If an LM commander was not happy with his spacecraft’s destination, he could change the LM’s trajectory by working a three-axis, pistol-grip controller in his right hand called the Attitude Controller Assembly. Like many controls in the LM, the controller assembly had more than one function. In orbit as well as closer to the moon’s surface, the astronaut could change the LM’s attitude in pitch, roll, and yaw with the controller. But during the approach phase, an astronaut could click the same assembly up, down, left, or right, incrementally changing the spacecraft flight path one degree laterally, up-range or down-range. The astronauts called it “redesignation.”
“I redesignated immediately four clicks to the right,” stated Scott. “And then shortly thereafter, after [Irwin] called me again with the numbers, I redesignated two more right and three up-range.”
Scott ended up making a record 18 redesignations, which collectively moved the LM’s landing 1,110 feet uprange and 1,341 feet north. When he landed, they were all of 2,000 feet from their intended touchdown spot, well within mission parameters.
The reason the LM pilot called out flight information for his commander was that the moon landings occurred before head-up displays had been invented. So on Apollo 17, one of Schmitt’s numerous jobs was to act as a human head-up display, feeding Cernan the rate of descent and altitude calls.
Schmitt: “Thirty-one feet per second, going down through 500. Twenty-five feet per second through 400. That’s a little high, Geno.”
Within seconds Challenger reached “low gate,” the point for making a visual assessment of the landing site to select either automatic or manual control. If the road looked clear, if the LM’s auto-targeting would make a safe landing, would Cernan let it do its job? Prior to Apollo 13, Jim Lovell asserted that if PGNS was heading for an acceptable landing spot, he would allow it to land the LM. But that was four missions and one abort ago. This was Apollo 17. The final mission. Back on Earth, Grumman’s engineers were minutes away from a case of the good stuff.
Schmitt: “Three hundred feet, 15 feet per second.”
Cernan: “Okay, I’ve got P-66.”
With those four words, the last commander of the last Apollo moon mission took over manual control of the landing of the lunar module. (P-66 was the computer program that would allow Cernan to work the controls all the way down to the lunar surface.)