DESTINATION MOON: A History of the Lunar Orbiter Program
The First Launch
[228] The launch of Surveyor I on May 31, 1966, and its need of the Deep Space Network, together with delivery problems of the photographic subsystem for the first flight Lunar Orbiter at Eastman Kodak, caused the tentative July 11 launch date to be slipped to August 9. By August 1 the photo subsystem had arrived and had been installed on board Lunar Orbiter I. On August 2 the spacecraft was transferred to Launch Pad 13 and mated with the Atlas-Agena launch vehicle. Following the mating, project personnel tested the compatibility of the spacecraft with the DSIF Station 71 at the Cape.4
On August 9 the Boeing-Lockheed-NASA team at the Eastern Test Range Launch Complex 13 and at support facilities near Hangar S counted the spacecraft down to T minus seven minutes. Then, with the launch only a short time away, an anomaly in the Atlas Propellent Utilization System caused a postponement of the mission until the launch window of the following day.5
Lunar Orbiter I, weighing 853 pounds, roared into space atop the Atlas-Agena D launch vehicle at 19:26 Greenwich Mean Time on August 10. Launch operations personnel injected the Agena and the spacecraft into a parking orbit [229] at 19:31 GMT, and at 20:04 the Agena fired its rocket once more to inject the Lunar Orbiter into a trajectory toward the Moon.6 Lunar Orbiter I deployed its solar panels and antennas as planned and acquired the Sun (the first celestial reference for establishing cruise attitude). The mission continued exactly according to the preflight plan until the time of initial acquisition of the second celestial reference, the star Canopus.7
The Canopus star tracker sensor proved to be one of two major problems during the Earth-Moon transit of the spacecraft. On August 11 at 02:14:57 GMT, flight operations personnel at the Deep Space Network facilities at JPL commanded the Canopus sensor to turn on. When it did, it indicated excess voltage 1.5 times stronger than the preflight calculated signal voltage. Acquisition of Canopus failed. The reason for the failure was thought to be excess light reflected from some part of the spacecraft's structure, stimulating undue response from the sensitive sensor. This problem should have been detected during system testing, but it had not been. However, flight operations attempted a number [230] of tests and experiments to correct or circumvent the anomaly.
The necessity for an attitude-stabilized spacecraft like Lunar Orbiter to acquire proper stabilization in reference to the Sun and the star Canopus cannot be overstressed. Unlike a spin-stabilized spacecraft, Lunar Orbiter I depended on proper orientation along its yaws pitch, and roll axes to arrive in the Moon's vicinity in the correct attitude to be injected into lunar orbit. After the failure of the Canopus sensor to acquire a fix on Canopus, flight operators were able to save Lunar Orbiter I's mission by developing an alternate procedure. At the time of the midcourse maneuver, they commanded the spacecraft to establish a roll reference by pointing the Canopus sensor at the Moon.8
This maneuver was executed successfully and after the sensor locked on the Moon, the flight controllers were reasonably sure that it was operating correctly. They developed a procedure that used the Canopus sensor during periods of occultation of the Sun to verify or correct the spacecraft's orientation.9
The other major problem encountered during the cislunar journey was overheating of the spacecraft. This did not [231] become serious until after the midcourse maneuver. To perform this manuever despite the trouble with the Canopus star tracker, Lunar Orbiter flight operators used the Moon as the roll reference. The midcourse maneuver was executed to correct the spacecraft's translunar trajectory in preparation for deboosting it into orbit around the Moon. A second manuever was executed to orient the spacecraft 36° off-Sun for a period of 8.5 hours.10 The purpose of this move was to lower the spacecraft's temperature on the equipment-mounting deck during transit.
The coating on the exterior of the deck was degrading under solar radiation at the expected rate, and no acute overheating was experienced until Lunar Orbiter I was already in orbit around the Moon. Nevertheless, the planned heat dissipation period when the spacecraft was flown 36° off-Sun did not seem to retard overall degradation of the thermal coating on the exterior of the equipment deck.
The need to regulate the spacecraft's temperature and to investigate the Canopus sensor anomaly necessitated pitch and yaw manuevers every few hours. These added small accelerations to the spacecraft, all approximately in the same direction. Their effect on the prediction of the spacecraft's position at the time of deboost was minimal, and the flight operators successfully worked around the effects of the [232] perturbations resulting from the off-Sun maneuvers. The position of Lunar Orbiter I at the time of the deboost maneuver into initial orbit around the Moon was estimated to be less than ten kilometers off the planned insertion point and presented little difficulty for flight controllers.11
Controllers began a series of commands at 15:22:56 GMT on August 14 to place the spacecraft in orbit. Before insertion the spacecraft executed another thermal relief maneuver, which lasted 7.5 hours. The maneuver provided the optimum temperature conditions before the critical insertion. The final sequence of commands for insertion was carried out without any problems, and Lunar Orbiter I was ready to begin the major work of its mission.12
The photographic mission of Lunar Orbiter I was entirely Apollo-oriented.13 Once the spacecraft had been placed in its initial orbit, with an apolune of 1,866.8 kilometers and a perilune of 189.1 kilometers, ground control checked out the subsystems. The necessity to fly off-Sun and the extra number of maneuvers required because of the Canopus sensor problem had affected the interrelationships of the spacecraft [233] subsystems, and flight controllers had to make compensations, especially in the power system to avoid overtaxing the batteries.
On August 15, during the sixth orbit, ground control successfully commanded Lunar Orbiter I to read out the Goldstone test film. This film, being the leader on the supply of film for the mission, had been pre-exposed and checked out through tests of the readout subsystem at the DSIF station in Goldstone, California, before the mission. The same data were now read out again and compared to the known results of the Goldstone tests in order to check the performance of the readout and communications subsystems on board the spacecraft.
At the time of the Goldstone test film readout the thermal problem became acute. The coating on the exterior of the equipment deck was supposed to radiate excess beat during periods of solar occultation. It did this approximately as predicted, but beat levels continued to rises probably because of more rapid degradation in the pigment Of the coating than had been expected. However, on August 18, during the twentieth orbit, a power transistor in the shunt regulator array failed, with a compensating effect on battery temperatures. The failure placed an extra load of 1.2 to 1.5 amperes on the power system, increasing the battery discharge rate during occultation of the Sun. The extra load meant that the off-Sun angle of 36° could [234] be reduced slightly at the time when sufficient power for readout was required of the power system.14 The analysis and compensatory action for this problem reflected outstanding flight operations.
After orbiting the Moon for four days and twenty-three hours Lunar Orbiter I began the first operation of its photo subsystem since the readout of the Goldstone test film. Eleven frames were advanced and processed during the twenty-fifth orbit at 12:12:13 GMT on August 18, bringing the unexposed film into position for the first photographic sequence, which was to begin on orbit 26.
The photographic subsystem, which Eastman Kodak had designed and built, was put together with the precision of a Swiss watch. Every component of the subsystem was tightly housed in an aluminum "bath tub" a little larger than a large round watermelon. A precision instrument with a very complex task to perform, the photo subsystem operated like a thrashing machine. The film, which had to go through three plane changes, was drawn from the supply spool, clamped in a movable platten, moved and exposed simultaneously, and advanced farther to make room for a new film-all in a matter of a few seconds.15
[235] The first site to be photographed, Site I-O (a portion of Mare Smythii), was covered by the Orbiter's dual lens camera as planned. Photo subsystem telemetry to Earth appeared to be normal. The photos were taken as follows. Ground control commanded the spacecraft to open the camera thermal door. Two photo sequences were then executed: one of sixteen frames in the high-resolution mode and one of four frames in the medium-resolution mode. They were made at an altitude of 246 kilometers above the Moon while the spacecraft's velocity relative to the lunar surface was 6,400 kilometers an hour. Exposure time for each shutter was 1/50 of a second, and simultaneous medium- and high-resolution pictures were made every ten seconds. After the sequences, the thermal door was closed and the film was processed.16
Five hours later the readout process began, at 19:50:52 GMT on August 18. All the medium-resolution frames were of excellent quality, but reconstruction of four high-resolution frames revealed severe image smearing. The first high-resolution frame contained some unsmeared data, but George Hage, the Boeing Lunar Orbiter Program Engineer, recognized it to be a double exposure. The first exposure [236] of the frame contained unsmeared data and proved to have been taken prematurely of a feature east of the planned target area when the V/H sensor was turned on.17 Apparently the shutter of the 610 mm lens was out of synchronization with the V/H sensor; further investigation demonstrated that this supposition was true.18
Flight operators in charge of mission photography set up an experiment to examine the possible causes of the smearing. After completion of the Site I-O photography ten more exposures were made with the 610 mm lens for purposes of evaluating exposure 26, the first picture of the four-frame sequence after photographing. Site I-O. One test involved the use of different exposure rates with and without the V/H sensor turned on. A second test was used to determine if, in fact, the V/H sensor was causing abnormal shutter operations. It consisted of three steps:
1) The camera thermal door was opened and the V/H sensor was turned on.
2) The sensor was left on for approximately 2 minutes and then turned off.
3) The camera thermal door was then closed and the camera shutter was commanded to take a picture with the door closed and to move fresh film into the camera for the next photograph.19
[237] The second test confirmed that the abnormal operation occurred when the V/H sensor was on; a high-resolution exposure was made with the thermal door open and no shutter command, but no medium-resolution picture was taken when the shutter command was given. Despite the problem, flight controllers made no deviations from the flight plan, and the spacecraft was transferred to its lower, final orbit at 09:49:58 GMT on August 21.20 The new orbital parameters were: apolune, 1,855 kilometers; perilune, 58 kilometers; inclination to the lunar equator, 12-32°.21
Just before the orbit transfer, Lunar Orbiter I took two frames of medium- and high-resolution pictures of the Moon's far side at an altitude of l,497 kilometers. The V/H sensor was off, because there was no need for image-motion compensation at such a high altitude. After the frames were read out, they revealed high-quality pictures of the lunar surface in both medium- and high-resolution modes, without smearing.22
Another problem occurred before the final orbit transfer, requiring the photo subsystem to take additional unplanned photographs. The Bimat apparently was sticking. [238] The original plan had called for fresh Bimat to be placed on the processing drum at least every 15 hours. This meant that two frames would be processed every four orbits. However, evidence of Bimat stick in the early frames precipitated the decision to use additional film which would permit processing during every orbit. Eight extra pictures were to be taken.23 This change and the extra diagnostic pictures taken to evaluate the high-resolution shutter problem forced a revision in the planned photographic coverage of the remaining sites. The result was that only eight exposures would be taken of Sites 4, 6, and 8 while the 24 other sites would receive the original 16-frame coverage.24
The trouble in the high-resolution camera lens shutter continued to plague photography when the V/H sensor was operating, despite the increase in output voltage which Eastman Kodak technicians bad recommended during analysis of the problem. Further analysis revealed that the logic-control circuitry of the 610-mm-lens focal-plane shutter was susceptible to electromagnetic interferences which caused it to trip at the wrong part of the image-motion compensation cycle. It was not possible to solve this problem by modifying procedures, and low-altitude high-resolution [239] photography on the first mission proved a failure despite further attempts to correct the problem.
Nitrogen gas, used by the attitude control subsystem to manuever the spacecraft, had been expended in greater amounts than originally planned because of the difficulties in the Canopus star tracker and alterations of planned photography caused by the shutter problems and the evidence of Bimat sticking. Moreover, thermal relief maneuvers and excess attitude update maneuvers, together with the failure of a gas regulator, increased the rate of nitrogen usage. Between August 23 and 31 an average of 0.17 kilograms of nitrogen was expended per day. Flight controllers tried an economizing procedure. They commanded the spacecraft to fly off-Sun on its pitch axis and to update its attitude on the pitch and yaw axes using the coarse Sun sensors and on its roll axis using the Canopus sensor. This change resulted in an expenditure of 0.04 kilograms per day between September 1 and 14.25
From the final orbit perilune of 58 kilometer, Lunar Orbiter I was deboosted successfully to a lower altitude of 40.5 kilometers for further photography on August 25. This move was the result of an analysis of the V/H sensor in a duplicate Lunar Orbiter photo subsystem on the ground [240] by Eastman Kodak engineers on August 24. They had concluded that there was a possibility that the camera would operate normally below an altitude of 51 kilometers.26 They reasoned that, since the ratio of velocity to height would be higher in the new, lower orbit, the image-motion compensation mechanism might be forced into synchronization with the 610 mm lens's focal-plane shutter. Synchronization was, unfortunately, never attained, but there was some reduction in smearing because a higher solar lighting angle permitted a change in shutter speed from 1/50 to 1/100 second.27
By August 29 Lunar Orbiter I had completed its photographic acquisition, with a total of 205 exposed frames. Of these, 38 frames had been taken in the initial orbit; 167 were made in the lower orbits. The spacecraft photographed all nine potential Apollo landing sites. Pictures of eleven sites on the far side of the Moon and two Earth-Moon pictures were also taken. The complete readout of the 28 photographs began on August 30.28
Despite the malfunctions in the photographic subsystem the spacecraft succeeded in taking many historic pictures. Command and maneuver requirements were developed to take, [241] in near real-time, such pictures as those of the morning and evening terminator on the lunar surface, the Earth as seen from the Moon's vicinity, numerous farside pictures, and additional photographs of sites of interest on the near side. Lunar Orbiter I photographed such areas as potential targets for Mission B, major craters, and mare and upland areas useful as Apollo navigation landmarks and was mostly able to satisfy the requirements to take these photographs.29
Of all the pictures which Lunar Orbiter I made, one of the most spectacular was the first photograph of the Earth taken from the vicinity of the Moon. This picture was not included in the original mission plan, and it required that the spacecraft's attitude in relation to the lunar surface be changed so that the camera's lenses were pointing away from the Moon. Such maneuvering meant a calculated risk and, coming early in the flight, the unplanned photograph of Earth raised some doubts among Boeing management about the safety of the spacecraft.
Robert J. Helberg, Boeing's Program Manager for Lunar Orbiter, opposed such a hazardous unnecessary risk. The spacecraft would be pointed away from the Moon so that [242] the camera's lenses could catch a quick view of Earth tangential to the lunar surface. Then, once the pictures were made (flight controllers would execute two photo sequences on two different orbits), Lunar Orbiter I would disappear behind the Moon where it would not be in communication with ground control. If, for some reason ground control failed to reestablish communications with it, the Apollo-oriented mission photography would probably remain undone, Moreover, Boeing had an incentive riding on the performance of the spacecraft, and Heiberg did not think it prudent to commit the spacecraft to a series of maneuvers for which no plans had been made.30
The understandably conservative Boeing stance was changed through a series of meetings between top NASA program officials, including Dr. Floyd L. Thompson, Clifford H. Nelson, and Lee R. Scherer. They convinced Heiberg that the picture was worth the risk and that NASA would make compensation in the event of an unexpected mishap with the spacecraft. After agreement had been reached, Lunar Orbiter flight controllers executed the necessary maneuvers to point the spacecraft's camera away from the lunar surface, and on two different orbits (16 and 26) it recorded two unprecedented, very useful photographs.
[243] The Earth-Moon pictures proved valuable for their oblique perspective of the lunar surface. Until these two photographs, all pictures had been taken along axes perpendicular or nearly perpendicular to the Moon's surface. On subsequent Lunar Orbiter missions oblique photography was planned and used more often.31
Lunar Orbiter I began its extended mission on September 16 after completion of photographic readout. During this period non-photographic data was telemetered to Earth at regular, planned intervals. Flight controllers monitored the orbital behavior of the spacecraft, the micrometeroid detectors, and the condition of the power, attitude control, and communications subsystems.
By October 28 the condition of Lunar Orbiter I had deteriorated significantly. Scherer issued a status report which pointed out the following: 1) very little gas remained for attitude control (0.4 kilograms at 7 kilograms per square centimeter-100 psi.-pressure); 2) estimated stabilized life of spacecraft was two to five weeks; 3) the battery was losing power because of prolonged overheating, and if it fell below 15 volts, the onboard flight programmer would lose essential [244] parts of its memory, 4) the transponder was responding erractically, and the inertial reference unit was losing its ability to keep the spacecraft stable. The program manager and his staff realized that loss of control over communication transmission from Orbiter I could jeopardize the mission of the second Lunar Orbiter. They conferred with members of the Langley Lunar Orbiter Project Office who, in turn, decided to command the spacecraft to impact on the far side of the Moon during its 577th orbit on October 29. This maneuver, successfully executed, brought the first mission to an end.32