Apollo 11 Descends to the Moon’s Surface

Based on measurements of the line-of-sight rate of landmarks, the estimates of altitudes converged on a predicted altitude of 52 000 feet at ignition.

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: Lunar Orbit

The abort guidance system calibration was accomplished at the preplanned spacecraft attitude. As the command and service modules maneuvered both spacecraft to the undocking attitude, a final switch and circuit breaker configuration check was accomplished, followed by donning of helmets and gloves.

Command module — Activities after lunar orbit circularization were routine, with the time being used primarily for photographing the lunar surface. The activation of the lunar module in preparation for descent was, from the viewpoint of the Command Module Pilot, a well-organized and fairly leisurely period. During the abort guidance system calibration, the command module was maintained at a fixed attitude for several minutes without firing thrusters. It was easy to stabilize the spacecraft with minimum-impulse control prior to the required period; therefore, no thruster firings were needed for at least 10 minutes.

The probe, drogue, and hatch all functioned perfectly; and the operations of closing out the tunnel, preloading the probe, and cocking the latches were done routinely. Previous practice with installation and removal of the probe and drogue during translunar coast was most helpful .

Two periods of orbital navigation (program P22) were scheduled with the lunar module attached. The first, at 83 hours , consisted of five marks on the Crater Kamp in the Foaming Sea. The technique used was to approach the target area in an inertial attitude hold mode, with the X-axis being roughly horizontal to the target when the spacecraft reached an elevation angle of 35° from the target, at which point a pitch-down of approximately 0.3 deg/sec was begun. This technique, which was necessary to assure a 2-1/2-minute mark period distributed evenly near the zenith, was performed without difficulty.

The second navigation exercise was performed on the following day, shortly prior to separation from the lunar module. A series of five marks was taken on a small crater on the inner north wall of crater 130. The previously described technique was used, except that two forward-firing thrusters (one yaw and one pitch) were inhibited to preclude thrust impingement on the deployed rendezvous-radar and steerable antennas. The reduced pitch authority doubled the time required (to approximately 3 seconds when using acceleration command) to achieve a 0.3-deg/sec pitch-down rate. Because the Command Module Pilot was in the lower equipment bay , where rate instrumentation is not available, it was necessary in both cases to achieve the pitch rate by timing the duration of acceleration-command hand controller inputs.

To prevent the two spacecraft from slipping and hence upsetting the docked lunar module platform alinement, roll thruster firings were inhibited after the probe preload until the tunnel had been vented to approximately 1 psi. Only single roll jet authority was used after the 1-psi point was reached and until the tunnel pressure became zero.

Undocking and Separation

Particular care was exercised in the operation of both spacecraft throughout the undocking and separation sequences to ensure that the lunar module guidance computer maintained an accurate knowledge of position and velocity.

The undocking action imparted a velocity of 0.4 ft/sec to the lunar module, as measured by the lunar module primary guidance system. The abort guidance system disagreed with the primary system by approximately 0.2 ft/sec, which is well within the preflight limit. The velocity was nulled, since the primary system was assumed to be correct. The command module undocking velocity was maintained until the desired inspection distance of 40 feet was reached. At this distance, the command module velocity was visually nulled with respect to the lunar module.

A visual inspection by the Command Module Pilot during a lunar module 360° yaw maneuver confirmed proper landing-gear extension. The lunar module maintained position with respect to the command module at relative rates believed to be less than 0.1 ft/sec. To enter the planned equiperiod separation orbit, the 2.5-ft/sec radially downward separation maneuver was performed at approximately 100-1/2 hours with the command and service modules.

Lunar Module Descent

The first optical alinement of the inertial platform, in preparation for descent orbit insertion, was accomplished shortly after entering darkness and following separation. The torquing angles were approximately 0,3°, indicating either an error in the docked alinement or platform drift. A rendezvous-radar lock was achieved manually, and the radar boresight coincided with that of the crew optical sight. Radar range was substantiated by the vhf ranging in the command module.

Descent orbit insertion — The descent orbit insertion maneuver was performed with the descent engine in the manual throttle configuration. Ignition at the minimum-throttle setting was smooth, with no noise or sensation of acceleration. After 15 seconds, the thrust level was advanced to 40 percent, as planned. Throttle response was smooth and free of oscillations. The guided cut-off left residuals of less than 1 ft/sec in each axis. The X and Z-axis residuals were reduced to zero by using the reaction control system. The computer-determined ephemeris was 9.1 by 57.2 miles, as compared with the predicted value of 8.5 by 57. 2 miles. The abort guidance system confirmed that the magnitude of the maneuver was correct. An additional evaluation was performed by using the rendezvous radar to check the relative velocity between the two spacecraft at 6 and 7 minutes subsequent to the maneuver. These velocity values corresponded to the predicted data within 0.5 ft/sec.

Alinement and navigation checks — Just prior to powered descent, the angle between the line of sight to the sun and a selected axis of the inertial platform was compared with the onboard computer prediction of that angle, and this comparison provided a check on inertial platform drift. Three such measurements were all within the specified tolerance, but the 0.08° spread between them was somewhat larger than expected.

Visual checks of down-range and cross-range position indicated that ignition for the powered descent firing would occur at approximately the correct location over the lunar surface. Based on measurements of the line-of-sight rate of landmarks, the estimates of altitudes converged on a predicted altitude of 52 000 feet at ignition. These measurements were slightly degraded because of a 10° to 15° yaw bias maintained to improve communications margins.

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