Encyclopedia Astronautica
1969.05.22 - Apollo 10 LM descends to within 15.4 km of the lunar surface

On May 22 activation of the lunar module systems began at 11:49 a.m. EDT. At 2:04 p.m. the spacecraft were undocked and at 4:34 p.m. the LM was inserted into a descent orbit. One hour later the LM made a low-level pass at an altitude of 15.4 kilometers over the planned site for the first lunar landing. The test included a test of the landing radar, visual observation of lunar lighting, stereo photography of the moon, and execution of a phasing maneuver using the descent engine. An error in switch postion brought a heart-stopping moment when the LM ascent stage went into wild gyrations after separation from the descent stage - possible a fatal error if it had occurred during take off from the surface on a landing mission. The ascent stage returned to dock successfully with the CSM following the eight-hour separation, and the LM crew returned to the CSM.

Flight control had planned to let the crew sleep until the last moment on 22 May, when Stafford and Cernan would leave Young and fly the lander down near the lunar surface. But, after playing "The Best Is Yet to Come" and sounding reveille, ground control found that the astronauts had stealthily risen, eaten breakfast, and quietly begun work on the flight plan checklist. Cernan removed the encumbrances from the tunnel and zipped over into the lunar module to get everything ready, while Young helped Stafford with his suit (a five-minute job even with assistance). Cernan then came floating back into the command module and jumped into his suit. When flight control heard from them at the start of the tenth circuit, the two pilots were in the lander and closing off the tunnel.

When Stafford and Cernan were ready for undocking, however, they found that the lunar module had slipped three and a half degrees out of line with the command module at the latching point, possibly because of loose mylar collecting on the docking ring. It might also have happened when Young, during docking, had forgotten to turn off the service module roll thrusters and flight control had been tardy in reminding him of the task. Whatever caused the problem, the crew feared separating the two craft might shear off some of the latching pins, possibly preventing redocking. Stafford and Cernan would be stranded in lunar orbit with no way back except by going out the lander hatch and making their way to the command module hatch - a dangerous undertaking. But Low, who was in the control room at the time, told Flight Director Lunney that as long as the misalignment was less than six degrees they could go ahead and undock.

Just before Apollo 10 rounded the corner to the back of the moon, flight control passed the good news to Stafford. The two crewmen in LM Snoopy heard a "pow" as they broke free. Young, all alone in what now seemed to be an unusually large command module, turned on the television camera so the flight controllers back on the earth could help him inspect the lander. Meanwhile the lunar module landing gear had deployed and was in place. The lander's systems checked out well, especially the radar, the abort guidance system, the antennas, and the pressurization of the descent propulsion system. Everything looked good, and everybody was ready to go. Telling Young not to get too lonesome and not to go off and leave them, Stafford and Cernan announced that they were ready to go down and snoop around the moon.

Young had used his service module thrusters to pull Charlie Brown nine meters away from the lunar module for the inspection. He then gave the same jets a spurt to thrust downward toward the moon until the two vehicles were three and a half kilometers apart. Stafford and Cernan were ready to try, for the first time, another of the operations with a significant Apollo abbreviation so cherished by the engineers - descent orbit insertion, or DOI. At nearly 100 hours into the mission, Stafford started the descent engine at minimum thrust - which slowly built up past 10 percent - and then 15 seconds later he increased it to 40 percent for 12 more seconds. The engine ran smoothly, with none of the chugging experienced on McDivitt's ride. Young tracked the burn optically and told the lunar module crewmen that they were moving away from him at more than 20 meters a second. Cernan did not think they were going that fast. "It's a very nice pleasant pace," he said. Now they could get a close look at a proposed landing site in the Sea of Tranquility, where Apollo 11 might set down in July. Stafford and Cernan had studied hard for what they were going to do. In a T-38 aircraft, they had simulated this trajectory above the earth. They had pored over charts and maps of the site, and they had scrutinized the area during their hours in lunar orbit. So the astronauts traveled easily down the approach path, calling out the names of craters, rilles, and ridges as they went along. They appeared to be traveling exactly over the track they wanted, reaching a low point of 14,447 meters above the surface. They took many pictures; then Stafford's camera failed as the film started to bind. He described the landing site as much like "the desert in California around Blythe." If a lander touched down on the near end, it would have a smooth landing, he said; but, if it wound up at the far end of the zone, extra fuel would be needed for maneuvering to a clear spot. Their landing radar worked perfectly when they tested it, and the pilots remarked that they had no visibility problems with lighting and sun angles.

Young caught sight of the lunar module at a distance of 120 kilometers; Snoopy appeared to be running across the lunar surface like a spider. At other times, using a sextant, he spotted the craft as far away as 550 kilometers. An hour after the first descent burn, Stafford and Cernan fired the engine again, to shape the trajectory for their return to the command module. Shoving the throttle forward for 40 seconds and 100 percent thrust, Stafford was happy to note that there was still no chugging. Young tried to see the flames from the engine but could not. Although the lander's speed had increased by 54 meters per second, the crew again had the impression that acceleration was slow. During these activities, the lunar module had a "hot [open] mike," which was fine with Young, since it kept him informed of what was happening in the lander. But whenever he talked, he had a feedback of his own voice. Somebody would have to fix that before the next mission, he said.

After Stafford's camera failed, he and Cernan had little to do except look at the scenery until time to dump the descent stage. Stafford had the vehicle in the right attitude 10 minutes early. Cernan asked, "You ready?" Then he suddenly exclaimed, "Son of a bitch!" Snoopy seemed to be throwing a fit, lurching wildly about. He later said it was like flying an Immelmann turn in an aircraft, a combination of pitch and yaw. Stafford yelled that they were in gimbal lock - that the engine had swiveled over to a stop and stuck - and they almost were. He called out for Cernan to thrust forward. Stafford then hit the switch to get rid of the descent stage and realized they were 30 degrees off from their previous attitude. The lunar module continued its crazy gyrations across the lunar sky, and a warning light indicated that the inertial measuring unit really was about to reach its limits and go into gimbal lock. Stafford then took over in manual control, made a big pitch maneuver, and started working the attitude control switches. Snoopy finally calmed down.

For this first lunar module flight to the vicinity of the moon, the pilots were supposed to use the abort guidance system instead of the primary guidance system, to test performance in the lunar environment. The abort system had two basic control modes, "attitude hold" and "automatic." In automatic, the computer would take over the guidance and start looking for the command module, which was certainly not what the crew wanted to do just then. In correcting for a minor yaw-rate-gyro disturbance, the pilots had accidentally switched the spacecraft to the automatic mode, and the frantic gyrations resulted. From Cernan's startled ejaculation to Stafford's report that everything was under control took only three minutes. Flight control told the crewmen they had made an error in switching, but the system was fine. They could fire the ascent engine. After the firing, the lander flew what Stafford called a "Dutch roll," yawing and pitching and snaking along. When the engine shut down, however, to the crew's surprise the attitude and flight path to the command module were correct. From a maximum distance of 630 kilometers, the thrust from the ascent engine moved the lunar module to within 78 kilometers of the mother ship.

As the lunar module approached, Young saw it through his sextant at a distance of 259 kilometers. Stafford and Cernan got a radar lock on the command module shortly after the insertion burn and watched with interest as the instrument measured the dwindling gap between the vehicles and demonstrated the theories of orbital mechanics in actual practice. Cernan especially liked the steady communications that kept both crews aware of what was happening. After watching the command module from as far away as 167 kilometers and then losing sight of it at sunset, the lunar module pilots saw Charlie Brown's flashing light with their unaided eyes at 78 kilometers. At last, the two craft were only eight meters apart, and the relative speed between them was zero. Stafford did find the ascent stage a little difficult to hold steady, just as Conrad had suspected, but Young slid the probe smoothly into the dead center of the drogue. Stafford rammed the lunar module forward, and the capture latches closed with a loud bang.

Stafford and Cernan had been gone for more than eight hours, and they were ready to get back into the command module and rest. Transfer ring equipment and closing the tunnel were easy. When all three were settled in, they cut the lander loose. Flight control then fired the ascent engine to fuel depletion (249 seconds) and sent the lunar module into solar orbit. The crew watched it move away; Snoopy was soon out of sight. Stafford and his crew went back to tracking landmarks on the surface below for the upcoming lunar landing mission.

After 31 circuits, the crew fired the service module engine to begin the return to the earth.

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