“It is not that you didn’t trust it,” recalls Cernan. “But you are only coming this way one time. And I’m sure not going to let some damn computer land it. The computer can give me all this information and I decide whether it’s useful or not, but I’m the guy who is going to land it.”
P-66 was about more than personal pride, however. The reconnaissance photographs that mission planners used to select the lunar landing sites had a resolution of 20 meters (65 feet), so a 19-meter boulder or crater that could easily tip the LM over may not have shown up. Even an appreciable slope could make unloading equipment next to impossible, and a one-meter-wide rock, if the LM managed to land on it just right, could overpressure the descent engine bell and cause one healthy explosion. So when Cernan looked out his window and saw that Challenger was headed for a boulder field, he did what Armstrong, Conrad, Shepard, Scott, and Young did before him. He flew his LM out of harm’s way.
To fly the LM, Apollo commanders found, was not an unnatural act. It was comparable to flying the helicopters that they trained in (both craft could hover; a helicopter would use the thrust generated by its rotor, whereas the LM would use the thrust of its descent engine). The commanders also spent hour upon hour in simulators. But early on, NASA knew that the simulators lacked fidelity in reproducing the final stages of landing, and real LMs were fragile and way too expensive to crater the home planet with, so they turned to the Lunar Landing Training Vehicle—the “Flying Bedstead.”
“Of all the aircraft I’ve flown over the years,” said Pete Conrad in a 1996 interview, “that was the one that scared the crap out of me.”
The training vehicle was a jet- and rocket-propelled craft that looked like it was designed by a hyperactive kid with the world’s biggest erector set. During lunar landing training, the astronaut would fly the LLTV to an altitude of several hundred feet, then switch to Lunar Simulation Mode. With five-sixths of the vehicle’s weight neutralized by its jet engine, the astronaut controlled the descent by throttling two rocket engines, and he adjusted attitude by working 16 control thrusters. While the craft quickly earned a reputation as a valuable training tool, it was also regarded as squirrelly and unforgiving. Despite the fact that three out of five Flying Bedsteads crashed (one with Neil Armstrong at the controls), LM commanders returning from the moon continued to give it a thumbs-up. “The LLTV is an excellent training vehicle for the final phases,” said Conrad. “I think it’s almost essential. I feel it really gave me the confidence that I needed.”
With 300 feet to go, Apollo 17’s Cernan needed all the confidence he could get since he was getting his first taste of real stick time after 250,000 miles. “You mostly controlled by changing your attitude,” says Cernan. “If I wanted to go a little left, I just roll to the left and the thrust vector would force me left. But now you’re still drifting left, so you’ve got to take it out. So you roll right. And so your attitude, which would change the direction of thrust from the descent engine, is what pretty much controlled your movements.”
The unique gravity and atmospheric conditions near the moon’s surface made attitude changes a dramatic event. “In a helicopter on Earth you can pull the nose up four or five degrees to stop forward motion,” says John Young, the commander on Apollo 16. “In the lunar module you’d pull it up 30 degrees.”
While Cernan was in charge of Challenger’s attitude and therefore its left/right, front/back velocity, he had to negotiate with PGNS to control the throttle. When P-66 kicked in, all Apollo commanders allowed PGNS to adjust the engine’s thrust to maintain a constant rate of descent. If the astronaut wanted to hasten his rate of descent, he could change it in one-foot-per-minute increments by clicking downward on the T-shaped thrust/translational controller in his left hand. Conversely, if he wanted to slow his descent, he would click up.
So on Apollo 17, Cernan pitched and rolled and flattened and steepened Challenger’s trajectory, and soon the boulders he could see through his window were well away from the reasonably level landing site ahead of him. But nothing was assured until touchdown. “Below the 200-foot level, you are in the dead man’s curve,” says Cernan. “Past that point, if the descent engine quit burning for any reason, you would fall to the surface and crash before you could manually abort.”
But Challenger’s engine kept burning as Cernan and Schmitt began their final descent.