Apollo 11 Lands on the Moon

After it became clear that an automatic descent would terminate in a boulder field surrounding a large sharp-rimmed crater, manual control was again assumed, and the range was extended to avoid the unsatisfactory landing area.

Continuing First Men on Moon,
our selection from Apollo 11 Mission Report by NASA Mission Evaluation Team and by The Astronauts: Neil Armstrong, Edwin “Buzz” Aldrin, and Michael Collins published in 1971. The selection is presented in eight easy 5-minute installments. For works benefiting from the latest research see the “More information” section at the bottom of these pages.

Previously in First Men on Moon

Time: July 20,1969
Place: Sea of Tranquility

Powered descent — Ignition for powered descent occurred on time at the minimum thrust level, and the engine was automatically advanced to the fixed-throttle point (maximum thrust) after 26 seconds . Visual position checks indicated the spacecraft was 2 or 3 seconds early over a known landmark, but with little cross-range error. A yaw maneuver to a faceup position was initiated at an altitude of about 45 900 feet approximately 4 minutes after ignition. The landing radar began receiving altitude data immediately. The altitude difference, as displayed from the radar and the computer, was approximately 2800 feet. At 5 minutes 16 seconds after ignition, the first of a series of computer alarms indicated a computer overload condition. These alarms continued intermittently for more than 4 minutes, and although continuation of the trajectory was permissible, monitoring of the computer information display was occasionally precluded. (See “Computer Alarms During Descent” in section 16.)

Attitude-thruster firings were heard during each major attitude maneuver and intermittently at other times. Thrust reduction of the descent propulsion system occurred nearly on time (planned at 6 minutes 24 seconds after ignition) and contributed to the prediction that the landing would probably be down range of the intended point, inasmuch as the computer had not been corrected for the observed down-range error.

The transfer to the final-approach-phase program (P64) occurred at the predicted time. After the pitch maneuver and the radar antenna position change, the control system was transferred from the automatic to the attitude hold mode, and control response checked in pitch and roll. Automatic control was restored after the pitch and yaw errors had been reduced to zero.

After it became clear that an automatic descent would terminate in a boulder field surrounding a large sharp-rimmed crater, manual control was again assumed, and the range was extended to avoid the unsatisfactory landing area. The rate-of-descent throttle control mode (program P66) was entered in the computer to reduce the altitude rate so as to maintain sufficient height for landing-site surveillance.

Both the down-range and the cross-range positions were adjusted to permit final descent in a small, relatively level area bounded by a boulder field to the north and by sizable craters to the east and south. Surface obscuration caused by blowing dust was apparent at 100 feet and became increasingly severe as the altitude decreased. Although visual determination of horizontal velocity, attitude, and altitude rate were degraded, cues for these variables were adequate for landing. Landing conditions are estimated to have been l or 2 ft/sec left, 0 ft/sec forward, and 1 ft/sec down; no evidence of vehicle instability at landing was observed.

Command Module Solo Activities

The Command Module Pilot consolidated all known documentation requirements for a single volume, known as the Command Module Pilot Solo Book, which was very useful and took the place of a flight plan, a rendezvous book, an updates book, a contingency extravehicular checklist, and so forth. Normally, this book was anchored to the Command Module Pilot by a clip attached to the end of his helmet tie-down strap. The sleep period was timed to coincide with that of the lunar module crew so that radio silence could be observed. The Command Module Pilot had complete trust in the various systems experts on duty in the Mission Control Center and therefore was able to sleep soundly.

The method used for target acquisition (program P22) while the lunar module was on the surface varied considerably from the method used when the spacecraft were docked. The optical alinement sight reticle was placed on the horizon image, and the resulting spacecraft attitude was maintained manually at the orbital rate in the minimum-impulse control mode. Once stabilized, the spacecraft maintained this attitude long enough to allow the Command Module Pilot to move to the lower equipment bay and take marks. He could also move from the equipment bay to the hatch window. in a few seconds to crosscheck the attitude. In general, this method of operation was satisfactory.

Despite the fact that the Command Module Pilot had several uninterrupted minutes each time he passed over the lunar module, he could never see the spacecraft on the surface. He was able to scan an area of approximately l square mile on each pass, and ground estimates of lunar module position varied by several miles from pass to pass. It is doubtful that the Command Module Pilot was ever looking precisely at the lunar module; he more likely was observing an adjacent area. Although it was not possible to assess the ability to see the lunar module from 60 miles, it was apparent there were no flashes of specular light to attract the Command Module Pilot’s attention.

The visibility through the sextant was good enough to allow the Command Module Pilot to acquire the lunar module (in flight) at distances of more than 100 miles. However, the lunar module was lost in the sextant field of view just prior to powered descent initiation (120-mile range) and was not regained until after ascent insertion (at an approximate range of 250 miles), when it appeared as a blinking light in the night sky.

In general, more than enough time was available to monitor systems and perform all necessary functions in a leisurely fashion, except during the rendezvous phase. During that 3-hour period when hundreds of computer entries, as well as numerous marks and other manual operations, were required, the Command Module Pilot had little time to devote to analyzing any off-nominal rendezvous trends as they developed or to cope with any systems malfunctions. Fortunately, no additional attention to these details was required.

Lunar Surface Operations

Postlanding checkout — The postlanding checklist was completed as planned. Venting of the descent oxidizer tanks was begun almost immediately. When the oxidizer tank pressure was vented to between 40 and 50 psi, fuel was vented to the same pressure level. Apparently, the pressure indications received on the ground were somewhat higher, and they increased with time. (See 11Hi gh Fuel Interface Pressure after Landi ng11 in section 16.) At ground request, the valves were reopened, and the tanks were vented to 15 psi.

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