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DESTINATION MOON: A History of the Lunar Orbiter Program

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CHAPTER VII: BUILDING THE SPACECRAFT: PROBLEMS AND RESOLUTIONS

 

Experiments for Lunar Orbiter

 

 

 

[133] The Lunar Orbiter spacecraft was designed not only to take photographs but also to carry out three non-photographic experiments. A summary of these experiments will help to explain the direction of program thinking on scientific investigations of the lunar environment and show how the experiments presented problems for the total spacecraft configuration. The requirements of the Apollo Program and the weight limitations of the Agena rocket restricted the scientific payload of Lunar Orbiter to four experiments: photography, selenodesy, micrometeoroid, and radiation.

 

During the period in which the Request for Proposals was being prepared, the Office of Space Science through its Space Sciences Steering Committee evaluated the kinds of experiments which would be most useful to the scientific investigation of the Moon as well as to immediate NASA objectives. The major work of this evaluation fell to the Planetology Subcommittee.¹

 

[134] The Subcommittee narrowed the field of experiments to be included on Lunar Orbiter early in the program's history. It found that one indispensable experiment the program should conduct was the recording of selenodetic information by tracking the spacecraft. The spacecraft would carry a transponder which would provide range and range-rate data, a necessity for mission control. Analysis of the data would establish a profile of the spacecraft's orbital behavior over a thirty-day period and longer. At a meeting of the Planetology Subcommittee on September 24, 1963, Gordon MacDonald of the University of California at Los Angeles had explained to Lunar Orbiter Program officials why the data were scientifically valuable as well as indispensable for the safety of the spacecraft on the first and subsequent missions.

 

He stated that if the Orbiters were to be flown in a low elliptical orbit around the moon, it would be mandatory to track the spacecraft on the first mission and [135] determine its behavior by accurate measurements.² A selenodesy experiment which could record data for a period of at least sixty days at an altitude of 256 kilometers above the Moon on the first mission could sufficiently confirm the safety of putting subsequent Orbiters into orbits which would go as low as 32 kilometers above the Moon. Moreover, [136] the selenodetic data gained in sixty days would be invaluable for the first Apollo lunar mission.³

 

Since its inception on May 4, 1962, the Lunar Sciences Subcommittee's Working Group on Selenodesy had developed information on lunar gravity and mass.⁴ Originally the Group had provided major technical guidance for the Surveyor Orbiter Project at JPL. It made a timely contribution to Lunar Orbiter mission planning as a result of this earlier experience. The Group's chief concern was the design of the trajectory and orbits which the Lunar Orbiter would fly. Its work confirmed the limited extent of knowledge about the selenodetic environment and the potential hazards inherent in certain kinds of orbit designs. In its work it could little imagine the discovery in 1967 through the analysis of tracking data from Lunar Orbiter V of mass concentrations under the great maria of the Moon. The Working Group on Selenodesy provided MacDonald with a firm basis of fact for his argument that selenodetic data gathered by monitoring the Lunar Orbiter spacecraft in orbit would be very valuable for future orbital Moon missions.⁵

 

[137] A group led by William H. Michael at the Langley Research Center designed the Lunar Orbiter selenodesy experiment, and its efforts were richly rewarded by the' data acquired during the five Orbiter missions.⁶ Indeed, the selenodetic information that the program obtained substantially aided in extending the exploration of the lunar gravitational environment. When taken with the data from the five successfully landed Surveyors, these data provided the Office of Manned Space Flight very reliable, indispensable information for the Apollo Program.

 

In addition to selenodesy the Planetology Subcommittee selected two other fields of scientific investigation for experiments on the first five Lunar Orbiters which made up Block I of the program.⁷ These were radiation and micrometeoroid flux in near lunar environment. The two experiments which Langley developed for the Orbiter were designed to measure the performance of the spacecraft as well as to provide useful data on potential hazards to manned missions to the Moon.

 

[138] The radiation experiment was designed by Dr. Trutz Foelsche and had two objectives as outlined by him:

 

The Principal purpose of the lunar orbiter radiation-measuring systems was to monitor, In real time, the high radiation doses that would accumulate on the unprocessed film in case of major solar cosmic ray events. In this way It would be possible for the mission control to minimize the darkening of the film by operational maneuvers, such as stopping the photographic operation and acceleration of development of the film in the loopers, and in case of more penetrating events, shielding the film in the cassette by the spacecraft itself and by the moon. Furthermore, the independent measurement of radiation doses would contribute to the diagnosis of film failure due to other reasons.

 

A second purpose was to acquire a maximum amount of information on radiation on the way to the moon and near the moon, insofar as this could e achieved within the weight limitation of 2 pounds.⁸

 

The danger that the film could be damaged by solar radiation had Dr. Foelsche and Dr. Samuel Ketzoff worried because the Eastman Kodak photographic subsystem provided only aluminum shielding at two grams per square centimeter at the film cassette and at two tenths of a gram per square centimeter in the rest of the system. Foelsche desired thicker shielding, but the contractors maintained that the film would be safe. The amount of shielding was a calculated risk, trading shielding weight against the probabilities of solar flare intensities.

 

[139] Although he would have preferred to mount a more sophisticated experiment, Foelsche designed a measuring system to carry out the objectives described above., remaining within a one-kilogram weight limit. The system's sensors, their arrangement and shielding, the measuring principle and dynamic ranges were all developed at Langley. The Lunar Orbiter Project Office at Langley and the Boeing Company then determined the specifications for the hardware, and Texas Instruments built and calibrated the experiment.⁹

 

The micrometeoroid experiment was the last non-photographic experiment which the Planetology Subcommittee approved for the Block I Orbiters. Designed by Charles A. Gurtler and William H. Kinnard of Langley, it consisted of twenty detectors mounted around the middle deck of the spacecraft, outside the thermal blanket. Each detector consisted of a pressurized semicylinder with a pressure-sensitive microswitch inside. The cylindrical surface of the detector was 0.025 mm beryllium copper test material. Inside the semicylinder, gas pressure held the switch closed. When a puncture of the surface material occurred, gas would escape, opening the microswitch, which would register the puncture electrically. Whenever the condition of the...

 

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[140] (Chart)

 

SCHEMATIC OF LUNAR ORBITER DOSIMETER SYSTEM

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[141] ...detectors was telemetered to Earth, any new punctures would be indicated and previously indicated ones would be verified (see diagrams on following pages).¹⁰

 

Gurtler and Kinnard presented their experiment to the OSSA Space Science Committee on October 5, 1964. After reviewing it, the Committee pointed out that the instrumentation was omnidirectional and limited in the quantity of data it could acquire. The Committee requested Gurtler and Kinnard to examine the kinds of similar instrumentation which the Surveyor and the Mariner C spacecraft had and to ask W. Merle Alexander at the Goddard Space Flight Center in Greenbelt, Maryland for specific assistance in the further study of the experiment's requirements, since Alexander was the principal investigator for micrometeoroid instrumentation on these two spacecraft.¹¹

 

In the end, however, Gurtler and Kinnard's experiment was implemented in the form originally presented to the Committee. While the instrumentation could provide only limited data, it had the advantages of simplicity and freedom...

 

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[142] (GRAPHIC)

 

GEOMETRY OF METEOROID DETECTORS ON SPACECRAFT

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[143] MICROMETEOROID PUNCTURE RATES

 

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[144] (GRAPHIC)

 

PRESSURIZED-CELL DETECTOR

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[145] ...from ambiguity.

 

The photographic experiment, which constituted the major means of implementing the program's objectives, has been discussed previously and will be referred to during the course of this narrative as the need arises.

 

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