Considering now the question of spacecraft performance, Figure 7 shows the dates in 1968 when certain of these sites can be achieved with the 7° to 20° sun elevation. In generating the performance scan, a 95,000-pound spacecraft was assumed loaded with 37,500 pounds of SPS propellant of which 1% was retained for reserve. It is apparent that changing the magnitude of sun elevation would shift the days that a particular site could be reached. Increasing the range of sun elevation would increase opportunities and decreasing the range of sun elevation would obviously decrease opportunities. Figure 7 (for Pacific injections) and Figure 8 (for Atlantic injections) show that availability of a considerable number of potential sites generally clustered in three groups which are separated in longitude such that a 48-hour recycle back at earth launch could be accommodated as planning is shifted westerly from one group to the next.
It is apparent from Figures 7 and 8 that Pacific injections (with daylight launches) are available during most of the year. During the latter part of the year, Atlantic injections would probably be used, and night launches would be necessary. The distributions for 1967 and 1969 are not markedly different from 1968. Figures 9 and 10 illustrate the landing site accessibility for 1969.
Figures 11 and 12 illustrate this mission planning concept for typical launch dates in 1968 and 1969 to three of the candidate sites. These figures show how the earth, moon, and sun cooperate to give a reasonable mission concept allowing two 48-hour recycle times at the launch pad and near optimum lighting at the moon. The precise targeting points would not have to be determined until about 6 months before the mission.
Data for site selection starts with earth-based photography, radar, IR, and other observations, which, at this point in time, have lead to the selection of a relatively large number of contender sites for Apollo landing. This number will decrease as Orbiter and Surveyor data lead to conclusions as to which are the better sites in the three groups. By 6 months before the mission it would be highly desirable to have narrowed these sites down to one in the east, one in the central portion, and one in the west.
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