really giving her heck on the RCS,'' but the thruster firings were actually normal. (NASA later determined that there had been a software problem in the simulator. The astronauts were used to a simulation that had less thruster activity than normal, so it seemed more active when they were in the actual LM. Also, as on Apollo 11, unanticipated fuel slosh was making for a rougher ride than expected).8

At 7,000 feet, the high-gate, the computer automatically switched to P64. The LM immediately began to pitch over. A full panorama of the lunar surface came into view.

This sight caught Conrad unprepared, with much greater field of view and detail than he experienced in the simulator. ''For the first couple of seconds, I had no recognition of where we were, although the visibility was excellent. It was almost like a black and white painting.'' Bean found this view, with all the craters, scary.9

Bean glanced down at the computer to enable the LPD. Before he could call out the number, Conrad noticed a reading of 42 degrees on the display and looked through the window at that angle. What had been undifferentiated gray, just moments before, now became a pattern.

Then came an excited shock of recognition. Conrad cried out: ''There it is! There it is! Son-of-a-gun! Right down the middle of the road!''

''Hey, it's targeted right for the center of the crater!'' Noun 69 had done its job;just a small redesignation would move them to the rim of the crater.

This was the critical moment missed on Apollo 11. The LM was pitched over, the commander could see out, and he recognized the features. Within about thirty seconds, Conrad began to redesignate the landing spot. He repeated seven times over the next minute (figure 10.1).

First Conrad moved the landing sight a bit to the right (north). One click. ''That's so fantastic. I can't believe it,'' he exclaimed of the targeting.

Conrad then redesignated forward (west), with two clicks. Then one click to the north. Then two clicks back (east). Then one more click north, one final click back (east). Conrad's seventh redesignation put the automatic landing algorithm targeted very near where Surveyor III had landed in 1967, but he would not use the auto mode to land.

At about four hundred feet he called, ''I got it.'' He then took over control, in P66, canceling the LPD mode (which would have allowed him to redesignate for forty-nine more seconds). Despite three northerly LPD designations, Conrad steered further to the right (north) when he took over. He also slowed the LM down to land closer up range than his last designation.

At about three hundred feet, Conrad banked toward the crater again and began to descend. Later he noted, ''I saw a suitable landing area between the Surveyor crater and head crater, which now meant I had to maneuver to my left and sort of fly around the side of the crater, which I started to do.''10

Actual landing point

P66 takeover

Actual landing point

P66 takeover

Initial automatic landing point
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